Anisotropic interactions of a single spin and dark-spin spectroscopy in diamond

Applied Physics Letters, 2009

New Journal of Physics, 2011

We present here both theoretical and experimental results on the fluorescence of single defect centers in diamond nanocrystals embedded in a planar dielectric microcavity. From a theoretical point of view, we show that the overall fluorescence collection efficiency using a moderate numerical aperture microscope objective can be enhanced by using a low-quality-factor microcavity. This could be used in particular for low-temperature applications, where the numerical aperture of collection microscope objectives is limited due to the experimental constraints. We experimentally investigate the control of the fluorescence spectrum of the emitted light from a single center. We show the simultaneous narrowing of the room temperature broadband emission spectrum and the increase in the fluorescence spectral density.

Physical Review Letters, 2011

We propose a scheme enabling controlled quantum coherent interactions between separated nitrogen-vacancy centers in diamond in the presence of strong magnetic fluctuations. The proposed scheme couples nuclear qubits employing the magnetic dipole-dipole interaction between the electron spins and, crucially, benefits from the suppression of the effect of environmental magnetic field fluctuations thanks to a strong microwave driving. This scheme provides a basic building block for a full-scale quantum information processor or quantum simulator based on solid-state technology.

Physical Review Letters, 2009

Applied Physics Letters, 2012

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 1-3 . Recently, it was shown that the optical detection of the electron spin resonance (ESR) associated with a single Nitrogen-Vacancy (NV) defect in diamond provides an unprecedented combination of magnetic sensitivity and spatial resolution under ambient conditions 4-9 . Here we demonstrate quantitative magnetic field mapping with nanoscale resolution, by applying a lock-in technique on the ESR frequency of a single NV defect placed at the apex of an atomic force microscope (AFM) cantilever. To illustrate the efficiency of the technique, we image a commercial magnetic hard disk and observe good agreement with the simulated magnetic field distributions. In addition, we demonstrate a novel all-optical magnetic imaging technique which is sensitive to large magnetic fields lying in a range usually considered as non accessible to diamond-based magnetometry. It relies on magnetic-fielddependent photoluminescence (PL) of the NV defect magnetic probe, induced by spin level mixing. This method allows us to directly resolve the nanoscale magnetic bit structure of the hard disk. Owing to the non-perturbing nature of the magnetic probe, this work should open up numerous perspectives in nanomagnetism and spintronics.

Applied Physics Letters, 2014

Spins associated with point-like defects in solids are at the heart of a broad range of emerging applications, from quantum information science 1-3 , to the development of highly sensitive quantum sensors 4-6 . In that context, optimal performance requires quantum grade purity crystals to ensure long spin coherence time, combined with a deterministic control of the defect orientation in the crystal lattice 7,8 . Although recent developments of chemical vapor deposition (CVD) techniques have allowed the production of high purity crystals with an unprecedented control over the growth environment 9,10 , controlling the orientation of point-like defects remains a challenging task in materials science. Here we show that the orientation of nitrogen-vacancy (NV) defects in diamond can be efficiently controlled through CVD growth on a (111)-oriented diamond substrate. More precisely, we demonstrate that spontaneously generated NV defects are oriented with a ∼ 97% probability along the [111] axis, corresponding to the most appealing orientation among the four possible crystallographic axes. Such a nearly perfect preferential orientation is explained by analyzing the diamond growth mechanism on a (111)-oriented substrate and could be extended to other types of defects. This work is a significant step towards the design of optimized diamond samples for quantum information and sensing applications.

New Journal of Physics, 2012

Magnetometry and magnetic imaging with nitrogen-vacancy (NV) defects in diamond rely on the optical detection of electron spin resonance (ESR). However, this technique is inherently limited to magnetic fields that are weak enough to avoid electron spin mixing. Here, we focus on the high off-axis magnetic field regime where spin mixing alters the NV defect spin dynamics. We first study, in a quantitative manner, the dependence of the NV defect optical properties on the magnetic field vector B. Magnetic-field-dependent timeresolved photoluminescence (PL) measurements are compared to a seven-level model of the NV defect that accounts for field-induced spin mixing. The model reproduces decreases in (i) ESR contrast, (ii) PL intensity and (iii) excited level lifetime with an increasing off-axis magnetic field. We next demonstrate that these effects can be used to perform all-optical imaging of the magnetic field component |B ⊥ | orthogonal on the NV defect axis. Using a scanning NV defect 2 microscope, we map the stray field of a magnetic hard disc through both PL and fluorescence lifetime imaging. This all-optical method for high magnetic field imaging at the nanoscale might be of interest in the field of nanomagnetism, where samples producing fields in excess of several tens of milliteslas are typically found.

Applied Physics Letters, 2011

We describe a technique for fabricating micro-and nano-structures incorporating fluorescent defects in diamond with a positional accuracy in the hundreds of nanometers. Using confocal fluorescence microscopy and focused ion beam (FIB) etching we initially locate a suitable defect with respect to registration marks on the diamond surface and then etch a structure using these coordinates. We demonstrate the technique here by etching an 8 µm diameter hemisphere positioned such that a single negatively charged nitrogen-vacancy defect lies at its origin. This type of structure increases the photon collection efficiency by removing refraction and aberration losses at the diamond-air interface. We make a direct comparison of the fluorescence photon count rate before and after fabrication and observe an 8-fold increase due to the presence of the hemisphere.

The European Physical Journal D, 2011

Many-body wavefunctions were utilized to calculate von Neumann's entropy as an entanglement measurement for neutral and negatively charged nitrogen vacancy (NV) centers in diamond. A generalized Hubbard Hamiltonian which considers e-e interaction terms completely was used to calculate many-electron wavefunctions of the ground and excited states. Correlation between entanglement and spin density distributed on neighboring atoms of NV is presented. The behavior of spin density and entanglement under relaxations of neighboring atoms is the same for all investigated ground and excited states. The results suggest that the spin density may be used to quantify the entanglemnt and vice versa.

New Journal of Physics, 2014

The realization of scalable arrangements of nitrogen vacancy (NV) centers in diamond remains a key challenge on the way towards efficient quantum information processing, quantum simulation and quantum sensing applications.

Applied Physics Letters, 2010

Optical detection of single defect centers in the solid state is a key element of novel quantum technologies. This includes the generation of single photons and quantum information processing. Unfortunately the brightness of such atomic emitters is limited. Therefore we experimentally demonstrate a novel and simple approach that uses off-the-shelf optical elements. The key component is a solid immersion lens made of diamond, the host material for single color centers. We improve the excitation and detection of single emitters by one order of magnitude, as predicted by theory.

Nature Nanotechnology, 2013

Electron and nuclear spins associated with point defects in insulators are promising systems for solid state quantum technology 1-3 . While the electron spin usually is used for readout and addressing, nuclear spins are exquisite quantum bits 4,5 and memory systems 3,6 . With these systems single-shot readout of nearby nuclear spins 5 as well as entanglement 4,7,8 aided by the electron spin has been shown. While the electron spin in this example is essential for readout it usually limits nuclear spin coherence 9 . This has set of the quest for defects with spin-free ground states 8,10 . Here, we isolate a hitherto unidentified defect in diamond and use it at room temperature to demonstrate optical spin polarization and readout with exceptionally high contrast (up to 45%), coherent manipulation of an individual excited triplet state spin, and coherent nuclear spin manipulation using the triplet electron spin as a meta-stable ancilla. By this we demonstrate nuclear magnetic resonance and Rabi oscillations of the uncoupled nuclear spin in the spin-free electronic ground state. Our study demonstrates that nuclei coupled to single metastable electron spins are useful quantum systems with long memory times despite electronic relaxation processes.

Physical Review Letters, 2008

Despite tremendous activity in employing the N À V À center in a host of quantum technology applications, the electronic and optical properties of the system are still not theoretically well understood. We have conducted density functional theory calculations of the N À V À system which show convergence at the 3 Â 3 Â 3 supercell level and for the first time produce a quantitatively accurate picture of the optical transition energy, excited-state lifetime, and optical polarization anisotropy taking into account all possible transitions within all contributing energy bands. These calculations were augmented by a group theoretical analysis, in sum providing a new ab initio understanding of this important solid-state quantum system.

Physical Review Letters, 2011

The nitrogen-vacancy (NV) center in diamond is supposed to be a building block for quantum computing and nanometer scale metrology at ambient conditions. Therefore, precise knowledge of its quantum states is crucial. Here, we experimentally show that under usual operating conditions the NV exists in an equilibrium of two charge states (70% in the expected negative (NV − ) and 30% in the neutral one (NV 0 )). Projective quantum non-demolition measurement of the nitrogen nuclear spin enables the detection even of the additional, optically inactive state. The nuclear spin can be coherently driven also in NV 0 (T1 ≈ 90 ms and T2 ≈ 6 µs).

Physical Review A, 2010

Quasi-TE and TM fundamental whispering gallery modes in a polymer coated silica microtoroid are theoretically investigated, and demonstrated to possess very high quality factors. The existence of nanometer thickness layer not only reduces the cavity mode volume evidently, but also draws the the maximal electric field's position of the mode to the outside of the silica toroid where single quantum dots or nanocrystals are located. Both effects result in a strongly enhanced coherent interaction between single dipole (for example, single defect center in diamond crystal) and the quantized cavity mode. Since the coated microtoroid is highly feasible and robust in experiment, it may offer an excellent platform to study strong-coupling cavity QED, quantum information and quantum computation.

Physical Review B, 2010

Controlled fabrication and identification of bright single photon emitters is at the heart of quantum optics and materials science. Here we demonstrate a controlled engineering of a chromium bright single photon source in bulk diamond by ion implantation. The Cr center has fully polarized emission with a ZPL centered at 749 nm, FWHM of 4 nm, an extremely short lifetime of ~1 ns, and a count rate of 0.5×10 6 counts/s. By combining the polarization measurements and the vibronic spectra, a model of the center has been proposed consisting of one interstitial chromium atom with a transition dipole along one of the <100> directions.

Physical Review X, 2014

We demonstrate theoretically and experimentally that the three-dimensional orientation of a single fluorescent nanoemitter can be determined by polarization analysis of the emitted light (while excitation polarization analysis provides only the in-plane orientation). The determination of the emitter orientation by polarimetry requires a theoretical description, including the objective numerical aperture, the 1D or 2D nature of the emitting dipole, and the environment close to the dipole. We develop a model covering most experimentally relevant microscopy configurations and provide analytical relations that are useful for orientation measurements. We perform polarimetric measurements on high-quality core-shell CdSe/CdS nanocrystals and demonstrate that they can be approximated by two orthogonal degenerated dipoles. Finally, we show that the orientation of a dipole can be inferred by polarimetric measurement, even for a dipole in the vicinity of a gold film, while in this case, the well-established defocused microscopy is not appropriate.

Applied Physics Letters

High frequency electron spin resonance (ESR) spectroscopy is an invaluable tool for identification and characterization of spin systems. Nanoscale ESR using the nitrogen-vacancy (NV) center has been demonstrated down to the level of a single spin. However, NVdetected ESR has exclusively been studied at low magnetic fields, where spectral overlap prevents clear identification of spectral features. Within this work, we demonstrate NVdetected ESR measurements of single-substitutional nitrogen impurities in diamond at a NV Larmor frequency of 115 GHz and the corresponding magnetic field of 4.2 Tesla. The NV-ESR measurements utilize a double electron-electron resonance sequence and are performed using both ensemble and single NV spin systems. In the single NV experiment, chirp pulses are used to improve the population transfer and for NV-ESR measurements.

Physical Review B, 2010

We report measurements of the optical properties of the 1042 nm transition of negativelycharged Nitrogen-Vacancy (NV) centers in type 1b diamond. The results indicate that the upper level of this transition couples to the m s = ±1 sublevels of the 3 E excited state and is short-lived, with a lifetime of < ∼ 1 ns. The lower level is shown to have a temperaturedependent lifetime of 462(10) ns at 4.4 K and 219(3) ns at 295 K. The light-polarization dependence of 1042 nm absorption confirms that the transition is between orbitals of A 1 and E character. The results shed new light on the NV level structure and optical pumping mechanism.

Journal of Physics: Condensed Matter, 2013

Nanoparticles have recently emerged as an important group of materials used in numerous disciplines within the life sciences, ranging from basic biophysical research to clinical therapeutics. Luminescent nanoparticles make excellent optical bioprobes significantly extending the capabilities of alternative fluorophores such as organic dyes and genetically engineered fluorescent proteins. Their advantages include excellent photostability, tunable and narrow spectra, controllable size, resilience to environmental conditions such as pH and temperature, combined with a large surface for anchoring targeting biomolecules. Some types of nanoparticles provide enhanced detection contrast due to their long emission lifetime and/or luminescence wavelength blue-shift (anti-Stokes) due to energy upconversion. This topical review focuses on four key types of luminescent nanoparticles whose emission is governed by different photophysics. We discuss the origin and characteristics of optical absorption and emission in these nanoparticles and give a brief account of synthesis and surface modification procedures. We also introduce some of their applications with opportunities for further development, which could be appreciated by the physics-trained readership.

New Journal of Physics, 2008

New Journal of Physics, 2009

The emission intensity of diamond samples containing nitrogenvacancy centres are measured as a function of magnetic field along a 111 direction for various temperatures. At low temperatures the responses are sample and stress dependent and can be modeled in terms of the previous understanding of the 3 E excited state fine structure which is strain dependent. At room temperature the responses are largely sample and stress independent, and modeling involves invoking a strain independent excited state with a single zero field splitting of 1.42 GHz. The change in behaviour is attributed to a temperature dependent averaging process over the components of the excited state orbital doublet. It decouples orbit and spin and at high temperature the spin levels become independent of any orbit splitting. Thus the models can be reconciled and the parameters for low and high temperatures are shown to be consistent.

New Journal of Physics, 2009

Using pulsed optically detected magnetic resonance techniques, we directly probe electron-spin resonance transitions in the excited-state of single nitrogen-vacancy (NV) color centers in diamond. Unambiguous assignment of excited state fine structure is made, based on changes of NV defect photoluminescence lifetime. This study provides significant insight into the structure of the emitting 3 E excited state, which is invaluable for the development of diamond-based quantum information processing. Over the last decade, the negatively charged nitrogen-vacancy (NV) color center in diamond has attracted a lot of interest because it can be optically addressed as single quantum system [1] and exhibits several important properties for quantum information science applications. Firstly, its perfect photostability at room temperature enables a practical NV-based single photon source [2, 3] to be realized for quantum cryptography applications [4, 5]. Secondly, NV color centers have a paramagnetic ground state whose spin can be optically polarized, read-out and exhibits long coherence time even at room temperature [6, 7]. Coherent manipulation of electron and 3 Author to whom any correspondence should be addressed.

Optics Express

Efficient collection of fluorescence from nitrogen vacancy (NV) centers in diamond underlies the spin-dependent optical read-out that is necessary for quantum information processing and enhanced sensing applications. The optical collection efficiency from NVs within diamond substrates is limited primarily due to the high refractive index of diamond and the non-directional dipole emission. Here we introduce a light collection strategy based on chirped, circular dielectric gratings that can be fabricated on a bulk diamond substrate to modify an emitter's far-field radiation pattern. Using a genetic optimization algorithm, these grating designs achieve 98.9% collection efficiency for the NV zero-phonon emission line, collected from the back surface of the diamond with an objective of aperture 0.9. Across the broadband emission spectrum of the NV (600-800 nm), the chirped grating achieves 82.2% collection efficiency into a numerical aperture of 1.42, corresponding to an oil immersion objective again on the back side of the diamond. Our proposed bulk-dielectric grating structures are applicable to other optically active solid state quantum emitters in high index host materials.

Nature Communications, 2013

Dynamic nuclear polarisation, which transfers the spin polarisation of electrons to nuclei, is routinely applied to enhance the sensitivity of nuclear magnetic resonance; it is also critical in spintronics, particularly when spin hyperpolarisation can be produced and controlled optically or electrically. Here we show the complete polarisation of nuclei located near the optically-polarised nitrogen-vacancy (NV) centre in diamond; by approaching the ground-state level anti-crossing condition of the NV electron spins, 13 C nuclei in the first-shell are polarised in a pattern that depends sensitively and sharply upon the magnetic field. Based on the anisotropy of the hyperfine coupling and of the optical polarisation mechanism, we predict and

Physical Review A

Applied Physics Letters

We demonstrate a robust, scale-factor-free vector magnetometer, which uses a closed-loop frequency-locking scheme to simultaneously track Zeeman-split resonance pairs of nitrogen-vacancy (NV) centers in diamond. Compared with open-loop methodologies, this technique is robust against fluctuations in temperature, resonance linewidth, and contrast; offers a three-order-of-magnitude increase in dynamic range; and allows for simultaneous interrogation of multiple transition frequencies. By directly detecting the resonance frequencies of NV centers aligned along each of the diamond's four tetrahedral crystallographic axes, we perform full vector reconstruction of an applied magnetic field.

Applied Physics B

We report on the characterization of the angular-dependent emission of single-photon emitters based on single nitrogen-vacancy (NV-) centers in nanodiamond at room temperature. A theoretical model for the calculation of the angular emission patterns of such an NV-center at a dielectric interface will be presented. For the first time, the orientation of the NV-centers in nanodiamond was determined from back focal plane images of NV-centers and by comparison of the theoretical and experimental angular emission pattern. Furthermore, the orientation of the NV-centers was also obtained from measurements of the fluorescence intensity in dependence on the polarization angle of the linearly polarized excitation laser. The results of these measurements are in good agreement. Moreover, the collection efficiency in this setup was calculated to be higher than 80% using the model of the angular emission of the NV-centers.

New Journal of Physics, 2014

We prove experimentally, upon polarization analysis performed on a large statistic of single nanoemitters, that high quality core/shell CdSe/CdS dot-in-rods behave as linear dipoles. Moreover, the dipole in-plane and out-of-plane orientations could be assessed. We demonstrate in particular that, contrary to expectations, the emitting dipole is not aligned with the elongated axis of the dot-in-rod. Besides, the polarimetric measurements prove that the excitation transition cannot be approximated by a single linear dipole, contrary to the emission transition. Finally, we highlight that non-radiative channels of charge carrier recombination do not affect the dipolar nature of the radiative transitions.

MRS Bulletin, 2021

Emerging quantum technologies require precise control over quantum systems of increasing complexity. Defects in diamond, particularly the negatively charged nitrogen-vacancy (NV) center, are a promising platform with the potential to enable technologies ranging from ultra-sensitive nanoscale quantum sensors, to quantum repeaters for long distance quantum networks, to simulators of complex dynamical processes in many-body quantum systems, to scalable quantum computers. While these advances are due in large part to the distinct material properties of diamond, the uniqueness of this material also presents difficulties, and there is a growing need for novel materials science techniques for characterization, growth, defect control, and fabrication dedicated to realizing quantum applications with diamond. In this review L. V. H. Rodgers Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA E-mail: lvhr@princeton.edu L. B. Hughes Materials Department,...

Nature Communications, 2020

Development of sensitive local probes of magnon dynamics is essential to further understand the physical processes that govern magnon generation, propagation, scattering, and relaxation. Quantum spin sensors like the NV center in diamond have long spin lifetimes and their relaxation can be used to sense magnetic field noise at gigahertz frequencies. Thus far, NV sensing of ferromagnetic dynamics has been constrained to the case where the NV spin is resonant with a magnon mode in the sample meaning that the NV frequency provides an upper bound to detection. In this work we demonstrate ensemble NV detection of spinwaves generated via a nonlinear instability process where spinwaves of nonzero wavevector are parametrically driven by a high amplitude microwave field. NV relaxation caused by these driven spinwaves can be divided into two regimes; one- and multi-magnon NV relaxometry. In the one-magnon NV relaxometry regime the driven spinwave frequency is below the NV frequencies. The dri...

Physical Review B, 2007

The nitrogen-vacancy (N-V) center in diamond is promising as an electron spin qubit due to its long-lived coherence and optical addressability. The ground state is a spin triplet with two levels (ms = ±1) degenerate at zero magnetic field. Polarization-selective microwave excitation is an attractive method to address the spin transitions independently, since this allows operation down to zero magnetic field. Using a resonator designed to produce circularly polarized microwaves, we have investigated the polarization selection rules of the N-V center. We first apply this technique to N-V ensembles in [100] and [111]-oriented samples. Next, we demonstrate an imaging technique, based on optical polarization dependence, that allows rapid identification of the orientations of many single N-V centers. Finally, we test the microwave polarization selection rules of individual N-V centers of known orientation.

Nature Nanotechnology, 2010

Physical Review B, 2017

We report T 2 spin coherence times for electronic states localized in Si vacancies in 4H-SiC. Our spin coherence study included two SiC samples that were irradiated with 2 MeV protons at different fluences (10 13 and 10 14 cm-2) in order to create samples with unique defect concentrations. Using optically detected magnetic resonance and spin echo, the coherence times for each sample were measured across a range of temperatures from 8 K to 295 K. All echo experiments were done at a magnetic field strength of 0.371 T and a microwave frequency of 10.49 GHz. The longest coherence times were obtained at 8 K, being 270 ± 61 µs for the 10 13 cm-2 proton-irradiated sample and 104 ± 17 µs for the 10 14 cm-2 sample. The coherence times for both samples displayed unusual temperature dependences; in particular, they decreased with temperature until 60 K, then increased until 160 K, then decreased again. This increase between 60 and 160 K is tentatively attributed to a motional Jahn-Teller effect. The consistently longer lifetimes for the 10 13 cm-2 sample suggest that a significant source of the spin dephasing can be attributed to dipole-dipole interactions between Si vacancies or with other defects produced by the proton irradiation. The lack of a simple exponential decay for our 10 14 cm-2 sample indicates an inhomogeneous distribution of defect spins.

Nature Communications

The recently discovered spin-active boron vacancy (V$${}_{{{{{{{{\rm{B}}}}}}}}}^{-}$$ B − ) defect center in hexagonal boron nitride (hBN) has high contrast optically-detected magnetic resonance (ODMR) at room-temperature, with a spin-triplet ground-state that shows promise as a quantum sensor. Here we report temperature-dependent ODMR spectroscopy to probe spin within the orbital excited-state. Our experiments determine the excited-state spin Hamiltonian, including a room-temperature zero-field splitting of 2.1 GHz and a g-factor similar to that of the ground-state. We confirm that the resonance is associated with spin rotation in the excited-state using pulsed ODMR measurements, and we observe Zeeman-mediated level anti-crossings in both the orbital ground- and excited-state. Our observation of a single set of excited-state spin-triplet resonance from 10 to 300 K is suggestive of symmetry-lowering of the defect system from D3h to C2v. Additionally, the excited-state ODMR has stron...

Frontiers in Physics, 2020

Fluorescence of the negatively charged nitrogen-vacancy (NV −) center of diamond is sensitive to external electromagnetic fields, lattice strain, and temperature due to the unique triplet configuration of its spin states. Their use in particulate diamond allows for the possibility of localized sensing and magnetic-contrast-based differential imaging in complex environments with high fluorescent background. However, current methods of NV − production in diamond particles are accompanied by the formation of a large number of parasitic defects and lattice distortions resulting in deterioration of the NV − performance. Therefore, there are significant efforts to improve the quantum properties of diamond particles to advance the field. Recently it was shown that rapid thermal annealing (RTA) at temperatures much exceeding the standard temperatures used for NV − production can efficiently eliminate parasitic paramagnetic impurities and, as a result, by an order of magnitude improve the degree of hyperpolarization of 13 C via polarization transfer from optically polarized NV − centers in micron-sized particles. Here, we demonstrate that RTA also improves the maximum achievable magnetic modulation of NV − fluorescence in micron-sized diamond by about 4x over conventionally produced diamond particles endowed with NV −. This advancement can continue to bridge the pathway toward developing nano-sized diamond with improved qualities for quantum sensing and imaging.

Proceedings of the National Academy of Sciences, 2012

Using an optical tweezers apparatus, we demonstrate three-dimensional control of nanodiamonds in solution with simultaneous readout of ground-state electron-spin resonance (ESR) transitions in an ensemble of diamond nitrogen-vacancy color centers. Despite the motion and random orientation of nitrogen-vacancy centers suspended in the optical trap, we observe distinct peaks in the measured ESR spectra qualitatively similar to the same measurement in bulk. Accounting for the random dynamics, we model the ESR spectra observed in an externally applied magnetic field to enable dc magnetometry in solution. We estimate the dc magnetic field sensitivity based on variations in ESR line shapes to be approximately . This technique may provide a pathway for spin-based magnetic, electric, and thermal sensing in fluidic environments and biophysical systems inaccessible to existing scanning probe techniques.

Physical Review X, 2018

Nitrogen vacancy (NV) centres in diamond are attractive as quantum sensors owing to their superb coherence under ambient conditions. However, the NV centre spin resonances are relatively insensitive to some important parameters such as temperature. Here we design and experimentally demonstrate a hybrid nano-thermometer composed of NV centres and a magnetic nanoparticle (MNP), in which the temperature sensitivity is enhanced by the critical magnetization of the MNP near the ferromagnetic-paramagnetic transition temperature. The temperature susceptibility of the NV center spin resonance reached 14 MHz/K, enhanced from the value without the MNP by two orders of magnitude. The sensitivity of a hybrid nano-thermometer composed of a Cu1-xNix MNP and a nanodiamond was measured to be 11 mK/Hz 1/2 under ambient conditions. With such high-sensitivity, we monitored nanometer-scale temperature variation of 0.3 degree with a time resolution of 60 msec. This hybrid nano-thermometer 2 provides a novel approach to studying a broad range of thermal processes at nanoscales such as nano-plasmonics, sub-cellular heat-stimulated processes, thermodynamics of nanostructures, and thermal remanent magnetization of nanoparticles. MAIN TEXT: Nanoscale temperature sensing is important for studying a broad range of phenomena in physics, biology, and chemistry, such as the temperature heterogeneities 1-3 in living cells, heat dissipation in nano circuits 4 , nano-plasmonics, and nano-magnetism (like thermal remanent magnetism of nanoparticles). There have been a number of nanoscale temperature detection schemes 5, 6 , such as scanning thermal microscopy (SThM) 7-9 , SQUID based nano-thermometer 10 , and fluorescence thermometers 11 based on rare

Physical Review B, 2013

We provide ab initio characterization of the negatively charged substitutional nickel (Ni − s) impurity in diamond using a hybrid density functional calculation. Ni − s is shown to carry a spin S = 3/2. The calculated hyperfine couplings on this defect support the identification of the W8 electron paramagnetic resonance center with Ni − s defect. We unambiguously determine the position of the Ni − s acceptor level in the gap. This level is located at about 2.0 eV above the valence band maximum and corresponds to a totally occupied triplet state responsible for the magnetization. We calculated the excited state properties of the defect. Our results may resolve the controversial assignments of Ni − s to different optical centers.

Nature Nanotechnology, 2011

Physical Review Letters

Negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (hBN) are promising spin defects in a van der Waals crystal. Understanding the spin properties of the excited state (ES) is critical for realizing dynamic nuclear polarization. Here, we report zerofield splitting in the ES of DES = 2160 MHz and an optically detected magnetic resonance (ODMR) contrast of 12% at cryogenic temperature. The ES has a g-factor similar to the ground state. The ES photodynamics is further elucidated by measuring the level anti-crossing of the VBdefects under varying external magnetic fields. In contrast to nitrogen vacancy (NV-) centers in diamond, the emission change caused by excited-state level anti-crossing (ESLAC) is more prominent at cryo-temperature than at room temperature. Our results provide important information for utilizing the spin defects of hBN in quantum technology. Color centers with optically addressable spins in wide bandgap materials (e.g., diamond 1, 2 and silicon carbide 3, 4) have been intensively studied in recent decades for applications in quantum sensing 5-8 and quantum information processing. 9, 10 For nanoscale sensing, it is preferable to bring the sensor close to the investigated object to enhance sensitivity. 11-13 Spin qubits in 2D materials naturally meet this requirement and present an extra opportunity for quantum sensing besides the remarkable spatial resolution and sensitivity achieved by diamond NVcolor centers. 14, 15 Among various 2D materials, hBN have attracted much attention for its capability to exhibit various bright single-photon emitters at room temperature (RT). 16-18 In addition, recent discoveries of optically addressable spin defects have further boosted the efforts to investigate

Nature Reviews Physics, 2021

Nanometer-scale imaging of magnetization and current density is the key to deciphering the mechanisms behind a variety of new and poorly understood condensed matter phenomena. The recently discovered correlated states hosted in atomically layered materials such as twisted bilayer graphene or van der Waals heterostructures are noteworthy examples. Manifestations of these states range from superconductivity, to highly insulating states, to magnetism. Their fragility and susceptibility to spatial inhomogeneities limits their macroscopic manifestation and complicates conventional transport or magnetization measurements, which integrate over an entire sample. In contrast, techniques for imaging weak magnetic field patterns with high spatial resolution overcome inhomogeneity by measuring the local fields produced by magnetization and current density. Already, such imaging techniques have shown the vulnerability of correlated states in twisted bilayer graphene to twist-angle disorder and revealed the complex current flows in quantum Hall edge states. Here, we review the state-of-the-art techniques most amenable to the investigation of such systems, because they combine the highest magnetic field sensitivity with the highest spatial resolution and are minimally invasive: magnetic force microscopy, scanning superconducting quantum interference device microscopy, and scanning nitrogen-vacancy center microscopy. We compare the capabilities of these techniques, their required operating conditions, and assess their suitability to different types of source contrast, in particular magnetization and current density. Finally, we focus on the prospects for improving each technique and speculate on its potential impact, especially in the rapidly growing field of two-dimensional materials.

Journal of Applied Physics, 2020

Nanodiamonds (NDs) hosting nitrogen-vacancy (NV) centers are promising for applications of quantum sensing. Long spin relaxation times (T 1 and T 2) are critical for high sensitivity in quantum applications. It has been shown that fluctuations of magnetic fields due to surface spins strongly influences T 1 and T 2 in NDs. However, their relaxation mechanisms have yet to be fully understood. In this paper, we investigate the relation between surface spins and T 1 and T 2 of single-substitutional nitrogen impurity (P1) centers in NDs. The P1 centers located typically in the vicinity of NV centers are a great model system to study the spin relaxation processes of the NV centers. By employing high-frequency electron paramagnetic resonance (EPR) spectroscopy, we verify that air annealing removes surface spins efficiently and significantly reduces their contribution to T 1 .

Physical Review B, 2017

We report a study of the magnetic field dependence of photoluminescence of NV − centers (negatively charged nitrogen-vacancy centers) in diamond single crystals. In such a magnetic field dependence characteristic sharp features are observed, which are coming from Level Anti-Crossings (LACs) in a coupled electron-nuclear spin system. For sensitive detection of such LAC-lines we use lock-in detection to measure the photoluminescence intensity. This experimental technique allows us to obtain new LAC lines. Additionally, a remarkably strong dependence of the LAC-lines on the modulation frequency is found. Specifically, upon decrease of the modulation frequency from 12 kHz to 17 Hz the amplitude of the LAC-lines increases by approximately two orders of magnitude. To take a quantitative account for such effects, we present a theoretical model, which describes the spin dynamics in a coupled electron-nuclear spin system under the action of an oscillating external magnetic field. Good agreement between experiments and theory allows us to conclude that the observed effects are originating from coherent spin polarization exchange in a coupled spin system comprising the spin-polarized NV − center. Our results are of great practical importance allowing one to optimize the experimental conditions for probing LAC-derived lines in diamond crystals comprising NV − centers and for indirect detection and identification of other paramagnetic defect centers.

Physical Review Letters, 2012

The zero-phonon transition rate of a nitrogen-vacancy center is enhanced by a factor of ∼ 70 by coupling to a photonic crystal resonator fabricated in monocrystalline diamond using standard semiconductor fabrication techniques. Photon correlation measurements on the spectrally filtered zero-phonon line show antibunching, a signature that the collected photoluminescence is emitted primarily by a single nitrogen-vacancy center. The linewidth of the coupled nitrogen-vacancy center and the spectral diffusion are characterized using high-resolution photoluminescence and photoluminescence excitation spectroscopy.

Nature Communications, 2014

The application of magnetic resonance spectroscopy at progressively smaller length scales may eventually permit 'chemical imaging' of spins at the surfaces of materials and biological complexes. In particular, the negatively charged nitrogen-vacancy (NV À) centre in diamond has been exploited as an optical transducer for nanoscale nuclear magnetic resonance. However, the spectra of detected spins are generally broadened by their interaction with proximate paramagnetic NV À centres through coherent and incoherent mechanisms. Here we demonstrate a detection technique that can resolve the spectra of electron spins coupled to NV À centres, in this case, substitutional nitrogen and neutral nitrogen-vacancy centres in diamond, through optically detected cross-relaxation. The hyperfine spectra of these spins are a unique chemical identifier, suggesting the possibility, in combination with recent results in diamonds harbouring shallow NV À implants, that the spectra of spins external to the diamond can be similarly detected.

Physical Review A, 2015

A negatively charged nitrogen vacancy (NV) center in diamond has been recognized as a good solid-state qubit. A system consisting of the electronic spin of the NV center and hyperfine-coupled nitrogen and additionally nearby carbon nuclear spins can form a quantum register of several qubits for quantum information processing or as a node in a quantum repeater. Several impressive experiments on the hybrid electron and nuclear spin register have been reported, but fidelities achieved so far are not yet at oor below the thresholds required for fault-tolerant quantum computation (FTQC). Using quantum optimal control theory based on the Krotov method, we show here that fast and high-fidelity single-qubit and two-qubit gates in the universal quantum gate set for FTQC, taking into account the effects of the leakage state, nearby noise qubits and distant bath spins, can be achieved with errors less than those required by the threshold theorem of FTQC.

Physical review, 2021

The nitrogen-vacancy center (NV center) in diamond at magnetic fields corresponding to the ground state level anticrossing (GSLAC) region gives rise to rich photoluminescence (PL) signals due to the vanishing energy gap between the electron spin states, which enables to have an effect on the NV center's luminescence for a broad variety of environmental couplings. In this article we report on the GSLAC photoluminescence signature of NV ensembles in different spin environments at various external fields. We investigate the effects of transverse electric and magnetic fields, P1 centers, NV centers, and the 13 C nuclear spins, each of which gives rise to a unique PL signature at the GSLAC. The comprehensive analysis of the couplings and related optical signal at the GSLAC provides a solid ground for advancing various microwave-free applications at the GSLAC, including but not limited to magnetometry, spectroscopy, dynamic nuclear polarization (DNP), and nuclear magnetic resonance (NMR) detection. We demonstrate that not only the most abundant 14 NV center but the 15 NV can also be utilized in such applications and that nuclear spins coupled to P1 centers can be polarized directly by the NV center at the GSLAC, through a giant effective nuclear g-factor arising from the NV center-P1 center-nuclear spin coupling. We report on new alternative for measuring defect concentration in the vicinity of NV centers and on the optical signatures of interacting, mutually aligned NV centers.

Applied Physics Letters, 2016

Broadband, large-area microwave antenna for optically detected magnetic resonance of nitrogen-vacancy centers in diamond Review of Scientific Instruments 87, 053904 (2016);

Nanophotonics, 2020

We designed a nanoscale light extractor (NLE) for the efficient outcoupling and beaming of broadband light emitted by shallow, negatively charged nitrogenvacancy (NV) centers in bulk diamond. The NLE consists of a patterned silicon layer on diamond and requires no etching of the diamond surface. Our design process is based on adjoint optimization using broadband timedomain simulations and yields structures that are inherently robust to positioning and fabrication errors. Our NLE functions like a transmission antenna for the NV center, enhancing the optical power extracted from an NV center positioned 10 nm below the diamond surface by a factor of more than 35, and beaming the light into a ±30°cone in the far field. This approach to light extraction can be readily adapted to other solid-state color centers.