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Combining gravity with quantum mechanics remains one of the biggest challenges of physics. In the past years, experiments with opto-mechanical systems have been proposed that may give indirect clues about the quantum nature of gravity. In a recent variation of such tests [D. Carney et al., Phys.Rev.X Quantum 2, 030330 (2021)], the authors propose to gravitationally entangle an atom interferometer with a mesoscopic oscillator. The interaction results in periodic drops and revivals of the interferometeric visibility, which under specific assumptions indicate the gravitational generation of entanglement. Here we study semi-classical models of the atom interferometer that can reproduce the same effect. We show that the core signature -- periodic collapses and revivals of the visibility -- can appear if the atom is subject to a random unitary channel, including the case where the oscillator is fully classical and situations even without explicit modelling of the oscillator. We also show that the non-classicality of the oscillator vanishes unless the system is very close to its ground state, and even when the system is in the ground state, the non-classicality is limited by the coupling strength. Our results thus indicate that deducing entanglement from the proposed experiment is very challenging, since fulfilling and verifying the non-classicality assumptions is a significant challenge on its own right., Comment: 7 pages, 1 figure

Engineering single atomic defects into wide bandgap materials has become an attractive field in recent years due to emerging applications such as solid-state quantum bits and sensors. The simplest atomic-scale defect is the lattice vacancy which is often a constituent part of more complex defects such as the nitrogen-vacancy (NV) centre in diamond, therefore an understanding of the formation mechanisms and precision engineering of vacancies is desirable. We present a theoretical and experimental study into the ultra-fast laser generation of vacancy-interstitial pairs (Frenkel defects) in diamond. The process is described by a set of coupled rate equations of the pulsed laser interaction with the material and of the non-equilibrium dynamics of charge carriers during and in the wake of the pulse. We find that a model for Frenkel defect generation via the recombination of a bound biexciton as the electron plasma cools provides good agreement with experimental data, reproducing an effective non-linearity of $\sim$ 40 for Frenkel defect generation with respect to laser pulse energy., Comment: 17 pages, 11 figures

The objective of the proposed macroscopic quantum resonators (MAQRO) mission is to harness space for achieving long free-fall times, extreme vacuum, nano-gravity, and cryogenic temperatures to test the foundations of physics in macroscopic quantum experiments at the interface with gravity. Developing the necessary technologies, achieving the required sensitivities and providing the necessary isolation of macroscopic quantum systems from their environment will lay the path for developing novel quantum sensors. Earlier studies showed that the proposal is feasible but that several critical challenges remain, and key technologies need to be developed. Recent scientific and technological developments since the original proposal of MAQRO promise the potential for achieving additional science objectives. The proposed research campaign aims to advance the state of the art and to perform the first macroscopic quantum experiments in space. Experiments on the ground, in micro-gravity, and in space will drive the proposed research campaign during the current decade to enable the implementation of MAQRO within the subsequent decade., Quantum Science and Technology, 8 (1), ISSN:2058-9565

There is significant interest in potential experimental tests of macroscopic quantum effects, both to test potential modifications to quantum theory and to probe the quantum nature of gravity. A proposed platform with which to generate the required macroscopic quantum spatial superposition is a nanodiamond containing a negatively charged nitrogen vacancy (${\text{NV}}^{-}$) centre. In this review, methods to fabricate nanodiamonds containing ${\text{NV}}^{-}$ suitable for these quantum applications are discussed. The proposed probes of the macroscopic limits of quantum theory are presented along with the spin physics of the ${\text{NV}}^{-}$ centre relevant to those tests., 12 pages, 2 figures

We demonstrate a simple, robust, and contactless method for nondestructive testing of magnetic materials such as steel. This uses a fiber-coupled magnetic sensor based on nitrogen-vacancy centers (NVCs) in diamond without magnetic shielding. Previous NVC magnetometry has sought a homogeneous bias magnetic field on the diamond to improve the sensitivity. In contrast, here we show that the spatial resolution for imaging is improved by applying an inhomogeneous magnetic field to the steel even though this leads to an inhomogeneous magnetic field on the diamond. Structural damage in the steel distorts the inhomogeneous magnetic field and by detecting this distortion we reconstruct the damage profile through quantifying the shifts in the NVC Zeeman splitting. With a 1-mm magnet as the source of our inhomogeneous magnetic field, we achieve a high spatial resolution of 1 mm in the plane parallel and 0.1 mm in the direction perpendicular to the surface of the steel. This works even when the steel is covered by a nonmagnetic material. The lift-off distance of our sensor head from the surface of 316 stainless steel is up to 3 mm.

We extend the concept of dynamical decoupling from spin to mechanical degrees of freedom of macroscopic objects, for application in interferometry. In this manner, the superposition of matter waves can be made resilient to many important sources of noise when these are driven along suitable paths in space. As a concrete implementation, we present the case of levitated (or free falling) nanodiamonds hosting a color center in a magnetic field gradient. We point out that these interferometers are inherently affected by diamagnetic forces, which restrict the separation of the superposed states to distances that scale with the inverse of the magnetic field gradient. Periodic forcing of the mechanical degree of freedom is shown to overcome this limitation, achieving a linear-in-time growth of the separation distance independent of the magnetic field gradient, while simultaneously protecting the coherence of the superposition from environmental perturbations.

Nitrogen-vacancy centers (NVCs) in diamond are being explored for future quantum technologies, and in particular ensembles of NVC are the basis for sensitive magnetometers. We present a fiber-coupled NVC magnetometer with an unshielded sensitivity of $(310\ifmmode\pm\else\textpm\fi{}20)\phantom{\rule{0.2em}{0ex}}\mathrm{pT}/\sqrt{\mathrm{Hz}}$ in the frequency range of 10--150 Hz at room temperature. This takes advantage of low-strain ${}^{12}\mathrm{C}$ diamond, lenses for fiber coupling and optimization of microwave modulation frequency, modulation amplitude, and power. Fiber coupling means the sensor can be conveniently brought within 2 mm of the object under study.

Three-dimensional arrays of silicon transistors increase the density of bits. Solid-state qubits are much larger so could benefit even more from using the third dimension given that useful fault-tolerant quantum computing will require at least 100,000 physical qubits and perhaps one billion. Here we use laser writing to create 3D arrays of nitrogen-vacancy centre (NVC) qubits in diamond. This would allow 5 million qubits inside a commercially available 4.5x4.5x0.5 mm diamond based on five nuclear qubits per NVC and allowing $(10 \mu m)^3$ per NVC to leave room for our laser-written electrical control. The spin coherence times we measure are an order of magnitude longer than previous laser-written qubits and at least as long as non-laser-written NVC. As well as NVC quantum computing, quantum communication and nanoscale sensing could benefit from the same platform. Our approach could also be extended to other qubits in diamond and silicon carbide., Comment: 8 pages, 4 figures

We present characterization of a lock-in amplifier based on a field programmable gate array capable of demodulation at up to 50 MHz. The system exhibits 90 nV/sqrt(Hz) of input noise at an optimum demodulation frequency of 500 kHz.The passband has a full-width half-maximum of 2.6 kHz for modulation frequencies above 100 kHz. Our code is opensource and operates on a commercially available platform.

MASERs are devices that exploit stimulated emission to amplify electromagnetic waves at microwave frequencies. Their principal advantage over conventional (semiconductor-based) electronic amplifiers is lower noise. An optically-pumped, solid-state pentacence maser operating at room temperature in pulsed mode was demonstrated [1]. This was followed by the recent demonstration of a continuous maser at room temperature using colour centres (NV) in diamond [2] paving the way for a new class of masers that could find new applications in medicine, security and sensing, quantum computing/information processing.

Atomic defects in wide-bandgap materials, such as the nitrogen-vacancy (NV) color center in diamond, show considerable promise for the development of a new generation of quantum information technologies, but progress has been hampered by the inability to produce and engineer the defects in a controlled manner. Here, we demonstrate an all-optical method for the deterministic writing of individual NV centers at selected locations with high positioning accuracy. Ultrashort pulse laser processing is used to both create and diffuse defects inside the crystal through local annealing. During the laser-annealing process, online fluorescence feedback provides a trigger to stop processing once the NV formation is detected. This method provides a new tool for the optical fabrication of engineered materials and devices for quantum technologies.

In this work, we demonstrate two developments, in device and material engineering respectively, towards an efficient spin-photon device using colour centres in diamond for scalable quantum computing networks: firstly, we report the emission enhancement of the coherent zero-phonon-line transistion of an nitrogen vacancy centre in a diamond membrane on coupling to a tunable open-cavity; secondly, we present a new method for deterministic writing of NV arrays in bulk diamond with a 96% yield of single defects, a 50 nm positional accuracy in the image plane, and NV electron spin coherence (T2) times of up to 170 microseconds.

Laser writing of colour centres in diamond offers a new method for the engineering of quantum materials and devices. Here we will describe our latest progress in the writing of highly coherent nitrogen-vacancy centres and in the deterministic writing of NV centres using laser annealing.

The negatively charged nitrogen vacancy centre in diamond is known for its coherent spin properties and optical interface, and thus is regarded a promising candidate for quantum information applications [1]. Realisation of an efficient spin-photon interface with the NV centre is made challenging however by the fact that, in bulk diamond, only 3-4% of spontaneously emitted photons occur in the zero phonon line (ZPL). Placing NV centre in an optical cavity is being explored by several groups [2][3][4] as an effective way to selectively enhance the coherent emission of NVs and thereby increase the efficiency of the coherent spin-photon coupling. Previous work reported successful coupling of the NV in nano-diamond to an open access micro-cavity and observed enhanced ZPL emission [5]. However the NV centres in nano-diamond suffer from broadened zero phonon transition and poor spin coherence. By fabricating NV centres in a ~micrometre thick membrane of high purity single crystal material we can take advantage of the tunability of open access cavities, and at the same time, provide close-to-bulk crystal environment to maintain the coherent spin properties of the NV centres. Here we report our work on the tunable cavity coupling of the ZPL of a NV centre in a 1.2micrometre-thick diamond membrane at 4K. The diamond membrane is fabricated from a 0.5mm-thick E6 CVD diamond plate where ion implantation is carried out on both surfaces to create NV centres at the depth of around 70nm. The plate is then machined into 30micrometre-thick slices, and thinned by ICP-RIE with a combination of Ar/Cl[6] and pure oxygen plasma etching recipes. The open cavity consists of a concave mirror (99.99% reflectivity) deposited on a template fabricated using Focused Ion Beam (FIB) milling[7] and a planar mirror (99.8% reflectivity) which supports the membrane. For bare cavities with mirror radii of curvature (RoC) of 12micrometre, we measured a finesse of F~2000 and mode volume as small as 0.75micrometre^3. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. When mounted in our bath cryostat the cavity modes have dominant Lorentzian line profiles which indicate a passive stability of the cavity length of better than 0.15nm. No active locking is currently deployed. With the presence of a diamond membrane inside the cavities, the measured finesse and mode volume of a cavity with 12micrometre RoC are found to be around 300 and 3 micrometre^3, respectively. We attribute the reduction in finesse to scattering at the membrane-air and membrane-mirror interfaces. On coupling to the ZPL of a target NV centre, we record a factor of 4 increase in the saturated intensity of ZPL fluorescence compared to that measured from the same NV centre in absence of the concave mirror. This result is consistent with the calculated Purcell factor of 16 combined with a relatively low efficiency of light extraction (estimated to be around 19%) from the cavity due to the scattering losses.

Interferometric position detection of levitated particles is crucial for the centre-of-mass (CM) motion cooling and manipulation of levitated particles. In combination with balanced detection and feedback cooling, this system has provided picometer scale position sensitivity, zeptonewton force detection, and sub-millikelvin CM temperatures. In this article, we develop an analytical model of this detection system and compare its performance with experimental results allowing us to explain the presence of spurious frequencies in the spectra.

A compact detector for space-time metric and curvature is highly desirable. Here we show that quantum spatial superpositions of mesoscopic objects, of the type which would in principle become possible with a combination of state of the art techniques and taking into account the known sources of decoherence, could be exploited to create such a detector. By using Stern-Gerlach (SG) interferometry with masses much larger than atoms, where the interferometric signal is extracted by measuring spins, we show that accelerations as low as $5\times10^{-15}\textrm{ms}^{-2}\textrm{Hz}^{-1/2}$ or better, as well as the frame dragging effects caused by the Earth, could be sensed. Constructing such an apparatus to be non-symmetric would also enable the direct detection of curvature and gravitational waves (GWs). The GW sensitivity scales differently from the stray acceleration sensitivity, a unique feature of MIMAC. We have identified mitigation mechanisms for the known sources of noise, namely Gravity Gradient Noise (GGN), uncertainty principle and electro-magnetic forces. Hence it could potentially lead to a meter sized, orientable and vibrational noise (thermal/seismic) resilient detector of mid (ground based) and low (space based) frequency GWs from massive binaries (the predicted regimes are similar to those targeted by atom interferometers and LISA)., Comment: 29 pages, 3 figures

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, a prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. It is fair to say that there are no feasible ideas yet to test the quantum coherent behaviour of gravity directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes., Comment: 7 pages

Recent demonstrations of entanglement between two remote Nitrogen-Vacancy centers, have opened the way for their use in distributed quantum networks. An efficient spin-photon interface will now be required to help realize this system as a technology. Here we demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy colour centre in nanodiamond at cryogenic temperatures. A plano-hemispherical open cavity, fabricated using focused ion beam milling provides mode volumes as small as 1.25 cubic microns and quality factor Q ~ 3000. It will be shown how the open geometry and independently adjustable mirrors allows for precise placement of the emitter in the centre of the cavity mode, and crucially enables in-situ tuning of the cavity resonances. At optimal coupling, the signal from individual zero phonon line transitions is enhanced by a factor of 6.25 through the Purcell effect and the overall emission rate of the NV- centre is increased by 40% compared with that measured from the same centre in the absence of cavity field confinement. This Purcell enhancement is mapped out as a function of cavity mode volume. These results represent a proof of principle for a tunable cryogenic spin-photon interface. However by far the best NV optical and spin coherences are to be found in bulk material and efforts towards the production of diamond membranes are currently being made, with dimensions suitable for open-cavity coupling. Efforts towards this and preliminary results will also be discussed.

Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time (T1) and the phase memory time (T2): T1 measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and T2 measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1 and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. T2 is more than two orders of magnitude greater than the duration of the spin manipulation pulses, which suggests that copper phthalocyanine holds promise for quantum information processing, and the long T1 indicates possibilities for medium-term storage of classical bits in all-organic devices on plastic substrates.

Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies., 21 pages including Supplementary information

Two types of the metallofullerene Nd@C82 have been isolated and characterized. HPLC was used to isolate Nd@C82(I, II). The two isomers were characterized by mass spectrometry and UV-Vis-N1R absorption spectroscopy. Nd@C82(I) was found to be similar in structure to the main isomer of other lanthanofullerenes such as La@C82. We assign Nd@C82(I) to have a C2v cage symmetry. Nd@C 82(II) showed a markedly different UV-Vis-NIR absorption spectrum to Nd@C82(I). Its spectrum is in good agreement with that of the minor isomer of metallofullerenes such as Pr@C82. We therefore assign Nd@C82(II) to have a Cs cage symmetry. In contrast to other metallofullerenes, both isomers appear to be equally abundant. Their molecular orbital structures have been studied by a combination of scanning tunnelling microscopy (STM) and density functional theory (DFT). Matching filled and empty-states STM images to DFT calculations allowed us to distinguish directly between the two isomers on a substrate. copyright The Electrochemical Society.

Dynamic nuclear polarization transfers spin polarization from electrons to nuclei. We have achieved this by a new method, simultaneously exciting transitions of electronic and nuclear spins. The efficiency of this technique improves with increasing magnetic field. Experimental results are shown for N@C60 with continuous-wave microwaves, which can be expected to produce even higher polarization than the corresponding pulsed techniques for electron spins greater than 1/2. The degree of nuclear polarization in this case can be easily monitored through the intensities of the well resolved hyperfine components in the EPR spectrum. The nuclear spin-lattice relaxation time is orders of magnitude longer than that of the electrons., Comment: 7 pages, 3 figures

Most experts agree that it is too early to say how quantum computers will eventually be built, and several nanoscale solid-state schemes are being implemented in a range of materials. Nanofabricated quantum dots can be made in designer configurations, with established technology for controlling interactions and for reading out results. Epitaxial quantum dots can be grown in vertical arrays in semiconductors, and ultrafast optical techniques are available for controlling and measuring their excitations. Single-walled carbon nanotubes can be used for molecular self-assembly of endohedral fullerenes, which can embody quantum information in the electron spin. The challenges of individual addressing in such tiny structures could rapidly become intractable with increasing numbers of qubits, but these schemes are amenable to global addressing methods for computation.

Spin Control Controlling and manipulating the spin of an electron is a central requirement for applications in spintronics. Some of the challenges researchers are facing include efficient creation of spin currents, minimization of Joule heating, and extending the lifetime of electronic spins, which is especially important for quantum information applications. Costache and Valenzuela (p. 1645 ) address the first challenge by designing and fabricating an efficient and simple superconducting-based single-electron transistor that can produce spin current with controlled flow. Key to the design is asymmetric tunneling, which leads to a ratchet effect (or diode-like behavior), allowing the separation of up and down spins. Jonietz et al. (p. 1648 ) use electric currents five orders of magnitude smaller than those used previously in nanostructures to manipulate magnetization in a bulk material, MnSi, pointing the way toward decreased Joule heating in spintronic devices. This so-called spin-torque effect causes the rotation of the skyrmion lattice of spins, characteristic of MnSi, which is detected by neutron scattering. Finally, McCamey et al. (p. 1652 ) extend the short lifetime of an electronic spin of a phosphorous dopant by mapping it onto the much longer lived nuclear spin of the atom. Mapping the nuclear spin back onto the electronic spin allows production of a spin memory with a storage time exceeding 100s, which should prove useful for future practical applications.

Optical trapping at high vacuum of a nanodiamond containing a nitrogen vacancy centre would provide a test bed for several new phenomena in fundamental physics. However, the nanodiamonds used so far have absorbed too much of the trapping light, heating them to destruction (above 800 K) except at pressures above approximately 10 mbar where air molecules dissipate the excess heat. Here we show that milling diamond of 1000 times greater purity creates nanodiamonds that do not heat up even when the optical intensity is raised above 700 GW/m2 below 5 mbar of pressure.

The scheme recently proposed in [M. Scala et al., Phys Rev Lett 111, 180403 (2013)], where a gravity-dependent phase shift is induced on the spin of a nitrogen-vacancy (NV) center in a trapped nanodiamond by the interaction between its magnetic moment and the quantized motion of the particle, provides a way to detect spatial quantum superpositions by means of spin measurements only. Here, the effect of unwanted coupling with other motional degrees of freedom is considered and we show that it does not affect the validity of the scheme. Both this coupling and the additional error source due to misalignment between the quantization axis of the NV center spin and the trapping axis are shown not to change the qualitative behavior of the system, so that a proof-of- principle experiment can be neatly performed. Our analysis, which shows that the scheme retains the important features of not requiring ground state cooling and of being resistant to thermal fluctuations, can be useful for the several schemes which have been proposed recently for testing macroscopic superpositions in trapped microsystems., Comment: 8 pages, 4 figures

We report on the design of a torsion pendulum that can resolve the mass loading from 5×10 17 3 He atoms (equivalent to a 1000A film) with a 0.1% resolution. The oscillator is fabricated from coin silver alloy, and the working surfaces are two highly polished coin silver discs, each with well-characterized surface roughness, that are diffusion welded together using a copper gasket. We report on the cell's temperature dependent background. The cell will be used to examine the evolution of the superfluid density and transition temperature as a function of film thickness as well as the normal fluid behavior.

Electrically detected magnetic resonance is a sensitive technique for studying electron spins bound to phosphorus atoms in silicon. Using a high magnetic field of 8.6 T increases this sensitivity by accessing a mechanism for spin-to-charge conversion which is different to that in the more commonly used magnetic field of ∼0.33 T. The higher-field regime benefits from a long spin coherence time of over 100 µs because the phosphorus spins that are detected are not coupled to dangling-bonds. Additionally, the high field permits the spin qubits to reach an equilibrium polarization of over 95% at a temperature of 2.8 K. We demonstrate electrically detected Rabi oscillations of these electron spins in the high-field regime. The Rabi frequency is proportional to the square root of the excitation power as expected.

Dopants in crystalline silicon such as phosphorus (Si:P) have electronic and nuclear spins with exceptionally long coherence times making them promising platforms for quantum computing and quantum sensing. The demonstration of single-spin single-shot readout brings these ideas closer to implementation. Progress in fabricating atomic-scale Si:P structures with scanning tunnelling microscopes offers a powerful route to scale up this work, taking advantage of techniques developed by the computing industry. The experimental and theoretical sides of this emerging quantum technology are reviewed with a focus on the period from 2009 to mid-2014.

We show how the interference between spatially separated states of the center of mass (COM) of a mesoscopic harmonic oscillator can be evidenced by coupling it to a spin and performing solely spin manipulations and measurements (Ramsey Interferometry). We propose to use an optically levitated diamond bead containing an NV center spin. The nano-scale size of the bead makes the motional decoherence due to levitation negligible. The form of the spin-motion coupling ensures that the scheme works for thermal states so that moderate feedback cooling suffices. No separate control or observation of the COM state is required and thereby one dispenses with cavities, spatially resolved detection and low mass-dispersion ensembles. The controllable relative phase in the Ramsey interferometry stems from a gravitational potential difference so that it uniquely evidences coherence between states which involve the whole nano-crystal being in spatially distinct locations., 5 pages, 2 figures, version accepted for publication in Physical Review Letters

Nanostructure and molecular orientation play a crucial role in determining the functionality of organic thin films. In practical devices, such as organic solar cells consisting of donor-acceptor mixtures, crystallinity is poor and these qualities cannot be readily determined by conventional diffraction techniques, while common microscopy only reveals surface morphology. Using a simple nondestructive technique, namely, continuous-wave electron paramagnetic resonance spectroscopy, which exploits the well-understood angular dependence of the g-factor and hyperfine tensors, we show that in the solar cell blend of C(60) and copper phthalocyanine (CuPc)-for which X-ray diffraction gives no information-the CuPc, and by implication the C(60), molecules form nanoclusters, with the planes of the CuPc molecules oriented perpendicular to the film surface. This information demonstrates that the current nanostructure in CuPc:C(60) solar cells is far from optimal and suggests that their efficiency could be considerably increased by alternative film growth algorithms.

We present pulsed electron-nuclear double resonance (ENDOR) experiments which enable us to characterize the coupling between bismuth donor spin qubits in Si and the surrounding spin bath of 29Si impurities which provides the dominant decoherence mechanism (nuclear spin diffusion) at low temperatures (< 16 K). Decoupling from the spin bath is predicted and cluster correlation expansion simulations show near-complete suppression of spin diffusion, at optimal working points. The suppression takes the form of sharply peaked divergences of the spin diffusion coherence time, in contrast with previously identified broader regions of insensitivity to classical fluctuations. ENDOR data suggest that anisotropic contributions are comparatively weak, so the form of the divergences is largely independent of crystal orientation., Added journal ref., minor improvements

Pulsed magnetic resonance is a wide-reaching technology allowing the quantum state of electronic and nuclear spins to be controlled on the timescale of nanoseconds and microseconds respectively. The time required to flip either dilute electronic or nuclear spins is orders of magnitude shorter than their decoherence times, leading to several schemes for quantum information processing with spin qubits. We investigate instead the novel regime where the eigenstates approximate 50:50 superpositions of the electronic and nuclear spin states forming "hybrid nuclear-electronic" qubits. Here we demonstrate quantum control of these states for the first time, using bismuth-doped silicon, in just 32 ns: this is orders of magnitude faster than previous experiments where pure nuclear states were used. The coherence times of our states are five orders of magnitude longer, reaching 4 ms, and are limited by the naturally-occurring 29Si nuclear spin impurities. There is quantitative agreement between our experiments and no-free-parameter analytical theory for the resonance positions, as well as their relative intensities and relative Rabi oscillation frequencies. In experiments where the slow manipulation of some of the qubits is the rate limiting step, quantum computations would benefit from faster operation in the hybrid regime., 20 pages, 8 figures, new data and simulations

The ability to probe the spin properties of solid state systems electrically underlies a wide variety of emerging technology. Here, we extend electrical readout of the nuclear spin states of phosphorus donors in silicon to the coherent regime with modified Hahn echo sequences. We find that, whilst the nuclear spins have electrically detected phase coherence times exceeding 2 ms, they are nonetheless limited by the artificially shortened lifetime of the probing donor electron., 10 pages, 3 figures

We investigate electron paramagnetic resonance spectra of bismuth-doped silicon, at intermediate magnetic fields B≃0.1-0.6 T, theoretically and experimentally (with 9.7 GHz X-band spectra). We identify a previously unexplored regime of "cancellation resonances," where a component of the hyperfine coupling is resonant with the external field. We show that this regime has experimentally accessible consequences for quantum information applications, such as reduction of decoherence, fast manipulation of the coupled electron-nuclear qubits, and spectral line narrowing.

The signal-to-noise ratio for room-temperature readout of individual qubits can be substantially improved by taking an indirect route through intermediate energy levels.

There is growing interest in bismuth-doped silicon (Si:Bi) as an alternative to the well-studied proposals for silicon based quantum information processing (QIP) using phosphorus-doped silicon (Si:P). We focus here on the implications of its anomalously strong hyperfine coupling. In particular, we analyse in detail the regime where recent pulsed magnetic resonance experiments have demonstrated the potential for orders of magnitude speedup in quantum gates by exploiting transitions that are electron paramagnetic resonance (EPR) forbidden at high fields. We also present calculations using a phenomenological Markovian master equation which models the decoherence of the electron spin due to Gaussian temporal magnetic field perturbations. The model quantifies the advantages of certain "optimal working points" identified as the $df/dB=0$ regions, where $f$ is the transition frequency, which come in the form of frequency minima and maxima. We show that at such regions, dephasing due to the interaction of the electron spin with a fluctuating magnetic field in the $z$ direction (usually adiabatic) is completely removed., Comment: 15 pages, 9 figures

A prerequisite for exploiting spins for quantum data storage and processing is long spin coherence times. Phosphorus dopants in silicon (Si:P) have been favoured as hosts for such spins because of measured electron spin coherence times (T2) longer than any other electron spin in the solid state: 14 ms at 7 K with isotopically purified silicon. Heavier impurities such as bismuth in silicon (Si:Bi) could be used in conjunction with Si:P for quantum information proposals that require two separately addressable spin species. However, the question of whether the incorporation of the much less soluble Bi into Si leads to defect species that destroy coherence has not been addressed. Here we show that schemes involving Si:Bi are indeed feasible as the electron spin coherence time T2 is at least as long as for Si:P with non-isotopically purified silicon. We polarized the Si:Bi electrons and hyperpolarized the I=9/2 nuclear spin of (209)Bi, manipulating both with pulsed magnetic resonance. The larger nuclear spin means that a Si:Bi dopant provides a 20-dimensional Hilbert space rather than the four-dimensional Hilbert space of an I=1/2 Si:P dopant.

Electron and nuclear spins are very promising candidates to serve as quantum bits (qubits) for proposed quantum computers, as the spin degrees of freedom are relatively isolated from their surroundings, and can be coherently manipulated e.g. through pulsed EPR and NMR. For solid state spin systems, impurities in crystals based on carbon and silicon in various forms have been suggested as qubits, and very long relaxation rates have been observed in such systems. We have investigated a variety of these systems at high magnetic fields in our multi-frequency pulsed EPR/ENDOR spectrometer. A high magnetic field leads to large electron spin polarizations at helium temperatures giving rise to various phenomena that are of interest with respect to quantum computing. For example, it allows the initialization of the both the electron spin as well as hyperfine-coupled nuclear spins in a well defined state by combining millimeter and RF radiation; it can increase the T2 relaxation times by eliminating decoherence due to dipolar interaction; and it can lead to new mechanisms for the coherent electrical readout of electron spins. We will show some examples of these and other effects in Si:P, SiC:N, and nitrogen-related centers in diamond., 13 pages, 5 figures. Confererence Proceedings of the 6th Asia-Pacific ESR/EPR Society Conference in Cairns, Australia (2008)

We assess routes to a diamond-based quantum computer, where we specifically look towards scalable devices, with at least 10 linked quantum gates. Such a computer should satisfy the deVincenzo rules and might be used at convenient temperatures. The specific examples we examine are based on the optical control of electron spins. For some such devices, nuclear spins give additional advantages. Since there have already been demonstrations of basic initialisation and readout, our emphasis is on routes to two-qubit quantum gate operations and the linking of perhaps 10-20 such gates. We analyse the dopant properties necessary, especially centres containing N and P, and give results using simple scoping calculations for the key interactions determining gate performance. Our conclusions are cautiously optimistic: it may be possible to develop a useful quantum information processor that works above cryogenic temperatures., 16 pages, 3 figures

The electron spin g- and hyperfine tensors of the endohedral metallofullerene Sc@C82 are anisotropic. Using electron spin resonance (ESR) and density functional theory (DFT), we can relate their principal axes to the coordinate frame of the molecule, finding that the g-tensor is not axially symmetric. The Sc bond with the cage is partly covalent and partly ionic. Most of the electron spin density is distributed around the carbon cage, but 5% is associated with the scandium dyz orbital, and this drives the observed anisotropy. © 2005 IOP Publishing Ltd.

${\text{Cr}}^{5+}$-doped ${\text{K}}_{3}{\text{NbO}}_{8}$, considered to be useful as an electron spin qubit, has been investigated by pulsed $X$ band $(\ensuremath{\sim}9.7\text{ }\text{GHz})$ and 240 GHz electron paramagnetic resonance and electron nuclear double resonance (ENDOR). Comparison of the low temperature electronic spin-lattice relaxation rate $1/{T}_{1}$ at 9.7 and 240 GHz shows that it is 250 times faster at 240 GHz than at $X$ band. On the other hand, spin-spin relaxation rate $1/{T}_{2}$ appears largely frequency independent and is very likely related to the superhyperfine (SHF) coupling of the ${\text{Cr}}^{5+}$ electron with the surrounding potassium and niobium nuclei. This coupling was investigated by hyperfine sublevel correlation spectroscopy at 9.7 GHz and pulsed Mims ENDOR at 240 GHz. The use of high frequency and field enabled us to unambiguously measure the hyperfine and quadrupole couplings of the $^{39}\text{K}$ in spite of its small magnetic moment. We find that the largest $^{39}\text{K}$ SHF coupling is positive, with 0.522 and 0.20 MHz as its isotropic and dipolar parts, respectively. $^{93}\text{N}\text{b}$ ENDOR was dominantly due to its quadrupolar interaction with a coupling of about 0.8 MHz and a SHF coupling of about 0.08 MHz. The significance of these data to spin qubit studies is pointed out.

We describe a pulsed electron paramagnetic resonance spectrometer operating at several frequencies in the range of 110-336 GHz. The microwave source at all frequencies consists of a multiplier chain starting from a solid state synthesizer in the 12-15 GHz range. A fast p-i-n-switch at the base frequency creates the pulses. At all frequencies a Fabry-Perot resonator is employed and the pi/2 pulse length ranges from approximately 100 ns at 110 GHz to approximately 600 ns at 334 GHz. Measurements of a single crystal containing dilute Mn(2+) impurities at 12 T illustrate the effects of large electron spin polarizations. The capabilities also allow for pulsed electron-nuclear double resonance (ENDOR) experiments as demonstrated by Mims ENDOR of (39)K nuclei in Cr:K(3)NbO(8).