Nitrogen-vacancy Centers Research Articles

Accurate Excitation Energies of Point Defects from Fast Particle-Particle Random Phase Approximation Calculations.

We present an efficient particle-particle random phase approximation (ppRPA) approach that predicts accurate excitation energies of point defects, including the nitrogen-vacancy (NV-) and silicon-vacancy (SiV0) centers in diamond and the divacancy center (VV0) in 4H silicon carbide, with errors of ±0.2 eV compared with experimental values. Starting from the (N + 2)-electron ground state calculated with density functional theory (DFT), the ppRPA excitation energies of the N-electron system are calculated as the differences between the two-electron removal energies of the (N + 2)-electron system. We demonstrate that the ppRPA excitation energies converge rapidly with a few hundred canonical active-space orbitals. We also show that active-space ppRPA has weak DFT starting-point dependence and is significantly cheaper than the corresponding ground-state DFT calculation. This work establishes ppRPA as an accurate and low-cost tool for investigating excited-state properties of point defects and opens up new opportunities for applications of ppRPA to periodic bulk materials.

Magnetic-field-assisted optical fiber quantum temperature sensor with enhanced sensitivity.

In recent years, utilizing nitrogen-vacancy color centers in diamond for temperature sensing has drawn great attention. However, increasing the sensitivity has encountered challenges due to the intrinsic temperature-dependent energy level shift, i.e., temperature responsivity, being limited to -74 kHz/K. In this Letter, we take advantage of the magnetic field to regulate the energy level to enhance temperature sensitivity. The sensor is formed by adhering a micron-sized diamond on the end face of an optical fiber, and a small magnet is mounted at a certain distance with the diamond exploiting a cured polydimethylsiloxane block as the bridge. The temperature change leads to the variation of the distance between the diamond and the magnet, thus affecting the magnetic strength felt by the diamond. This finally contributes an additional temperature-induced energy level shift, giving rise to an enhanced sensitivity. Experimental results demonstrated the proposed scheme and achieved a 4.2-fold improvement in the temperature responsivity and a 2.1-fold enhancement in sensitivity. Moreover, the diamond and the fiber-optic integrated structure improve the portability of the sensor.

Integrated coplanar waveguide coil on diamond for enhanced homogeneous broadband NV magnetometry.

Nitrogen-vacancy (NV) centers in diamond have emerged as promising quantum sensors due to their highly coherent and optically addressable spin states with potential applications in high-sensitivity magnetometry. Homogeneously addressing large ensembles of NV centers offers clear benefit in terms of sensing precision as well as in fundamental studies of collective effects. Such experiments require a spatially uniform, intense, and broadband microwave field that can be difficult to generate. Previous approaches, such as copper wires, loop coils, and planar structures, have shown limitations in field homogeneity, bandwidth, and integration in compact devices. In this paper, we present a coplanar waveguide (CPW) gold coil patterned on a 3 × 3 mm 2 diamond substrate, offering full integration, enhanced stability, and broad bandwidth suitable for various NV sensing applications. Coil fabricated on diamond offers several advantages for magnetometry with NV centers ensemble, including enhanced heat dissipation, seamless integration, scalability, and miniaturization potential. We optimize critical geometrical parameters to achieve a homogeneous magnetic field with a coefficient of variation of less than 6% over an area of 0.5 mm 2 and present experimental results confirming the performance of the proposed CPW coil.

Spin-State Control of Shallow Single NV Centers in Hydrogen-Terminated Diamond.

The ability to control the charge and spin states of nitrogen-vacancy (NV) centers near the diamond surface is of pivotal importance for quantum applications. Hydrogen-terminated diamond is promising for long spin coherence times and ease of controlling the charge states due to the low density of surface defects. However, it has so far been challenging to create negatively charged single NV centers with controllable spin states beneath a hydrogen-terminated surface because atmospheric adsorbates that act as acceptors induce surface holes. In this study, we optically detected the magnetic resonance of shallow single NV centers in hydrogen-terminated diamond through precise control of the nitrogen implantation fluence. Furthermore, we found that the probability of detecting the resonance was enhanced by reducing the surface acceptor density through passivation of the hydrogen-terminated surface with hexagonal boron nitride without air exposure. This control method opens up new opportunities for using NV centers in quantum applications.

The concentration estimation for ensemble of NV– color centers with spin coherent manipulation

The concentration and distribution of negatively charged nitrogen-vacancy color centers (NV−) directly affect the sensitivity and resolution of quantum sensing. In this paper, a simple method is proposed to estimate the concentration of the ensemble of NV− centers with spin coherent manipulation. The stray fluorescence is eliminated by separating the fluorescence near the NV− zero-phonon line. By introducing a single NV− fluorescence quantitative model, the number of NV− centers in the diamond under test can be obtained. The average concentration of the ensemble of NV− centers in the sample can be obtained by combining the detection volume. Compared with the results measured by electron spin resonance, the concentration of NV− centers measured by our method can be verified, and the maximum error between the two methods is less than 4.3%. The results show that the method provides an effective reference for accurately characterizing the concentration distribution characteristics of the NV− ensemble.

Measurement of Microdisplacement by Spin Manipulated Magnetometer of Nitrogen Vacancy Centers With Magnetic Flux Concentration Structure

Measurement of Microdisplacement by Spin Manipulated Magnetometer of Nitrogen Vacancy Centers With Magnetic Flux Concentration Structure

A noise-robust quantum dynamics learning protocol based on Choi–Jamiolkowski isomorphism: theory and experiment

With the rapid development of quantum technology, the growing manipulated Hilbert space makes learning the dynamics of the quantum system a significant challenge. Machine learning technique has brought apparent advantages in some learning strategies, therefore, we introduce it to indirect learning in this paper. Based on Choi–Jamiolkowski isomorphism, we propose a protocol that learns the dynamics of an inaccessible quantum system using a quantum device at hand. For an n-qubit system, the learning task can be done iteratively, with operational complexity O(poly(n,L)/ϵ2) in each iteration, where L is the circuit depth and ε is the measurement error. Then we theoretically prove its noise resilience to global depolarization, state preparation and measurement noise, and unitary noise in gates implementation, where we find the learned dynamics stay invariant. Finally, we investigate the protocol experimentally on a nitrogen-vacancy center system with a natural noise source. The results show that the behavior of a relatively intractable nuclear spin can be learned through an easily accessible electron spin under different noise models, demonstrating the protocol’s feasibility.

All-optical determination of one or two emitters using polarization-enhanced photon correlations of nitrogen-vacancy centers in diamond

All-optical determination of one or two emitters using polarization-enhanced photon correlations of nitrogen-vacancy centers in diamond

Detecting nitrogen-vacancy-hydrogen centers on the nanoscale using nitrogen-vacancy centers in diamond

In diamond, nitrogen defects like the substitutional nitrogen defect (Ns) or the nitrogen-vacancy-hydrogen complex (NVH) outnumber the nitrogen-vacancy (NV) defect by at least one order of magnitude creating a dense spin bath. While neutral Ns has an impact on the coherence of the NV spin state, the atomic structure of NVH reminds of a NV center decorated with a hydrogen atom. As a consequence, the formation of NVH centers could compete with that of NV centers possibly lowering the N-to-NV conversion efficiency in diamond grown with hydrogen-plasma-assisted chemical vapor deposition (CVD). Therefore, monitoring and controlling the spin bath is essential to produce and understand engineered diamond material with high NV concentrations for quantum applications. While the incorporation of Ns in diamond has been investigated on the nano- and mesoscale for years, studies concerning the influence of CVD parameters and the crystal orientation on the NVH formation have been restricted to bulk N-doped diamond providing high-enough spin numbers for electron paramagnetic resonance and optical absorption spectroscopy techniques. Here, we investigate submicron-thick (100)-diamond layers with nitrogen contents of (13.8±1.6)ppm and (16.7±3.6)ppm, and exploiting the NV centers in the layers as local nanosensors, we demonstrate the detection of NVH− centers using double electron-electron resonance (DEER). To determine the NVH− densities, we quantitatively fit the hyperfine structure of NVH− and confirm the results with the DEER method usually used for determining Ns0 densities. With our experiments, we access the spin bath composition on the nanoscale and enable a fast feedback loop in CVD recipe optimization with thin diamond layers instead of resource- and time-intensive bulk crystals. Furthermore, the quantification of NVH− plays a very important role for understanding the dynamics of vacancies and the incorporation of hydrogen into CVD diamond optimized for quantum technologies. Published by the American Physical Society 2024

Sensing of magnetic field effects in radical-pair reactions using a quantum sensor

Magnetic field effects (MFE) in certain chemical reactions have been well established in the last five decades and are attributed to the evolution of transient radical pairs whose spin dynamics are determined by local and external magnetic fields. The majority of existing experimental techniques used to probe these reactions only provide ensemble averaged reaction parameters and spin chemistry, hindering the observation of the potential presence of quantum coherent phenomena at the single molecule scale. Here, considering a single nitrogen-vacancy (NV) center as quantum sensor, we investigate the prospects and requirements for the detection of MFEs on the spin dynamics of radical pairs at the scale of a single and small ensembles of molecules. We employ elaborate and realistic models of radical pairs, considering their coupling to the local spin environment and the sensor. For two model systems, we derive signals of MFE detectable even in the weak coupling regime between a radical pair and an NV quantum sensor and observe that the dynamics of certain populations, as well as coherence elements, of the density matrix of the radical pair are directly detectable. Our investigations will provide important guidelines for the potential detection of spin chemistry of biomolecules at the scale of a single and small ensembles of molecules. Published by the American Physical Society 2024

Imaging the Meissner effect in hydride superconductors using quantum sensors.

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena1. The megabar regime represents an interesting frontier, in which recent discoveries include high-temperature superconductors, as well as structural and valence phase transitions2-6. However, at such high pressures, many conventional measurement techniques fail. Here we demonstrate the ability to perform local magnetometry inside a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach uses a shallow layer of nitrogen-vacancy colour centres implanted directly within the anvil7-9; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the nitrogen-vacancy centre to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH9 (ref. 10). By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed-loop optimization of superhydride materials synthesis.

Infrared vertical external cavity surface emitting laser threshold magnetometer

Nitrogen-vacancy (NV) centers have considerable promise as high sensitivity magnetometers; however, they are commonly limited by inefficient collection and low contrasts. Laser threshold magnetometry (LTM) enables efficient collection and high contrasts, providing a path toward higher sensitivity magnetometry. We demonstrate an infrared LTM using an ensemble of NV centers in a single-crystal diamond plate integrated into a vertical external cavity surface emitting laser. The laser was tuned to the spin-dependent absorption line of the NV centers, allowing for optical readout by monitoring the laser output power. We demonstrate a magnetic sensitivity of 7.5 nT/Hz in the frequency range between 10 and 50 Hz. Furthermore, the contrast and the projected photon shot noise limited (PSNL) sensitivity are shown to improve significantly by operating close to the lasing threshold, achieving 18.4% and 26.6 pT/Hz near the threshold. In addition, an unexpected saturable absorption phenomenon was observed near the threshold, which enhanced the contrast and projected PSNL sensitivity.

Four-Order Power Reduction in Nanoscale Electron-Nuclear Double Resonance with a Nitrogen-Vacancy Center in Diamonds.

Detecting nuclear spins using single nitrogen-vacancy (NV) centers is of particular importance in nanoscale science and engineering but often suffers from the heating effect of microwave fields for spin manipulation, especially under high magnetic fields. Here, we realize an energy-efficient nanoscale nuclear-spin detection using a phase-modulation electron-nuclear double resonance scheme. The microwave field can be reduced to 1/250 of the previous requirements, and the corresponding power is over four orders lower. Meanwhile, the microwave-induced broadening to the line-width of the spectroscopy is significantly canceled, and we achieve a nuclear-spin spectrum with a resolution down to 2.1 kHz under a magnetic field at 1840 Gs. The spectral resolution can be further improved by upgrading the experimental control precision. This scheme can also be used in sensing microwave fields and can be extended to a wide range of applications in the future.

Qudit-based high-dimensional controlled-not gate.

High-dimensional quantum systems expand quantum channel capacity and information storage space. By implementing high-dimensional quantum logic gates, the speed of quantum computing can be practically enhanced. We propose a deterministic 4 × 4-dimensional controlled-not (CNOT) gate for a hybrid system without ancillary qudits required, where the spatial and polarization states of a single photon serve as a control qudit of four dimensions, whereas two electron-spin states in nitrogen-vacancy (NV) centers act as a four-dimensional target qudit. As the control qudits are easily operated employing simple optical elements and the target qudits are available for storage, the CNOT gate works in a deterministic way, and it can be flexibly extended to n × n-dimensional (n > 4) quantum gates for other hybrid systems or different photonic degrees of freedoms. The efficiency and fidelity of the CNOT gate are analyzed aligning with current technological capabilities, finding that they have satisfactory performances.

Fiber-taper collected emission from NV centers in high-Q/V diamond microdisks.

Fiber-coupled microdisks are a promising platform for enhancing the spontaneous emission from color centers in diamond. The measured cavity-enhanced emission from the microdisk is governed by the effective volume (V) of each cavity mode, the cavity quality factor (Q), and the coupling between the microdisk and the fiber. Here we observe room temperature photoluminescence from an ensemble of nitrogen-vacancy centers into high Q/V microdisk modes, which when combined with coherent spectroscopy of the microdisk modes, allows us to elucidate the relative contributions of these factors. The broad emission spectrum acts as an internal light source facilitating mode identification over several cavity free spectral ranges. Analysis of the fiber taper collected microdisk emission reveals spectral filtering both by the cavity and the fiber taper, the latter of which we find preferentially couples to higher-order microdisk modes. Coherent mode spectroscopy is used to measure Q ∼ 1 × 105 - the highest reported values for diamond microcavities operating at visible wavelengths. With realistic optimization of the microdisk dimensions, we predict that Purcell factors of ∼50 are within reach.

Application of machine learning to experimental design in quantum mechanics

The recent advances in Machine Learning hold great promises for the field of quantum sensing and metrology. With the help of reinforcement learning, we can tame the complexity of quantum systems and solve the problem of optimal experimental design. Reinforcement learning is a powerful model-free technique that allows an agent, which is typically a neural network, to learn the best strategy to reach a certain goal in a completely a priori unknown environment. However, in general, we know something about the quantum system the agent is interacting with, at least that it follows the rules of quantum mechanics. In quantum metrology, we typically have a model for the system and only some parameters of the evolution or of the initial state are unknown. We present here a general Machine Learning technique that can optimize the precision of quantum sensors, and in doing so it exploits the knowledge we have on the system. We have developed a Python package to automate a broad class of optimizations that can be found in the tasks of quantum parameter estimation, quantum metrology and quantum hypothesis testing. What the agent is learning here is an optimal adaptive strategy, that, on the basis of the previous outcomes, decides the next measurements to perform. It works both for Bayesian estimation and for frequentist estimation. The user is required to implement the physics of the system to be studied and state which parameters in the experiment are controllable and which are unknowns. The functions of the library allow then to easily train a neural network agent for optimizing the precision of the sensor, by simulating the experiment. We have explored some applications of this technique to magnetometry on NV centers (both DC and AC), to state discrimination in quantum optics and to phase estimation. So far, we were able to certify better results than the current state-of-the-art controls for many examples. The Machine Learning technique developed here can be applied in all those scenarios where the quantum system is well characterized and relatively simple and small. In these cases, we can squeeze every last bit of information from a quantum sensor by controlling it with a neural network appropriately trained.

Microwave optical limiting via an acoustic field in a diamond mechanical resonator.

We investigate the generation and control of the reverse saturable absorption (RSA) and optical limiting (OL) at microwave (mw) range in high-Q single-crystal diamond mechanical resonator (DMR) embedded with many nitrogen-vacancy (NV) centers. The strain-induced acoustic modes enable mechanical manipulation of NV centers. On the basis of strain-coupling mechanism, it is shown that the saturable absorption (SA) switches to the RSA by applying the acoustic field, leading to induce the OL in the diamond through the cross-Kerr effect. We demonstrate that the OL characteristics such as, threshold, efficiency, and dynamic range can be controlled by changing either the intensity or frequency of the acoustic field. Moreover, we show that this optical limiter can amplify noiselessly the low intensity of the mw field input to the sensors and also attenuate any gain-induced noise and increase in the intensity of the mw field if it exceeds the intensity threshold. In addition, it is shown that by increasing either the number of NV centers or length of the diamond, the optical limiter can be more efficient. The physical mechanism of the OL establishment is explained using the analytical expressions, which are in good agreement with the numerical results. Our proposed acoustic-induced optical limiter can be a scheme for protecting different optical and electronic devices in mw range, remote sensing, navigation, communications, microwave heating and thermo/laser therapy.

Design of compact integrated diamond nitrogen–vacancy center quantum probe

An integrated quantum probe for magnetic field imaging is proposed, where the nitrogen–vacancy (NV) center fixed at the fiber tip is located on the periphery of flexible ring resonator. Using flexible polyimide (PI) as the substrate medium, we design a circular microstrip antenna, which can achieve a bandwidth of 140 MHz at Zeeman splitting frequency of 2.87 GHz, specifically suitable for NV center experiments. Subsequently, this antenna is seamlessly fixed at a three-dimensional-printed cylindrical support, allowing the optical fiber tip to extend out of a dedicated aperture. To mitigate errors originating from processing, precise tuning within a narrow range can be achieved by adjusting the conformal amplitude. Finally, we image the microwave magnetic field around the integrated probe with high resolution, and determine the suitable area for placing the fiber tip (SAP).

Fast coherent control of nitrogen-14 spins associated with nitrogen-vacancy centers in diamonds using dynamical decoupling

Abstract A nitrogen-vacancy (NV) center in a diamond enables the access to an electron spin, which is expected to present highly sensitive quantum sensors. Although exploiting a nitrogen nuclear spin improves the sensitivity, manipulating it using a resonant pulse requires a long gate time owing to its small gyromagnetic ratio. Another technique to control nuclear spins is a conditional rotation gate based on dynamical decoupling, which is faster but unavailable for nitrogen spins owing to the lack of transverse hyperfine coupling with the electron spin. In this study, we generated effective transverse coupling by applying a weak off-axis magnetic field. An effective coupling depends on the off-axis field; the conditional rotation gate on the nitrogen-14 spins of an NV center was demonstrated within 4.2 μs under an 1.8% off-axis field and a longitudinal field of approximately 280 mT. We estimated that a population transfer from the electron to nitrogen spins can be implemented with 8.7 μs. Our method is applicable to an ensemble of NV centers, in addition to a single NV center.

Sensing the Spin State of Room-Temperature Switchable Cyanometallate Frameworks with Nitrogen-Vacancy Centers in Nanodiamonds.

Room-temperature magnetically switchable materials play a vital role in current and upcoming quantum technologies, such as spintronics, molecular switches, and data storage devices. The increasing miniaturization of device architectures produces a need to develop analytical tools capable of precisely probing spin information at the single-particle level. In this work, we demonstrate a methodology using negatively charged nitrogen vacancies (NV-) in fluorescent nanodiamond (FND) particles to probe the magnetic switching of a spin crossover (SCO) metal-organic framework (MOF), [Fe(1,6-naphthyridine)2(Ag(CN)2)2] material (1), and a single-molecule photomagnet [X(18-crown-6)(H2O)3]Fe(CN)6·2H2O, where X = Eu and Dy (materials 2a and 2b, respectively), in response to heat, light, and electron beam exposure. We employ correlative light-electron microscopy using transmission electron microscopy (TEM) finder grids to accurately image and sense spin-spin interacting particles down to the single-particle level. We used surface-sensitive optically detected magnetic resonance (ODMR) and magnetic modulation (MM) of FND photoluminescence (PL) to sense spins to a distance of ca. 10-30 nm. We show that ODMR and MM sensing was not sensitive to the temperature-induced SCO of FeII in 1 as formation of paramagnetic FeIII through surface oxidation (detected by X-ray photoelectron spectroscopy) on heating obscured the signal of bulk SCO switching. We found that proximal FNDs could effectively sense the chemical transformations induced by the 200 keV electron beam in 1, namely, AgI → Ag0 and FeII → FeIII. However, transformations induced by the electron beam are irreversible as they substantially disrupt the structure of MOF particles. Finally, we demonstrate NV- sensing of reversible photomagnetic switching, FeIII + (18-crown-6) ⇆ FeII + (18-crown-6)+ •, triggered in 2a and 2b by 405 nm light. The photoredox process of 2a and 2b proved to be the best candidate for room-temperature single-particle magnetic switching utilizing FNDs as a sensor, which could have applications into next-generation quantum technologies.