Symposium Organizers
Marco Fanciulli CNR-INFM MDM National Laboratory &
The University of Milano-Bicocca
John Martinis University of California-Santa Barbara
Mark Eriksson University of Wisconsin-Madison
J1: Semiconductors I
Session Chairs
Monday PM, December 01, 2008
Room 308 (Hynes)
9:30 AM - **J1.1
Spin Coherence and Spin Computation in Semiconductor Nanostructures.
S. Das Sarma 1
1 , University of Maryland, College Park , Maryland, United States
Show Abstract10:00 AM - J1.2
Solid State Quantum Memory using the 31P Nuclear Spin: Coherent Electron Spin Storage Beyond a Second.
John Morton 1 2 , Alexei Tyryshkin 3 , Richard Brown 1 , Shyam Shankar 3 , Brendon Lovett 1 , Arzhang Ardavan 2 , Thomas Schenkel 4 , Joel Ager 4 , Eugene Haller 4 5 , Stephen Lyon 3
1 Department of Materials, Oxford University, Oxford United Kingdom, 2 Clarendon Laboratory, Oxford University, Oxford United Kingdom, 3 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 4 , Lawrence Berkeley National Lab, Berkeley, California, United States, 5 Materials Science Department, UC Berkeley, Berkeley, California, United States
Show AbstractThe transfer of information between different physical forms is a central theme in communication and computation, for example between processing entities and memory. Nowhere is this more crucial than in quantum computation, where great effort must be taken to protect the integrity of a fragile quantum bit. Nuclear spins are known to benefit from long coherence times compared to electron spins, but are slow to manipulate and suffer from weak thermal polarisation. A powerful model for quantum computation is thus one in which electron spins are used for processing and readout while nuclear spins are used for storage [1]. Here we demonstrate the coherent transfer of a superposition state in an electron spin 'processing' qubit to a nuclear spin 'memory' qubit, using a combination of microwave and radiofrequency pulses applied to 31P donors in an isotopically pure 28Si crystal. The electron spin state can be stored in the nuclear spin on a timescale that is long compared with the electron decoherence time and then coherently transferred back to the electron spin, thus demonstrating the 31P nuclear spin as a solid-state quantum memory. We perform density matrix tomography on the original and recovered electron spin states and demonstrate a total memory fidelity of 90%, which we attribute to systematic imperfections in radiofrequency pulses which can be improved through the use of composite pulses [2]. We apply dynamic decoupling to protect the nuclear spin quantum memory element from sources of decoherence. The coherence lifetime of the quantum memory element is studied as a function of donor concentration and temperature, and is found to exceed one second at 5.5K.[1] BE Kane Nature 393, 133 (1998))[2] JJL Morton et al. Phys Rev Lett 95, 200501 (2005)
10:15 AM - **J1.3
Addressing the Charge and Spin of a Single Dopant Atom in a Nano MOSFET.
Sven Rogge 1
1 Kavli Institute of Nanoscience, Delft University of Technology, Delft Netherlands
Show Abstract10:45 AM - J1.4
Microwave Effects in a Silicon Single Donor Quantum Dot.
Enrico Prati 1 , Rossella Latempa 1 , Marco Fanciulli 1 2
1 Laboratorio Nazionale MDM, CNR-INFM, Agrate Brianza Italy, 2 Department of Material Science, Università degli Studi di Milano - Bicocca, Milano Italy
Show AbstractAn isolated, diffused dopant embedded in the Si inversion layer of the nanostructure acts as a quantum dot (Donor Quantum Dot, DQD) at cryogenic temperature. In dopant based quantum dots, like in lithographically defined and split gate quantum dots where the spatial confinement into a small channel MOSFET gives a discrete energy spectrum, the electronic states can be coupled by an electromagnetic field in order to realize qubits [1,2,3]. In DQD the electron spin qubits are based on Zeeman state doublets, coupled by microwave irradiation at the resonance frequency. Microwave irradiation should drive spin resonance and manipulation with pulses of appropriate duration as already observed in GaAs systems [4]. In Si the observation and the control of single electron spin resonance is still unaddressed because of the extremely small linewidth (expected to be less than one gauss) if compared to GaAs where the spin orbit effect broadens the line to tens of gauss. The quantum transport characterization of a single Donor Quantum Dot (DQD) under microwave irradiation is studied. Such a system consists of an isolated dopant lying in the channel of a silicon decanano flash memory. The dopant provides two well resolved energy levels below the conduction band edge measured as two distinct elastic tunneling dc current peaks [5]. The device is a decanano MOSFET with an impurity in the channel, characterized at 300 mK with and without microwave illumination in Coulomb-blockade condition. The microwave irradiation causes photon assisted tunneling as in ordinary lithographically defined and split gate defined quantum dots. Here we show that the microwave irradiation is able to produce photon assisted tunneling (the fluctuations of the energy levels into the leads acting as an electromotive force [6,7,8] combined with photoionization of the confined electrons [9] and/or electron state excitations in the leads due to single and multiple photon absorption [8]), and to perform a spectroscopy of the excited energy levels of the donor at sufficiently high power. Our measurements clarify the strength and the limits of the interaction of microwave irradiation with a single donor in a Si nanostructure in view of developing spin control of the electron in donor qubits.[1] B.E. Kane, Nature 393 133 (1998)[2] D. Loss and D.P. DiVincenzo, Phys. Rev. A 57, 120 (1998)[3] R. Vrijen, E. Yablonovitch, K. Wang, H. W. Jiang, A. Balandin, V. Roychowdhury, T. Mor, and D. DiVincenzo, Phys. Rev. A 62, 12306 (2000)[4] F. H. L. Koppens et al.:Nature 442 766 (2006)[5] H. Sellier et al. Phys. Rev. Lett. 97, 206805 (2006)[6] G. Ferrari, L. Fumagalli, M. Sampietro, E. Prati and M. Fanciulli, J. Appl. Phys. 98, 044505 (2005)[7] E. Prati, M. Fanciulli, A. Calderoni, G. Ferrari, M. Sampietro: J. Appl. Phys. 103, 104503 (2008)[8] L. P. Kouwenhoven et al.: Phys. Rev. B 50, 2019 (1994)[9] T. Fujisawa and S. Tarucha: Superl. and Microstr. 21, 247 (1997)
11:30 AM - **J1.5
Current Status and Future Prospects for QIP in Si:P MOS Architectures Incorporating Single Atom Spintronics.
Robert Clark 1
1 Australian Research Council Centre for Quantum Computer Technology, University of New South Wales, Sydney, New South Wales, Australia
Show AbstractThe Australian Research Council Centre for Quantum Computer Technology (CQCT: see www.qcaustralia.org for publications) has a major focus on single-atom electron spin qubit devices in silicon.A highly compact, fully-MOS Si:P single-spin qubit device architecture has been developed, in which the key features are a silicon radiofrequency single electron transistor [1],[2] (Si RF-SET) for single-shot qubit readout, via spin-dependent single-electron tunneling between an implanted single-P-atom [3],[4] and the induced-electron-reservoir formed by the RF-SET island, that also incorporates local ESR for qubit control.Characterisation of the Si MOS qubit devices, and related Si MOS single-atom resonant tunneling diode structures will be reported, as will measurements of the Si:P electron spin coherence times at mK temperatures. The incorporation of Si:P qubits into a fault-tolerant, bi-linear architecture [5] involving the coherent transport (by adiabatic passage) of electron spin via P-atom arrays [6] will also be described, together with proving experiments in this regime.This project involves collaboration between a number of CQCT researchers at the Universities of New South Wales and Melbourne (see also related MRS 2008 abstract by D.N.Jamieson). The work is supported by the Australian Research Council (Centres of Excellence scheme), the Australian Government, and the US National Security Agency and Army Research Office under contract number W911NF-04-1-0290.[1] S.J.Angus, A.J.Ferguson, A.S.Dzurak and R.G.Clark, Nano Letters 7, 2051 (2007).[2] S.J.Angus, A.J.Ferguson, A.S.Dzurak and R.G.Clark, Applied Physics Letters 92, 112103 (2008).[3] D.N.Jamieson, C.Yang, T.Hopf, S.M.Hearne, C.I.Pakes, S.Prawer, M.Mitic, E.Gauja, S.E.Andresen, F.E.Hudson, A.S.Dzurak and R.G.Clark, Applied Physics Letters 86, 202101 (2005).[4] S.E.Andresen, R.Brenner, C.J.Wellard, C.Yang, T.Hopf, C.Escott, R.G.Clark, A.S.Dzurak, D.N.Jamieson and L.C.L. Hollenberg, Nano Letters 7, 2000 (2007).[5] L.C.L.Hollenberg, A.D.Greentree, A.G.Fowler and C.J.Wellard, Physical Review B 74, 045311 (2006).[6] A.D.Greentree, J.H.Cole, A.R.Hamilton and L.C.L. Hollenberg, Physical Review B 70, 235317 (2004).
12:00 PM - J1.6
Hyperpolarization of Phosphorus Donors in Silicon.
Dane McCamey 1 , Johan van Tol 2 , Gavin Morley 3 , Christoph Boehme 1
1 Department of Physics, University of Utah, Salt Lake City, Utah, United States, 2 Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory at Florida State University, Tallahassee, Florida, United States, 3 London Centre for Nanotechnology, University College London, London United Kingdom
Show AbstractPhosphorus doped crystalline silicon (Si:P) is a model system for investigating spin effects in the solid state and at the same time is a point defect with great technological importance. Si:P has been used since the beginning of the semiconductor industry in the early 1950's for applications ranging from the ubiquitous (MOSFETs) to the conceptual (single electron transistors). The ability to hyperpolarize the spins in this material is important for a number of its applications. Utilizing the nuclear spin of phosphorus donors as quantum bits [1,2] relies on the ability to obtain a well characterized initial state [3], which can be obtained by hyperpolarization.Here, we experimentally demonstrate a method for obtaining nuclear spin hyper-antipolarization, that is, antipolarization significantly in excess of that expected for a thermal equilibrium [4]. By exploiting a modified Overhauser process, we obtain more than 68% nuclear antipolarization of phosphorus donors in silicon. This polarization is reached with a time constant of ~150 seconds, at a temperature of 1.37 K and a magnetic field of 8.5 T. Unlike dynamic nuclear polarization, the scheme does not require the application of resonant microwaves fields, but only that hot carriers be introduced, which we achieve using white light photoexcitation. We verify the polarization using both conventional and electrically-detected magnetic resonance [5]. The polarization as a function of various experimental parameters are found to agree with a model for this effect, which predicts polarization of near 100% should be achievable, which we will discuss.Finally, we note that this effect may have applications outside the QIP community, for example, in magnetic resonance imaging [6].[1] Kane, B. E. A silicon-based nuclear spin quantum computer. Nature, 393, 133 (1998). [2] Morton, J. J. L. et al. Solid state quantum memory using the 31P nuclear spin. arXiv:0803.2021v1 (2008).[3] DiVincenzo, D. P. The Physical Implementation of Quantum Computation. Fortschritte der Physik, 48, 771 (2000).[4] McCamey, D. R. et al. Fast nuclear spin hyperpolarization of phosphorus in silicon. arXiv:0806.3429v1 (2008).[5] McCamey, D. R. et al. Spin-dependent processes at the crystalline Si-SiO2 interface at high magnetic fields. Phys. Rev. B (in press) (2008) arXiv:0802.0230v2.[6] Dementyev, A. E. et al. Dynamic Nuclear Polarization in Silicon Microparticles. Phys. Rev. Lett., 100, 127601 (2008).
12:15 PM - J1.7
Quantum Confinement and Role of Dimension in Silicon Nanostructures Doped with P: a Parameter-Free Calculation.
Alberto Debernardi 1 , Marco Fanciulli 1 2
1 , Laboratorio Nazionale MDM, CNR-INFM, Agrate Brianza (MI) Italy, 2 Scienze dei Materiali, Universita' di Milano Bicocca, Milano Italy
Show AbstractBy means of the effective mass approximation we have computed without any adjustable parameter, the electronic states of substitutional donors in silicon nanostructures. In order to obtain the correct ground state energy, we include in the computational scheme used [1] the inhomogeneous band structure, the valley-orbit coupling in a non-perturbative way, and the scattering effects of impurity core states.Studying the energy levels and hyperfine splitting of a P impurity in silicon spherical nanocrystals as a function of the nanocrystal radius we found that the confinement effects produce an enhancement of the energy levels and of the hyperfine splitting (a quantity of paramount importance for quantum computation) as the crystal radius is decreased.Our results are in good agreement with available experimental data taken from literature. We discuss the effect due to the dimensionality by studying the quantum confinement in quantum dots and quantum wires, and we investigate how the shape and the size of the nanostructures can affect the energy levels of shallow states and the related hyperfine coupling between electron and nuclear spins.It has been predicted [1] that the ground state of P-doped bulk silicon in uniform electric field slightly decreases with increasing electric field up to a critical field of 2.5 MV/m, at which the donor is ionized by tunneling.We investigate how this situation is modified when the shallow electrons are confided by a potential barrier, by computing the Stark effect in Si:P dots of different radius and in other Si-based nanostructures of reduced dimensionality.[1] A. Debernardi, A.Baldereschi, and M.Fanciulli, "Computation of the Stark effect in P impurity states in silicon", Phys. Rev. B, 74, 035202 (2006)
12:30 PM - **J1.8
Valley Interference Effects on Bound and Gated Electrons in Silicon.
Belita Koiller 1
1 Physics Institute, Universidade Federal do Rio de Janeiro, Rio de Janeiro Brazil
Show AbstractJ2: Semiconductors II
Session Chairs
Monday PM, December 01, 2008
Room 308 (Hynes)
2:30 PM - **J2.1
Spin Qubits in Quantum Dots.
Daniel Loss 1
1 Department of Physics, University of Basel, Basel Switzerland
Show AbstractElectron spins in quantum dots are promising candidates for qubits and scalable quantum computers in solid state systems. I will address a few important physics aspects in this field, the most important one being the decoherence problem due to the interaction of spins with the surrounding environment. The dominant sources are typically spin-orbit induced spin-phonon couplings and the interaction of nuclear spins with electron spins via the hyperfine interaction. The resulting spin dynamics and associated decoherence are rather complex quantum phenomena, quite often dominated by non-Markovian behavior. I will give a brief overview of various spin candidate systems, such as electrons or holes in GaAs, nanowires, nanotubes, graphene, etc.
3:00 PM - J2.2
Millisecond Spin Coherence for Fe3+ in ZnO: A Promising Qubit.
Jerome Tribollet 1 , Jan Behrends 1 , Klaus Lips 1
1 Silicon Photovoltaics, Helmholtz Centre Berlin for Materials and Energy, Berlin Germany
Show AbstractOver the last decades, a variety of quantum systems have been proposed as possible quantum bits (qubits) for quantum computers (QC), like trapped ions and atoms, superconducting devices, electron and nuclear spins of molecules or electron and nuclear spins in solid state materials. Among the last class of qubits, spins in silicon (Si), are believed to be the most promising qubits for large scale quantum information processing due to the fact that silicon has exceptional spin properties and the most advanced technology. Pure silicon can be prepared practically nuclear spin free, thereby eliminating superhyperfine interactions between the electron spins and the host matrix. The small spin-orbit interaction of silicon reduces the coupling between the spin degree of freedom and the vibrations of the host crystal lattice. These two properties provide a very quiet environment for electron spins shown to result in spin coherence times of phosphorous donor electrons in the millisecond range in isotopically purified Si at low temperature. The main drawback of Si is its indirect band gap which limits the use of optical methods for single spin qubit readout.In search for new materials for scalable spin based QC concepts, Zinc Oxide (ZnO) can be an interesting alternative. ZnO provides the same quiet environment for spin qubits as Si, however, combined with superior optical properties. The superhyperfine interactions can be easily eliminated in ZnO since nuclear spin-free isotopes of Zn are highly abundant (96%). In addition, the technological relevance of ZnO has strongly increased during which induced a rapid improvement of its growth and doping methods within the past few years. The fact that ZnO has a direct band gap suggests that the combination of optical and magnetic excitation could allow more elaborate schemes for spin preparation, manipulation and read-out. Another advantage of ZnO results from the spin properties of transitions metal ions (TMI) with spin S >1/2 which allows to encode each qubit on a multilevel spin system, due to the zero field splitting created by the crystal field in wurtzite ZnO. Such a high spin system can be relevant for quantum information processing due to the large number of auxiliary spin states available for manipulations or encoding.In this presentation, we demonstrate experimentally by means of pulsed electron paramagnetic resonance (EPR) that traces of TMI in natural ZnO (here: Fe3+ ions) can have spin coherence times, T2, exceeding 100 microseconds at low temperature. Through a temperature dependent study of their relaxation properties, we predict for highly diluted TMI in chemically and isotopically purified ZnO crystals spin coherence times similar to those reported for phosphorous in purified Si. We show that for Fe3+ the spin coherence at low temperature is limited by direct spin-lattice relaxation which is about 1.4 ms at T = 6 K. Implication of these results for quantum computing will be discussed.
3:15 PM - J2.3
Electrical Detection of the Bell States of Phosphorus in Silicon.
Hiroki Morishita 1 , Hirotaka Tanaka 2 , Kouichi Semba 2 , Leonid Vlasenko 3 , Kentarou Sawano 4 , Yasuhiro Shiraki 4 , Mikio Eto 1 , Kohei Itoh 1
1 School of Fundamental Science and Technology, Keio Univ., Yokohama Japan, 2 , NTT Basic Research Laboratories,NTT, Atsugi Japan, 3 , A. F. Ioffe Physico-Technical Institute of Russian Academy of Sciences, St Petersburg Russian Federation, 4 Advanced Research Laboratories, Musashi Institute of Technology, Tokyo Japan
Show AbstractWe demonstrate electrical detection of the Bell states formed by phosphorus electron and nuclear spins embedded in silicon bulk single crystals. The Hamiltonian of the phosphorus in silicon is composed as the sum of the 1) electronic Zeeman, 2) nuclear Zeeman, and 3) hyperfine interaction terms. Extensive studies on Si:P have been performed recently using the conventional electron spin resonance (ESR) [1,2] and electrically detected magnetic resonance (EDMR) [3,4] with moderately high magnetic fields that realize four well defined states |++>, |+->, |-+>, and |--> where left and right symbols in each ket correspond to the states of the electron spin and nuclear spin, respectively. However, it is well known that in the limit of the low magnetic field, especially below 200G as shown in this study, the hyperfine term dominates the Hamiltonian and the four states become |++>, a |+->+b |-+>, -b|+->+a|-+>, and |-->, i. e., two of them being well-known Bell states. The present study succeeded in the EDMR spectroscopy of ensembles of phosphorus in silicon under B<200G observing the transitions between these four states. The direct comparison of the experimentally observed peak positions and theory in the plot of the magnetic field vs. irradiated microwave frequency shows excellent quantitative agreement between our experiment and theory confirming the involvement of the two Bell states. The coefficients “a” and “b” of the Bell states change with the applied magnetic field B and become a≈b as B→0. The microwave power dependence of the EDMR signal intensity is qualitatively similar to the recent result for high B [5]. This work was supported in part by Grant-in-Aid for Scientific Research by MEXT, Specially Promoted Research #18001002 and in part by Special Coordination Funds for Promoting Science and Technology.[1]E. Abe et al, cond-mat/0512404.[2]A. M. Tyryshkin et al, J. Phys. Cond. Matt., vol. 18, S783 (2006).[3]A. R. Stegner et al., Nature Physics, vol. 2, 835 (2006).[4]H. Huebl et al, Phys. Rev. Lett., vol. 100, 177602 (2008).[5]B. Stich et al, J. Appl. Phys., vol. 77. 1546 (1994).
3:30 PM - **J2.4
Electric Field Noise in Cryogenic Microfabricated Ion Traps.
I. Chuang 1 2 , J. Labaziewicz 1 , Y. Ge 1 , D. Leibrandt 1 , P. Antohi 1 , S. Wang 1 , R. Shewmon 1 , K. Brown 1
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 EECS, MIT, Cambridge, Massachusetts, United States
Show Abstract4:30 PM - **J2.5
Readout and Initialization Schemes of 31P Nuclear Spin Qubits in Silicon.
Christoph Boehme 1
1 Department of Physics, University of Utah, Salt Lake City, Utah, United States
Show AbstractMore than a decade after the proposal of Kane’s silicon quantum computer [1] phosphorous (31P) nuclear spin qubits are still considered to be among the most coherent spin qubits in a solid state environment. In spite of this, there has been limited progress on the utilization of this property for quantum information applications, mainly because there has still not been the demonstration of a single 31P readout, inhibiting the pursuit of experimental research on controlled qubit interactions, initialization, and single qubit operations which require the availability of the former. Our ideas for possible readout schemes evolved from the investigation of spin-dependent electronic transitions in silicon with pulsed electrically detected magnetic resonance spectroscopy that was developed for materials defect spectroscopy. The realization that coherent electron spin motion can govern electric currents [2] lead to the proposal of a recombination readout for 31P utilizing hyperfine controlled spin-dependent recombination of 31P donor electrons as spin to charge-conversion mechanism [3]. When we demonstrated this approach experimentally using silicon dangling bonds at the silicon to silicon dioxide interface as probe spins [4], it became clear that strong limitations on the readout and therefore the qubit coherence times exist due to the inability to manipulate the geometry between qubits and probe spins. In order to overcome this problem we have therefore investigated electric readout schemes based on a spin trap mechanism which encodes the electron donor spin state into a current by means of spin-dependent trapping and reemission of free charge carriers. Initial experiments demonstrate that this approach allows very long readout times and therefore qubit coherence times and they suggest that [5] these times are electrically controllable. Finally, I will briefly show the experimental demonstration of a fast and simple initialization scheme for 31P nuclear spin qubits in silicon. Using a magnetic field induced quenching of a non-equilibrium Overhauser relaxation time with intermediate magnetic fields (B0 ≈ 8.5T) it is possible to pump significant 31P nuclear hyper-antipolarization [6] without the utilization of dynamic nuclear polarization or other magnetic resonance induced pumping processes.[1] B. E. Kane, Nature, 393, 133 (1998).[2] C. Boehme, K. Lips, Phys. Rev. Lett., 91 (24), 246603 (2003).[3] C. Boehme, K. Lips, Phys. Stat. Sol.(b), 233 (3), 427 (2002).[4] A. R. Stegner, C. Boehme, H. Huebl, M. Stutzmann, K. Lips, M. S. Brandt, Nature Physics 2, 835 (2006).[5] G. W. Morley, D. R. McCamey, H. A. Seipel, L.-C. Brunel, J. van Tol, C. Boehme, http://arxiv.org/abs/0806.3431 (2008).[6] D. R. McCamey, J. van Tol, G. W. Morley and C. Boehme, http://arxiv.org/abs/0806.3429 (2008).
5:15 PM - **J2.7
Arrays of Single Atoms in Silicon by Controlled Ion Implantation for Quantum Computer Components.
David Jamieson 1
1 School of Physics, University of Melbourne, Parkville, Victoria, Australia
Show AbstractThe challenge of building the two most essential components of a solid state quantum computer: a qubit and a qubit transport mechanism are addressed here by use of controlled single ion implantation. These components are part of a comprehensive 2D scalable architecture which employs single phosphorus atoms in silicon, Si:P, which can act as charge or spin qubits together with arrays of single P atoms for the Charge Transport by Adiabatic Passage (CTAP) protocol. CTAP employs adiabatic tunnelling to transport donor electron spin qubits along arrays of charged P donors to interaction sites. Construction of these components requires placing single phosphorus donor atoms into silicon with sub-20 nm precision. The architecture also includes MOS gate-induced 2DEG reservoirs for spin-dependent tunneling, together with local electron spin resonance and a silicon single electron transistor detector for spin readout. The process flow for the construction of these ancillary components is compatible with controlled single ion implantation which employs on-chip detector electrodes that produce signals sensitive to the electron-hole pairs induced by a single ion impact. To implant P atoms at the required depth below a 5 nm thick pre-existing, high temperature, low interface trap density gate oxide requires sub-14 keV P implant energy and the detection of a charge transient of less than 1,000 electron-hole pairs. Ion straggling of 14 keV P ions limits the implant placement precision to about 10 nm with higher precision for lower energy and thinner oxides. Localisation of the implanted atom is by either an electron beam lithography defined PMMA mask with a 15 nm diameter aperture, or a scanned nano-stencil incorporating a nanoaperture machined by a focused ion beam system and associated alignment markers. One to four atom devices fabricated in this way have exhibited a rich array of phenomena consistent with controlled single electron transfers between the implanted donors. CTAP requires a device with three or more P atoms in a row spaced with sub-15 nm precision. Ion beam lithography with the nanostencil has demonstrated this capability to a precision of better than 30±10 nm, with higher precision possible in the near future. These results, together with the foreseeable precision improvements and theoretical models, suggest that controlled ion implantation is a viable strategy to build proof-of-principle components of solid state quantum computers using technologies readily available in the early 21st Century.This work is a collaboration between workers at the Universities of Melbourne and New South Wales within the Australian Research Council Centre of Excellence for Quantum Computer Technology. The work is supported by the Australian Research Council, the Australian Defence Science and Technology Organisation and the USA Army Research Office under contract number under contract number W911NF-04-1-0290.
J3: Poster Session
Session Chairs
Tuesday AM, December 02, 2008
Exhibition Hall D (Hynes)
9:00 PM - J3.1
Electronic Transport in Suspended Graphene Devices.
Britt Baugher 1 , Fangfei Shen 1
1 Physics, MIT, Cambridge, Massachusetts, United States
Show AbstractGraphene’s linear dispersion relation and theoretically ballistic transport of Dirac-like particles makes it an ideal system for the study of exotic quantum phenomena. Some of these phenomena, most notably the half-integer quantum Hall effect, have already been well vetted. Others, such as Klein tunneling and Veselago lensing, have proved more elusive. Current graphene fabrication procedures involve wet etching as well as the presence of an imperfect substrate. Such techniques and conditions result in samples with hindered mobility due to the presence of nearby impurities, either in the substrate or as wet processing residue. It has been shown that suspending graphene above the substrate and annealing it can dramatically improve mobility, opening up a new regime to graphene research. We will present results from measurements on our high mobility suspended graphene devices.
9:00 PM - J3.10
Electronic Transport Through Narrow Barriers in Graphene.
Leonardo Campos 1 , Andrew Gamalski 1 , Pablo Jarillo-Herrero 1
1 Physics, MIT, Boston, Massachusetts, United States
Show AbstractGraphene is a single layer of carbon atoms arranged as a (2D) honeycomb lattice. It has received considerable attention by the scientific community for its remarkable properties such as mean free path in excess of 1 micrometer (in bulk flakes), the possibility of tuning the charge density and the linear dispersion relation. The long mean free path and the long phase breaking length (at low temperature) in combination with the characteristic graphene dispersion relation allow the possibility of testing exotic quantum mechanical properties in ballistic conductors. One of these unusual properties, predicted to occur for Dirac massless particles, is Klein tunneling: particles can tunnel through high barrier with probability 1. Although experimental claims of observations of the Klein tunneling have been made, there is still significant disagreement with theory, due to the diffusive nature of electronic transport in most realistic sample geometries. We propose to test this effect by using ultra-narrow barriers, so as to be in the ballistic regime, required for an unambiguous observation of Klein tunneling.
9:00 PM - J3.11
Structure and Magnetic Induced Anisotropy in Zn1-xMnxSe.
Mario Saenger 1 , Michael Hetterich 2 , Xinyu Liu 3 , Jacek Furdyna 3 , Tino Hofmann 1 , Ralph Skomski 4 , David Sellmyer 4 , Mathias Schubert 1
1 Department of Electrical Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska - Lincoln, Lincoln, Nebraska, United States, 2 Institut für Angewandte Physik and Center for Functional Nanostructures (CFN), Universität Karlsruhe (TH), Karlsruhe Germany, 3 Department of Physics, University of Notre Dame, Notre Dame, Indiana, United States, 4 Department of Physics, University of Nebraska - Lincoln, Lincoln, Nebraska, United States
Show AbstractZnSe has attracted increasing interest due to the possibility of combining semiconducting properties with spin functionality by alloying ZnSe with Mn ions. Generalized spectroscopic ellipsometry in the near infrared to ultraviolet spectral range (1 to 3.4 eV) is used to study the optical properties of Zn
1-xMn
xSe (0 < x < 0.28) thin films deposited by molecular beam epitaxy (MBE) onto (001) GaAs. We observe an intrinsic uniaxial optical anisotropy in all samples containing manganese. The anisotropy has a characteristic photon energy dependence, and we determine from our ellipsometry analysis ordinary and extraordinary dielectric functions, i.e., for polarizations perpendicular and parallel to the optical axis, respectively. Upon application of standard critical point lineshape models, both functions reveal different bandgap energies and suggest valence band splitting. Our investigations suggest that this birefringence is common in MBE grown Zn
1-xMn
xSe, and we propose spontaneous atomic ordering as its origin. We also employ magnetooptic generalized ellipsometry in a spectral range covering the fundamental bandgap transition energy (1.8 to 3 eV) to investigate the magnetic field induced anisotropy in dependence of the Mn concentration at room temperature. We present a dielectric function tensor model expanded from our previous work [1], which accounts for the dielectric and for the magnetic field induced chiral anisotropy. By virtue of this model, the effective Mn concentration parameter is identified means the magnetooptic induced birefringence and, can be measured in dependance of the Mn concentration at room temperature. Research funded by NSF in MRSEC QSPIN at the University of Nebraska-Lincoln.
[1] J. Kvietkova, B. Daniel, M. Hetterich, M. Schubert, D. Spemann, D. Litvinov, and D. Gerthsen, Phys. Rev. B, 70, 045316, (2004).
9:00 PM - J3.2
Concentration Dependence of Inter-dot Coupling in CdSe Quantum Dot Assembly.
Akira Sugimura 1 2 , Wei Lu 2 , Kohei Tai 1 , Ikuro Umezu 1 2
1 Department of Physics, Konan University, Kobe Japan, 2 Quantum Nanotechnology Laboratory, Konan University, Kobe Japan
Show Abstract9:00 PM - J3.3
Single-Photon Emission from Single Nanostructures Consisting of Fluorescent Molecules.
Sadahiro Masuo 1 2 , Akito Masuhara 3 , Mai Muranushi 1 , Takumi Murakami 1 , Yoshihisa Matsuda 3 , Shinjiro Machida 1 , Hitoshi Kasai 3 2 , Hidetoshi Oikawa 3 , Akira Itaya 1
1 Department of Macromolecular Science and Engineering, Kyoto Institute of Technology, Kyoto Japan, 2 , PRESTO-JST, Kawaguchi Japan, 3 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai Japan
Show AbstractThe advent of photon-based quantum cryptography and quantum information processing is increasing the need for such light sources that produce individual single photons. It is of particular importance that these single photons are produced in as controlled a manner as possible. Generally, single-photon emission has been observed from so-called “single-quantum system” such as single fluorescent molecules, single quantum dots, single atom and ion, and so on. In addition to single quantum systems, even multi-quantum systems have been induced to exhibit single-photon emission by controlling their size on the nanometer scale, as has been demonstrated with isolated multichromophoric dendrimers. In this work, we demonstrate the emission from single nanostructures consisting of fluorescent molecules exhibit single-photon behavior. That is, even molecular assemblies can also be made to behave as single-photon sources by controlling the size at nanometer scale. As fluorescent molecules, perylenediimide derivatives and conjugated polymers (poly[2-methoxy,5-(2’-ethylhexyloxy)-p-phenylene-vinylene]:MEH-PPV) with different molecular weights were used. Their nanostructures were prepared by the reprecipitation method. The size of the prepared nanostructures was estimated from a SEM measurement. The emission properties of individual nanostructures were measured using a Hanbury-Brown and Twiss type photon correlation set-up in combination with pulsed laser excitation. By combining the photon correlation measurement with the excitation laser-triggered time-correlated single photon counting technique, the time traces of the emission intensity and lifetime, and the photon correlation were measured simultaneously.From SEM images, the size of the nanostructures was ~ 70 nm for perylenediimide derivatives, and ~ 20 nm for MEH-PPVs. In the case of MEH-PPVs, the size of nanostructures was controlled using MEH-PPVs with different molecular weight. The single nanostructures on a substrate were measured by above set-up. As results, it was revealed that most of the single nanostructures exhibit single-photon behavior even when more than one exciton is generated by an intense single excitation pulse. In the case of MEH-PPVs, it was observed that probability of single-photon emission increases with decreasing the size of nanostructures. The present results indicate that molecular assemblies can also be considered as candidates for single-photon sources.
9:00 PM - J3.4
Photon Storage in Optically Driven Color Centers in Diamond.
Jin Hui Wu 1 , Maurizio Artoni 2 3 , Giuseppe La Rocca 4
1 College of Physics, Jilin University, Changchung China, 2 , European Laboratory for Non Linear Spectroscopy, Sesto Fiorentino (Firenze) Italy, 3 , School of Engineering, University of Brescia, Brescia Italy, 4 , Scuola Normale Superiore, Pisa Italy
Show AbstractAn ever increasing effort has been devoted over the years to develop techniques for manipulating light in optical devices. Electromagnetic induced transparency (EIT) is one of these techniques that has recently led to an astonishing control on light wave propagation in ultracold clouds of alkali atoms. EIT may be employed to realize a photonic band gaps that are controllable through the parameters of an external standing light wave pattern [1]. Such gaseous approaches are not however suitable for on-chip implementation. Early work on control over photonic band-gap comprises, e.g., structures built from the periodic complex susceptibility of quantum well excitons whose optical properties can be dynamically modified through the Stark effect. Other interesting proposals to control photonic band-gaps in semiconductor heterostructures have been brought forward and where control over the band-gap is achieved through EIT in conduction intersubband transitions of a n-doped quantum well. EIT effects have also been observed in a class of solid materials exhibiting defect states, following either familiar or less familiar schemes, and among which presodimium doped Y2SiO5 and diamond containing nitrogen vacancies (N-V) color centers are perhaps the most ubiquitous ones. Color centers in diamond, in particular, have attracted over the past few years a renewed interest for their potential as single-photon sources and are attractive qubit candidates as they behaves a bit like an atom trapped in the diamond lattice. These centers can have extremely long-lived spin coherence because the diamond lattice is composed primarily of 12C, which has zero nuclear spin. In addition, N-V color centers also have interesting optical properties as they exhibit a configuration with two ground state levels connected to a common excited state by optical transitions of moderate strength leading to a lambda-type level configuration required for the observation of EIT [2,3]. This has been exploited to devise a novel photonic band-gap mechanism [4]. We here study the propagation of a very week optical pulse in the band-gap region of N-V diamond crystals. Our calculations show that adopting realistic parameters, as taken from recent experiments on coherent population trapping in N-V color centers, nearly complete reflectivities can be attained in a mm long diamond sample. This occurs when most probe frequency components lie inside the band-gap, yielding instead controllable loss and distortion as the incident probe pulse falls outside the gap. The relevant photonic band-gap may be all optically controlled while its well developed structure is seen to arise from the reduced values of residual absorption in the EIT region.[1] M. Artoni et al., Phys. Rev. Lett. 96, 073905 (2006).[2] C. Wei et al., Phys. Rev. A 60, 2540 (1999). [3] P. Hemmer, et al.,Opt. Lett. 26, 361 (2001).[4] Q.-Y. He, et al., Phys. Rev. B 73, 195124 (2006).
9:00 PM - J3.5
Laser Scattering: a Fast, Sensitive, In-Line Technique for Quantum Dots Process Characterization and Monitoring.
Emmanuel Nolot 1 , Joël Dufourcq 1 , Sylvie Favier 1
1 , CEA-LETI-MINATEC, Grenoble France
Show AbstractIn-line characterization and monitoring of quantum dots process, which consist in fast full-wafer mapping measurement of both the size and the surface density of nanodots, still remain a challenge.In order to compare different in-line characterization techniques, we used a set of 200mm wafers, with silicon nanodots grown by Low Pressure Chemical Vapor Deposition (LPCVD) on top of a 5nm-thick thermal SiO2 tunnel oxide. Process conditions were tuned to vary both the dots size in the 5-9nm range and the dots density in the 3.1011-1.1012 dots/cm2 range.Then, the wafers were measured with variable angle spectroscopic ellipsometry (VASE) on a Woollam M2000 tool, X-ray reflectometry (XRR) on a Jordan Valley JVX5200 tool and laser scattering on a KLA Tencor SP2UV tool. Lastly, different areas of the wafers were measured by high-resolution scanning electron microscopy (SEM) that has been considered the reference technique to get both size and surface density. SEM is of course unable to fulfill the requirement of a fast full-wafer mapping measurement.Laser scattering on unpatterned wafers is a common technique, mainly used in semiconductor fabs for optical wafer inspection (i.e. defectivity). Previously, the low frequency part of the scattered light (called ``haze'') was viewed mainly as a noise source that limits the particle detection sensitivity. However, the haze signal of SP2UV tool contains enough information to improve in-line surface characterization and to give, in less than one minute for a full-wafer mapping, very encouraging results. As far as dots process is concerned, haze signal is sensitive enough to detect small changes in dots size and/or density, between two wafers and within a specific wafer.This paper will present the results obtained with VASE, XRR and laser scattering techniques on the set of wafers.
9:00 PM - J3.6
Optical Signatures of Coherent Couplings in Coupled Quantum Dots.
Avinash Kolli 1
1 Department of Materials, University of Oxford, Oxford United Kingdom
Show AbstractQuantum dots are a promising candidate host system for the storage and manipulation of quantum information. This information may be encoded in the spin state of a confined electron within a quantum dot, or as the presence or absence of a electron-hole pair (an exciton). A spin encoding benefits from weak couplings to the environment and so long decoherence times. However, such weak coupling is also observed between the spin-qubits themselves, thus leading to slow gate operation times. Excitonic qubits benefit from strong coupling to optical fields enabling fast manipulation, however they also couple strongly to their environment, which quickly decoheres the quantum information. We therefore see that neither encoding on its own is suitable for a scalable quantum computation architecture. Hybrid schemes have therefore emerged that propose the encoding of information within the spin, but then use spin-selectively created excitons to mediate interactions.A crucial component of many of these proposals is strong coherent interactions between the excitons. Theoretical studies have predicted both electron/hole tunnel coupling, and also dipole-dipole interactions of excitons. Although there have been some early experimental results to suggest such coherent coupling, they have not been conclusive. In this work we aim to predict signatures of such couplings in the photoluminescence spectra and the photon statistics of the light emitted from these systems.We first consider a system driven by a common non-resonant laser field. We assume that we are able to apply a static electric field to our system, thus enabling us to Stark shift the creation energies of the two bare excitons. We are able to calculate analytically the spectra and correlation functions using the Quantum Regression Theorem. From our results, a simple signature for coupling emerges - if we look at the cross-correlation between different lines in the spectra we observe photon bunching and anti-bunching effects, a clear sign of a coupled quantum system. However, such a scenario does not yield any information about the form of the coupling, most importantly if indeed the coupling is a coherent process. This is a result of the non-resonant nature of the driving. We therefore expand our study to include a resonant driving of the system. We numerically analyze the system and present plots for both photoluminescence spectra and the auto- and cross-correlation functions.
9:00 PM - J3.7
Control of Fano Coupling in Semiconductor Quantum Wells by Tuning the Density of Neighboring Extended Wannier-Stark States.
Jongseok Lim 1 , Woo-Ram Lee 1 , Heung-Sun Sim 1 , Richard Averitt 2 , Joshua Zide 3 , Arthur Gossard 4 , Jaewook Ahn 1
1 Department of Physics, KAIST, Daejeon Korea (the Republic of), 2 Department of Physics, Boston University, Boston, Massachusetts, United States, 3 Electrical and Computer Engineering, University of Delaware, Newark, Delaware, United States, 4 Materials Department, University of California , Santa Barbara, California, United States
Show AbstractThis paper reports a study of excitonic Fano resonance in GaAs/AlGaAs superlattice where the resonance is tuned not only by the external electric field but also by the density of extended Wannier-Stark states. While Fano Resonance (FR) has been reported ubiquitously in a variety of quantum mechanical systems including atoms, molecules, semiconductors and even in optical resonators, a direct control of Fano coupling, thus a controllable FR, has been very recently performed with a semiconductor system. Controllable FR has a certain similarity to the optical coherent control experiments for an atomic system where the interference between resonant and non-resonant transitions of atomic energy levels are controlled by spectrally encoded broadband laser pulses. In semiconductor quantum wells, resonant exciton transitions generate a FR via the interference between the two channels, a discrete state of one subband of an electron-hole pair and energetically degenerate exciton continua pertaining to other subbands. In semiconductors, if the densities of the continuum energy states are tunable, a similar but new kind of coherent control of optical transitions, in conjunction with the interference in FR, may be possible.For our study, we have used an MBE-grown GaAs/Al0.3Ga0.7As superlattice with 35 GaAs quantum wells of 9.7-nm thickness (34 mono-layers) separated by 1.7-nm thick (6 mono-layers) Al0.3Ga0.7As barriers, clad on both side by 250-nm Al0.3Ga0.7As buffer layers. Photo-reflectance measurements are carried out at a temperature of 4 K using a Fourier Transform Infrared Spectrometer. We have characterized the resonance appearing in photo-reflectance spectra as the Stark field varies and found an anomalous behavior. When Fano resonance of an exciton state with neighboring extended Wannier-Stark states is investigated, between the resonant couplings with the next nearest neighboring WS state and with the nearest neighboring WS state, the exciton state resonantly sweeps through the energy interval, having the minimal coupling somewhere in between. The effective density of states felt by the exciton states as a function of bias electric field shows a parabolic behavior, which has not been carefully considered in previous FR study. By extending the original Fano formula to include the contribution of the energy dependent density of state (DOS), we obtain the bare coupling parameter Γo with which the decreasing behavior of Fano coupling without the DOS contribution is then recovered. Implications of these findings to the development of abilities in semiconductor quantum device for tunable resonance and engineered interference are to be addressed.
9:00 PM - J3.8
Spectroscopic Study of InAs/InP Quantum Wires.
Vitaliy Dorogan 1 , Yu. Mazur 1 , O. Bierwagen 2 , G. Tarasov 3 , E. DeCuir 4 , S. Noda 1 , Z. Zhuchenko 3 , M. Manasreh 4 , W. Masselink 2 , G. Salamo 1
1 Physics Department, University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Physics, Humboldt-Universität zu Berlin, Berlin Germany, 3 , Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kyiv Ukraine, 4 Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractA detailed photoluminescence (PL) and Fourier transformed infrared absorption study of InAs/InP(001) quantum wires (QWrs) and comparison with parent quantum well (QWs) system both grown under the same conditions of 4 monolayers of InAs on InP have been performed. PL temperature dependence of both QWrs and QWs shows unusual two-branch switching of the excitonic PL peaks. This behavior is explained as the thermal activation of excitonic ground state of the confined nanostructures. We observe strong modification of the absorption spectrum line-shape (flat regions formation) at room temperature for a QWr sample. This kind of behavior is interpreted as thermally induced change of the dimensionality from 1D to anisotropic 2D. Absorption measurements using polarized light also conform the dimensionality changes, which can be seen in disappearance or significant reduction of difference between the absorption spectra at different polarizations in the regions of the hh1-e1 and lh1-e1 transitions in QWr sample.
9:00 PM - J3.9
Peculiarities of Carrier Tunneling in Coupled Quantum Dot – Quantum Well Nanostructure.
Vitaliy Dorogan 1 , Yuriy Mazur 1 , Euclydes Marega 1 , Gregory Salamo 1
1 Physics Department, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractNew advances in nanoelectronics requires better understanding of coupling mechanisms in various nanostructures, especially quantum dots (QDs). In light emitting diodes, lasers and detectors based on quantum dots, lateral and vertical coupling plays very important role in carrier redistribution and overall device performance. Future quantum computing devices will be built of multiple nanostructures that interact with each other exchanging carriers and energy. Convenient system to study abovementioned phenomena is layer of semiconductor quantum dots coupled with a quantum well (QW).System that consists of single layer of InAs QDs and an InGaAs QW separated by a GaAs spacer, thickness of which is varied in a series of samples, was grown by a molecular beam epitaxy. In some cases, AlAs or AlGaAs barriers were introduced inside the GaAs spacer to vary the barrier height. The QD-QW system was investigated by means of continuous-wave (cw) and time-resolved photoluminescence (PL) methods. Carrier tunneling between QW and QDs has been studied as a function of spacer thickness and barrier height showing that at the 20 nm spacer there is almost no tunneling from QW to QDs. Power dependence of PL spectra clearly reveals the state filling and saturation effects at higher excitation intensities. Time-resolved PL measurements provided information about the carrier relaxation mechanisms, lifetimes at different energy states in the QDs and the tunneling time from QW to QDs.
Symposium Organizers
Marco Fanciulli CNR-INFM MDM National Laboratory &
The University of Milano-Bicocca
John Martinis University of California-Santa Barbara
Mark Eriksson University of Wisconsin-Madison
J4: Semiconductors III
Session Chairs
Tuesday AM, December 02, 2008
Room 308 (Hynes)
9:30 AM - **J4.1
Coherence and Control of a Single Electron Spin in a Quantum Dot.
Lieven Vandersypen 1
1 Kavli Institute of NanoScience, TU Delft, Delft Netherlands
Show AbstractFollowing our earlier work on single-shot read-out and relaxation of a single spin in a quantum dot, we now demonstrate coherent control of a single spin (detection is done using a second spin in a neighbouring dot). First, we manipulate the spin using conventional magnetic resonance. Next, we show that we can also rotate the spin using electric fields instead of magnetic fields. In both cases, 90 rotations can be realized in about 50 ns or less. We use these control techniques to probe decoherence of an isolated electron spin. The spin dephases in about 30 ns, due to the hyperfine interaction with the uncontrolled nuclear spin bath in the host material of the dot. However, since the nuclear spin dynamics is very slow, this dephasing can be largely reversed using a spin-echo pulse. Echo decay times of about 0.5 us are obtained at 70 mT. Studies to suppress the relevant fluctuations in the nuclear spin bath are underway. In parallel, we have started work on quantum dots in graphene, which is expected to offer superior coherence times. As a first step, we have succeeded in opening a bandgap in bilayer graphene, necessary for electrostatic confinement of carriers.
10:00 AM - J4.2
Isotopically Engineered Silicon for Quantum Information Technology.
Joel Ager 1 , H. Riemann 2 , E. Haller 1 3
1 , Lawrence Berkeley Nat. Lab, Berkeley, California, United States, 2 , Institut für Kristallzüchtung, Berlin Germany, 3 , University of California at Berkeley, Berkeley, California, United States
Show AbstractFor a number of solid-state quantum computation (QC) schemes, the composition of stable isotopes must be carefully controlled. For example, natural silicon consists of 28Si (92.23%), 29Si (4.67%), and 30Si (3.10%). The nuclear spin carrying isotope 29Si can be regarded as an “impurity” which must be minimized, as in the use of a pure 28Si matrix for 31P electron spin-based QC approaches [1], or precisely controlled, as in the proposal of an ordered array of 29Si [2]. Simultaneous control of both chemical (e.g., dopants) and isotopic composition thus adds a new dimension to bulk and thin film Si technology. The synthesis of isotopically enriched dislocation-free bulk single crystals and thin film structures will be described. For bulk crystal synthesis, a silane-based process compatible with the relatively small available amounts of isotopically enriched precursors was used. Silane was decomposed to poly crystalline in a recirculating flow reactor with a silane to Si conversion efficiency exceeding 95%. Subsequent float zone processing produced single crystals of Si enriched in all three stable isotopes: 28Si, 99.92%; 29Si, 91.37%; and 30Si, 89.8%. Thin films of the same isotopic composition were grown by chemical vapor deposition. High chemical purity was maintained throughout the processing. The concentrations of shallow dopants P and B are as low as mid-1013 cm−3 and concentrations of C and O can be lower than 1016 and 1015 cm-3, respectively. It has been known since the invention of electron spin resonance spectroscopy that the T2 of the electron bound to 31P is increased in 28Si enriched material due to depletion of 29Si. In this context, pulsed ESR measurements on a 28Si-enriched single crystal sample produced in this work holds the current “world record” with a T2 at 10 K of 5 ms [3]. We have recently lowered the P concentration in our 28Si-enriched bulk crystals by a factor of ~10 by repeated zone-refining. This material has been used to demonstrate quantum memory storage with a T2 of over 1 second using coupling of the 31P electron and nuclear spins [4]. Using highly 28Si enriched single crystals, the hyperfine coupling of the electron and nuclear spins for the 31P donor has been observed optically [5]. While the P bound exciton lines with 150 neV widths are the narrowest ever measured, they are not yet at the limit predicted by the photoluminescence lifetime, showing that inhomogeneous effects are still present. This indicates a clear role for advanced synthesis of isotopically controlled structures with even higher chemical and structural perfection. [1] B. E. Kane, Nature 393, 133 (1998).[2] T. D. Ladd et al., Phys. Rev. Lett. 89, 017901 (2002). [3] A. M. Tyryshkin et al., J. Phys. Cond. Mat. 18 S783 (2006).[4] J. J. L. Morton et al., arxiv.org/abs/0803.2021v1.[5] A. Yang et al., Phys. Rev. Lett. 97, 227401 (2006).
10:15 AM - J4.3
Transport and Charge Sensing in Silicon/silicon-germanium Double-quantum Dots.
Christie Simmons 1 , Nakul Shaji 1 , Madhu Thalakulam 1 , E. Sackmann 1 , B. Van Bael 1 , D. Savage 1 , M. Lagally 1 , R. Joynt 1 , M. Friesen 1 , R. Blick 1 , S. Coppersmith 1 , M. Eriksson 1
1 Physics, University of Wisconsin, Madison, Wisconsin, United States
Show AbstractGated quantum dots in silicon/silicon-germanium are of interest because spins in silicon are weakly coupled to the host material. Here we present the results of transport measurements and charge sensing in silicon/silicon-germanium quantum dots. We demonstrate that Coulomb blockade measurements through a single quantum dot are well correlated with charge sensing in a nearby quantum point contact. We present data demonstrating charge sensing in both single and double quantum dots. Charge sensing enables the determination of the absolute number of electrons in the system, and we present data demonstrating a one-electron single quantum dot in silicon/silicon-germanium.Charge motion through double quantum dots can be a sensitive function of the spin of the carriers. We present measurements of transport through a double quantum dot in the spin blockade regime, and we observe two effects. First, we observe Pauli spin blockade: a strong reduction of current due to long spin relaxation times between triplet and singlet states. As expected, Pauli blockade is lifted due to spin exchange when the triplet is resonant with the Fermi-level of the leads, and it is lifted when both the singlet and triplet spin states are energetically accessible.Reversing the flow of current in the spin blockade regime leads to a new effect, lifetime-enhanced transport, in which long spin lifetimes enhance, rather than suppress, the flow of current through the dots. This effect depends equally on long spin lifetimes and preferential loading of states that are close to the Fermi level of the leads. From analysis of transport through the double dot the loading and unloading rates of triplet and singlet states can be extracted. We present these rates and discuss the implications for spin readout in quantum dots.[1] Nakul Shaji, C. B. Simmons, Madhu Thalakulam, Levente J. Klein, Hua Qin, H. Luo, D. E. Savage, M. G. Lagally, A. J. Rimberg, R. Joynt, M. Friesen, R. H. Blick, S. N. Coppersmith, M. A. Eriksson, arXiv:0708.0794, appearing in Nature Physics, doi:10.1038/nphys98.[2] “Single-electron quantum dot in Si/SiGe with integrated charge sensing,” C.B. Simmons, Madhu Thalakulam, Nakul Shaji, Levente J. Klein, Hua Qin, R.H. Blick, D.E. Savage, M.G. Lagally, S.N. Coppersmith, and M.A. Eriksson, Appl. Phys. Lett. 91, 213103 (2007).
10:30 AM - J4.4
Electron-Phonon Interaction Induced Decoherence of Exchange-Coupled Spin Qubits in Semiconductor Nanostructures.
Xuedong Hu 1
1 Department of Physics, University at Buffalo, SUNY, Buffalo, New York, United States
Show AbstractA crucial issue in spin-based quantum information processing is spin coherence. Single spin decoherence in confined states (whether by a quantum dot or by a donor ion) has been studied extensively, with hyperfine interaction to the the environmental nuclear spins being identified as the most important channel of spin decoherence [1]. For coupled spin qubits, there are new decoherence channels beyond those for single spins. For example, coupling of spin qubits in solids is based on exchange interaction, which is electrostatic in nature. This means that charge fluctuations in the environment could lead to gate errors and dephasing [2]. In addition, when two electron are coupled through exchange interaction, their eigenstates are singlet and triplet states, where the orbital wave functions are symmetric and anti-symmetric respectively. This differentiation of electron charge distribution in the double quantum dot also leads to different couplings to the lattice fluctuations.Here we study theoretically the decoherence of exchange-coupled electron spin qubits through interaction with the lattice fluctuations [3]. We first establish how electron-phonon interaction can affect two exchange-coupled electron spins, and show that its main effect is to cause pure dephasing between the two-electron singlet and triplet states because of their different charge distributions. We then investigate the different types of electron-phonon interactions and the phonon modes involved for quantum dots in both GaAs and Si. In particular, we show that two-spin dephasing in GaAs is dominated by piezo-electric coupling to transverse acoustic phonons, while in Si it is dominated by deformation potential coupling to longitudinal acoustic phonons. An interesting aspect of electon-phonon interaction induced pure dephasing is that it does not lead to a complete loss of coherence between the singlet and triplet states. Instead, it leads to a constant loss of contrast that is dependent on wave function overlap between the two quantum dots. Since the dephasing and the exchange coupling have different dependence on the overlap, there exists a regime where dephasing due to electron-phonon interaction is still small while exchange coupling is reasonably large. [1] S. Das Sarma, R. de Sousa, X. Hu, and B. Koiller, "Spin quantum computation in silicon nanostructures", Solid State Communications 133, 737 (2005); X. Hu, "Quantum Dot Quantum Computing", cond-mat/0411012. Published in Quantum Coherence: From Quarks to Solids, Springer Lecture Notes in Physics 689, ed. by W. Poetz, J. Fabian, and U. Hohenester (Springer, Berlin, 2006), pp 83-114. [2] X. Hu and S. Das Sarma, "Charge fluctuation induced dephasing of exchange coupled spin qubits", Phys. Rev. Lett. 96, 100501 (2006).[3] X. Hu, in preparation.
10:45 AM - J4.5
Spin-Pair Formation in Phosphorus-Doped Silicon.
Felix Hoehne 1 , Hans Huebl 1 , Bastian Galler 1 , Andre Stegner 1 , Martin Stutzmann 1 , Martin Brandt 1
1 , Walter Schottky Institut, Technische Universität München, Garching Germany
Show AbstractVia electronic transport measurements, paramagnetic centers such as defects can be detected very sensitively in semiconductor devices [1]. Furthermore, also the spin state can be determined via time-resolved spin-dependent transport measurements [2]. These experiments rely on the Pauli principle, which governs the transitions of electrons between paramagnetic states. As an example, consider the capture of on electron from a Phosphorus-donor state by a Si/SiO2-interface state, recently used for the readout of donor-based qubits. Since the final state is doubly occupied, the Pauli principle demands that state to be in a spin singlet state. Only Phosphorus/interface state pairs in the same singlet state can undergo the capture process, pairs in a triplet state cannot. However, until now only circumstantial evidence existed demonstrating that the observed spin-dependent transport is in fact caused by that spin pair. Using electron double resonance in a pulsed electrically detected magnetic resonance (pEDMR) scheme, we show here that indeed Phosphorus/interface state pairs are formed and that they constitute the dominant spin-dependent recombination process under the bias conditions investigated. The experiment is based on monitoring the Rabi oscillations induced on one component of the spin pair as a function of the orientation of the second component using a total of three different microwave frequencies. The technique also allows the quantitative determination of the singlet recombination time, in perfect agreement with recent results obtained by electrically detected spin echos [3].[1] D.R. McCamey, H. Huebl, M.S. Brandt, W.D. Hutchison, J.C. McCallum, R.G. Clark, and A.R. Hamilton, Appl. Phys. Lett. 89, 182115 (2006)[2] A.R. Stegner, C. Boehme, H. Huebl, M. Stutzmann, K. Lips, and M. S. Brandt, Nature Phys. 2, 835 (2006)[3] H. Huebl, F. Hoehne, B. Brolik, A.R. Stegner, M. Stutzmann, and M. S. Brandt, Phys. Rev. Lett. 100, 177602 (2008)
11:30 AM - **J4.6
Quantum State Tomography of a Two-Electron Spin Qubit in GaAs.
A. Yacoby 1
1 , Harvard University, Cambridge , Massachusetts, United States
Show Abstract12:00 PM - J4.7
Low-Frequency Noise and Lateral Transport Studies of In0.35Ga0.65As/GaAs Quantum Dot Heterostructures.
Vasyl Kunets 1 , Timothy Morgan 1 , Yuriy Mazur 1 , Vitaliy Dorogan 1 , Peter Lytvyn 1 , Morgan Ware 1 , Dorel Guzun 1 , John Shultz 1 , Gregory Salamo 1
1 Physics Department, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractIn0.35Ga0.65As/GaAs remotely-doped quantum well and quantum dot heterostructures were grown by MBE on (100) GaAs. The nucleation of quantum dots was observed in situ by RHEED and confirmed by AFM and low temperature photoluminescence techniques. Low-frequency noise studies have been performed to probe defects in such heterostructures throughout the transition from a highly strained quantum well to quantum dots. Results were compared to a bulk n-type GaAs reference sample. We revealed three dominant defects in our GaAs samples with activation energies of 0.8 eV, 0.54 eV and 0.35 eV. These defects were found in all samples with the same activation energies. However, structures containing In0.35Ga0.65As QDs show an additional peak at low temperatures due to the presence of defects which are not observed for the bulk reference GaAs or the In0.35Ga0.65As quantum well sample. Detailed analysis shows that for deposition of 9 ML and 11 ML In0.35Ga0.65As QD samples, the additional peak corresponds to the well known M1 defect in GaAs with an activation energy of 0.18 eV, while for a coverage of 13 ML a not as well known defect was found to have an activation energy of 0.12 eV. All defects were characterized quantitatively in terms of their activation energy, capture cross section and density. Hall effect studies of these heterostructures at high temperatures also reveal an unusual trend for a curve of electron density vs. temperature when compared to bulk GaAs. The observed behavior is discussed based on the presence of In0.35Ga0.65As quantum dots in the GaAs matrix.
12:15 PM - J4.8
Electric Field Tunable Exchange Interaction in InAs/GaAs Coupled Quantum Dots.
Kushal Wijesundara 1 , Mauricio Garrido 1 , Swati Ramanathan 1 , Michael Scheibner 2 , Allan Bracker 2 , Dan Gammon 2 , Eric Stinaff 1
1 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractSpin manipulation in coupled quantum dots is of interest for quantum information applications. Control of the exchange interaction between electrons and holes in coupled quantum dot systems via an applied electric field may provide a promising technique for such spin control. Polarization dependent photoluminescence spectra were used to investigate the spin dependent interactions in coupled quantum dot systems. Vertically stacked, self-assembled InAs quantum dots grown by molecular beam epitaxy (MBE) on GaAs substrate were used with different dot heights of 2, 4, and 6 nm and were embedded in a Schottky diode to control the electric field and selectively charge them. By varying the electric field, the ground state hole energy levels are brought into resonance, resulting in the formation of molecular orbitals observed as anticrossings between the direct and indirect transitions in the spectra. The indirect and direct transitions of the neutral exciton demonstrate high and low circular polarization memory respectively due to variation in the exchange interaction. The ratio between the polarization values as a function of energy, electric field, and the barrier height was measured. These results indicate a possible method of tuning between indirect and direct configurations to control the degree of exchange interaction.
12:30 PM - J4.9
Magnetic Interactions of Cold Atoms with Anisotropic Conductors: Suppressing Decoherence and Corrugations for Quantum Information at Room Temperature.
Ron Folman 1 , Tal David 1 , Yonathan Japha 1 , Valery Dikovsky 1 , Ran Salem 1 , Carsten Henkel 2
1 Physics, Ben-Gurion University, Be'er Sheva Israel, 2 , Potsdam University, Potsdman Germany
Show AbstractJ5: Semiconductors IV
Session Chairs
Tuesday PM, December 02, 2008
Room 308 (Hynes)
2:30 PM - **J5.1
Is Room Temperature, Silicon-based, Quantum Computing Fantasy?
Marshall Stoneham 1
1 London Centre for Nanotechnology, University College London, London United Kingdom
Show AbstractMany studies of quantum behaviour are made at low temperatures. Yet this is not always necessary. Quantum phenomena show in two main ways. The first is through quantum statistics, where the quantal h appears as hω/kT. When statistics matter most (e.g., at or near equilibrium), high T opposes quantum effects. The second is through quantum dynamics, where h appears in various ways, some unassociated with T, and may open new channels of behaviour. Quantum information processing relies on staying away from equilibrium: dynamics dominate. There is no intrinsic problem with high temperatures. Practical problems are quite another matter, of course. Quantum operations have already been demonstrated at room temperature in diamond, but progress towards an integrated system with tens or hundreds of linked quantum gates is a far harder challenge. Diamond is what one might call silicon-compatible. But using silicon directly, especially if fabrication were practical in a a fab plant reasonably similar to the ones that exist, then that would make practical QIP more likely. My talk will address some of the problems and possible solutions in the context of the optically-controlled spintronics ideas of Stoneham, Fisher and Greenland (2003 J Phys Cond Matt 15 L447).
3:00 PM - J5.2
Non-Markovian Dephasing in Solid-state Single Photon Sources.
Ahsan Nazir 1 , Sean Barrett 2
1 Department of Physics and Astronomy, University College London, London United Kingdom, 2 Blackett Laboratory, Imperial College London, London United Kingdom
Show AbstractIndistinguishable photons lie at the heart of linear optical and hybrid light-matter quantum information processing (QIP). In particular, interference effects such as photon bunching and 'which-path' erasure are central to a range of QIP protocols, leading to a recent drive for highly efficient single photon sources. Solid-state systems such as quantum dots, individual molecules, or nitrogen-vacancy centres in diamond are emerging at the forefront of this new technology due to the potential they offer for realizing not only efficient but also controllable, deterministic single photon devices. However, solid-state systems are noisy and their optical transitions dephase. This limits the degree of indistinguishability within the photon emission, degrading subsequent interference effects and diminishing the entangling power of distributed QIP schemes. In this presentation I will describe the behaviour of solid-state single photon sources within the quantum jump formalism, accounting for a realistic dephasing environment. The model I adopt is analytically solvable in both the Markovian and non-Markovian cases, providing a great deal of insight into the detrimental effects of various forms of source dephasing on photon interference. I will show that by post-selecting photon detection events within a narrow time window environmental effects may be overcome, leading to an increase in interference visibility (or entangling power) at the expense of effective source efficiency. This is made possible with fast photodetectors due to the intrinsic dependence of the visibility on the timing of the detection events themselves. Moreover, the dynamics of charge carriers within a solid-state environmentis an interesting problem in its own right. For example, the relative importance of Markovianand non-Markovian processes in disentanglement dynamics or in impeding coherent qubit control remains an important question. Here, too, time-resolved two-photon interference can offer considerable insight, beyond that expected when no timing information is available. In fact, I will show that by varying the size of the post-selection window, the dynamics of dephasing process may be probed in real time, leading to specific signatures for Markovian and non-Markovian effects.
3:15 PM - J5.3
Resonant Optical Excitation of Excited States in Coupled Quantum Dots.
Mauricio Garrido 1 , Kushal Wijesundara 1 , Swati Ramanathan 1 , Eric Stinaff 1 , Daniel Gammon 2 , Allan Bracker 2 , Michael Scheibner 2
1 Physics and Astronomy, Ohio University, Athens, Ohio, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractExcited states in single quantum dots (QDs) have been shown to be useful for spin state initialization and manipulation. For quantum information processing it is desirable to have multiple spins interacting, which has been demonstrated in coupled quantum dot (CQD) systems. Therefore, an understanding of the excited states in these systems is a crucial step towards the realization of quantum information applications involving CQDs. Photoluminescence excitation (PLE) studies were performed on InAs/GaAs CQDs to probe their excited states. CQDs are embedded in a Schottky diode structure and are individually addressed by means of submicron apertures in an Al shadow mask. Samples with barrier thicknesses of 2, 4 and 6 nm between the vertically stacked QDs were employed. Using a new method to visualize the PLE, the optical signatures of excited states in the CQDs show a rich structure, revealing information on indirect and direct absorption and recombination that takes place. Polarization and power dependence of the photoluminescence give further insight into the processes that give rise to such signatures. An interesting example that was observed is resonant charge-state creation, which may provide a possible initialization mechanism.
3:30 PM - J5.4
Electric Field Control of Hole Spins in Optically Excited Quantum Dot Molecules.
Matthew Doty 1 , Juan Ignacio Climente Plasencia 2 , Marek Korkusinski 4 , Michael Scheibner 3 , Allan Bracker 3 , Pawel Hawrylak 4 , Dan Gammon 3
1 Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, 2 , CNR-INFM National Center on nanoStructures and bioSystems at Surfaces (S3), Modena Italy, 4 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada, 3 , Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractSpins confined in quantum dots have long been considered a promising qubit candidate and have been the subject of intense research. We have studied spin properties of coupled pairs of InAs quantum dots (quantum dot molecules) using optical spectroscopy.[1,2] Coherent tunneling of electron or hole spins between the two quantum dots leads to the formation of delocalized molecular states. In the case of a hole spin, the formation of delocalized molecular states results in a very large and resonant enhancement or reduction of g-factor that is controlled with an applied electric field.[3] This effect arises because of the corresponding enhancement or suppression of the hole wavefunction in the tunnel barrier for the bonding (symmetric) and anti-bonding (anti-symmetric) states, respectively. Using this effect to identify the symmetry of the wavefunction, we have now found that the spin-orbit interaction induces a reversal of the energetic order of the bonding and anti-bonding molecular states as a function of tunnel barrier thickness.[4] That is, for barriers thicker than 3 nm the anti-bonding state becomes the molecular ground state, in direct contrast to the predictions of the simple particle-in-a-box model or the one-band effective mass theory. Recent experiments have shown that holes in quantum dots have surprisingly long spin relaxation times[5] and extremely weak hyperfine interactions with nuclei.[6] The weak hyperfine interaction suppresses a primary decoherence mechanism and makes holes very promising for the development of spin-based quantum information processing. The results presented here provide a powerful new tool for spin control that is unique to holes. By taking advantage of the spin-orbit interaction, hole molecular ground states with either bonding or anti-bonding orbital character can be engineered. This, in turn, enables the design of structures where the hole g factor resonantly increases or decreases with applied electric field and provides opportunities for the coherent control of single confined hole spins using electric fields[7] or g tensor modulation.[8][1] Stinaff, E. A. et al., Science 311, 636-639 (2006).[2] Scheibner, M. et al., Phys. Rev. B 75 (24), 245318 (2007).[3] Doty, M. F. et al., Phys. Rev. Lett. 97 (19), 197202 (2006).[4] Doty, M. F. et al., in review, cond-mat arXiv:0804.3097v1 (2008).[5] Heiss, D. et al., Phys. Rev. B 76, 241306 (2007).[6] Gerardot, B. D. et al., Nature 451 (7177), 441-444 (2008).[7] Nowack, K. C. et al., Science 318 (5855), 1430-1433 (2007).[8] Kato, Y. et al., Science 299 (5610), 1201-1204 (2003).
3:45 PM - J5.5
Coherent Access of a Quantum Dot Strongly Coupled to a Nanocavity.
Dirk Englund 1 , Andrei Faraon 1 , Ilya Fushman 1 , Jelena Vuckovic 1
1 Applied Physics, Stanford University, Stanford, California, United States
Show AbstractCavity quantum electrodynamics in solid state systems offers a scalable and robust platform for quantum optics research and the development of quantum information processing applications. In this approach, researchers aim to create a quantum network [1], which combines the utility of the photon as an information carrier with the nonlinearity of an atomic system for interacting more than one quantum bit (qubit) in a gate. The quantum network requires a way to coherently probe an atom or quantum dot in a cavity. It is also possible to perform quantum information processing without storing information in quantum dots, but instead follow and all-optical approach where quantum information is stored only in photons [2,3]. Quantum gates, in this case, could be realized by employing very large optical nonlinearities that occur in coherent scattering from quantum dots strongly coupled to photonic crystals. To implement quantum information processing in the solid state, we have recently demonstrated a novel method to probe a single InAs quantum dot (QD) that is strongly coupled to a photonic crystal (PC) cavity, using a laser beam resonant with the quantum dot[4]. We show that the quantum dot strongly modifies the cavity reflection amplitude and phase [4,5]. At high probe intensity, the quantum-dot induced reflectivity feature vanishes as the quantum dot becomes saturated. This giant optical nonlinearity occurs at a level of one photon per quantum dot lifetime. We also show that depending on the frequency of the probe beam, the reflected beam’s statistics are either bunched or antibunched compared to the Poissonian statistics of the probe beam. These measurements represent the first demonstrations of photon blockade [6] and photon-induced tunneling in solid state [7].REFERENCES:[1] J. I. Cirac, P. Zoller, H. J. Kimble, and H. Mabuchi, Phys. Rev. Lett. 78, 3221 (1997).[2] Q. Turchette, C. Hood, W. Lange, H. Mabuchi, and H. J. Kimble, Phys. Rev. Lett. 75, 4710 (1995).[3] K. Nemoto and W. J. Munro, Phys. Rev. Lett. 93, 250502 (2004).[4] D. Englund, A. Faraon, I. Fushman, N. Stoltz, P. Petroff, and J. Vuckovic, Nature, 450(6):857–61, 2007. [5] I. Fushman, D. Englund, A. Faraon, N. Stoltz, P. Petroff, and J. Vuckovic, Science 320, 769 (2008).[6] K. M. Birnbaum, A. Boca, R. Miller, A. D. Boozer, T. E. Northup, and H. J. Kimble. Nature 436, 87 (2005).[7] A. Faraon, I. Fushman, D. Englund, N. Stoltz, P. Petroff, and J. Vuckovic, to be published in Nature Physics (2008).
4:30 PM - **J5.6
Two Dimensional Electron Systems with Mobility Exceeding 105 cm2/Vsec on Hydrogen-terminated Silicon Surfaces.
B. Kane 1 , Robert McFarland 1 , Tomek Kott 1
1 Lab for Physical Sciences, University of Maryland, College Park , Maryland, United States
Show Abstract5:00 PM - J5.7
Improved Gate Frequency Performance above 1 MHz for 1310 nm Single Photon Detection using Germanium Avalanche Photodiodes in Geiger Mode.
John Seamons 1 , Malcolm Carroll 1 , Kenton Childs 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractCurrently InGaAs/InP avalanche photodiodes (APDs) are the leading choice for single photon avalanche photodiodes (SPADs) in the eye-safe near-infrared (NIR) due to their preeminence in optical networking and their subsequent optimization as SPADs. A primary challenge in operating these SPADs at high frequencies is after-pulsing[1]; which limits detection bit rates to ~5 MHz, even when used in combination with the fastest active-quenching circuitry. A higher bit rate is very desirable for several applications including: quantum computation with linear optics[2], linear quantum cryptography[3], eye-safe light detection and ranging (LIDAR)[4] and quantum key distribution (QKD)[5]. In order to enhance the bit rate at 1310 nm we examine Ge SPADs, which have been reported to have more than an order of magnitude smaller charge trap densities than InGaAs/InP SPADs and were successfully operated in free-running mode by passively quenching[6]. We present 77 K dark count rate (DCR) measurements at peak detection efficiency (DE) of a commercially available InGaAs/InP APD and compare to a Judson Technologies Ge APD[7] and Ge SPADs, that have been fabricated at Sandia National Laboratories. The InGaAs/InP APD shows significant increase in the DCR for gate frequencies greater than 200 kHz, the typical signature of after-pulsing, while the Ge APDs maintain a more ideal flat DCR up to the measured 5 MHz. This shows a potential benefit that Ge SPADs might provide in increasing bit rates and operating frequency past the ~5MHz InGaAs/InP limit by maintaining a flatter DCR profile at higher frequencies. This work has been supported by the IC Postdoctoral Fellowship Program. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.[1]Z. L. Yuan, et al., Appl. Phys. Lett. 91(4), 041114 (2007).[2]E. Knill, et al., Nature 409(6816), 46-52 (2001).[3]N. Gisin, et al., Rev. Mod. Phys. 74(1), 145-195 (2002).[4]T. Maruyama, et al., Opt. Eng. 41(2), 395-402 (2002).[5]C. H. Bennett, et al., IEEE Proc. Int. Conf. on Comp., Sys. and Sig. Proc., Bangalore, India, 175-179 (1984).[6]G. Rigordy, et al., Appl. Opt. 37(12), 2272-2277 (1998).[7] J. A. Seamons, et al., Proc. SPIE. 6976, 697607 (2008).
5:15 PM - J5.8
Nanophotonic Structures Fabricated in Single Crystal Diamond.
Thomas Babinec 1 , Murray McCutcheon 1 , Mughees Khan 1 , Irfan Bulu 1 , Kirsten Smith 1 2 , Philip Hemmer 3 , Marko Loncar 1
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Physics and Astronomy, Vrije Universiteit, Amsterdam Netherlands, 3 Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas, United States
Show AbstractWe present the design, fabrication, and characterization of a variety of novel nanophotonic structures fabricated in single-crystal diamond, including nanowire waveguides, solid immersion lenses, and free-standing photonic crystals. Finite-difference time-domain (FDTD) simulations indicate that diamond photonic crystal cavities can possess quality factors as high as Q=45,000, while possessing mode volumes as small as V=0.39(λ/n)^3. The corresponding Purcell Factor of F=13,000 indicates that this device may significantly enhance light-matter interaction. In order to obtain the nanometer-scale precision and smoothness requisite for high-Q cavities, we use standard focused ion beam (FIB) milling techniques to obtain thin diamond membranes suitable for photonic crystal patterning. We assess the effect of this physical milling procedure on the optical properties of the diamond crystal. These nanophotonic structures represent a new class of quantum information devices that may achieve increased photon collection efficiencies, enhanced spontaneous emission rates, and even enable photonic channels for long-distance communication between registers in single-crystal diamond.
5:30 PM - J5.9
Optical Emission of Isoelectronic Beryllium Pairs in Silicon toward Quantum Information Processing.
Toyofumi Ishikawa 1 , Kei Yoshizawa 1 , Katsuhiko Naito 1 , Takeharu Sekiguchi 2 , Mike L.W. Thewalt 2 , Kohei Itoh 1
1 School of Fundamental Science and Technology, Keio Univ., Yokohama Japan, 2 Department of Physics, Simon Fraser University, Burnaby , British Columbia, Canada
Show AbstractNuclear spins in semiconductor are promising candidates for practical quantum bits [1,2]. Recently, we have demonstrated successfully the optical readout of the nuclear spin states of 31P impurities (I=1/2) in silicon [3]. However, it was also shown that the photocurrent readout of the 31P nuclear spins was likely to be more efficient than the direct photon readout due to the low quantum efficiency of the photon emission from 31P [3]. Toward quantum information networking mediated by photons in silicon, a very efficient photon emitter in silicon - the isoelectronic Be pair - is investigated in the present study. Photoluminescence (PL) measurements of Be-doped Si samples with externally applied magnetic fields have been performed to reveal the microstructure of the Be pair in Si. Host Si isotope effects on the PL spectra have been also studied toward the optical networking (formation of the cluster states) of the 9Be nuclear spin quantum states (I=3/2). Beryllium was introduced by thermal diffusion into the isotopically natural silicon of 92.23% 28Si purity (Si-nat) and isotopically enriched silicon of 99.93% 28Si purity (Si-28). The PL measurements were performed with the BOMEM DA8 Fourier transform interferometer using Nd:YLF laser (1047nm) for excitation. PL spectra of the Be-doped Si samples in the absence of magnetic field show three no-phonon (NP) lines indicating low symmetry of the Be-pair defect, as reported in the previous work [4]. Quantitative analysis of these NP lines in magnetic fields shows that the Be pair is symmetric around the <111> axis. This conclusion is consistent with the ab initio calculation [5]. Furthermore, the NP lines of the Be-doped Si-28 sample shows significant energy shift (-0.10 meV) and drastic narrowing of the lowest energy line (26 to 2.6 μeV) in comparison with those of the Be-doped Si-nat sample. These isotope effects are qualitatively the same to those observed in the PL spectra of P and B in Si [6,7]. Currently, near-field optical microscopy/spectroscopy is employed in our effort to detect single photon emission from a single Be pair. It is our final goal to detect a photon entangled with the 9Be nuclear spin state of the Be pair. This work was supported in part by Grant-in-Aid for Scientific Research by MEXT, Specially Promoted Research #18001002 and in part by Special Coordination Funds for Promoting Science and Technology.[1] B. E. Kane, Nature 393, 133 (1998). [2] T. D. Ladd et al., Phys. Rev. B 71, 014401 (2005). [3] A. Yang et al., Phys. Rev. Lett. 97, 227401 (2006). [4] M. L. W. Thewalt et al., Solid State Comun. 44, 573 (1982). [5] E. Tarnow et al., Phys. Rev. B 42, 11252 (1990). [6] M. Cardona and M. L. W. Thewalt, Rev. Mod. Phys. 77, 1173 (2005). [7] D. Karaiskaj et al., Solid State Comun. 123, 87 (2002).
5:45 PM - J5.10
Tuning the InAs Quantum Dot Emission Wavelength by Aplying AlAs Cap Layer and Post-Growth Annealing.
Vitaliy Dorogan 1 , Jihoon Lee 1 , Yuriy Mazur 1 , Zhiming Wang 1 , Gregory Salamo 1
1 Physics Department, University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractSuccessful use of semiconductor quantum dots (QDs) in optoelectronic device and quantum computation applications requires the ability of controlling the emission wavelength or, in other words, the energy position of electron states in QDs. There are two types of methods that allow to affect the QD emission energy. First type is the change of particular parameters during the growth (substrate temperature, cap material, thickness of cap), and the second one is some kind of post-growth treatment (annealing). In this work, we investigate the effect of both during-the-growth and post-growth processes to control the emission wavelength of InAs QDs grown by molecular beam epitaxy (MBE) by means of photoluminescence (PL) technique. The influence of different capping materials (GaAs and AlAs) as well as post-growth rapid thermal annealing (RTA) on the QD emission has been studied. AlAs-capped QDs exhibit a red-shifted PL peak compared to that of GaAs-capped sample which is explained by a suppressed In-Ga intermixing due to AlAs layer. Application of RTA process results in a blue shift and narrowing of the full width at the half maximum of PL peak for GaAs-capped sample which occurs as a result of In-Ga inter-diffusion between GaAs layers and InAs QDs changing the content and slightly the size of QDs. In case of AlAs-capped sample, the PL emission also shifts to the blue side of the spectrum with increase of RTA temperature; however, it has more complicated structure revealing different size distributions of QDs.
Symposium Organizers
Marco Fanciulli CNR-INFM MDM National Laboratory &
The University of Milano-Bicocca
John Martinis University of California-Santa Barbara
Mark Eriksson University of Wisconsin-Madison
J6: Semiconductors V
Session Chairs
Wednesday AM, December 03, 2008
Room 308 (Hynes)
9:30 AM - **J6.1
Fast Optical Control of Spins in Quantum Dots for Quantum Technology Applications.
Duncan Steel 1
1 Physics and EECS, University of Michigan, Ann Abor, Michigan, United States
Show Abstract10:00 AM - J6.2
Optically Probing and Manipulating Hole-Spin in a Single Quantum Dot.
Brian Gerardot 1 , Daniel Brunner 1 , Paul Dalgarno 1 , Richard Warburton 1 , Khaled Karrai 2 , Nick Stoltz 3 , Pierre Petroff 3
1 Physics Department, Heriot-Watt University, Edinburgh United Kingdom, 2 Center for NanoScience, Ludwig-Maximilians University, Munich Germany, 3 Materials Department, University of California , Santa Barbara, California, United States
Show AbstractA new paradigm stimulated by quantum information processing is the application of quantum optics to solid-state media. Due to their heterostructure functionality and potential robustness against dephasing mechanisms, self-assembled quantum dots (QDs) are at the forefront of this domain. The strong quantization, which leads to atom-like properties for QDs, additionally inhibits phonon-related spin decoherence mechanisms commonly found in the solid-state. However, a conduction-level electron has an s-like atomistic wavefunction and the coupling between its spin and the fluctuating magnetic moments of the host nuclei leads to fast spin relaxation at low magnetic fields and spin dephasing. On the other hand, valence-band holes have a p-like atomistic wavefunction. This leads to a negligible hole wavefunction at the nuclei positions, strongly suppressing the contact hyperfine interaction. We report here optical manipulation of a single hole-spin in a QD with high fidelity demonstrating its robustness against relaxation mechanisms [1]. We perform resonant laser spectroscopy on a single hole in an InAs QD embedded in a charge-tunable structure. Optical spin pumping is achieved with two different mechanisms. In the first experiment, performed at zero external magnetic field, excitation with σ+ (σ-) polarization selectively excites the hole-spin down (up) transition only. The hyperfine interaction leads to fast Larmor precession of the excited state electron-spin. This, combined with spontaneous emission, transfers the population to the hole-spin state orthogonal to that excited by the laser. The fidelity of this spin initialization process is better than 99%. We confirm this interpretation by applying an external magnetic field parallel to the growth direction. This decreases the electron-spin precession frequency, slowing the spin pumping mechanism and reducing the hole-spin initialization fidelity. By monitoring optical pumping as a function of magnetic field we determine T1 ~ 1ms.In the second experiment, we apply a magnetic field perpendicular to the growth direction to break the rotational symmetry of the QD. This mixes the spin states, creating optical dipoles between the ground (hole-spin) and excited (positively charged exciton) states. This effectively forms two lambda-systems. Hence, resonant excitation of one dipole can project the hole-spin into the orthogonal state via spontaneous emission. We achieve high-fidelity optical pumping with this mechanism. By exciting with two orthogonal linearly polarized lasers, the spin-cooling mechanism is frustrated and both the electron and hole g-factors can be accurately measured. In the framework of quantum optics, the hole-spin based lambda system is promising for probing the spin dephasing mechanisms and for exploiting spin resonance or coherent population trapping to achieve an arbitrary superposition of spin states. [1] B.D. Gerardot et al., Nature 451, 441 (2008).
10:15 AM - J6.3
Optical Approaches to Quantum Computing Using Spin Qubits.
Brendon Lovett 1 , Erik Gauger 1 , Ahsan Nazir 2 , Marshall Stoneham 2 3 , Simon Benjamin 1 , Thomas Stace 4 , Peter Rohde 1
1 Department of Materials, University of Oxford, Oxford, Oxon, United Kingdom, 2 Department of Physics and Astronomy, University College London, London United Kingdom, 3 , London Centre for Nanotechnology, London United Kingdom, 4 Department of Physics, University of Queensland, Brisbane, Queensland, Australia
Show AbstractElectron spins confined within solid state nanostructures are preferred candidates for embodying quantum information. A popular idea is to use optically active nanostructures such as self assembled quantum dots. The so-called Pauli blocking effect can lead to the spin-dependent optical creation of trion (electron spin plus exciton) states - and in this presentation I shall discuss how these can be used to implement fast single and two qubit gates. This approach has the potential to achieve the "best of both worlds", i.e. marrying the long spin decoherence times with rapid gating. Two broad classes of gate operations will be discussed. First, a dynamic strategy, based on the application of sudden laser pulses will be introduced. In effect, the application of the laser dresses and modifies the system eigenstates. I will show that the time evolution of the system can lead to a universal set of gates on the spin qubits, by taking the system through the trion states that lie outside the computational Hilbert space. [1]Second, an adiabatic approach will be presented where a slowly varying laser pulse allows adiabatic following of the modified eigenstates. Both single and two qubit variants will again be discussed. [2, 3]The adiabatic strategy has crucial advantages over the dynamic approach. In particular, we analyse all of the principal decoherence mechanisms in this system, including photon emission and phonon coupling, by employing a Markovian master equation approach. We find that the adiabatic approach can be extremely robust against all of these sources of decoherence even at finite temperature. [3, 4]In the final part of the presentation, we will discuss a method for creating entanglement in a pair of remote spins that are coupled to an optically active 'mediator' spin. Again, dynamic and adiabatic approaches are presented and we find that the adiabatic approach can be applied to a much broader class of physical interaction mechanisms. [5][1] A. Nazir, B. W. Lovett, S. D. Barrett, T. P. Spiller and G. A. D. Briggs Phys. Rev. Lett. 93 150502 (2004)
[2] B. W. Lovett, A. Nazir, E. Pazy, S. D. Barrett, T. P. Spiller and G. A. D. Briggs, Phys. Rev. B 72 115324 (2005)
[3] E. M. Gauger, S. C. Benjamin, A. Nazir and B. W. Lovett Phys. Rev. B 77 115322 (2008)
[4] E. M. Gauger, P. P. Rohde, A. M. Stoneham and B. W. Lovett, to appear in New Journal of Physics (2008) http://arxiv.org/abs/0802.3670
[5] E. M. Gauger, A. Nazir, S. C. Benjamin, T. M. Stace and B. W. Lovett, to appear in New Journal of Physics (2008)http://arxiv.org/abs/0804.2139
10:30 AM - **J6.4
Fluctuations and Dissipation in a RF Nanomechanical Structure.
Keith Schwab 1
1 , Cornell University, Ithaca , New York, United States
Show AbstractJ7: Superconductors I
Session Chairs
Wednesday PM, December 03, 2008
Room 308 (Hynes)
11:30 AM - **J7.1
Materials.
D. Pappas 1
1 , National Institute of Standards and Technology, Louisville, Colorado, United States
Show Abstract12:00 PM - J7.2
Crystalline Silicon Dielectrics in Superconducting Qubit Circuits.
David Hover 1 , Weina Peng 1 , Steven Sendelbach 1 , Mark Eriksson 1 , Robert McDermott 1
1 Physics, University Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractSuperconducting circuits comprising Josephson junctions are a leading candidate for scalable quantum information processing in the solid state. Qubit energy relaxation times are limited by microwave loss induced by a continuum of two-level state (TLS) defects in the dielectric materials of the circuit. State-of-the-art phase qubit circuits employ a micron-scale Josephson junction shunted by an external capacitor. In this case, the qubit T1 time is directly proportional to the quality factor (Q) of the capacitor dielectric. The amorphous capacitor dielectrics that have been used to date display intrinsic Q of order 1,000 to 10,000. Shunt capacitors with Q ~ 1,000,000 are required to extend qubit T1 times well into the microsecond range. Crystalline dielectric materials are an attractive candidate for qubit capacitor dielectrics, due to the extremely low density of TLS defects. However, the robust integration of crystalline dielectrics with superconducting qubit circuits remains a challenge. Here we describe a novel approach to the realization of high-Q crystalline capacitor dielectrics for superconducting qubit circuits. The capacitor dielectric is a crystalline silicon nanomembrane. The membrane is liberated from a silicon-on-insulator (SOI) wafer and manually transferred to the metallic base electrode; subsequent metal deposition and dry etching completes the capacitor structure. We discuss characterization of crystalline silicon capacitors with low-power microwave transport measurements at millikelvin temperatures. In addition, we report progress on integrating the crystalline capacitor process with Josephson qubit fabrication.This work was supported in part by the NSF MRSEC program. Some devices were fabricated at the Cornell NanoScale Facility, a member of the NSF-funded National Nanotechnology Infrastructure Network.
12:15 PM - J7.3
Physical Vapor Deposition Synthesis of Ferromagnetic Rutile CrO2 Through Substitutional Alloying Techniques.
Kevin West 1 , Jiwei Lu 1 , Stuart Wolf 1
1 , University of Virginia, Charlottesville , Virginia, United States
Show AbstractCrO2 is a known half metal with nearly 100% spin polarization making it an excellent candidate for a host of spintronic applications. However, CrO2 is metastable at ambient conditions and readily irreversibly decomposes to Cr2O3 at temperatures between 250°C and 450°C. To date, CrO2 thin films have only been prepared using high pressure decomposition, or atmospheric chemical vapor deposition (CVD). Most proposed spintronic devices utilizing CrO2 thin films require multilayer epitaxial growth, and present growth methods are not well suited for this. Ideally a physical vapor deposition (PVD) technique is desired, so that high quality interfaces can be engineered to preserve the highly spin polarized nature of CrO2. In this talk an alternative PVD substitutional alloying fabrication technique is described to prepare Ru doped CrO2 thin films. CrO2 crystallizes in the tetragonal rutile structure with lattice constants a = 4.422 Å and c = 2.920 Å. Ruthenium dioxide (RuO2) crystallizes in the tetragonal rutile structure with lattice constants a = 4.497 Å and c = 3.105 Å. In addition, RuO2 is a highly conductive metal and has a very stable structure at room temperature, making it an excellent candidate material for substitutional alloying with CrO2. This technique utilizes the reactive bias target ion beam deposition (RBTIBD) method to fabricate RuxCr1-xO2 (0.04 ≤ x ≤ 0.18) thin films on single crystalline TiO2 substrates of various orientations. Here we demonstrate that the addition of Ru substitutionally alloyed stabilizes the CrO2 tetragonal rutile structure while, retaining most of the transport and magnetic properties of pure CrO2 at room temperature. Point Contact Andreev Reflection (PCAR) shows that RuxCr1-xO2 has a spin polarization of ~70 %.
12:30 PM - **J7.4
Superconducting Optical Photon Detectors for Quantum Information Applications.
Sae Woo Nam 1 , Adriana Lita 1 , Burm Baek 1 , Brice Calkins 1
1 , National Institute of Standards and Technology, Boulder, Colorado, United States
Show AbstractJ8: Superconductors II
Session Chairs
Wednesday PM, December 03, 2008
Room 308 (Hynes)
2:30 PM - **J8.1
Improving Charge Qubit Coherence by Design.
Robert Schoelkopf 1
1 Applied Physics, Yale University, New Haven, Connecticut, United States
Show Abstract3:00 PM - J8.2
Single Ion Detection Using Avalanche Diodes in Geiger Mode.
Edward Bielejec 1 , J. Seamons 1 , M. Carroll 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractWe present experimental results and fabrication details for single ion Geiger mode avalanche diode (SIGMA) detectors. The SIGMA extends Geiger mode operation of conventional avalanche diodes developed for the single photon detection community. These detectors have been designed to detect a single electron-hole (e-h) pair generated by a single incident photon. Detection of low energy ions (less than 10 keV) is desired for the single donor implantation required for quantum information processing. Significant progress has been made in the area using the combination of a built-in Si p-i-n diode and lateral masking of the surface using a polymethyl methacrylate (PMMA) mask which is electron beam lithography patterned (D. N. Jamieson et al., Appl. Phys. Lett. 86, 202101 (2005)). The current state of the art for this method of single ion detection is signal-to-noise limited to a lower energy bound of ~10 keV phosphorus ions, generating ~1000 e-h pairs. The avalanche diodes underlying the SIGMA detector has been shown to have a detection sensitivity to single e-h pairs, we have extended this proven detector technology to single ion detection. Furthermore, this work takes advantage of a complementary metal oxide semiconductor (CMOS) foundry the Microelectronics Development Laboratory at Sandia National Laboratories to build these devices providing a path towards fabrication of novel single donor semiconductor devices. We observe single ion detection sensitivity for both room temperature and 77 K operation with the SIGMA detector to a 250 keV H+ beam using gated, passively quenched Geiger mode avalanche photodiodes. We determine the detector efficiency to be ~100% with a dark count probability of 15% at room temperature. The detector efficiency remains constant and the dark count dramatically reduces with decreasing temperature. The details of the measurement technique as well as the experimental setup will be presented. We find detector efficiency remains high for single ion implantation both inside and outside the active region (high fielded region) of the detector indicating the ability to use this detector outside of the pristine regions required for quantum information processing. A comparison between the measured detector efficiency and the ion beam induced charge (IBIC) signal will be discussed. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.
3:15 PM - J8.3
Out-Of-Equilibrium Josephson Effect in Superconducting Tunnel Nanojunctions.
Francesco Giazotto 1 , Stefano Tirelli 1 , Alexander Savin 2 , Cesar Pascual Garcia 1 , Jukka Pekola 2 , Fabio Beltram 1
1 Physics, NEST INFM-CNR & Scuola Normale Superiore, Pisa Italy, 2 Low Temperature Laboratory, Helsinki University of Technology, Helsinki Finland
Show AbstractNonequilibrium dynamics in superconducting nanocircuitsis currently in the focus of an intense experimentaland theoretical effort. In this context, the controlof the Josephson current in superconductor-normalmetal-superconductor (SNS) weak links is receiving muchattention. In these systems supercurrent is manipulatedby modifying the quasiparticle energy distributionin the N region via current injection from externalterminals. There have been some successfuldemonstrations of such out-of-equilibrium SNS junctions. On the other hand, it was recently predicted that supercurrent can be controlled in all-superconductingtunnel structures as well. In this casethe operation stems from nonequilibrium induced in thesuperconducting state, which may lead to intriguing featurespeculiar to out-of-equilibrium superconductors.In this contribution we report the fabrication and characterizationof all-superconducting Al-AlOx-Ti tunnel nanostructures which allow controlof the Josephson coupling in a small Tiisland driven far from equilibrium. This is accomplishedby injecting quasiparticles from tunnel-coupled Alsuperconducting leads. Both supercurrent enhancementand suppression with respect to the equilibrium, as wellas generation at temperatures well above the island criticaltemperature were achieved by changing the quasiparticleinjection rate. Our findings are explained within amodel relating the superconducting state of the island tothe heat flux driven through it upon injection. Furthermore, our results suggest that these structures can be used as a prototype system for the investigation of nonequilibrium Josephson dynamics in mesoscopic superconductors.
3:30 PM - **J8.4
Noise and Dephasing from Surface Magnetic States in Superconducting Circuits.
Robert McDermott 1 , Steven Sendelbach 1 , David Hover 1 , John Martinis 2 , Michael Mueck 3
1 Dept. of Physics, University of Wisconsin, Madison, Wisconsin, United States, 2 Dept. of Physics, University of California, Santa Barbara, California, United States, 3 Institute for Applied Physics, University of Giessen, Giessen Germany
Show AbstractSuperconducting qubits are a leading candidate for scalable quantum information processing. In order to realize the full potential of these qubits, it is necessary to develop a more complete understanding of the microscopic physics that governs dissipation and dephasing of the quantum state. In the case of the Josephson phase and flux qubits, the dominant dephasing mechanism is an apparent low-frequency magnetic flux noise with a 1/f spectrum and a magnitude of several microPhi_0/sqrt(Hz) at 1 Hz, where Phi_0 = h/2e is the magnetic flux quantum. Recent qubit results are compatible with the excess low-frequency noise measured by researchers at Berkeley more that 20 years ago in a series of experiments on SQUIDs cooled to millikelvin temperatures. The origin of this excess noise was never understood. Here we describe studies of flux noise and temperature-dependent magnetization in SQUIDs cooled to millikelvin temperatures. We observe that the flux threading the SQUIDs increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data is compatible with the thermal polarization of unpaired surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin-glass freezing at low temperature. These results suggest a microscopic explanation for the universal 1/f flux noise in SQUIDs and superconducting qubits, and suggest that suitable surface treatments of the superconducting films will lower the density of magnetic states, leading to superconducting devices with lower noise and solid-state qubits with improved coherence times.
4:30 PM - **J8.5
Quantum Information Storage using Tunable Flux Qubits.
Matthias Steffen 1 , Frederico Brito 1 , DiVincenzo David 1 , Matthew Farinelli 1 , George Keefe 1 , Shwetank Kumar 1 , Frank Milliken 1 , Mary Rothwell 1 , Jim Rozen 1 , Mark Ketchen 1
1 , IBM, Yorktown Heights, New York, United States
Show AbstractWe provide an overview of our experimental efforts at IBM using superconducting qubits. Recently, we observed quantum information storage of a few microseconds in a coplanar waveguide resonator coupled to a tunable flux qubit. The qubit itself, however, has short coherence times which we believe are due to capacitive coupling of the qubit to bias leads. We present a theoretical model for this loss mechanism and show preliminary results of experiments that aim at reducing its impact. In addition we also show data on phase qubits with a novel microwave read-out scheme.
5:00 PM - J8.6
Susceptibility of Magnetic Surface States in Superconducting Circuits.
Steven Sendelbach 1 , David Hover 1 , Robert McDermott 1 , Michael Mueck 2
1 Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Institut fuer Angewandte Physik, Justus-Leibig-Universitaet, Giessen Germany
Show AbstractRecent experiments indicate that there is a high density of unpaired spins residing on the surfaces of superconducting thin films used to implement SQUIDs and superconducting qubits. Fluctuations of these spins give rise to low frequency flux noise and dephasing of the qubit state. Realization of phase and flux qubits with improved dephasing times will require a deeper understanding of the microscopic physics that governs fluctuations of the surface spins. Here we describe experiments that probe the ac spin susceptibility of the surface magnetic states. The detector is a dc SQUID-based susceptometer optimized for the study of surface spins. We discuss the temperature and frequency dependence of the spin susceptibility, and relate these to interactions between spins, the distribution of spin relaxation times, and possible spin-glass freezing. In addition, we describe experiments to probe microwave dissipation induced by magnetic surface states in superconducting circuits. This work was conducted in part at the Cornell NanoScale Facility, a member of the NSF-funded National Nanotechnology Infrastructure Network.
5:15 PM - J8.7
Correlation Between Tunnel Barrier Characteristics and Atomic Structure in Al/AlOx/Al Tunnel Barriers.
Samira Nik 1 , Per Delsing 2 , Tine Greibe 2 , Henrik Pettersson 1 , Emrah Yucelen 3 , Eva Olsson 1
1 Department of Applied Physics, Chalmers University of Technology, Göteborg Sweden, 2 Department of Microtechnology and Nanoscience, Chalmers University of Technology, Göteborg Sweden, 3 , FEI Company, Eindhoven Netherlands
Show AbstractIn most cases the microscopic structure of Al/AlOx/Al tunnel barriers is not important for the qualitative properties of the device and it is often treated as an ideal homogeneous barrier described with some tunnelling strength. However when the noise of the device is measured the microscopic nature of the tunnel barrier does matter. For example the low frequency noise in charge sensors based on single electron tunnelling junctions (SETs) is believed to originate from charge traps either in the tunnel barrier or in the substrate of the device. It is clear that there is both charge noise and resistance noise present in the SETs. Since most superconducting qubits are made with the same aluminium technology, this will be true also for qubits.The present work concerns the effect of oxidation process parameters on the tunnel barriers characteristics and the atomic structure of Al/AlOx/Al tunnel barriers. High resolution analytical transmission electron microscopy (TEM) using a Cs probe corrected Fei Titan 80-300 field emission gun with a monochromator, a high resolution Gatan Tridiem for spectroscopy and energy filtered imaging and a high angle annular dark field scanning TEM (STEM) detector provides information about both atomic and electronic structure.