The nitrogen-vacancy (NV) center is a remarkable point defect in diamond that is at the frontier of quantum technology. In particular, the NV center has many exciting applications as a quantum sensor in nano-metrology, including magnetometry, electrometry, thermometry, piezometry and gyroscopy. Indeed, it is possible for a single NV center to measure the complete three-dimensional vector of a local electric field or the position of a single elementary charge in ambient conditions. The promising applications of the center have driven fundamental inquiry into its properties, which has yielded a rich understanding of its physics. However, there remain several unresolved issues that bear implications for the center&’s existing applications and whose resolution may facilitate the innovation of new applications.
The unresolved issues include: a first-principles model of the NV spin&’s temperature susceptibility, the pressure and mechanical stress response of the NV spin, the properties of the intermediate spin-singlet levels and their role in the NV optical spin polarization and readout mechanism, the electronic structure and optical spin polarization mechanism of the neutral NV center, and the requirement for orientation calibration of a NV sensor to fully realize the capabilities of NV vector electrometry.
A first-principles model of the NV spin&’s temperature susceptibility has implications for the optimization of NV nanothermometry techniques. Precise measurements of the pressure and stress response of the NV spin will facilitate the innovation of a diverse range of NV spin-mechanical sensors. New knowledge of the spin-singlet levels and their role in the NV optical spin polarization and readout mechanism may provide the requisite insight to engineer enhanced spin preparation and readout fidelities. Beyond enabling NV vector electrometry, the design of a technique to measure the orientation of a single NV center&’s structure will potentially find other applications in nanoscale multi-axis rotation sensing. Finally, the establishment of key aspects of the neutral NV center&’s electronic structure will enable an improved understanding of its optical spin polarization mechanism and, perhaps, the first insights into the spin-charge dynamics of the NV center.
In this presentation, I will report recent experimental and theoretical advances in the resolution of each of the above issues and further discuss the implications of these advances for the center&’s quantum sensing applications.

Nitrogen-vacancy center (NV) in diamond exhibits superior optical and spin properties that can be harnessed to detect magnetic and electric fields as well as temperature. These properties were mostly demonstrated in bulk diamond that made NV very promising in sensor applications operating at the nanoscale. To this end, NV centers are engineered very close to the surface of diamond in order to detect external signals. However, the properties of NV center may be deteriorated by the presence of the surface of diamond which stems to harness NV centers in this very important field. Relatively little is known about the properties of diamond surfaces, particularly, how the surface may interact with close-by NV centers. Ab initio quantum mechanical simulations with predictive power can be very useful in understanding these phenomena. We applied advance density functional theory calculations on (001) diamond surfaces with using different combination of terminators, in order to determine their electronic structure and their interaction with a near-surface NV center. We identify the combinations of surface terminators that are ideal to host NV centers for nanoscale sensing. This work was financed by EU FP7 grants DIAMANT and DIADEMS, as well as the Lendület program of the Hungarian Academy of Sciences.

Nitrogen-vacancy (NV) defect centers formed a few nanometers from the surface of diamond can serve as sensitive, high-resolution tools to probe and image spins external to the diamond. In this work, we apply NV-based magnetic sensing to two types of electronic spin systems: 1) spins naturally residing at the surface of diamond and 2) gadolinium spins affixed to a scanning probe tip. Diamond surface spins are now known to be largely responsible for the decoherence of NV centers formed near the diamond surface and hence present a significant obstacle to high-sensitivity nanoscale magnetic imaging using current protocols. We characterize the magnitude and timescale of magnetic noise intrinsic to diamond by measuring the spin coherence of NVs formed at several nanoscale distances from the surface, and we show that the depth dependent part of this noise is consistent with the source being a two-dimensional electronic surface spin bath [1]. These results can then be used to determine the NV&’s magnetic sensitivity under various magnetic sensing schemes. Importantly we find that, even though the longitudinal relaxation time T1 of shallow NVs decreases with proximity to the surface, the T1 remains sufficiently long to make NV-based relaxometry a viable technique for imaging spins of single ions with nanometer resolution.
To this end, using individual shallow NVs as T1 relaxometry sensors, we have recently demonstrated scanning probe imaging of gadolinium spins with < 20 nm lateral resolution in ambient conditions [2]. Gadolinium (Gd3+) ions are particularly interesting spin systems with applications in biology such as spin labels and MRI contrast agents. We discuss ongoing efforts towards single Gd3+ spin imaging sensitivity, including the chemical modification of silicon atomic force microscopy tips with a sparse layer of Gd3+ end#8209;functionalized polymers, and reductions in NV-Gd separation via shallower NVs to reduce pixel acquisition time and hence mitigate the effects of long-term thermal drift. With practical improvements, this scanning probe relaxometry technique exhibits potential for imaging the structure of individual spin-labeled biomolecules.
[1] B.A. Myers, A. Das, M.C. Dartiailh, K. Ohno, D.D. Awschalom, A.C. Bleszynski
Jayich, Phys. Rev. Lett. 113, 027602 (2014).
[2] M. Pelliccione, B.A. Myers, L.M.A Pascal, A. Das, A.C. Bleszynski Jayich,
Arxiv:1409.2422 (under review).

Atomic-scale defects in bulk and nanocrystalline silicon carbide (SiC) are promising for quantum information processing, photonics and sensing. Their spin state can be initialized, manipulated and readout by means of optically detected magnetic resonance [1,2]. Using this technique we recently demonstrated that the ground state has spin S = 3/2 and a population inversion can be generated using optical pumping, leading to stimulated microwave emission even at room temperature [1]. By controlling the neutron irradiation fluence, the defect concentration can be varied over several orders of magnitude down to a single defect level [3]. Here we demonstrate that these atomic-scale defects can be also attractive for local or environment sensing. We identified several, separately addressable spin-3/2 centers in the same crystal, which can be used either for magnetic field, or temperature measurements. Some of these defects are characterized by nearly temperature independent axial crystal fields, making these centers very attractive for vector magnetometry. Contrarily, the zero-field splitting of another center exhibits a giant thermal shift, which can be used for thermometry applications [4].
[1] H. Kraus, et al., Nat. Phys. 10, 157 (2014).
[2] A. Muzha, et al., arXiv:1409.0756.
[3] F. Fuchs, et al., arXiv:1407.7065.
[4] H. Kraus, et al., Sci. Rep. 4, 5303 (2014).

Recent results will be presented on manipulation methods of nitrogen-vacancy (NV) centres in diamond which significantly improve the sensitivity of single-spin magnetometers. These methods are also beneficial for applications in quantum information, and can be applied to similar crystalline defects. They can furthermore be used to simplify device structures for the control of arrays of defects. In this context, progress on the integration of NV centres into scalable, silicon-based microcavity arrays will be shown [1]. A theoretical scheme for large-scale quantum computation based on this system will be described which takes into account the known imperfections of the NV centre and places realistic requirements on the experimental apparatus [2].
Finally, findings on the use of NV ensembles in magnetometry will be outlined, including the use of optimal control techniques for improved sensitivity, and the implementation of NV-based sensors in practical, robust devices.
[1] “Arrays of open, independently tunable microcavities”, C. Derntl et al., Optics Express 22 (2014)
[2] “Photonic Architecture for Scalable Quantum Information Processing in Diamond“, K. Nemoto et al., Phys. Rev. X 4 (2014)

Surface plasma waves on metals arise from the collective oscillation of many free electrons in unison. Since the surface plasma waves can be quantized similarly to electromagnetic waves in free space, their quanta—surface plasmons—should exhibit the same quantum phenomena as photons do. Classical electromagnetism captures the essential physics of these “surface plasma” waves using simple models with only macroscopic features, accounting for microscopic interactions in metals. In quantum theory microscopic interactions should be taken into account, as any substantial environmental interactions could decohere quantum superpositions of surface plasmons.
Here we report on the two experiments that test the quantum properties of plasmons and study the decoherence of their path-entangled state. The first experiment is a plasmonic version of the Hong-Ou-Mandel experiment in which we observe two-photon quantum interference (TPQI) between plasmons with a visibility of 93%, comparable to the visibility of TPQI between dielectrically-guided photons. To make this measurement, we produce pairs of single photons by spontaneous parametric down-conversion and couple them into low-loss silicon nitride waveguides that deliver them to and collect them from plasmonic directional couplers. This hybrid dielectric-plasmonic platform enables us to couple single photons into and out of plasmonic components with relatively high efficiency, resulting in high count rates and error bars of order 1%.
In the second experiment, we extend this platform to investigate path entanglement in circuits that involve plasmonic elements. We use TPQI at a dielectric 50-50 directional coupler to prepare a path-entangled two-photon state, then send the photons through plasmonic waveguides, and finally let them interfere at a second dielectric coupler to determine whether they remain entangled. We report a measurement of path entanglement between surface plasmons with 95% contrast, confirming that a path-entangled state can indeed survive without measurable decoherence. Our measurement suggests that elastic scattering mechanisms of the type that might cause pure dephasing in plasmonic systems must be weak enough not to significantly perturb the state of the metal under the experimental conditions we investigated.

Quantum interference (QI) in the spontaneous emission from nearly degenerate excited states to a common ground state leads to a variety of remarkable effects, such as coherent population trapping, quantum beats, ultranarrow spectra lines, phase control, lasing without inversion, efficient quantum photo engine, to name a few. In free space for QI to rigorously manifest itself a stringent criterion of non-orthogonal transition dipole moments, corresponding to the decay channels, must be satisfied (Ρ₁. P₂ ne;0). This constraint, stems from the fact that in free space spontaneous emission occurs in two mutually-orthogonal modes of polarization, is rarely met in atomic physics. However, by breaking the isotropic nature of quantum vacuum one can overcome this limitation and restore QI even for orthogonal transitions. Several proposals for creating such an anisotropic quantum vacuum (AQV) to induce and enhance QI have recently been put forward, but these are either limited to confined geometries (sub-wavelength optical cavities) or the near-field of plasmonic metamaterials. Loading quantum emitters to the near field (within a few tens of nm) of such configurations is extremely challenging from an experimental perspective; therefore, ideally we would like to create a strong AQV over remote distances (from the material boundary) but this has never been accomplished until now. Here, profiting from recent advances in the field of plasmonic metasurfaces we introduce and theoretically demonstrate a long-sought-after solution to this problem. We harness the phase-control ability and polarization-dependent response of a judiciously designed gradient metasurface to tailor the quantum vacuum and induce strong anisotropy for quantum emitters (d #8811; lambda;₀) at macroscopic distances. On the basis of rigorous calculations, we show that for our considered configuration the decay rate of a remote quantum emitter is strikingly different in orthogonal directions (i.e., highly anisotropic). Such a quantum vacuum engineering can, among others, serve as an important step towards bringing metasurfaces, applications of which to date have mainly focused on classical fields, into the realm of quantum electrodynamics (QED). QED based on gradient plasmonic metasurfaces may open new possibilities for long-range interactions be- tween quantum emitters in many-body physics, decoherence-free subspace for quantum information processing, solid-state quantum optics and spintronics.

We report a novel growth strategy to achieve defect free mono-modal In(Ga)As/GaAs quantum dot (QD) distribution in strain coupled heterostructures for infrared detector applications. In conventional multilayer strain coupled InAs/GaAs structure, grown by molecular beam epitaxy, typically 2.7 ML QD is used in seed layer and kept it (2.7 ML) constant in subsequent layers. Therefore the overgrowth percentage is changing layer by layer which caused defects and also produce multimodal distribution of dots and effectively degrade device performances.
In our proposed structure, we introduced a seed In(Ga)As QD layer of 8 ML separated from the next QD layer by 25 Å GaAs. In this layer, the 2D to 3D transition commences at 4.7 ML and the overgrowth thickness is 3.3 ML (41.25%). Now when the subsequent QD layer of 5 ML is grown instead of 8 ML, transition occurs at 3 ML and we have overgrowth thickness of 2 ML (40%). Hence with this approach we were able to maintain constant overgrowth percentage throughout the layers. A high intense photoluminescence (PL) spectrum is observed at 1050 nm wavelength which indicates good optical quality of the material. Power dependent PL spectra confirmed that we have obtained mono-modal structure compared to conventional coupled sample where we observed multimodal distribution.
To check the luminescence properies of QDs at high temperature, the samples were further subjected to rapid thermal annealing treatment for 30 s each at 650 °C, 700 °C, 750 °C, 800 °C and 850 °C temperature in an argon atmosphere.Thermal stability in PL peak position is observed till 750^oC annealing temperature. The activation energy (calculated from temperature dependent PL spectra) for as-grown sample was 163.78 meV, and for its annealed counterparts, the activation energy was 166.36 meV, 164.64 meV, 161.2 meV, 143.09 meV, and 135.33 meV for annealing temperatures of 650 0C, 700 0C, 750 0C, 800 0C, and 850 0C, respectively. No significant change in activation energy is observed till 750 0C annealing temperature indicating stability in QD confinement at high temperature. With this proposed structure, we have fabricated infrared photodetector, where low dark current density (~2.67 x 10^-6 A/cm² ) and two color spectral responses in mid-IR wavelength at 4.54 and 3.92 µm were achieved. Therefore, this newly proposed coupled QD structure can be used as an alternative to commercially available photodetectors and cameras for imaging applications in the mid-IR region (3-5 µm).
DST, India and Riber, France are acknowledged.

There has been long-standing interest in and use of silicon carbide (SiC) due to its excellent electronic, mechanical, and thermal properties. Recently, spin-active, optical defects have been identified and studied in the 3C, 4H, and 6H polytypes of SiC, which has made SiC an intriguing material for quantum information and sensing applications. In particular, six defects that emit in the near infrared and show long coherence times have been studied in 4H-SiC. High quality optical cavities, engineered to be resonant with the frequency and spatial location of the defects, would be of tremendous importance in developing these new applications. Cavities that couple to the local spin states of these defects would allow longer-range, photon-mediated transmission of that information.
Although several groups have demonstrated optical cavities in SiC, these approaches utilized non-lattice-matched materials such as SiC-on-insulator or SiC grown on Si. Such approaches allow the selective etch-removal of the underlying substrate, important in achieving optical isolation of the cavity. However, the mismatch between SiC and the underlying material can potentially result in the formation of additional defects which could then compromise the quality of the cavity as well as the performance of the material itself.
Our earlier work thus focused on homo-epitaxial 4H-SiC materials and employed a dopant selective photoelectrochemical (PEC) etch with a P-type epilayer grown on an N-type substrate. The PEC etch in KOH allows for removal of the N-type substrate, thereby undercutting devices formed in the epilayer. Using this technique, we demonstrated microdisks with Q > 9000.
This work describes the use of the PEC undercut technique to form one-dimensional SiC photonic crystal cavities (PCC), which have a few advantages in achieving coupling to spin-active defects in the SiC. Indeed, these nanobeam PCCs are distinguished by their very small mode volume (V) < (lambda;/n)³ and very high simulated Q (> 10⁶). The wavelength of the fundamental TE mode can also be easily controlled by tuning the lattice constant of the PCC. The first nanobeam cavities were designed to exhibit modes at a wavelength of ~700 nm, in the visible. The cavities consist of Bragg mirror regions on either side of an inner region of 8 or 16 holes where both lattice constant and hole size are linearly tapered. Cavities with both circular and elliptical inner holes were fabricated. The cavities were fabricated using electron beam lithography, reactive ion etch-transfer, and PEC etching to form the undercut structures. Photoluminescence measurements revealed cavity Q values of > 5000. We also compared the Q and resonant wavelength of >200 cavities to better understand which cavity geometries are optimal. The high values of Q obtained for these SiC visible-range nanobeam cavities give confidence in the ability to form nanobeam PCCs with modes in the near IR, resonant with 4H-SiC spin defects.

Colloidal carbon dots (C-dots), as one class of photoluminescent (PL) materials have attracted considerable attention. Compared to colloidal semiconductor quantum dots (QDs), C-dots exhibit relatively good bio-compatibility with low toxicity, and surface modification could be performed readily. In this presentation, we report our newly-developed one-step microwave approach to water-dispersible C-dots. A water-soluble molecule folic acid (FA) and a water-soluble polymer polyethylenimine (PEI) were used to facilitate our approach which is simple and low-cost. The resulting C-dots exhibit excitation-independent emission behaviors in water (with the excitation wavelength in the range of 300 nm - 460 nm). Also, we report our preliminary efforts on gene transfection assisted by our C-dots. These C-dots are positively charged due to the presence of the cationic polymer PEI. We argue that the positively-charged nature could enhance the efficiency of gene transfection. It seems that the positively-charged nature is essential for plasmid immobilization, condensation, and escape from endosome and lysosome. Our study suggests that positively-charged nanoparticles may have potential in gene therapy.

ABSTRACT
Resonator-Quantum Well Infrared Photo detectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). In order to improve thermal sensitivity, integration time, and operating temperature, a theoretical optimization of R-QWIPs has been performed. For 25 micron pitch, 10 Me^- integrated charge, F/2 optics, and 9.2 micron cutoff, NETD can be 19 mK at Tint = 1.43 ms and T = 77 K. For 6 micron pitch, 6.8 Me^- integrated charges, and F/1 optics, NETD = 19 mK at 4.0 ms and 77 K. To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements (DE) and the thickness of the active resonator must be uniformly and accurately realized to within 0.05 mm accuracy and the substrates of the detectors have to be removed totally to prevent the escape of unabsorbed light in the detectors. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Using these etching techniques, we have studied single detectors and fabricated FPAs with pixel pitches from 6 to 30 microns and formats from 256 x 256 to 1920 x 1080. Both contact and step and repeat projection systems were used to pattern the wafers. The detail of the FPA fabrication and their characteristics will be presented

Most industrial fields concentrate on resource efficient manufacturing as well as cost and time reduction in the production process. The integration of sensors in lightweight structures to ensure structural safety in the automotive or aircraft engineering has great potential to meet the mentioned requirements.
Hence, novel concepts for the manufacturing of highly sensitive sensors and their integration into process lines are necessary. With regard to a fast and efficient manufacturing of nanocomposite-based sensors the employment of printing technologies offers great potential.
In our paper we present a sensor device consisting of a piezoelectrical layer, a silver bottom electrode, a composite layer with embedded quantum dots (QDs) and a transparent top electrode. The sensor is dedicated to detect mechanical stresses and indicates it by a color change in the photoluminescence (PL) of the integrated QDs.
The inkjet printing technology as a high accurate depositioning method is used to print the conductive electrodes of the sensor. For the bottom electrode a nanoparticle silver ink and for the transparent top electrode a PEDOT:PSS ink is used. The different inks were optimized in terms of printability and opportunities to sinter the nanoparticles without influencing the active layer of the sensor. Different pre-treatment methods of the substrate (UV and plasma) are investigated to achieve optimal printed layers regarding exact contours. Additionally, suitable functionality formation techniques, especially thermal and NIR sintering, which do not affect the functionality of the active composite layer are investigated. Finally the printed patterns are morphologically and electrically characterized in detail.
The quantum dot-based sensor manufactured on flexible substrates by inkjet printing demonstrates remarkable potential for a large scale production process which is time and cost effective.

Recent diamond nanophotonics work has shown tremendous promise in using color centers in diamond as the basis for quantum qubits and single photon sources for quantum key distribution [I. Aharonovich and N. Elke Advanced Optical Materials (2014)]. We present a pathway to deterministically create color centers in diamond using counted top-down ion implantation. This work would combine focused ion beam implantation using the Sandia National Laboratories (SNL) nanoImplanter (nI) and in-situ ion beam induced charge (IBIC) detection. The nI is a 100 kV focused ion beam system combining a <10 nm spot with a mass velocity filter allowing the user to choose the ion species and energy from a liquid metal alloy ion source (LMAIS). This allows for user access of approximately one-third of the periodic table of elements. The in-situ IBIC detector builds on SNL recent success in producing deterministically counted single ion implanted devices for quantum qubit research using low energy heavy ions and the on-going work showing the ability to detect single ion strikes using IBIC [J. Forneris et al., EPL 108 18001 (2014)]. In sum, we see a path forward to creating counted color centers in diamond. The largest unanswered question is the observed yield of color centers for low energy heavy ion implantation, although recent work [S. Tamura et al., App. Phys. Exp. 7, 115201 (2014)] has indicates up to a 15% yield of SiV with low energy implanted Si potentially pointing the direction to improved yields.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Using natural abundance Si as source material, we use mass filtered ion beam deposition to grow epitaxial films of ²⁸Si with an isotope fraction of 99.99996 % (0.3 ppm residual ²⁹Si). Highly enriched ²⁸Si is a critical material for solid state quantum information because it interacts weakly with the spin states of electrons in quantum dots or embedded single atom optical qubits such as ³¹P donors. Removal of the 4.7 % ²⁹Si nuclear spins allows for exceedingly long coherence (T₂) times, approaching an hour at room temperature for ³¹P nuclear spins. Additionally, the mass homogeneity of enriched ²⁸Si produces resolvable hyperfine transitions for optically addressing various dopants. Despite the importance of ²⁸Si to quantum information systems, it is an extremely scarce material. We use scanning tunneling microscopy (STM) and transmission electron microscopy (TEM) to investigate the crystalline quality of our epitaxial ²⁸Si films. Scanning electron microscopy (SEM) is also used to study the growth morphology at different temperatures. Electrical measurements using MOSCAPS and electron spin resonance (ESR) measurements are also being pursued to characterize our films. Numerous experimental systems can take advantage of ²⁸Si as a medium for qubits including STM-based hydrogen lithography devices, which we are investigating, single donors coupled to single electron transistors, and enriched SiGe quantum wells. These research efforts show a clear need for an additional source of ²⁸Si such as the one we demonstrate.

Nitrogen-vacancy (NV) center in diamond is a luminescent poin defect with various applications ranging from quantum information processing, high sensitivity magnetometry to biomedical sensing and imaging. In many applications it is important to control the charge state of the NV centre, because the negatively charged NV (NV-) centres have favourable optical and spin properties. In this work we investigate the effect of fluorine surface termination on stabilization of NV- centres close to the surface of nanodiamond. We show that for various size ranges of nanodiamond particles, the optical properties of fluorinated ND are the most stable in comparison with oxygen and hydrogen termination.

In this talk we show very specific properties of NV centers engineered in diamond to be used as nanoscale sensors operating in cells. High biocompatibility of nanodiamond (FND), and stable luminescence from its NV centers make FND an attractive alternative to molecular dyes. We present a principle of a novel method that can be used for remote monitoring of chemical processes in biological environment based on color changes from photo-luminescent (PL) NV centers in FND . Developed nanosensors provide real-time information about nucleic acid (NA) binding, delivery, and release into cytoplasm. The proposed method can be also used for the optical detection of variously charged polymers in buffer solution, showing the possibility of tracking charged molecules in biological environment.

Recently there has been great progress in implementing nitrogen vacancy centers in diamond and defects in SiC as sources of steady single photon emitters for application in quantum communication science. And report on photon anti-bunching effect has been made for neutron irradiated SiC materials. A single element photodiode (typically one or two SPAD detectors), biased above the breakdown voltage, is employed to measure the emitted single photons from these materials. Although, the SPAD detectors are sensitive to single photons and have reasonable, fast response time, they are unable to resolve multiple photon events (which could result from the photon emitting sites in these promising materials). At this conference poster session, we present our work on photon-number-resolving detectors, the silicon photomultiplier (SiPM), which are widely used in high energy physics and medical imaging research. We describe our investigation of characterizing the SiPM, the multipixel photon counters (MPPC) from Hamamatsu. To produce a few photons, suitable for single photon experiments, we make use of classical light, from small plastic scintillators, induced by energetic cosmic ray particles. Our experimental results include measurements on quantized photon number distribution, the dependence of detector gain on temperature and bias voltage. In addition, we describe our recent observation of the band-to-band light emission from the SiPM and its implication for secure communication. Our experimental setup consists of a 5 Giga sample/second domino ring waveform digitizer, the DRS4, in combination with standard NIM electronic modules. To isolate true photon signals from noise, the SiPM detector waveforms are triggered by coincidence between a pair plastic scintillator coupled to photomultipliers. A scintillator sheet embedded with blue to green wavelength shifting fiber is optically coupled to the SiPM detector and serves as the detector for characterization. Analysis of the digitized detector waveforms using signal processing techniques would also be presented.

An important emerging application of near-surface nitrogen-vacancy (NV) centers is the detection of nuclear magnetic resonance (NMR). NV centers located within a few nanometer of the diamond surface readily detect proton NMR signals from nanoscale samples placed on the diamond surface. Proton signals are detected even when no sample is present, indicating the presence of a proton-rich adsorbate layer due to physisorbed water molecules, hydroxyl (-OH), carboxyl (-COOH), or aliphatic (-CH₂) sources. Here, we describe the reduction of proton NMR background by application of deuterated glycerol that replaces the water layer or chemically exchanges surface protons with deuterium. We also demonstrate a significant, unexpected improvement to near-surface NV spin coherence.

Measuring and understanding the charge carrier dynamics and transport properties is a key requirement for further optimization of efficient organic light emitting devices (OLEDs) and for understanding their degradation processes on a microscopic scale. Up to now, however, standard characterization techniques such as time-of-flight and space charge limited current measurements are neither capable of in-situ characterization during operation nor can they provide information on the local variation of the electronic transport inside the device. Therefore, we propose an optical method based on single guest molecules which are doped into the emissive layer of a polymeric OLED. Monitoring the fluorescence of the probe molecules upon charge carrier injection we observe a quenching of the intensity due to the formation of long-lived, i.e. dark triplet states. Considering bimolecular Langevin recombination combined with an extended set of rate equations we are able to determine the current densities with a resolution of 10^-4 A/cm² in the proximity of a single molecule. In addition, the charge carrier mobility within the polymeric host can be derived in dependence of applied electric field according to the Poole-Frenkel law. This is successfully demonstrated for two commercially relevant OLED polymers SY-PPV and SPB-02T which are doped with the squaraine dye monomer M. Our results are in accordance with macroscopic characterization methods and yield zero-field mobilities µ₀ on the order of 10^-7 cm²/Vs for SY-PPV and 10^-9 cm²/Vs for SPB-02T. The degree of quenching at high electric fields is more pronounced in case of the SPB-02T OLEDs due to higher current densities. This fact is supported by an enhanced coefficient γ describing the field dependence and extracted to 7.5 x 10^-4 (m/V)^1/2 in contrast to 4 x 10^-4 (m/V)^1/2 determined for SY-PPV. With our single molecule based optical technique we are able to determine non-invasively the charge carrier dynamics during OLED operation with a local resolution of 15-20 nm corresponding to the Langevin capture radius in organic solid states.

I will describe progress on precision measurements using atom-like quantum defects known as Nitrogen-Vacancy (NV) color centers in diamond. In particular, NV-diamond provides an unprecedented combination of magnetic field sensitivity and spatial resolution in a room-temperature solid due to the remarkable properties of NV centers, including long electronic spin coherence times, optical spin polarization and read-out, a large Zeeman shift of the spin transitions, and the robust physical properties of diamond in a wide variety of forms (bulk crystals, films, nanocrystals, etc.). I will present recent results in areas such as sensing, imaging, and control of individual electronic and nuclear spins, and nanoscale imaging of magnetic fields from a wide range of samples in both the physical and life sciences -- from meteorites to biological cells under ambient conditions.

Over the past several decades, silicon carbide has evolved from being a simple abrasive to a versatile material platform for high-power electronics, extreme condition structural systems, optoelectronics, and nanomechanical devices. These technologies have been driven by advanced doping, CMOS compatible device processing, and the ready availability of large-area, single-crystal wafers. Recent advances have also established silicon carbide as a promising host for a novel class of quantum technologies based on the quantized spin of intrinsic color centers [1], with the potential of leveraging existing device platforms alongside solid-state quantum control. One family of color centers that stands out in particular is the family of divacancies, which are in many ways analogous to nitrogen-vacancy centers in diamond. These color centers emit near the optical telecom bands and have ground-state spin triplets that can be optically polarized, manipulated with microwaves, and have long spin coherence times that persist up to room temperature [2, 3]. We will present recent advances in this rapidly developing field including electrically driven spin resonance [4], coherent spin-strain coupling [5], the incorporation of silicon carbide defects into photonic crystal cavities [6], as well as the identification and manipulation of single spins [7]. In particular, the addressability of single spins in a material amenable to advanced growth and microfabrication techniques is an exciting route to technologies such as quantum repeaters, nuclear gyroscopes, and precision sensors inside of cells. This work is supported in part by the AFOSR and the NSF.
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. 107, 8513 (2010)
2. W. F. Koehl, B. B. Buckley, F. J. Heremans, G. Calusine, and D. D. Awschalom, Nature 479, 84 (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, Nature Comm. 4, 1819 (2013 )
4. P.V. Klimov, A. L. Falk, B. B. Buckley, and D. D. Awschalom, Phys. Rev. Lett. 112, 087601 (2014)
5. A. L. Falk, P. V. Klimov, B. B. Buckley, V. Ivády, I. A. Abrikosov, G. Calusine, W. F. Koehl, Á. Gali, and D. D. Awschalom, Phys. Rev. Lett. 112, 187601 (2014)
6. G. Calusine, A. Politi, and D. D. Awschalom. Appl. Phys. Lett. 105, 011123 (2014)
7. D. J. Christle, A. L. Falk, P. Andrich, P. V. Klimov, J. Hassan, N. T. Son, E. Janzén, T. Ohshima, and D. D. Awschalom, Nature Mat., in press (2014); arXiv:1406.7325.

Diamond based quantum technologies that utilise the nitrogen vacancy (NV) centre in diamond has seen rapid growth in diamond research over the past decade. The initial growth was driven by the fact the NV centre provides an ‘easy&’ to manipulate quantum system at room temperature along with opening up the possibility of a new material to deliver a solid state quantum computer. Since this time there are now a host of potential applications for the NV defect moving it from a quantum curiosity to a commercial development platform. These technologies are pushing the development needs of the material, and the processing of that material.
This paper will describes the advances in CVD diamond synthesis and diamond processing with special attention to producing engineered diamond for magnetometry applications.

Single electronic spins offer unique opportunities for sensing applications in fields ranging from mesoscopic physics over material-science to biology. Key to performance for such sensors are quantum coherent spins, which can be efficiently initialised, manipulated and read out.
The electronic spin system of the Nitrogen-Vacancy (NV) center in diamond offers these properties and is a leading candidate for practical implementations of many proposed quantum sensing tasks. Prime examples include nanoscale magnetometry, electric field imaging or force sensing.
In this talk, I will discuss recent experiments in my group towards the goal of promoting such diamond-based quantum sensors to practical, real-life applications. In particular, I will present our activities in the context of scanning NV magnetometry and hybrid systems consisting of NV spins coupled to diamond nanomechanical oscillators. Our experiments rely on dedicated and unique diamond nanofabrication processes, through which we obtain high-performance quantum sensors in form of all-diamond scanning probes or single-crystalline mechanical resonators. These structures allow for highly efficient optical interfacing of single NV spins, yield long NV spin coherence times and robust sensory devices - all essential requirements to obtain powerful NV quantum sensors.
I will present two classes of examples for sensing applications we recently realised. On one hand, we implemented a high-performance nanoscale scanning NV magnetometer, with which we investigated various magnetic nanostructures, such as thin-film magnetic domains and magnetic nanoparticles. On the other hand, I will present our studies of strain-induced coupling between a single NV spin and a diamond nanomechanical oscillator. These include the first quantitative determination of the corresponding strain coupling strengths and the demonstration of resolved sideband operation in our devices. Our results on this hybrid system demonstrate first essential steps towards further experiments in the quantum regime, such as spin-based oscillator sideband cooling or the recently proposed generation of spin-squeezing in nanomechanical oscillators, which in itself could form a valuable resource for future quantum sensors.

With atom-like properties and millisecond-scale coherence times at room temperature, nitrogen-vacancy (NV) centers in diamond are promising candidates for nanoscale sensors as well as hybrid quantum systems comprising spins and phonons or spins and photons. Single-crystal diamond mechanical resonators in particular are a versatile platform for spin-coupled mechanical systems [1] as well as for scanning probes for high resolution sensing [2]. The formation of NV-containing nanostructures in diamond that retain the NVs&’ high quality is key to many of these applications; however, challenges in diamond nanofabrication have limited the realization of such applications. Here, I describe advances in diamond fabrication that result in high-quality factor SCD mechanical structures. Using a combination of etching and a wafer bonding-based technique, we form thin films of SCD in the form of diamond on insulator (DOI), a versatile starting point for diamond mechanics and photonics [3]. I will present our results on strain-mediated coupling between a single NV spin and the high quality mechanical structures formed from DOI [4]. Current efforts towards reaching the quantum regime of coupling will be presented. Lastly, I will present results in which we use the DOI platform to fabricate a diamond-based scanning probe magnetometer with a single NV sensor at the apex of scanning tip, which promises non-invasive imaging with single spin sensitivity and nanoscale resolution.
[1] S. Bennett et al.Phys. Rev. Lett.110, 156402 (2013).
[2] P. Maletinsky et al.Nat. Nanotechnol.7, 320-4 (2012).
[3] P. Ovartchaiyapong et al. Appl. Phys. Lett.101, 163505 (2012).
[4] P. Ovartchaiyapong et al. Nat. Commun.5, 4429 (2014).

The nitrogen-vacancy (NV) center in diamond has proven to be a sensitive atomic-scale magnetometer capable of detecting nuclear magnetic resonance (NMR) signals from hydrogen and other spin species external to the diamond. Here, we demonstrate two dimensional imaging of ¹H NMR from an organic test object composed of poly(methylmethacrylate) using a single NV center as the sensor.[1] The NV center, which was created by shallow implantation into a layer of isotopically pure ¹²C diamond, detects the oscillating magnetic field from precessing protons in the test object as it is scanned past the NV center. A spatial resolution of roughly 12 nm was achieved, limited primarily by the scan accuracy. Improvements in resolution and signal-to-noise ratio should be possible with shallower NV centers and the use of optical nanostructures for enhanced photon collection efficiency.
[1] D. Rugar, H. J. Mamin, M. H. Sherwood, M. Kim, C. T. Rettner, K. Ohno and D. D. Awschalom, Nature Nanotechnol. (submitted).

Color centers in diamond can be used for implementing extremely powerful sensors as well as opening the pathway to a solid state quantum information processing. The key technology to fabricate these devices is the addressing of single atoms in a solid with high lateral resolution. Whereas the manipulation of single atoms at the surface is possible since several years, the three-dimensional addressing and placement requires substantial efforts. The combination of surface manipulation and overgrowth is one possibility but it is technically very challenging.
Ion beam implantation allows deposition of single countable atoms inside a given solid with nanometer precision. The two requirements to meet this goal are first to focus or collimate the ion beam and secondly to count the ions.
Mainly focused (FIB) [1] or collimated ion beams [2,3] are combined with a secondary electron detection or the detection of holes and electrons using a pn-junction structure [4]. Both methods depend on the target material. Counting single ions before they hit the surface is even more challenging - the development of a single ion source based on an ion trap is one solution [5] and realized by the Mainz group [6]. A micro-structured ion trap is capable of placing an exact number of atoms or molecules into solid state substrates with sub-nanometer precision for both depth and lateral position. Recent results proofing resolutions less than 10 nm are promising for the generation of coupled spin systems with nitrogen vacancy color centers in diamond. We have also implemented a single ion microscope for alignment of
implantation sites with respect to transparent surface markers [7].
Currently an alternative concept is under development at the Leipzig group, based on a commercial FIB lithography system equipped with a high brightness gas source.
The talk will discuss the state of the art of single ion nano-implantation methods and compare the results and prospects.
[1] T. Shinada et al. Nature, 437, 1128 (2005)
[2] T. Schenkel et al. J. Vac. Sci. Tech. 21, 2720 (2003)
[3] S. Pezzagna et al. Small 6, 2117 (2010)
[4] T. Hopf et al. J. Phys.: Condens Matter 20, 415205 (2008)
[5] W. Schnitzler et al. Phys. Rev. Lett. 102, 070501 (2009)
[6] K. Singer et al. Rev. Mod. Phys. 82, 2609 (2010)
[7] G. Jacob et al. arxiv.org:1405.6480 (2014)

The nitrogen - vacancy (NV) center in diamond is a promising platform for hybrid quantum systems, nanoscale magnetic imaging, and quantum photonics. Realization of these diamond-based technologies requires the deterministic placement and formation of near-surface NVs, which remains an outstanding challenge in the NV community. To address this challenge, we have developed a method to form localized vacancies in diamond via electron irradiation by a transmission electron microscope (TEM). By combining nanoscale lateral control of the TEM with nanoscale depth control afforded by nitrogen delta-doping during diamond growth [1], we demonstrate three-dimensional placement of NV centers. By varying the electron energy between 120 and 200 keV, we characterize the depth extent of NV formation as a function of energy and find that shallow NVs can be formed down to 145 keV. We also characterize the effects of electron dose (1e14 - 1 e19 e/cm²) on both the NV density and NV spin properties. We find that spin coherence times vary with dose (lower doses are more favorable) and importantly, we measure long spin coherence times in excess of 150 µs for 20 nm deep NVs. Lastly we discuss our work on leveraging the TEM&’s imaging capabilities to form NVs with 3-d deterministic positioning in nanofabricated diamond structures such as nano-mechanical resonators and photonic crystal cavities. The ability to place an NV center precisely in the maximum of an optical field or a strain field is an important enabling step towards the development of hybrid quantum systems that integrate high-quality, strongly-coupled NV centers.
[1] Kenichi Ohno, F. Joseph Heremans, Lee C. Bassett, Bryan A. Myers, David M. Toyli, Ania C. Bleszynski Jayich, Christopher J. Palmstroslash;m and David D. Awschalom Appl. Phys. Lett. 101, 082413 (2012)

Nitrogen vacancy centers in diamond are important due their great promise as quantum bits and for novel sensor applications. The reliable formation of NV centers of high quality and with high spatial resolution is still very challenging. We present results from studies of NV formation in the interplay of local electronic excitation and global thermal annealing. We find that NV centers can be formed with low efficiency by electronic excitation alone, without thermal annealing. Exposure to electronic excitations from low energy electrons (e. g. from a focussed, few keV electron beam in an SEM) or from swift, heavy ions also increases the NV formation efficiency during consecutive thermal annealing steps, compared to thermal annealing alone. We discuss insights into the formation kinetics of NV centers from these studies and outline possible directions for process development towards reliable formation of defect centers with desired properties (NV's and others).
Acknowledgments:
This work was performed in part at the Molecular Foundry and the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory and was supported by the Office of Science, Office of Basic Energy Sciences, Scientific User Facilities Division, of the U.S. Department of Energy under Contract No. DE-AC02—05CH11231 and by the Laboratory Directed Research and Development Program.

Negatively charged nitrogen-vacancy (NV) centers in diamond are a promising solid-state spin system for applications in quantum information, sensing and metrology. However, a key challenge for such solid-state systems is to realize a spin coherence time that is much longer than the time for quantum spin manipulation protocols. Our previous work [1] demonstrates an improvement of more than two orders of magnitude in the spin coherence time of NV centers compared with the measurements at room temperature once we removed the longitudinal spin relaxation T₁ limitation by cooling the sample to 80 K.
Here we extend the previous work to systematic studies on preserving arbitrary spin states, characterization of the effect of pulse errors and mitigating other experimental imperfections. We present comparisons of the experimental result obtained by implementing various dynamical decoupling multipulse sequences and show that the concatenated version of the XY8 pulse sequence is so far the best candidate for preserving an arbitrary electron spin state in high-density ensembles of NV centers in diamond. The results of these studies have an immediate impact on improvements of the sensitivities of AC magnetometry with NV ensembles.

A main source of noise in bulk diamond NV^- centers is the nuclear spins of C¹³, which can be mostly removed through isotopic purification. When used as quantum bits, these NV^- centers are placed in the bulk. When used as atomic-scale sensors of magnetic and electric fields, and temperature, the NV^- is very close to the surface of a nanodiamond. The surface can affect the energy levels of the NV^- center and change the charge state. Another source of noise is unpaired spins that can couple to the NV^- center. Using first-principles simulations of the OH-terminated (001) diamond surface, we characterize the exchange coupling between the NV^- center and defects with unpaired electrons on the surface of diamond, considering also the effect on the energy levels of the NV^- center. The exchange coupling between spins on the surface is also considered, as these defects can be correlated.
Prepared by LLNL under Contract DE-AC52-07NA27344.

Nitrogen vacancy (NV^-) color centers in diamond are a prime candidate for use in quantum information devices, owing to their spin-1 ground state, straightforward optical initialization and readout, and long intrinsic coherence times in a room-temperature solid. In practice, the coherence time is limited by the presence of fluctuating magnetic fields, usually from an electron or nuclear spin bath also present in the diamond. When the NV^- center is coupled strongly to several ¹³C nuclear spins, the system exhibits a higher level of complexity where transitions are of a mixed electron and nuclear spin character. Some of these transitions have effective gyromagnetic ratios approaching zero, isolating them from the relaxation effects of the fluctuating spin bath. Using optically-detected magnetic resonance, we identify at least nine of these transitions when the magnetic field is aligned along the NV^- axis that are observed over a range of field from 60 G to 350G. In a 99.99% ¹³C diamond, the inhomogeneous linewidth of observed transitions is reduced by factors as high as 60 over pure electron spin transitions. These results provide a method by which quantum information may be stored in long-lived coherences via a choice of magnetic field and RF pulses.

The nitrogen vacancy (NV) center is an atom-sized defect in diamond that is a remarkably good sensor of magnetic, electric, thermal, and strain fields on the nanoscale. Because of these sensitivities, the NV can be easily coupled to external degrees of freedom and in particular, its strain sensitivity makes the NV a promising element of a hybrid quantum system comprising spins and phonons. We have recently characterized the sensitivity of the NV&’s ground state spin to strain by controllably applying dynamical strain to NV centers embedded inside high quality factor diamond mechanical resonators[1]. Through strain coupling, we show that coherent mechanical control of individual spins in diamond is possible. These results are encouraging for proposals to use such a spin-mechanical platform for spin-squeezing, and phonon-mediated spin-spin interactions[2]. We discuss the necessary steps needed to reach these goals and current progress including improvements in diamond fabrication, NV formation, and readout techniques.
[1] P. Ovartchaiyapong, K.W. Lee, et al. Nat. Commun.5, 4429 (2014).
[2] S. Bennett et al.Phys. Rev. Lett.110, 156402 (2013).

Coherent control of the nitrogen vacancy (NV) center in diamond&’s triplet spin state has traditionally been achieved with ac magnetic fields. Although the selection rules governing this magnetic control forbid direct access to the |-1>harr;|+1> spin transition, we show that gigahertz-frequency lattice strain resonant with the spin state splitting can be used to coherently control NV center spins within this subspace. Moreover, this mechanical coupling arises from direct spin-phonon interactions that could enable non-classical interactions between NV center spins and mechanical resonators in future experiments. To achieve coherent mechanical spin control, we fabricate a bulk-mode mechanical microresonator from single-crystal diamond to apply large amplitude ac stress to the diamond substrate. Using this resonant stress, we mechanically drive Rabi oscillations between the |-1> and |+1> states of an NV center spin ensemble. We then measure the spin dephasing time of a mechanically driven {-1,+1} spin qubit and compare this to the dephasing time of the three magnetically-driven spin qubits that can be constructed within the NV center ground state. These results demonstrate coherent mechanical control of the magnetically forbidden |-1>harr;|+1> spin transition, thus closing the loop on NV center ground state spin control and enabling the creation of a coherent Δ-system within the NV center ground state.

Single defects in diamond, especially the nitrogen vacancy impurity (NV center), can serve as sensitive probes for magnetic fields with nanoscale spatial resolution. Our group&’s goal is to utilize NV centers for applications in nanoscale magnetic resonance imaging and nanoscale condensed matter physics.
In this talk we will discuss recent advances in pursuit of these goals, including the detection of small nuclear spin ensembles deposited on the surface of a diamond chip, and attempts to NV centers to measure the nanoscale texture and phase transition of magnetic materials. I will also highlight the importance of the diamond material, and in particular the diamond surface, for improving the properties of shallow (<10 nm) NV defects.

I will present recent results of our experiments to demonstrate nuclear magnetic resonance (NMR) on nanoscale samples, using a single NV center as a detector for nuclear magnetization.
This will include a review of the first proof-of-principle experiments detecting the nuclear magnetization of nanoscale samples [1,2], to introduce the experimental concept and spectroscopy protocols.
Mainly, however, I will focus on a recent demonstration of magnetic resonance imaging, using a single NV center as a scanning probe to detect nuclear spin ensembles with chemical contrast and a spatial resolution in the 10nm range [3].
[1] T. Staudacher et al., Science 339, 561 (2013)
[2] H.J. Mamin et al., Science 339, 557 (2013)
[3] T. Häberle et al., arXiv:1406.3324 (2014)
[4] D. Rugar et al., arXiv:1406.2983 (2014)

The nitrogen-vacancy (NV) color center in diamond is an atom-like system in the solid-state which specific spin properties can be efficiently used as a sensitive magnetic sensor. An external magnetic field induces Zeeman shifts of the NV center levels which can be measured using Optically Detected Magnetic Resonance (ODMR). In this work, we exploit the ODMR signal of an ensemble of NV centers in order to quantitatively map the vector structure of a magnetic field produced by a sample close to the surface of a CVD diamond slab hosting a thin layer of NV centers. The NV centers luminescence is collected by a microscope objective and imaged on a CMOS camera. Each image gives rise to a complete data acquisition over the whole object. The pump beam propagates towards the active layer thanks to total internal reflections on the main faces of the diamond slab, thus avoiding heating of the sample located underneath. The reconstruction of the magnetic field is based on a maximum-likelihood technique which exploits the response of the four intrinsic orientations of the NV center inside the diamond lattice. The sensitivity associated to a 1 mu;m² area of the doped layer, equivalent to a sensor consisting of approximately 10 ⁴ NV centers, is of the order of 2 mu;T / Hz^1/2. The spatial resolution of the imaging device is 400 nm, limited by the numerical aperture of the optical microscope which is used to collect the photoluminescence of the NV layer. The versatility of the sensor is illustrated by the accurate reconstruction of the magnetic field created by a DC current inside a copper wire deposited on the diamond sample.
Several improvements can be considered to increase the sensitivity of the device. Using a camera with a higher dynamics would allow increasing the signal to noise ratio. The CW pumping of our device may induce power broadening of the OMDR line that could be avoiding pumping in pulsed regime. The sample can also be improved. Increasing the yield of the N to NV conversion would allow decreasing the number of residual Nitrogen atoms surrounding the NV centers. This may avoid a possible broadening of the OMDR linewidth due to the nuclear spin bath.
The possible applications of such a device can go into several directions. The magnetic measurement based on NV centers is non perturbative and thus would be useful for applications such as spintronics where weak magnetic fields have to be detected, e.g. monitoring spin currents in graphene. As diamond is biocompatible, the present apparatus is noninvasive and biomedical applications are possible. For example, investigating the weak magnetic field induced by action potential propagating in neurons laying on the diamond crystal would allow better understanding of neural circuits response to external stimuli.

Nitrogen-vacancy (NV) centers in diamond, owing to their long spin coherence time at room temperature, have been extensively studied for quantum information processing and ultrasensitive metrology. In this work we propose NV centers as a promising system for room-temperature masers and microwave amplifiers. Maser is coherent stimulated emission in the microwave waveband (0.3~300 GHz frequency, or 10³~1 mm wavelength). Maser has played an important role in the early stage of studying lasers. Its applications, however, has not been as ubiquitous as lasers, due to the requirements of vacuum for atomic or free-electron maser and low temperature for solid-state masers. Room-temperature solid-state masers are highly desirable, but the difficulty lies in the fact that the lifetime of spins (emitters) under such conditions is usually too short to achieve population inversion and hence stimulated emission under practical pump condition. Long spin lifetime in organic materials has enabled a room-temperature solid-state maser, which, however, operates only at the pulse mode with a low repetition rate (~1 Hz) due to the high pump power needed. NV centers feature the longest known spin lifetime at room temperature among all solid-state systems (~10 ms, versus ~0.1 ms in organic materials). They also have high optical pumping efficiency (~10⁷ s^-1, versus ~10³-10⁵ s^-1 in organic materials). Large single crystals of diamond can be grown by CVD and the NV centers can be created by ion implantation with the density well tuneable. The coupling between a large number of NV center spins and a high-quality microwave cavity has been demonstrated. Our numerical simulation demonstrates that under readily accessible conditions (cavity Q-factor ~10⁵, pump rate ~10⁵ s^-1), a room-temperature diamond maser in the continuous wave mode is feasible. The maser, via superradiation, can sustain macroscopic quantum coherence among the spins with coherence time ~10⁵ s (corresponding to linewidth ~1 mu;Hz). When configured as a room-temperature microwave amplifier, the noise temperature is as low as ~0.1 K (versus ~1 K for the state-of-the-art ruby amplifier working at liquid-helium temperature). This work may facilitate a broad range of microwave technologies such as ultrasensitive magnetic resonance spectroscopy, astronomy observation, space communication, radar, and high-precision clocks.
This work was supported by Hong Kong RGC and CUHK Focused Investments Scheme.

The properties of the NV- center have been used in recent years for developing NV^--based scanning probe and wide-field magnetometers with high sensitivity and spatial resolution, capable of magnetic imaging of electron spins on the nanoscale. Their performance with respect to sensitivity and spatial resolution needs, however, significant improvements to image single electron and nuclear spins, needed for applications in future high-density magnetic storage disks or for studying proteins and cellular structures.
In this presentation we report about the realization of single crystalline diamond tips from ultra-pure diamond with typical geometrical dimensions of 75 -100 nm tip diameter, tip-lengths of ca. 1 µm, with conical shapes where the base diameter is about 300 nm. Each tip contains a single NV-center at the apex, realized by shallow (5 to 7 nm) implantation of nitrogen with a flux of about 10⁸ cm^-3 using 5 kV acceleration voltage. After implantation we applied thermal annealing at 800°C for 1h in vacuum to form NV centers. To optimize the optical excitation and read-out properties through the tips we have applied Comsol® calculations in order to determine required tip geometries. The geometrical properties of tips are characterized with scanning electron microscopy (SEM) and the optical properties with confocal micro-photoluminescence spectroscopy (µ-PL). For the fabrication of diamond NMR-tips we apply inductive coupled plasma (ICP) etching using oxygen reactive gas and titanium as mask material (ca. 80 nm diameter per tip). The titanium is deposited on diamond by application of a “two resist technique” in combination with e-beam lithography which will be introduced in detail. Finally, the tips are characterized by µ-PL experiments and anti-bunching measurements to investigate the yield of NMR-tip formation as well as the number of NV centers in each tip.

Single crystal diamond (SCD) is one of the most promising materials for photonics, optomechanics, MEMS/NEMS, quantum information processing and sensing devices. Other than its well-known exceptional mechanical and thermal properties, diamond is also a high refractive index and wide band gap material, unique in that it has known to host many colour centers ranging from UV to infrared.
To fully exploit fabrication of nanostructures in an integrated optical network, the SCD membrane needs to have thickness of the order of hundreds of nanometer (<1 µm). The fabrication of such thin SCD slabs is not a straight forward process. Commercially available Chemical Vapour Deposited (CVD) SCD plates are typically in the order of hundreds of micrometres. Recently, free standing thin SCD membranes as thin as 5 µm can be obtained commercially (Element Six, Applied Diamond Inc.). However, these membranes tend to have gradients in the thickness profile and their surface roughness is not ideal due to limitations of mechanical polishing. Furthermore, handling of these thin membranes has traditionally been a major obstacle to full scale fabrication of optical structures. Researchers have therefore used relatively thick 20-50mm SCD plates for their initial material which requires significant reactive ion etching.
Other techniques, such as ion implantation and lift off, may also be used to fabricate free-standing thin SCD membranes. However, these techniques introduce a residual built-in strain and damage resulting in membranes that are vulnerable to cracks, bowing and difficulty with handling which undermines subsequent processing of the membranes and degrades the optical quality of the membranes.
Many applications in the field of photonics and sensing technologies require high quality thin diamond membranes containing colour centres. However, for waveguiding the thickness of the SCD membrane is critical in satisfying the single mode condition in the vertical direction. Our aim is to fabricate optically active ultra thin SCD membranes with the thickness of the order of lambda;/n, where n~2.4 is the refractive index of the diamond. For example, the negatively charged Nitrogen Vacancy(NV) with the zero phonon line at lambda;=637 nm will thus require thickness in the order of ~300nm.
We demonstrate an entirely novel, reproducible, scalable and low cost method has been developed for fabricating optically active ultra thin SCD membranes. This method enables fabrication of thin and perfectly flat SCD membranes which are easily transferable and flip able for further fabrication of photonic structures, for instance microcavities, micro-ring resonators, waveguides. This present method of fabricating optically active ultra thin SCD membranes is a breakthrough in the field of diamond fabrication and serves as a user friendly platform for variety of quantum applications such as photonic, sensing, NEMS/MEMS and large scale integrated electro-optomechanic devices

The control of water quality, and more specifically radioactive contamination is a topic of ever-growing importance. Actinides are particularly harmful to the human body when ingested and state of the art methods for alpha radiation detection take days of time. Therefore, the development of fast in-situ measurement technology is of great interest.
A significant innovation in this field is the electrochemical precipitation of actinide atoms by local pH variations achieved with electrochemical grade diamond electrodes on top of Si-based p-i-n alpha particle sensor. In order to combine diamond and Si for sensor applications several technology steps had to be implemented and optimized.
In a first step, electronical decoupling of the electrochemical activities (boron-doped diamond electrodes) from Si-p-i-n sensing was achieved by depositing a roughly 100 nm thick SiO₂ layer. Next, a dense diamond nanoparticle seeding was applied to overgrow the SiO₂ layer with a diamond film of about 200 nm thickness without damaging the insolating layer. After overgrowth, Aluminum contacts have been evaporated and structured to integrate the hybrid sensor into a housing which was optimized for use with aqueous actinide solutions.
In this presentation we will summarize the fabrication details and discuss sensing results achieved with the presented device. For characterization we have applied FIB etching/SEM, current voltage measurements and cyclic voltammetry to reveal electrochemical as well as alpha-particle related sensing. This data will be compared with results achieved by conventional sensing techniques to elucidate the advantages of this new sensing device.

Nanocrystalline diamond (NCD) - based biosensors are being developed in recent years as tools to record and stimulate excitable cells¹. Whereas the use of bare NCD surfaces for adhesion and growth of primary neurons is debatable ² NCD exhibits beneficial chemical and biochemical inertness, high corrosion resistance, excellent mechano-optical properties and large surface area. Boron doped NCD form semiconducting or metallically conducting films with tunable surface chemistry. These properties make NCD and Boron-Doped NCD attractive material for electrodes or electrode coatings for the development of implantable electrodes to electrically communicate between neurons or muscles and peripheral prosthesis.
Our laboratories developed bio-inspired methodologies to improve the electrical coupling between neurons and electronic devices by introducing the protruding gold mushroom-shaped microelectrodes (gMuE) that are actively engulfed by neurons and thereby increase the seal resistance ^3,4. The aim of this work is to present technology enabling fabrication of diamond coated gMuE that is expected to enhance the electrode functional characteristics. Using focused ion beam milling^5,6 we prepared cross-sections of cultured rat hippocampal neurons on NCD coated gMuEs, cultured on NCD functionalized-gMuE matrix. After 6 DIV, the neurons were chemically fixed, dehydrated and sputtered by a 5 nm thick gold layer. Examination of cross sections revealed that the cultured neurons engulfed well the NCD coated gMuEs. The borderlines interfacing the gold surface of the gMuE and the NCD layer as well as that between the NCD and the cells plasma membrane are clearly visible. We estimate the space between the plasma membrane and the NCD-gMuE to be ~ 10 nm, indicating that the NCD-gMuE provides a very good cell adhesion surface. The approach described above constitutes the first step toward future studies focused on the understanding of how the cell's plasma membrane deforms in response to different NCD conformation, chemical surface functionalizations, electrode sizes and geometry.
1. Roy, R. K. & Lee, K.-R. Biomedical applications of diamond-like carbon coatings: A review. J. Biomed. Mater. Res. B Appl. Biomater.83B, 72-84 (2007).
2. Ojovan, S. M. et al. Nanocrystalline diamond surfaces for adhesion and growth of primary neurons, conflicting results and rational explanation. Front. Neuroengineering7, (2014).
3. Spira, M. E. & Hai, A. Multi-electrode array technologies for neuroscience and cardiology. Nat. Nanotechnol.8, 83-94 (2013).
4. Santoro, F. et al., On Chip Guidance and Recording of Cardiomyocytes with 3D Mushroom-Shaped Electrodes. Nano Lett.13, 5379-5384 (2013).
5. Santoro, F. et al., FIB section of cell-electrode interface: An approach for reducing curtaining effects. Microelectron. Eng.124, 17-21 (2014).
6. Santoro, F. et al. Interfacing Electrogenic Cells with 3D Nanoelectrodes: Position, Shape, and Size Matter. ACS Nano8, 6713-6723 (2014).

Single color centers in diamond have gained increased attention during recent years due to their relevance in spin physics. The applications of diamond in quantum devices include magnetic imaging on the nano-scale, single photon emitters for quantum cryptography and Qbits for quantum computing at room temperature [1]. The electron spin lifetime in diamond ranges up to milliseconds at room temperature which is superior to all other materials for quantum physics applications [2]. The spin decoherence time strongly depends on impurities and on ¹³C in diamond which needs to be optimized for such quantum devices. In addition, for quantum computing and magnetometry only the negatively charge NV center (NV^-) is important, which requires to control and fix the Fermi-level position with respect to the NV center.
NV centers can be generated either by implantation or by gas phase doping adding nitrogen to the CH₄/H₂ mixture. While implantation allows to control the spatial incorporation it also generates defects which cannot be annealed by post treatments. Gas phase doping on the other side allows to dope diamond with minimized compensating defects but the incorporation of nitrogen is random.
In this work, we present and discuss the fabrication of nitrogen delta-doped diamond in isotopically enriched (¹²C) layers. The homoepitaxy of (111) oriented diamond is carried out using microwave plasma enhanced chemical vapor deposition (MWPECVD) optimized for delta doping. The diamond seed can be moved into and out of the plasma without switching the plasma off. In addition, the diamond seed temperature is actively controlled, decoupling heating by the plasma from heating of the holder. The reactor is equipped with a load-lock system to realize UHV conditions (p < 10^-7 mbar), contains gas purification systems for methane (Zirconium) and hydrogen (Palladium) as well as an optical emission spectroscope to characterize the plasma properties.
We will introduce the homoepitaxial growth of isotopically enriched (¹²C) ultra-pure diamond. In order to reduce surface roughness, the growth is carried out on off-angle polished mesa-structures fabricated on (111)-substrates which are then overgrown to form atomically smooth surfaces. These layers have shown to contain nitrogen concentrations below 10¹⁴ cm^-3. Then, nitrogen delta-doped (10¹⁸ cm^-3) layers are grown on top of the mesas with thicknesses of a few nanometers. Finally, we add a thin (5-10 nm) cap-layer of ultra-pure diamond to generate long spin-coherence times of the NV- centers. The properties of these films have been characterized using SEM, SIMS, AFM, micro-Raman, confocal-micro-PL and Hanbury-Brown and Twiss interferometry.
[1] Koizumi, S., Nebel, C. E., & Nesladek, M. (2008). Physics and applications of CVD diamond. Weinheim: Wiley-VCH.
[2] Balasubramanian, G. et al. (2009) Ultralong spin coherence time in isotopically engineered diamond, Nat Mater, 8, 383-7.

Due to its unique properties, the Nitrogen-Vacancy (NV) color center in diamond is now at the heart of an increasing panel of applications, for quantum sensing and for the development of spin-based quantum information processing. In the past ten years, progresses achieved in chemical vapour deposition (CVD) techniques have allowed the production of high-purity single-crystal diamonds in which the quantum mechanical properties of the hosted NV defects can be optically manipulated and exhibit long coherence times even at room temperature. Generally the NV color center dipole is randomly aligned along one of the four possible [111] crystallographic axis, due to the C_3v symmetry of this point defect which is a one of the serious issue for the development of NV-based quantum technologies.
Recently, it was shown that preferential orientation of NV color centers can be partially obtained in diamond layers which are synthetized by CVD in a step-flow mode either on (110)- or (100)-oriented substrates [1,2] and that by growing a single-crystal layer on the non-conventional (111)-orientation, natively occurring NV defects are almost perfectly oriented (~97%) [3-5] perpendicularly to the surface [4] which is ideal to efficiently couple NV luminescence to photonic or magnetic nanostructures [7]. However growth conditions on (111) substrates are very difficult to control and the material quality is usually lower than for conventional (100) orientation. Moreover, (111) orientation is well known to be difficult to polish and process while impurities can be easily incorporated.
(113) is another possible orientation for making single crystal diamond growth and in this paper, we show that high quality thick and smooth diamond layers can be grown at growth rates as high as 20 µm/h . No residual stress has been measured allowing easier processing and polishing than (111) layers. Finally, the NV characteristics of these (113) oriented samples have been characterized; it has been found that 75% of the grown-in NV centers have a preferential orientation along one of the 4 directions. Since most of the applications involving NV defects rely on the long coherence of its electron spin state, the spin coherence properties was measured and compared to reference values for native NV centers in ultrapure layers grown on a (100)- and (111)-oriented substrates. The coherence times of grown-in NV centers were found be around 200 µs which is comparable to high-quality bulk (100) layers which highlights that this material would be suitable for quantum sensing applications.
[1] A. M. Edmonds et al. Phys. Rev. B86, 035201 (2012).
[2] L. M. Pham et al. Phys. Rev. B86, 121202 (2012).
[3] A. Tallaire et al. Diam. Relat. Mat.41, 34 (2014).
[4] M. Lesik et al. Appl. Phys. Lett.104: 113107 (2014).
[5] J. Michl et al. Appl. Phys. Lett. 104, 102110 (2014).
[6] T. Fukui et al., Applied Physics Express 7, 055201 (2014).
[7] E. Neu et al., Applied Physics Letters 104, 153108 (2014)

The negatively charged nitrogen vacancy centre (NV^-) in diamond has been extensively study in the last 5-10 years. This defect consists of a lattice vacancy, where one of the nearest neighbour carbon atoms has been replaced with a nitrogen atom, which has trapped an electron. The ground and low-energy excited states of NV^-/0 defects are derivable from the four sp³ hybrid orbitals on atoms bordering the vacancy; this is the so called “vacancy model” pioneered by the work of Coulson and Kearsley (Proc. R. Soc. London, Ser. A 241, 433 1957), that has been remarkably successful in leading to an understanding of the magnetic and optical properties of many defects. The ground state has S=1 and can be ~100% spin polarised by optical pumping with green light. It is well known that the spin state of a single NV^- defect can be detected optically at room temperature and this property has been exploited in demonstrations of high precision magnetic field imaging with a single spin sensor and quantum information processing.
The aggregation of nitrogen in diamond is believed to be driven by both vacancy and interstitial mechanisms but there is still some controversy over the details of the mechanisms. In recent years considerable progress has been made in identifying the single and di-nitrogen interstitial defects in diamond (S. Felton et al., J. Phys. Condens. Matter.
21(36), 364212, 2009), S. Liggins et al., PRB 81(8), 085214, 2010) and a good deal is known about the family of N_nV defects, where n=1-4. The charge states of the complexes in this family are determined by the availability of acceptors and donors. N₂V^- is believed to have a S=1/2 ground state, but until recently eluded detection by Electron Paramagnetic Resonance (EPR). N₃V⁰ has a S=1/2 ground state, but the complexity of the spectrum has previously precluded detailed study.
In this paper we present new EPR results on both N₂V^- and N₃V⁰ obtained by studying suitably processed diamonds which are doped almost exclusively with ¹⁵N. Spin polarisation results are also presented that upon first inspection are somewhat surprising. We show that the spin polarisation can be explained by a failure of the “vacancy model” and that a shallow electron trap, localized outside the vacancy, gives rise to the excited state(s) responsible for spin polarisation. The results stimulate discussions on the use of both N₂V^- and N₃V⁰ as photochromic memory or qubits.
Support from the EPRSC (EP/J007951/1: Quantum Information with NV Centres), De Beers UK Ltd and the Gemological Institute of America is gratefully acknowledged.

Since applications of NV centers in the field of bio-imaging (Balasubramanian, et al. 2014), solid#8208;state quantum information processing (Childress en Hanson 2013), or nanoscale sensing of magnetic fields (Rondin, et al. 2014) (Hong, et al. 2013), electric fields and temperatures (Schirhagl, et al. 2014) are based on the particular optical and magnetic properties of NV^- centers, controlling the charge state of NV centers is of crucial importance. The objective of our work was to study NV^- ground state occupation and to characterize NV centers charge transitions by a combination of sensitive optical techniques. For this purpose, single crystal HPHT diamond plates containing a high density of NV centers (~ 20 ppm) have been characterized by photocurrent spectroscopy, photoluminescence spectroscopy and optically detected magnetic resonance (ODMR). The influence of sample illumination with blue light of varying intensity (420 nm, up to 10 mW) during these measurements has been investigated and compared with the characteristics of photocurrent spectra. Blue illumination of the sample has been observed to induce a decrease in the absolute value of the ODMR signal, and to affect the intensity of photoluminescence associated to NV^-. The correlations established between these results have given information about NV center charge dynamic and NV^- ground state occupation.
Balasubramanian, G., A. Lazariev, S. R. Arumugam, en D. Duan. Curr. Opin. Chem. Biol. 20 (2014): 69-77.
Childress , L., en R. Hanson. MRS Bulletin 38 (February 2013): 134-138.
Hong, S., et al. MRS Bulletin 38 (2013): 155-161.
Rondin, L., J. P. Tetienne, T. Hingant, J. F. Roch, P. Maletinsky, en V. Jacques. Rep. Prog. Phys. 77, nr. 5 (2014): 056503.
Schirhagl, R., K. Chang, M. Loretz, en C. L. Degen. Annu. Rev. Phys. Chem. 65 (2014): 83-105.

New opportunities exist for super-resolution imaging and lithography using diamond color centers like the nitrogen-vacancy (NV) and the silicon-vacancy (SiV). One of these is magnetic resonance lithography (MRL) which is the inverse of magnetic resonance imaging (MRI). Another is light shift imaging (LSI), which is a variation of MRI wherein the gradient field is produced by optical light shifts instead of DC or microwave fields. Experimentally the NV remains the system of choice for room temperature applications. However at low temperatures the SiV has a number of important advantages including a strong zero phonon line, low spectral diffusion and inhomogeneous broadening, and strongly allowed optical Raman transitions suitable for electromagnetically induced transparency (EIT). This presentation will review the performance limitations of the NV and SiV in the context of super-resolution imaging and lithography with emphasis on the LSI and MRI techniques.

The relevance of super-resolution fluorence imaging techniques in modern optics and photonics is demonstrated by the recent awarding of the Nobel Prize in Chemistry for 2014 to E. Betzig, S. W. Hell and W. E. Moerner [1]. Super-resolution techniques such as STimulated Emission Depletion (STED [2]), Ground State Depletion (GSD [3]) and PhotoActivated Localization Microscopy (PALM [4]) allow a new class of microscopy studies in both biological and material science fields, among which it is worth mentioning the high-resolution imaging of single color centers in diamond [5].
In this work we demonstrate the application of a novel imaging technique based on the exploitation of photon autocorrelation statistics [6, 7] to the super-resolution imaging of isolated nitrogen#8209;vacancy (NV) emitters in single-crystal diamond [8].
Confocal photoluminescence (PL) microscopy images of NV centres in bulk CVD diamond were acquired on a pixel-by-pixel basis using a scanning laser beam. For each pixel, the PL intensity (i.e. I_PL(x, y)) was acquired, together with the values of second- and third-order autocorrelation functions in correspondence of zero delay time (i.e. g⁽²⁾(Δt=0, x, y) and g⁽³⁾(Δt=0, x, y)).
A substantial resolution enhancement was achieved by mapping proper polynomial combinations of the above-mentioned quantities, according to the theoretical predictions. In particular, the results from the mapping of 2- and 3-center clusters show that the increase in lateral resolution is in agreement with the theoretical model that predicts a narrowing of the point spread function proportional to the inverse of the square root of the highest order of the autocorrelation function measured.
We critically compare the advantages (broader applicability, experimental setup availability, hellip;) and potential disadvantages (limitations in the resolution and acquisition time) of this technique with respect to state-of-the-art super-resolution techniques.
References
[1] http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2014/
[2] S. W. Hell et al., J. Opt. Lett. 19, 780 (1994)
[3] S. W. Hell et al., Appl. Phys. B 60, 495 (1995)
[4] E. Betzig et al., Science 313, 1642 (2006)
[5] E. Rittweger et al., Nat. Photonics 3, 144 (2009)
[6] O. Schwarts et al., Phys. Rev. A 85, 033812 (2012)
[7] O. Schwarts et al., Nano Lett. 13, 5832 (2013)
[8] D. Gatto Monticone et al., Phys. Rev. Lett. 113, 143602 (2014)

The ability to map magnetic field distributions with high sensitivity and nanoscale resolution is of crucial importance for fundamental studies ranging from material science to biology, as well as for the development of new applications in spintronics and quantum technology. In that context, we follow a recently proposed approach to magnetic sensing based on optically detected electron spin resonance (ESR). It was shown that this method applied to a single nitrogen-vacancy (NV) defect in diamond could provide an unprecedented combination of spatial resolution and magnetic sensitivity under ambient conditions. In this talk, I will show how scanning-NV magnetometry can be used as a powerful tool for fundamental studies in nanomagnetism, focusing on domain wall structure in ultrathin ferromagnetic wires [1].
[1] J.-P. Tetienne, T. Hingant, J.-V. Kim, L. Herrera Diez, J.-P. Adam, K. Garcia, J.-F. Roch, S. Rohart, A. Thiaville, D. Ravelosona, V. Jacques, Nanoscale imaging and control of domain-wall hopping with a nitrogen-vacancy center microscope, Science 344, 1366 (2014).

Defects in the lattice of diamond lead to unique properties such as characteristic luminescence as well as magnetic properties. Many of these have been explored in detail for bulk diamond as defects can be generated by postproduction implantation of foreign atoms and/or vacancies.
For nanoparticles however, the generation of suitable defects in already existing nanoobjects is more challenging. High energy beam treatment typically leads to lattice damage and even partial or complete graphitization of the material.
Here, we report on the production and characterization of diamond nanoparticles from bulk and CVD diamond materials that already contain defect centers such as SiV,^[1,2] or NV centers^[3] and dopants, e.g. boron^[4] in the starting material. We could show that the properties of the defects are not altered upon mechanochemical treatment and suitable surface modification and show comparable spectroscopic properties. However, it was found that the concentration of defects is reduced as locations with high defect concentrations are also more easily broken apart in the mechanical treatment.
Nanodiamond particles with controlled defects are suitable for a variety of applications such as labelling, seeding of new diamond growth and quantum applications.
[1] E. Neu, C. Arend, E. Gross, F. Guldner, C. Hepp, D. Steinmetz, E. Zscherpel, S. Ghodbane, H. Sternschulte, D. Steinmüller-Nethl, Y. Liang, A. Krueger, C. Becher, Appl. Phys. Lett.2011, 98, 243107.
[2] E. Neu, F. Guldner, C. Arend, Y. Liang, S. Ghodbane, H. Sternschulte, D. Steinmüller-Nethl, A. Krueger, C. Becher, J. Appl. Phys.2013, 113, 203507.
[3] L. Liebermeister, F. Petersen, A. v. Münchow, D. Burchardt, J. Hermelbracht, T. Tashima, A. W. Schell, O. Benson, T. Meinhardt, A. Krueger, A. Stiebeiner, A. Rauschenbeutel, H. Weinfurter, M. Weber, Appl. Phys. Lett.2014, 104, 031101.
[4] S. Heyer, W. Janssen, S. Turner, Y.-G. Lu, W.g S. Yeap, J. Verbeeck, K. Haenen, A. Krueger, ACS Nano2014, 8, 5757-5764.

Fluorescent diamond nanoparticles (FNDs) represent a key component in recent development of ultra-high precision optical resolution techniques. FNDs can accommodate nitrogen-vacancy (NV) centers - an extremely photostable crystal lattice defect emitting in near-infrared region. Electron transitions among NV quantum states can be influenced by very weak external electric or magnetic fields, which have been utilized for construction of various types of probes and nanosensors. For application of FNDs in biological systems, a precise and better control of particles&’ surface and electronic properties is still required.
Different needs coming from chemistry and physics sides will be discussed and synthetic approaches towards bioapplicable FNDs will be presented. Specifically, boosting the emission intensity and narrowing its distribution within the FNDs, decreasing the polydispersity of particles and shaping them to become pseudospherical, creation of antifouling polymeric coating on FNDs and its bioorthogonal modification with various (bio)molecules using click chemistry, and targeting the cancer cells using these conjugates will be shown. Coating of FNDs with a thin gold layer providing plasmonic nanodiamonds and application of these nanoarchitectures as highly effective opto-thermal converters in cancer thermoablation will be also discussed.

Diamond NV centers have been shown to sensitively measure temperature with nanoscale resolution. Temperature sensitivities of down to few mK/sqrt(Hz) in bulk diamond and 0.1K/sqrt(Hz) have been reached. This sensitive is sufficient to measure heat production by chemical reactions with nanoscale resolution. A particularly interesting case is cellular metabolism. I will show how to use diamond nanocrystals to locally measure temperature changes as function of metabolic activity. The method has the potential to answer the long standing quest for reliable measurements of intra cellular temperature gradients. Such measurements can be complemented by additional sensor capabilities von the NV center. I will comment on our recent attempts of detecting single proteins in vivo and in vitro.

While all-optical logic has historically been difficult to implement, recent progress in the fabrication of nanoscale photonic devices and circuits promises to reduce the energy required per operation by orders of magnitude. Recently, devices containing semiconductor quantum dots have achieved switching near the single-photon level at low temperature [1], and switching energies below 1 fJ have been achieved in semiconductor devices at room temperature [2]. All-optical logic will most likely be used in optical communications applications such as network routing, where the energy and latency costs of converting optical signals to electrical signals and back could be avoided.
This talk will describe semiclassical simulations investigating the effects of quantum noise in coherent nonlinear photonic circuits operating with switching energies in the 10-1000 photon regime [3]. We also address the problem of fabrication variation, and describe a design approach and tuning algorithm to enable these circuits to tolerate a much higher degree of parameter variation. Experimental progress towards realizing photonic logic circuits in the laboratory will also be described.
[1] R. Bose, D. Sridharan, H. Kim, G. S. Solomon, and E. Waks, Phys. Rev. Lett. 108, 227402 (2012).
[2] K. Nozaki et al., Nature Photonics 4, 477 (2010).
[3] C. Santori, J. S. Pelc, R. G. Beausoleil, N. Tezak, R. Hamerly, and H. Mabuchi, Phys. Rev. Applied 1, 054005 (2014).

In contrast to classical physics, quantum measurements exhibit an intriguing interplay between information gain and system disturbance. Although the outcome of the measurement is probabilistic, the back-action imparted on the measured system is accurately described by quantum theory, given the measurement outcome. Therefore, quantum measurements can in principle be used to manipulate a quantum system without the need for control fields.
Here we report on the experimental realization of quantum control using sequential partial measurements with real-time feedback on a nuclear spin qubit in diamond [1]. We control the measurement strength via an ancilla qubit (electron spin of a nitrogen-vacancy center). Using post-selection, controlled wavefunction collapse and quantum weak values (spin exceeding 10) are observed. We then exploit a dynamical-stop quantum non-demolition measurement on the ancilla qubit to enable repetitive measurements without post-selection. By incorporating real-time feedback, we demonstrate measurement-only control of a quantum system.
On the application side, adaptive measurements enable the estimation of phase in interferometry with a variance scaling at the Heisenberg limit by using only a single qubit, relaxing the need of fragile entangled states. We will discuss applications of real-time feedback to adaptive sensing of magnetic field with high dynamic range [2].
[1] M. S. Blok et al., Nature Physics (2014)
[2] P. Cappellaro, Phys Rev A 85, 030301 (2012)

Because quantum states inevitably suffer from errors, scalable quantum computing requires quantum error correction [1,2]. The information of a single qubit is encoded in multiple physical qubits. Errors are detected by comparing these qubits and are subsequently corrected. Capitalizing on recently achieved control over multiple individual solid-state nuclear spin qubits in the environment of the nitrogen-vacancy (NV) color center [1,3] we present progress towards the implementation of quantum-error-correction [1].
We use the NV electron spin to detect individual nuclear carbon-13 spins within its unique spin bath [3]. We realize a method to control these weakly coupled spins using the electron spin. In this way we selectively initialize and control individual nuclear spins and construct high-fidelity gates. By implementing a three-qubit quantum-error-correction protocol at room temperature, we have shown that we can in principle protect a logical quantum state encoded in three nuclear spin qubits against errors [1].
Our current work is geared towards combining the quantum gates with high-fidelity projective readout of the NV electron spin [4]. The main outstanding challenge is to preserve nuclear spin superposition states while reading out the electron spin.
We will present our most recent results towards measurement-based error correction. This work allows for the precise mapping of the unique NV electron spin environment and subsequently transforms weakly coupled nuclear spins from a source of decoherence into a reliable resource. Our results enable control of multi-qubit registers, which together with long-distance communication between two NV centers [4,5] paves the way towards complex error-corrected network protocols and network-based surface code quantum computing [6].
[1] T.H. Taminiau et al., Nature Nanotech. 9, 171 (2014)
[2] G. Waldherr et al., Nature 506, 204-207 (2014)
[3] T.H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012)
[4] L. Robledo et al., Nature 477, 547 (2011)
[5] W. Pfaff et al., Science 345, 532-535 (2014)
[6] N.H. Nickerson et al., Nature Commun. 4, 1756 (2013)

Entanglement is one of the most intriguing phenomena in quantum science. The outcomes of independent measurements of the quantum state of entangled objects are correlated, even if the objects are space-time separated. This property, often called quantum non-locality, cannot be explained by classical physics and is a unique resource for quantum information processing and quantum communication.
Fifty years after its derivation, many experiments have violated Bell&’s famous inequality [1] and in this way proven quantum non-locality. However, all of these experiments rely on additional assumptions, most prominently the absence of signaling between the entangled particles, and fair-sampling of the full dataset when using inefficient detectors. Closing the ‘loopholes&’ that arise from these assumptions is one of the major research goals of experimental quantum physics, with applications ranging from device independent quantum key distribution [2] to the generation of provably-random numbers [3].
Towards the first experimental realization of a loophole-free Bell test, we employ two Nitrogen-vacancy (NV) centers in diamond that are located in independent setups at a distance of 1.3km. Remote entanglement is established via a robust protocol based on a joint measurement of single photons that are entangled with the electron spins of the two NV centers [4, 5]. The efficiency of our protocol is maximized by using micro-fabricated, anti-reflection coated diamond samples. At cryogenic temperatures, the resulting high photon-collection efficiency enables fast (<3 us) single-shot readout of the NV electron spin with a fidelity above 97%, which is in principle sufficient for a loophole-free test of Bell&’s inequality. We will present our latest results towards this goal.
References
[1] J.S. Bell, 1964, Physics 195. 1
[2] J. Barrett, L. Hardy, and A. Kent, Phys. Rev. Lett. 95, 010503 (2005)
[3] S. A. Pironio et al. Nature 464, 1021 (2010)
[4] H. Bernien et al. Nature 497, 86-90 (2013)
[5] W. Pfaff et al. Science 345, 532 (2014)