Nitrogen-vacancy Centers Research Articles

Laser-written waveguide-integrated coherent spins in diamond

Quantum emitters, such as the negatively charged nitrogen-vacancy center in diamond, are attractive for quantum technologies, such as nano-sensing, quantum information processing, and as a non-classical light source. However, it is still challenging to position individual emitters in photonic structures while preserving the spin coherence properties of the defect. In this paper, we investigate single and ensemble waveguide-integrated nitrogen-vacancy centers in diamond fabricated by femtosecond laser writing followed by thermal annealing. Their spin coherence properties are systematically investigated and are shown to be comparable to native nitrogen-vacancy centers in diamond. This method paves the way for the fabrication of coherent spins integrated within photonic devices.

High dynamic-range and portable magnetometer using ensemble nitrogen-vacancy centers in diamond.

Nitrogen vacancy (NV) centers in diamonds have been explored for a wide range of sensing applications in the last decade due to their unique quantum properties. In this work, we report a compact and portable magnetometer with an ensemble of NV centers, which we call the Quantum MagPI (Quantum Magnetometer with Proportional Integral control). Our fully integrated compact sensor assembly and control electronics fit inside a 10 × 10 × 7cm3 box and a 30 × 25 × 5cm3 rack-mountable box, respectively. We achieve a bandwidth normalized sensitivity of ∼10 nT/Hz. Using closed-loop feedback for locking to the resonance frequency, we extend the linear dynamic range to 200 μT (20× improvement compared to the intrinsic dynamic range) without compromising the sensitivity. We report a detailed performance analysis of the magnetometer through measurements of noise spectra, Allan deviation, and tracking of nT-level magnetic fields in real-time. In addition, we demonstrate the utility of such a magnetometer by real-time tracking of the movement of an elevator car and door opening events by measuring the projection of the magnetic field along one of the NV-axes under ambient temperature and humidity conditions.

Thresholds for the distributed surface code in the presence of memory decoherence

In the search for scalable, fault-tolerant quantum computing, distributed quantum computers are promising candidates. These systems can be realized in large-scale quantum networks or condensed onto a single chip with closely situated nodes. We present a framework for numerical simulations of a memory channel using the distributed toric surface code, where each data qubit of the code is part of a separate node, and the error-detection performance depends on the quality of four-qubit Greenberger–Horne–Zeilinger (GHZ) states generated between the nodes. We quantitatively investigate the effect of memory decoherence and evaluate the advantage of GHZ creation protocols tailored to the level of decoherence. We do this by applying our framework for the particular case of color centers in diamond, employing models developed from experimental characterization of nitrogen-vacancy centers. For diamond color centers, coherence times during entanglement generation are orders of magnitude lower than coherence times of idling qubits. These coherence times represent a limiting factor for applications, but previous surface code simulations did not treat them as such. Introducing limiting coherence times as a prominent noise factor makes it imperative to integrate realistic operation times into simulations and incorporate strategies for operation scheduling. Our model predicts error probability thresholds for gate and measurement reduced by at least a factor of three compared to prior work with more idealized noise models. We also find a threshold of 4×102 in the ratio between the entanglement generation and the decoherence rates, setting a benchmark for experimental progress.

Exploring High-Spin Color Centers in Wide Band Gap Semiconductors SiC: A Comprehensive Magnetic Resonance Investigation (EPR and ENDOR Analysis).

High-spin defects (color centers) in wide-gap semiconductors are considered as a basis for the implementation of quantum technologies due to the unique combination of their spin, optical, charge, and coherent properties. A silicon carbide (SiC) crystal can act as a matrix for a wide variety of optically active vacancy-type defects, which manifest themselves as single-photon sources or spin qubits. Among the defects, the nitrogen-vacancy centers (NV) are of particular importance. This paper is devoted to the application of the photoinduced electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques at a high-frequency range (94 GHz) to obtain unique information about the nature and properties of NV defects in SiC crystal of the hexagonal 4H and 6H polytypes. Selective excitation by microwave and radio frequency pulses makes it possible to determine the microscopic structure of the color center, the zero-field splitting constant (D = 1.2-1.3 GHz), the phase coherence time (T2), and the values of hyperfine (≈1.1 MHz) and quadrupole (Cq ≈ 2.45 MHz) interactions and to define the isotropic (a = -1.2 MHz) and anisotropic (b = 10-20 kHz) contributions of the electron-nuclear interaction. The obtained data are essential for the implementation of the NV defects in SiC as quantum registers, enabling the optical initialization of the electron spin to establish spin-photon interfaces. Moreover, the combination of optical, microwave, and radio frequency resonant effects on spin centers within a SiC crystal shows the potential for employing pulse EPR and ENDOR sequences to implement protocols for quantum computing algorithms and gates.

Toward Programmable Quantum Processors Based on Spin Qubits with Mechanically Mediated Interactions and Transport.

Solid-state spin qubits are promising candidates for quantum information processing, but controlled interactions and entanglement in large, multiqubit systems are currently difficult to achieve. We describe a method for programmable control of multiqubit spin systems, in which individual nitrogen-vacancy (NV) centers in diamond nanopillars are coupled to magnetically functionalized silicon nitride mechanical resonators in a scanning probe configuration. Qubits can be entangled via interactions with nanomechanical resonators while programmable connectivity is realized via mechanical transport of qubits in nanopillars. To demonstrate the feasibility of this approach, we characterize both the mechanical properties and the magnetic field gradients around the micromagnet placed on the nanobeam resonator. We demonstrate coherent manipulation of a spin qubit in the proximity of a transported micromagnet by utilizing nuclear spin memory and use the NV center to detect the time-varying magnetic field from the oscillating micromagnet, extracting a spin-mechanical coupling of 7.7(9)Hz. With realistic improvements, the high-cooperativity regime can be reached, offering a new avenue toward scalable quantum information processing with spin qubits.

Quantitative measurement of graphitic sp[formula omitted] on single nanodiamonds with sub-monolayer sensitivity using differential interference contrast and photo-thermal microscopy

The surface of nanodiamonds is prone to be decorated with a graphitic layer due to the thermodynamic instability of diamond versus graphite. Notably, this surface layer affects the zeta-potential of nanodiamonds controlling their colloidal stability, modulates the charging of embedded nitrogen vacancy centres used for sensing and quantum technology, and introduces an optical absorption which leads to heating and destruction of nanodiamonds under intense laser irradiation. Physical and chemical treatments to reduce the graphitic carbon are available, but a sensitive method to measure this graphitic layer on single nanodiamonds is lacking. Here, we demonstrate a non-destructive method to quantitatively determine the graphitic carbon correlatively with the nanodiamond size, on individual nanodiamonds. The method allows determining the fraction of graphitic sp2 to diamond sp3 bonded atoms, and estimating the sp2 surface layer thickness. We investigated milled nanodiamonds, both untreated and chemically treated to remove sp2. We found that untreated nanodiamonds were covered by the equivalent of a single sp2 monolayer, which was reduced to 10% of a monolayer for treated nanodiamonds. In both cases, significant variations between individual nanodiamonds were observed. The minimum number of sp2 bonded atoms detectable under the measurement conditions reported here was found to be 2×104.

Understanding Central Spin Decoherence Due to Interacting Dissipative Spin Baths.

We propose a new approach to simulate the decoherence of a central spin coupled to an interacting dissipative spin bath with cluster-correlation expansion techniques. We benchmark the approach on generic 1D and 2D spin baths and find excellent agreement with numerically exact simulations. Our calculations show a complex interplay between dissipation and coherent spin exchange, leading to increased central spin coherence in the presence of fast dissipation. Finally, we model near-surface nitrogen-vacancy centers in diamond and show that accounting for bath dissipation is crucial to understanding their decoherence. Our method can be applied to a variety of systems and provides a powerful tool to investigate spin dynamics in dissipative environments.

Optimizing off-axis fields for two-axis magnetometry with point defects

Vector magnetometry is an essential tool for characterizing the distribution of currents and magnetization in a broad range of systems. Point defect sensors, like the nitrogen vacancy center in diamond, have demonstrated impressive sensitivity and spatial resolution for detecting these fields. Measuring the vector field at a single point in space using single defects, however, remains an outstanding challenge. We demonstrate that careful optimization of the static bias field can enable simultaneous measurement of multiple magnetic field components with enhanced sensitivity by leveraging the nonlinear Zeeman shift from transverse magnetic fields, realizing an improvement in transverse sensitivity from >200 μT/Hz (no bias field) to 30 μT/Hz. This work quantifies the trade-off between the increased frequency shift from second-order Zeeman effects with decreasing contrast as off-axis field components increase, demonstrating the measurement of multiple components of the magnetic field from an exemplar antiferromagnet with a complex magnetic texture.

Projective measurements can probe nonclassical work extraction and time correlations

Quantum correlation functions are a natural way to encode multitime information, as they are ubiquitous in analysis from fluctuation theorems to information scrambling. Correlation functions can be identified with quasiprobabilities associated to quantum processes. In this work we show how these can be measured via error-cancellation techniques, using projective measurements only and no ancillae. The scheme is implemented in a nitrogen-vacancy center in diamond undergoing a unitary quantum work protocol. We reconstruct quantum-mechanical time correlations encoded in the Margenau-Hills quasiprobabilities by observing work extraction peaks five times those of sequential projective energy measurement schemes and in violation of newly derived stochastic bounds. We interpret the phenomenon via anomalous energy exchanges due to the underlying negativity of the quasiprobability distribution. Published by the American Physical Society 2024

Quantum control and Berry phase of electron spins in rotating levitated diamonds in high vacuum

Levitated diamond particles in high vacuum with internal spin qubits have been proposed for exploring macroscopic quantum mechanics, quantum gravity, and precision measurements. The coupling between spins and particle rotation can be utilized to study quantum geometric phase, create gyroscopes and rotational matter-wave interferometers. However, previous efforts in levitated diamonds struggled with vacuum level or spin state readouts. To address these gaps, we fabricate an integrated surface ion trap with multiple stabilization electrodes. This facilitates on-chip levitation and, for the first time, optically detected magnetic resonance measurements of a nanodiamond levitated in high vacuum. The internal temperature of our levitated nanodiamond remains moderate at pressures below 10−5 Torr. We have driven a nanodiamond to rotate up to 20 MHz (1.2 × 109 rpm), surpassing typical nitrogen-vacancy (NV) center electron spin dephasing rates. Using these NV spins, we observe the effect of the Berry phase arising from particle rotation. In addition, we demonstrate quantum control of spins in a rotating nanodiamond. These results mark an important development in interfacing mechanical rotation with spin qubits, expanding our capacity to study quantum phenomena.

Near zero-field microwave-free magnetometry with ensembles of nitrogen-vacancy centers in diamond

Near zero-field microwave-free magnetometry with ensembles of nitrogen-vacancy centers in diamond

Microwave power self-coherent reference measurement based on ensembles of nitrogen-vacancy centers in diamond.

Microwave detection based on optical detection magnetic resonance technology (ODMR) of nitrogen-vacancy (NV) centers is simple and non-invasive. However, in high microwave power ranges, saturation appears and cannot be used for accurate power measurement. The self-coherent reference measurement for high-power microwave based on ODMR of NV centers has been demonstrated. Firstly, by introducing the principle of microwave self-coherent reference, that is, by adjusting the phase difference to achieve power regulation of microwave, a conversion model by phase modulation between enhancement and attenuation of microwave power is introduced. Then, the microwave self-coherent reference measurement is established under combinations of microwave power with different phase settings. Combined with the frequency modulation technology, the sensitivity of measurement is significantly improved from 4.59 nT/Hz1/2 to 67.69 pT/Hz1/2. The maximum measurement range of microwave power can be extended to 2×104 times the initial saturated power of direct measurement with ODMR. The results show that the method efficiently overcomes saturation under the direct measurement of ODMR and provides useful technical assistance for near-field detection, performance monitoring, and problem diagnostics for microwave devices.

Manipulating carbon related spin defects in boron nitride by changing the MOCVD growth temperature

A common solution for precise magnetic field sensing is to employ spin-active defects in semiconductors, with the NV center in diamond as a prominent example. However, the three-dimensional nature of diamond limits the obtainable proximity of the defect to the sample. Two-dimensional boron nitride, which can host spin-active defects, can be used to overcome those limitations. In this work, we study spin properties of sp2-bonded boron nitride layers grown using Metal Organic Chemical Vapor Deposition at temperatures ranging from 700 °C to 1200 °C. With Electron Spin Resonance (ESR) we show that our layers exhibit spin properties, which we ascribe to carbon related defects. Supported by photoluminescence and Fourier-transform infrared spectroscopy, we distinguish three different regimes: (i) growth at low temperatures with no ESR signal, (ii) growth at intermediate temperatures with a strong ESR signal and a large number of spin defects, (iii) growth at high temperatures with a weaker ESR signal and a lower number of spin defects. The observed effects can be further enhanced by an additional annealing step. Our studies demonstrate wafer-scale boron nitride that intrinsically hosts spin defects without any ion or neutron irradiation, which may be employed in spin memories or magnetic field detectors.

Discovery of atomic clock-like spin defects in simple oxides from first principles

Virtually noiseless due to the scarcity of spinful nuclei in the lattice, simple oxides hold promise as hosts of solid-state spin qubits. However, no suitable spin defect has yet been found in these systems. Using high-throughput first-principles calculations, we predict spin defects in calcium oxide with electronic properties remarkably similar to those of the NV center in diamond. These defects are charged complexes where a dopant atom — Sb, Bi, or I — occupies the volume vacated by adjacent cation and anion vacancies. The predicted zero phonon line shows that the Bi complex emits in the telecommunication range, and the computed many-body energy levels suggest a viable optical cycle required for qubit initialization. Notably, the high-spin nucleus of each dopant strongly couples to the electron spin, leading to many controllable quantum levels and the emergence of atomic clock-like transitions that are well protected from environmental noise. Specifically, the Hanh-echo coherence time increases beyond seconds at the clock-like transition in the defect with 209Bi. Our results pave the way to designing quantum states with long coherence times in simple oxides, making them attractive platforms for quantum technologies.

Accurate hyperfine tensors for solid state quantum applications: case of the NV center in diamond

The decoherence of point defect qubits is often governed by the electron spin-nuclear spin hyperfine interaction that can be parameterized by using ab inito calculations in principle. So far most of the theoretical works have focused on the hyperfine interaction of the closest nuclear spins, while the accuracy of the predictions for distinct nuclear spins is barely discussed. Here we demonstrate for the case of the NV center in diamond that the absolute relative error of the computed hyperfine parameters can exceed 100% using an industry standards first-principles code. To overcome this issue, we implement an alternative method and report on significantly improved hyperfine values with O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{{{{{{\\mathcal{O}}}}}}}}$$\\end{document}(1%) relative mean error at all distances. The provided accurate hyperfine data for the NV center enables high-precision simulation of NV quantum nodes for quantum information processing and positioning of nuclear spins by comparing experimental and theoretical hyperfine data.

Plasmonically engineered nitrogen-vacancy spin readout.

Ultra-precise readout of single nitrogen-vacancy (NV) spins holds promise for major advancements in quantum sensing, computing, and communication technologies. Here we present a rigorous open quantum theory capable of simultaneously capturing the optical, vibronic, and spin interactions of the negatively charged NV center, both in the presence and absence of plasmonic interaction. Our theory is verified against existing experiments in the literature. We predict orders of magnitude brightness and contrast enhancements in optically detected magnetic resonance (ODMR) and NV spin qubit readout arising from plasmonic interaction. Such optimal enhancements occur in carefully engineered parameter regions, necessitating rigorous modelling prior to experimentation. Our theory equips the community with a tool to identify such regions.

Temperature dependence of magnetic sensitivity in ensemble NV centers

The magnetic sensitivity of the ensemble NV centers is directly related to temperature. In this study, we systematically investigated the temperature dependence of photoluminescence properties and optical detection magnetic resonance in ensemble NV centers from 1.6 K to 300 K. The magnetic sensitivity of the ensemble NV centers increases with the temperature rising in the range of 1.6 K to 75 K due to changes in contrast and linewidth, reaching a minimum near 40 K. Furthermore, the decrease in sensitivity is attributed to laser intensity overload at low temperatures by studying the influence of laser power on contrast and linewidth. These results offer valuable insights into NV magnetic sensing applications.

Near-zero-field microwave-free magnetometry with nitrogen-vacancy centers in nanodiamonds

We study the fluorescence of nanodiamond ensembles as a function of static external magnetic field and observe characteristic dip features close to the zero field with potential for magnetometry applications. We analyze the dependence of the feature’s width and the contrast of the feature on the size of the diamond (in the range 30 nm–3000 nm) and on the strength of a bias magnetic field applied transversely to the field being scanned. We also perform optically detected magnetic resonance (ODMR) measurements to quantify the strain splitting of the zero-field ODMR resonance across various nanodiamond sizes and compare it with the width and contrast measurements of the zero-field fluorescence features for both nanodiamonds and bulk samples. The observed properties provide compelling evidence of cross-relaxation effects in the NV system occurring close to zero magnetic fields. Finally, the potential of this technique for use in practical magnetometry is discussed.

Terahertz emission from diamond nitrogen-vacancy centers.

Coherent light sources emitting in the terahertz range are highly sought after for fundamental research and applications. Terahertz lasers rely on achieving population inversion. We demonstrate the generation of terahertz radiation using nitrogen-vacancy centers in a diamond single crystal. Population inversion is achieved through the Zeeman splitting of the S = 1 state in 15 tesla, resulting in a splitting of 0.42 terahertz, where the middle Sz = 0 sublevel is selectively pumped by visible light. To detect the terahertz radiation, we use a phase-sensitive terahertz setup, optimized for electron spin resonance (ESR) measurements. We determine the spin-lattice relaxation time up to 15 tesla using the light-induced ESR measurement, which shows the dominance of phonon-mediated relaxation and the high efficacy of the population inversion. The terahertz radiation is tunable by the magnetic field, thus these findings may lead to the next generation of tunable coherent terahertz sources.

Deep polarization of the ensemble nitrogen-vacancy centers in diamonds induced by an Airy beam with a long focal depth

Solid-state spin systems with nitrogen-vacancy (NV) centers in diamonds constitute an increasingly popular platform for quantum sensing. However, most existing platforms designed with ensemble NV centers exhibit a sensitivity that is significantly less than the theoretical maximum. This low sensitivity limits the expansion of the experimental results and application areas. In this study, the sensitivity is improved by increasing the pumping depth of the excitation beam to increase the number of particles involved in spin polarization at a given laser intensity. Compared with the proposed Airy beam with a long focal depth (25.46 λ) and the widely utilized Gauss beam pumping ensemble NV centers, the spin resonance factor fSR can be improved by 10.02%. This sensitivity-optimized approach enhances the functionality of sensors with NV centers.