The silicon technology can deliver sub-10 nm devices where ‘every atom counts&’. Manipulation of atoms with high precision on such a scale, in principle, can lead to technological innovations, such as transistors with extremely short gate length, quantum computing components and optoelectronic devices. One possible strategy to create this next generation of devices is to precisely place individual discrete dopants (such as phosphorous atoms) in a nanoscale transistor.
In this paper, we report the first systematic study of quantum transport simulation of the impact of precisely positioned dopants on the performance of ultimately scaled gate-all-around silicon nanowire transistors (SNWT) designed for digital circuit applications. Due to strong inhomogeneity of the self-consistent electrostatic potential, a full 3-D real-space NonEquilibrium Green&’s Function (NEGF) formalism is used. The simulations are carried out for an n-channel NWT with 2.2 x 2.2 nm2 cross-section and 6 nm channel length, where the locations of the precisely arranged dopants in the source drain extensions and in the channel region have been varied. The individual dopants act as localized scatters and, hence, impact of the electron transport is directly correlated to the position of the single dopants. As a result, a large variation in the ON-current and modest variation of the subthreshold slope are observed in the ID-VG characteristics when comparing devices with microscopically different discrete dopant configuration. The variations of the current-voltage characteristics are analyzed with reference to the behaviour of the transmission coefficients.

Quantum confinement can turn thin silicon nanowires (SiNWs) to wide band gap material where the large surface-to-volume ratio indicates that its electronic structure may be tailored by surface termination. Here we show an example how these properties of thin SiNWs may be utilized to host quantum bits.

A phosphorus (P) donor has been extensively studied in bulk silicon to realize the concept of Kane quantum computers. In most cases the quantum bit was realized as an entanglement between the donor electron spin and the nonzero nuclei spin of the donor impurity mediated by the hyperfine coupling between them. The donor ionization energies and the spin-lattice relaxation time limited the temperatures to a few kelvin in these experiments. Here, we demonstrate by means of ab initio density functional theory calculations that quantum confinement in thin SiNWs results in (i) larger excitation energies of donor impurity and (ii) a sensitive manipulation of the hyperfine coupling by external electric field. We propose that these features may allow to realize the quantum bit (qubit) experiments at elevated temperatures with a strength of electric fields applicable in current field-effect transistor technology. We also show that the strength of quantum confinement and the presence of strain induced by the surface termination may significantly affect the ground and excited states of the donors in thin SiNWs, possibly allowing an optical read-out of the electron spin. [1] Binghai, Rurali, Gali, Nano Letters, 12, 3460 (2012).

Bistable behavior of single Si dopants in the (110) surface layer of GaAs was studied with a scanning tunneling microscope (STM). The Si atom acts as either a positively charged substitutional donor or a negatively charged interstitial. Its configuration can switch under influence of a local biased STM tip. To independently manipulate the charge state, the sample was illuminated by a laser during STM operation. The Si atom can be reversibly switched between its positive and negative charge state by turning the laser on and off, respectively. This process occurs mostly with the photon energy tuned above the bandgap of GaAs, indicating that photogenerated electron-hole pairs play an important role in the process. The occupation of the donor and interstitial configurations depends on the carrier dynamics, i.e. the possibility of the electrons to escape or to be captured. If the tip induced band bending (TIBB) is large enough, it is possible for electrons to tunnel into the conduction band and the donor configuration is observed. Another escape path is created when the sample is illuminated and photogenerated holes can recombine with the bound electrons of the dopant.
The bistability of the Si dopant was explored to demonstrate memory operations on a single atom, using the STM tip as a static electrical gate for “reading” and “writing” the information. Three different tunneling conditions were applied that enabled (a) writing the negative charge state “0”, (b) writing the positive charge state “1” and (c) reading the charge state without affecting it. This shows the potential for using properties of single dopants in nano-scale devices.

We present recent work characterizing the ultrafast dynamics of self-assembled nanoparticles in a GaAs matrix. The dynamics are markedly different from semi-intrinsic GaAs systems; charge transfer, sub-picosecond recombination times and magnetic responses are observed. The results shown here indicate the systems may be candidates for designer electronics compatible with existing semiconductor technology.

Spin qubits in silicon are excellent candidates for scalable quantum information processing (QIP) due to their long coherence times and the enormous investment in silicon MOS technology. Here I discuss qubits based upon single phosphorus (P) dopant atoms in Si [1]. Projective readout of such qubits had proved challenging until single-shot measurement of a single donor electron spin was demonstrated [2] using a silicon single electron transistor (Si-SET) and the process of spin-to-charge conversion. The measurement gave readout fidelities > 90% and spin lifetimes T1 > 6 seconds [2], opening the path to demonstration of electron and nuclear spin qubits in silicon.
Integrating an on-chip microwave transmission line enabled single-electron spin resonance (ESR) of the P donor electron. We used this to demonstrate Rabi oscillations of the electron spin qubit, while a Hahn echo sequence revealed electron spin coherence times T2 > 0.2 ms [3]. This time is expected to become much longer in isotopically enriched 28Si devices. We also achieved single-shot readout of the 31P nuclear spin (with fidelity > 99.6%) by monitoring the two hyperfine-split ESR lines of the P donor system. By applying (local) NMR pulses we demonstrated coherent control of the nuclear spin qubit, giving a coherence time T2 > 60 ms [4].
[1] B.E. Kane, Nature 393, 133 (1998).
[2] A. Morello et al., Nature467, 687 (2010).
[3] J.J. Pla et al., Nature 489, 541 (2012).
[4] J.J. Pla et al., Nature, in press, arXiv:1302.0047.

Remarkable progress in single donor qubits has recently been achieved using surface gated MOS nanostructures placed near timed implants. An important next step is deterministic fabrication of one and two donor implants aligned to surface gates. We will present results on single ion implantation of nanostructures using integrated detectors combined with a focused ion beam.
High precision placement of donors requires tight lateral confinement of the ion beam and low energy for small vertical straggle. The low energy introduces strict requirements for detector signal to noise. To achieve high signal to noise, single ion Geiger mode avalanche (SIGMA) detectors are integrated into the nanostructure process flow. However, dark count probabilities in the SIGMAs are relatively high for robust repeatable single ion implants when using large spot size beams, d=10-30 um, even at detector operation temperatures of approximately 120K.
A high intensity, high resolution focused donor ion beam with a reduced temperature stage enhances single ion implant accuracy through (a) increased probability of an ion arrival before a dark count occurs (i.e., increased flux); (b) tight lateral confinement allowing both high precision placement with minimal impact on ion flux; and (c) reduced dark current in the detector when operating at reduced temperature. We will present development of a new Nanoimplanter with a focused ion beam system capable of a 10 nm spot size that has been designed to provide donor sources and a cold stage. The development of the Nanoimplanter represents the ultimate test of the top-down ion implantation technique. We will highlight a series of machine improvements (spot size on target, development of reliable P and Sb liquid metal ion sources and low temperature stage development) necessary to build qubit structures. We will finally discuss detection efficiency and dark count probability of nanostructures implanted by the Nanoimplanter.
Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

The use of the nuclear spin of a single dopant in a semiconductor host to store and manipulate quantum information is being widely studied due to its very large spin coherence time, preserved thanks to the weak coupling to its environment [1,2]. However, this weak coupling also makes detection of a single nuclear spin an outstanding problem in different fields of physics, not only in quantum computing but also in single spin magnetic imaging. Here we consider the case of a single Bi dopant in a silicon nanotransistor. We study how the transport characteristics of this single dopant device can provide information about its nuclear spin state. We consider both the sequential and the cotunneling transport regimes. In the sequential regime case, the dI/dV curve yields the single electron spectral function, while in the cotunneling regime, it provides information about the electronic spin spectral function [3,4]. The hyperfine coupling to the nuclear spin results in a modification of the electronic spin spectral function which, in turn, could be probed by Inelastic Electron Tunneling Spectroscopy (IETS)[3,4], provided that the spectral resolution is high enough. We find that the hyperfine coupling opens new transport channels which can be resolved at experimentally accessible temperatures. Our simulations also evince that IETS yields information about the occupations of the nuclear spin states, paving the way towards transport-detected single nuclear spin resonance.
[1] B. E. Kane 393, 133 (1988)
[2] T. D. Ladd et al., Nature 464, 45 (2010)
[3] F. Delgado, R. Aguado, and J. Fernández-Rossier, Appl. Phys. Lett. 101, 072407
(2012)
[4] F. Delgado and J. Fernández-Rossier, Phys. Rev. Lett. 107, 076804 (2011)

Optical access to a qubit in silicon has been an important goal for both quantum computation and communication but has to date only been achieved in the ensemble limit. Here, we present the photoionization of an individual erbium dopant in silicon. A single-electron transistor is used as a single shot charge detector to observe the resonant ionization as a function of photon energy. This allows for optical addressing and electrical detection of individual erbium dopants with exceptionally narrow line width. The hyperfine coupling for 167Er is clearly resolved which allows single shot readout of the nuclear spin. Moreover, large g-factor differences are obtained from the Zeeman splitting of individual atoms. In the experiment about 30 ions are implanted into the active region of a nano transistor. Individual erbium centers can be resolved by different gate coupling and photon energy, which allows optical addressing and electrical detection of several single erbium atoms.

Incorporation of a single dopant atom into nanoscale semiconductors usually suffers from statistical fluctuations in the number and position of the dopant atoms, which can have a dramatic effect on the electronic and optical properties of the devices. Hence, it is crucial to quantify and locate the position of dopant atoms through atomic visualization technique. Annular dark-field scanning transmission electron microscopy (ADF-STEM) is a powerful technique for such purpose, as demonstrated previously on various combinations of dopant atoms and host materials. However, those experiments require that the difference in atomic numbers between the host crystals and dopant atoms (ΔZ) be sufficiently large to yield a visible contrast in the image. In this work, we detected light Mn dopant atoms in ZnSe nanocrystals (NCs) with low ΔZ by performing simultaneous acquisition of electron energy loss spectra (EELS) and ADF-STEM image [1]. This technique can become an alternative in identifying individual dopant atoms inside materials whereby conventional ADF-STEM imaging is difficult to perform due to low ΔZ. The ZnSe NCs were synthesized using high temperature organometallic solution based route. The Mn doping was achieved by injecting dimethylmanganese into a mixture containing diethylzinc and Se. The number of incorporated Mn atoms can be tuned by varying the concentration of dimethylmanganese. Three types of samples were examined: 2.9 nm diameter NCs with an average of 0.7 incorporated Mn atom per NC (singly doped), 3.7 nm NCs with an average of 6.2 Mn atoms per NC (highly doped), and 2 nm undoped NCs used for reference. The first step of the data processing is to construct 2-D EELS maps Mn L_2,3- EELS edge spectrum. Pixels with high intensity in these maps are indicative of the presence of Mn. Spatial correlations of the EELS maps with the simultaneously acquired ADF-STEM images could then be used to locate the position of Mn dopant atoms.
[1] A. A. Gunawan, A. Wills, M.G. Thomas, D. J. Norris, K. A. Mkhoyan, Nano Letters, 11 (2011) 5553.

The study of the spin of individual transition-metal dopants in a semiconductor host is an emergent field in nanoscience known as magnetic solotronics, which promises novel spintronic devices at the atomic scale. Advances in STM-based techniques allow experimentalists to investigate substitutional dopants at a semiconductor surface with unprecedented accuracy and degree of detail [1]. In this talk we will first review recent theoretical studies based on both microscopic tight-binding (TB) models and density-functional theory techniques, which have shed light on several of the experimental findings. In particular for the case of substitutional Mn dopants on the (110) GaAs surface, TB models have provided a successful description of the electronic and magnetic properties of the associated acceptor states and the exchange interaction that these mediate between two Mn atoms. We will discuss a Chern number theory aimed at treating the Mn spin and the hole spin and orbital magnetic moments on equal footing[3]. We will also present results for a microscopic treatment of the magnetic-dopant d-levels for different TM impurities positioned on the layers near the GaAs surface. Electronic transitions within the d-level shell of the dopants seem to be responsible for unusual features observed in recent STM experiments on Fe impurities in GaAs[4]. Electronic excitations of the d levels might also be accessible to inelastic electron tunneling spectroscopy similar to the one successfully carried out for magnetic dopants adsorbed on insulating surfaces.
In the second part of the talk we will present results of ongoing theoretical work addressing the properties of substitutional magnetic dopants on the surface of three-dimensional topological insulators (TIs), such as Bi2Se3 and Bi2Te3. In this case the gap of the host material is traversed by surface gapless states with linear dispersion around the Dirac point. A finite density of magnetic ions substituted for e.g. Bi in the parent compound are expected to cause a breaking of time-reversal symmetry with the opening of a gap at the Dirac point. In analogy with the case of itinerant acceptors in dilute MnGaAs, we will discuss the role of the conducting surface Dirac electrons in mediating a ferromagnetic exchange interaction among the localized spin of the dopants. Using a tight-binding model we will focus in particular on the electronic structure inside the TI bulk gap and the local density of states around the magnetic impurity, which can provide a guide for ongoing and future STM experiments on single magnetic dopants in TIs.
[1] A. M. Yakunin et al., Phys. Rev. Lett. 92, 216806 (2004); D. Kitchen et al., Nature 442, 436
(2006).
[2] J.-M. Tang et al., Phys. Rev. Lett. 92, 047201 (2004); T.O. Strandberg et al., Phys. Rev. B 80, 024425 (2009).
[3] T.O. Strandberg et al. Phys. Rev. Lett. 106, 017202 (2011).
[4] J. Bocquel et al., arXiv:1203.6293v.1.

The scaling of electronic devices such as transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations can impact device performance and functionality. Toward this end, the scanning tunneling microscope (STM) is emerging as a useful tool for its capabilities of atomic manipulation, imaging and tunneling spectroscopy. I will discuss our STM studies of Mn acceptors within the surface layer of a p-doped GaAs crystal [1-3]. We start by sublimating Mn adatoms onto the GaAs (110) surface, prepared by cleavage in ultrahigh vacuum. A voltage pulse applied with the STM tip allows us to replace a Ga atom in the surface with the Mn atom, thus forming a single Mn acceptor and a Ga adatom. We find that the properties of Mn acceptors can be tuned by control of the local electrostatic landscape. For example, the STM tip can be used to position As vacancies [1] and other adatoms (e.g. Mn, Ga) [4], all of which are charged +1e. Direct Coulomb repulsion causes a reduction in the hole-Mn binding energy as the defects are moved nearby. Tunneling spectroscopy allows us to quantify this effect, through the shift of an in-gap acceptor resonance toward lower energy. In addition, defect-induced band bending provides a method for tuning the ionization state of the Mn acceptors, as evidenced by a ring-like feature in STM images. We have extended this work to dimers of Mn acceptors, in the hopes of tuning the spin-spin interaction between them [5]. Tunneling spectra of the dimers reveal six dimer states, representing bonding and anti-bonding combinations of the individual Mn acceptor states. These states systematically shift in energy as charged defects are brought nearby. Comparison of these experimental results with density functional theory calculations provides further insight into the electronic states and properties of the defects. These studies show that tunable control over single dopants in semiconductors is becoming a realistic route for next-generation classical- and quantum-based information technologies.

[1] D H Lee and J A Gupta, “Tunable Field Control Over the Binding Energy of Single Dopants by a Charged Vacancy in GaAs,” Science 330, no. 6012 (December 23, 2010): 1807-1810.
[2] Dong-Hun Lee and Jay A Gupta, “Tunable Control Over the Ionization State of Single Mn Acceptors in GaAs with Defect-Induced Band Bending,” Nano Letters 11, no. 5 (May 11, 2011): 2004-2007.
[3] D H Lee, N M Santagata, and J A Gupta, “Influence of the Local Environment on Zn Acceptors in the GaAs(110) Surface,” Applied Physics Letters 99, no. 5 (2011): 053124.
[4] D. Gohlke et al., “Tuning the electrostatic landscape in semiconductors with charged adatoms” (submitted)
[5] D. Gohlke et al., (in preparation)

Investigating the nature and properties of individual dopants in semiconductors is required to make possible new “solotronic” devices where a single impurity determines the functionality of the complete device [1].
Scanning tunneling microscopy and I(V) spectroscopy were used to map the spatial structure of the electronic states of Fe impurities buried below the (110) GaAs surface. Like most transition metals in GaAs, Fe is a deep acceptor and has a binding energy of 510 meV. As expected we found asymmetric bow-tie like shapes for the acceptor state wavefunction of Fe. This shape, arising from the cubic symmetry of the host crystal, is a signature of deep acceptors [2].
However additional features were detected which we suggest are related to the multivalent nature of Fe in GaAs. Fe can be in the isoelectronic Fe3+ valence state or in its acceptor Fe2+ valence state. This acceptor state can be in two different charge states, A0 or Aminus;., where Aminus; corresponds to the ionized acceptor (Fe2+)minus; and A0 to the neutral acceptor [Fe2+, h+].
A reduced conductivity is observed around each Fe atom when imaging filled states at negative voltages. The spatial structure of this phenomenon is found to be strongly anisotropic. We interpret this as the result of interference between two tunnel channels, in which one path virtually excites an electronic transition, from E to T2 crystal-field-split states in the d-shell of the Fe impurity.
When imaging empty states at positive voltages, the asymmetric bow-tie like shape of the Fe acceptor can be surrounded by a dark and isotropic depression in the topographic map. This corresponds to a charging ring closing around the acceptor position in the dI/dV map.
[1] P. M. Koenraad and M. E. Flatté, Nature Materials 10, 91 (2011).
[2] A. Yakunin, A. Silov, P. Koenraad, J. Wolter, W. Van Roy, J. De Boeck, J.-M. Tang, and M. Flatté, Physical Review Letters 92, 1 (2004).

Zinc oxide (ZnO) is a wide-band-gap semiconductor (E_g ~3.4 eV), is a possible dilute-magnetic semiconductor and supports high carrier concentrations while remaining highly transparent to visible light. ZnO has attracted much interest recently because of this wide variety of functional properties.
Transition metal and rare earth dopants have been observed to enhance the magnetic properties of ZnO and could potentially produce robust room temperature ferro-magnetism. Such properties would enable spintronic applications for a new class of devices. The formation volume of such point defects is an important property that influences their position and diffusion around extended defects and in nano-scale particles. However, it is still rarely analysed for many systems.
Here, we report a comparative study on the properties of d- and f-block dopants in ZnO. Specifically, we have investigated the defect levels and accommodation of Mn2+, Fe2+, Eu2+, Mn3+, Fe3+, Eu3+ and Gd3+ dopants in the ZnO host lattice. These dopants have been chosen to give clearer comparisons between dopant properties (charge, size, spin state and orbital shape) and the resulting lattice relaxation, magnetic properties and density of states.

The unique properties of diamond- very high thermal conductivity, high charge mobility and bio-compatibility make it ideal material for various applications - like quantum information processing and bio-markers. The diamond crystal can also hold a large number of optically active defect centers and many of them have been observed at the single level. Among them, the nitrogen-vacancy (NV) center seems to be the most promising one due to its stability during laser illumination, the ability to optically read-out its electron spin and its long coherence and relaxation times. It this talk the recent progress in using this system as a qubit, magnetic and electric field sensor will be reviewed. New methods for controlling the coherence and the spin environment will be presented.

The decrease of the structure size in semiconductor electronics and magnetic information storage devices has dramatically reduced the number of atoms necessary to process and store one bit of information: An individual magnetic atom would represent the ultimate size limit for storing and processing information. With semiconductor quantum dots (QDs) doped with Mn atoms, the probing of a single atomic spin in a solid state environment become possible using optical techniques. The state of a photon emitted or absorbed by a II-VI semiconductor QD containing a Mn atom is directly related to the spin state of the magnetic atom (S=5/2): the exciton acts as an effective magnetic field along the QD growth axis that lifts up the degeneracy between the six (2S+1) Mn spins states. Depending on the Mn spin orientation, the recombination of an exciton emits a photon with a given energy and polarization¹. We demonstrated that a Mn atom embedded in a QD may act as an optically addressable spin based memory². The next steps will be to perform a coherent control of an individual atom and to tune the coupling between such ultimate memories: a carrier mediated interaction between two Mn spins inserted in the same QD is one step towards these challenging goals.
For a QD containing two Mn atoms, the optical spectra are in general quite complex, nevertheless each collective state of the two Mn spins can be optically addressed³. We have shown that the resonant optical injection of spin polarized carriers can be used to initialize these localized spins⁴. We then demonstrated that under a strong resonant optical field, the energy of any spin state of one or two Mn atoms can be independently tuned using the optical Stark effect⁵. Under optical excitation resonant with an absorption transition, we can enter the strong coupling regime where hybrid states of matter and light are created. The ground state of the magnetic atoms is "dressed" with light. The spin dependent strong coupling with the laser field modifies the Mn fine structure (interaction with the crystal field) and hyperfine structure (interaction with the 55Mn nuclear spin I=5/2). We demonstrated that the optically induced modification of the fine structure of the Mn atom significantly affects its spin dynamics and its initialization under resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, for particular conditions of the laser detuning and excitation intensity, the spin population can be trapped in the state which is resonantly excited. This mechanism is modelled considering the coherent dynamics of the coupled electronic and nuclear spins of the Mn atom optically dressed by a resonant laser field. It will be used in future experiments for a coherent manipulation of the coupled electronic and nuclear spins of Mn atoms.

Localization of optical energy on zero-dimensional objects such as molecules is critical for the manipulation of excitonic energy, such as in excitonic switches. However, the large size mismatch between the visible photon wavelength and the absorption cross-section of single molecules limits the efficiency of localization. We present a scheme in which single acceptor molecules are coupled by Förster energy transfer to a thin film of J-aggregates, enhancing the effective absorption cross-section and the subsequent fluorescence of the acceptor molecules by a factor of ~5,000. The enhancement is accomplished by strong absorption (10⁶ cm^-1) in the J-aggregate thin film, long-range (~100 nm) singlet exciton diffusion in the J-aggregates, and subsequent energy transfer to the acceptor molecule. We also show fluorescence enhancement of nanocrystal quantum dots and discuss the J-aggregate thin films as a general excitonic antenna for localizing excitons on single nanoscale acceptors.

With its long-lived electronic spin coherence and optical addressability, the nitrogen-vacancy (NV) center in diamond has attracted much interest as a potential solid-state qubit for quantum information applications. However, a difficult challenge in this system is how to connect many NV centers together, as needed for large-scale computation. One possible route, which we have pursued at HP Labs for several years, uses photonic networks combined with measurement-based schemes based on optical interference to create long-distance entanglement. Two of the many requirements for this approach to succeed are control over the NV center&’s energy level structure and the ability to place NV centers into optical structures such as microcavities to enable efficient coupling to an optical channel.
This talk will mainly describe our work on coupling NV centers to optical microcavities and waveguides using various geometries, including structures made entirely from diamond [1,2] as well as hybrid structures [3]. We have measured Purcell enhancement of the zero-phonon emission rate by factors estimated to be as high as 70, such that the zero-phonon fraction of spontaneous emission increases from 3% to 70%. We are currently searching for process improvements to increase the spectral stability of the optical transitions in these structures, which are affected by interactions between the NV center and nearby surfaces. Recent high-resolution spectroscopy results from NV centers near diamond surfaces subjected to various processing steps will be presented. While we hope to find a materials-based solution, alternatively, one may actively stabilize the optical transition frequency using externally applied dc electric fields, and recent results [4] using this technique will be discussed.
[1] A. Faraon, P. E. Barclay, C. Santori, K.-M. C. Fu, and R. G. Beausoleil, Nat. Phot. 5, 301 (2011).
[2] A. Faraon, C. Santori, Z. Huang, V. M. Acosta, and R. G. Beausoleil, Phys. Rev. Lett. 109, 033604 (2012).
[3] P. E. Barclay, K.-M. C. Fu, C. Santori, A. Faraon, and R. G. Beausoleil, Phys. Rev. X 1, 011007 (2011).
[4] V. M. Acosta, C. Santori, A. Faraon, Z. Huang, K.-M. C. Fu, A. Stacey, D. A. Simpson, K. Ganesan, S. Tomljenovic-Hanic, A. D. Greentree, S. Prawer, and R. G. Beausoleil, Phys. Rev. Lett. 108, 206401 (2012).

Many proposals for quantum information technologies require quantum states that can be easily manipulated by an outside observer, while remaining sheltered from the destructive influences of the surrounding environment. Semiconductor defects, while generally considered undesirable in traditional electronic devices, can in certain cases be extremely well-suited for this purpose [1]. We discuss recent experimental results that identify several defects capable of performing as spin quantum bits (qubits) in the 4H, 6H, and 3C crystal polymorphs of silicon carbide (SiC). The spins of these defects can be optically addressed using infrared light at near-telecom wavelengths, and gigahertz microwaves can be used to coherently manipulate the quantum states of these spins at temperatures ranging from 20 K to room temperature. Furthermore, the 4H and 6H polymorphs of SiC each contain multiple distinct forms of defect spin qubit that can be independently addressed and manipulated due to their distinct optical and spin transition energies. We show that ensembles of these distinct spin species can then be coherently coupled via magnetic dipole interactions, suggesting that crystal polymorphism in SiC offers a unique opportunity to engineer networks of separately addressable spins. Together with the industrial scale crystal growth and advanced microfabrication techniques that already exist for SiC, these results make SiC a promising candidate for various photonic, spintronic, and quantum information applications that merge quantum degrees of freedom with classical electronic and optical technologies [2, 3].
This work is funded by AFOSR and DARPA.
[1] J. R. Weber*, W. F. Koehl*, J. B. Varley*, A. Janotti, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, Proc. Natl Acad. Sci. USA 107, 8513 (2010).
[2] W. F. Koehl, B. B. Buckley, F. J. Heremans, G. Calusine, and D. D. Awschalom, Nature 479, 84 (2011); A. Dzurak, Nature 479, 47 (2011).
[3] A. L. Falk, B. B. Buckley, G. Calusine, W. F. Koehl, V. V. Dobrovitski, A. Politi, C. A. Zorman, P. X.-L. Feng, and D. D. Awschalom, submitted (2012).

One of the most intriguing phenomena in quantum physics is the entanglement of spatially separated objects. This leads to such counterintuitive effects as “spooky action at a distance” where measurement of one object instantaneously affects the state of the other object. In addition to its importance to the fundamentals of physics, entanglement is also an extremely valuable resource in quantum information technology.
Quantum networks have been envisioned in order to distribute quantum information that is processed and stored in local nodes [1]. Their applications would range from secure quantum communication to distributed quantum computation. Setting up a quantum network requires the generation of entanglement between widely separated qubits combined with local long-lived quantum registers. Remote entanglement could so far be only shown between individual trapped ions and atoms [2, 3, 4].
Here we present a key experiment towards the realization of scalable quantum networks with solid-state qubits. We have entangled the electron spins of two individual nitrogen vacancy centers in diamond, separated by a distance of three meters. We establish this entanglement using a robust protocol based on a joint measurement on single photons that are entangled with the electron spins of the two NV centers. The detection of the photons projects the spins into an entangled state. We verify the high quality of the generated quantum correlations by performing single-shot readout [5] on both NV spins individually in different bases.
The NV center electron spin is an exceptional qubit with long coherence times, high fidelity control and readout. Furthermore the NV can act as a bus in a quantum register formed by surrounding nuclear spins [6]. These nuclear spin register can provide the required long-lived local memory that enables deterministic teleportation, quantum repeaters and extended quantum networks. Our results therefore open the door to a new class of experiments with solid-state qubits that have the potential to enable large-scale quantum networks.
[1] H. J. Kimble, Nature, 453, 1023 (2008)
[2] D. L. Moehring et al., Nature, 449, 68 (2007)
[3] S. Ritter et et al., Nature, 484, 195 (2012)
[4] J. Hofmann et al., Science, 337, 72 (2012)
[5] L. Robledo et al., Nature, 574, 477 (2011)
[6] W. Pfaff et al., Nature Physics, AOP, DOI: 10.1038/NPHYS2444

The exchange interaction between two widely separated localized electron spins plays a key role in spin-dependent logic and quantum computation[1]. Advances in single-ion implantation as well as electronic and optical spectroscopy have permitted direct observation of the interaction between two such spins[2,3]. The separations of relevance to such devices are often several nanometers, which would require tens of thousands of atoms to be simulated in an ab initio calculation. Tight-binding calculations of dopants in tetrahedrally-coordinated semiconductors[4] have been shown to produce accurate results for dopant-dopant exchange over long distances[2,5], so we use this formalism to calculate the interaction between dopant pairs. We fix the atomic-scale potentials for the dopants to match the energies of the deep levels from ab initio calculations[6] and construct the wave functions of the dopant pairs using a tight-binding calculation of spatial Green&’s functions in an infinite crystal[5]. The spin-orbit splitting is obtained from atomic spin-orbit splitting values[7]. We then evaluate the exchange interaction from the calculated spectra of the tight-binding model for the dopant pairs.
Transition metal dopants in tetrahedrally-bonded semiconductors are good candidates for single-spin manipulation and spin-spin interaction because they offer both highly-localized states (e symmetry states do not overlap with the conduction or valence band wave functions) and much more extended states (t2 symmetry states do overlap effectively with the valence band wave functions). To properly consider these two types of states we generalize from Ref. 5 to an spds* tight-binding model for the host semiconductor. Here we examine the exchange interaction between pairs of Ni and Cr dopants for different orientations and pair spacings. Ni and Cr provide good contrast because the exchange interaction in Ni pairs is determined by extended states, while in Cr pairs it is determined by localized states. A clear dependance of the exchange interaction on direction is observed, as expected due to the crystal symmetry. Additionally, for both sets of dopants there are pair orientations for which, at a distances beyond 80 A, the exchange interaction remains experimentally relevant.
This work was supported by an AFOSR MURI.
[1] Semiconductor Spintronics and Quantum Computation, ed D.D. Awschalom, N. Samarth & D. Loss (Springer Verlag, Heidelberg, 2002).
[2] D. Kitchen et al., Nature 442, 436 (2006).
[3] P. Koenraad & M.E. Flatté, Nat. Mat. 10, 91 (2011).
[4] H.P. Hjalmarson, et. al., PRL 44, 810 (1980).
[5] J.-M. Tang & M.E. Flatté, PRL 92, 047201 (2004).
[6] T. Chanier, C. Pryor & M.E. Flatté, PRB 86, 085203 (2012).
[7] C. E. Moore, Atomic Energy Levels. As Derived From the Analyses of Optical Spectra. vol. I - III (1949, 1952, 1958).

Scanning tunneling microscopy is a powerful tool for studying the electronic and magnetic properties of magnetic nanostructures on surfaces. Over the last decade, inelastic tunneling spectroscopy has been used to probe discrete energy levels of quantum spin systems. These states can often be described as solutions of simple spin Hamiltonians. In spin excitation spectroscopy, a spin system is kicked from the ground into excited spin states at discrete energy increments.
In this talk we will focus on the dynamics of quantum spin systems on surfaces. STM can measure tunnel currents in the range of pico amps with millisecond time resolution. This time resolution is well matched to observing transition between spin states of artificial magnetic nanostructures on surfaces that can be built and measured with STM. We will highlight an example of extended, artificial antiferromagnets on a Cu2N surface (Science 2012). Smaller magnetic clusters relax much faster but their dynamics can be measured with pump probe techniques. A pump voltage pulse drives the spin system into excited states and a subsequent probe pulse measures the resulting population of spin states. An exponential decay back to the ground state is observed when averaging over many pump-probe cycles (Science 2010). We will show results down to nanosecond time resolution with an ultimate limit set by modern electronics at about 100 pico seconds.
Individual atoms on Cu2N relax their spin states even faster. Hence, another technique is employed to determine spin relaxation times: small tunnel currents always leave the spin system in the ground state while high currents can create non-equilibrium distributions of spin states. This approach relies on some modeling but allows time domain measurements down to about 1 pico second (Nature Physics 2010). Transition metal atoms on metal surfaces relax even faster, on time scales of about 100 femtoseconds. This fast relaxation manifests itself as a measurable lifetime broadening of spin excitation spectra.
Combining these approaches allows measurements of spin relaxation times over about 16 orders of magnitude for spins on surfaces - while maintaining the atomic scale spatial resolution of STM!

Scanning tunnelling spectroscopy (STS) enables to access the spin of an individual atom adsorbed onto or doped into the surface of metals [1] or semiconductors [2]. On nonmagnetic metallic substrates, arrays of atomic spins can be assembled in almost any geometry one atom at a time using the tip of the scanning tunnelling microscope as a tool [3]. Within such arrays, the magnetic atoms are coupled by direct or Ruderman-Kittel-Kasuya-Yosida exchange via the substrate electrons, which can be tailored in strength and sign by controlling the inter-atomic distances [3]. Therefore, such assemblies of atomic spins form an ideal playground to study fundamental questions of spin ground states and dynamics.
We use spin-resolved STS [1] and inelastic STS [2,4] in order to measure the magnetic-field dependent ground states [3] and dynamics [5] of such arrays in real space and real time. The results are compared to models which use parameters from ab-initio calculations. Distinct ground states of linear chains, depending on even or odd numbers of constituent atoms, and magnetic frustration within complex two-dimensional arrays have been observed directly [3]. Moreover, we have investigated the magnetization dynamics of clusters of direct-exchange coupled atoms and found a strong spin-transfer torque effect [5]. Finally, the obtained knowledge was used in order to realize model systems of all-spin based logic gates [6].
[1] J. Wiebe, L. Zhou, and R. Wiesendanger, J. Phys. D: Appl. Phys. 44, 464009 (2011).
[2] A. A. Khajetoorians, B. Chilian, J. Wiebe, S. Schuwalow, F. Lechermann, and R. Wiesendanger, Nature 467, 1084 (2010).
[3] A. A. Khajetoorians, J. Wiebe, B. Chilian, S. Lounis, S. Blügel, and R. Wiesendanger, Nature Physics 8, 497 (2012).
[4] A. A. Khajetoorians, S. Lounis, B. Chilian, A. T. Costa, L. Zhou, D. L. Mills, J. Wiebe, and R. Wiesendanger, Phys. Rev. Lett. 106, 037205 (2011).
[5] A. A. Khajetoorians, B. Baxevanis, Chr. Hübner, T. Schlenk, S. Krause, T. O. Wehling, S. Lounis, A. Lichtenstein, D. Pfannkuche, J. Wiebe, and R. Wiesendanger, Science (accepted 2012).
[6] A. A. Khajetoorians, J. Wiebe, B. Chilian, and R. Wiesendanger, Science 332, 1062 (2011).

Understanding the emergence of classical behavior in a world governed by quantum mechanics at the microscopic scale is one of the main fundamental open problems in physics. The radical difference between the two behaviors is dramatically represented by quantum systems that are, at the same time, in two classically different states. The quantum to classical transition is empirically linked to the size of the systems and conceptually related to the concept of environmental decoherence, but no general and clear rules have been determined. In this talk we address this fundamental question in the context of a single magnetic atom in an otherwise non-magnetic solid. Whereas we focus more on the case of magnetic atoms on a conducting substrate, our discussion applies for magnetic atoms in doped semiconductors such as Mn on III-V semiconductors. We find that the quantum to classical transition depends on the relative strength of its exchange coupling to the electrons in the surface and the zero field splitting induced by quantum spin tunneling[1]. Thus, spin parity plays an important role [2] and integer spins are more robust towards environmental decoherence, the ultimate responsible of the emergence of the classical behavior. We discuss how the quantum to classical transition can be inferred using a Scanning Tunneling Microscope (STM) to perform either inelastic electron tunneling spectroscopy or, in the case of spin polarized STM, spin imaging.
[1] F. Delgado, S. Loth, J. Fernández-Rossier, in preparation.
[2] F. Delgado, J. Fernandez-Rossier, Phys. Rev. Lett. 108, 196602 (2012)

Magnetic coupling between transition metal atoms that are linked through ligand p-orbitals relies on the virtual exchange of electrons between neighboring sites. The intricate characteristics of the resulting superexchange coupling rely on a complex interplay between electron hopping and Coulomb interaction. In this talk I will review recent experiments on individual superexchange coupled d-metal atoms placed inside a covalent surface network. Using low-temperature scanning tunneling microscopy the atoms are positioned with atomic precision and their quantum-magnetic properties investigated locally. Our experiments reveal novel insights into the resulting hybridized orbital configuration that are of importance in the fields of molecular magnetism and strongly correlated electron systems.