Single Nitrogen-vacancy Center Research Articles

High-resolution spectroscopy of a single nitrogen-vacancy defect at zero magnetic field

We report a study of high-resolution microwave spectroscopy of nitrogen-vacancy (NV) centers in diamond crystals at and around zero magnetic field. We observe characteristic splitting and transition imbalance of the hyperfine transitions, which originate from level anti-crossings (LACs) in the presence of a transverse effective field. We use pulsed electron spin resonance spectroscopy to measure the zero-field spectral features of single NV centers for clearly resolving such LACs. To quantitatively analyze the magnetic resonance behavior of the hyperfine spin transitions in the presence of the effective field, we present a theoretical model, which describes the transition strengths under the action of an arbitrarily polarized microwave magnetic field. Our results are of importance for the optimization of the experimental conditions for the polarization-selective microwave excitation of spin-1 systems in zero or weak magnetic fields.

Variable bandwidth, high efficiency microwave resonator for control of spin-qubits in nitrogen-vacancy centers.

Nitrogen-Vacancy (NV) centers in diamond are attractive tools for sensing and quantum information. Realization of this potential requires effective tools for controlling the spin degree of freedom by microwave (mw) magnetic fields. In this work, we present a planar microwave resonator optimized for microwave-optical double resonance experiments on single NV centers in diamond. It consists of a piece of wide microstrip line, which is symmetrically connected to two 50Ω microstrip feed lines. In the center of the resonator, an Ω-shaped loop focuses the current and the mw magnetic field. It generates a relatively homogeneous magnetic field over a volume of 0.07 × 0.1mm3. It can be operated at 2.9GHz in both transmission and reflection modes with bandwidths of 1000 and 400MHz, respectively. The high power-to-magnetic field conversion efficiency allows us to produce π-pulses with a duration of 50ns with only about 200 and 50 mW microwave power in transmission and reflection, respectively. The transmission mode also offers capability for efficient radio frequency excitation. The resonance frequency can be tuned between 1.3 and 6GHz by adjusting the length of the resonator. This will be useful for experiments on NV-centers at higher external magnetic fields and on different types of optically active spin centers.

Radio-Frequency Electric Field Sensing Based on a Single Solid-State Spin

Nanoscale amplitude and phase sensing of a rf electromagnetic field can benefit broad frontiers in science and technology. The imaging applications in these research areas demand a nanoscale sensor able to detect rf electric fields over a wide frequency range under ambient conditions, which remains a challenge. Here, we present a rf electrometry based on the forbidden magnetic dipole transition of a single shallow nitrogen-vacancy center in diamond and experimentally demonstrate the detection of rf electric fields at frequencies ranging from 13.85 MHz to 2.02 GHz. A sensitivity of $265\phantom{\rule{0.2em}{0ex}}{\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{cm}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{Hz}}^{\ensuremath{-}1/2}$ for amplitude measurement and a standard deviation of ${0.2}^{\ensuremath{\circ}}$ for phase measurement are achieved. The potential applications of nanoscale sensing and imaging of electromagnetic field are discussed.

Spin Dynamics of a Solid-State Qubit in Proximity to a Superconductor.

A broad effort is underway to understand and harness the interaction between superconductors and spin-active color centers with an eye on hybrid quantum devices and novel imaging modalities of superconducting materials. Most work, however, overlooks the interplay between either system and the environment created by the color center host. Here we use a diamond scanning probe to investigate the spin dynamics of a single nitrogen-vacancy (NV) center proximal to a superconducting film. We find that the presence of the superconductor increases the NV spin coherence lifetime, a phenomenon we tentatively rationalize as a change in the electric noise due to a superconductor-induced redistribution of charge carriers near induced redistribution of charge carriers near the NV. We then build on these findings to demonstrate transverse-relaxation-time-weighted imaging of the superconductor film. These results shed light on the dynamics governing the spin coherence of shallow NVs, and promise opportunities for new forms of noise spectroscopy and imaging of superconductors.

Nanoscale covariance magnetometry with diamond quantum sensors

Nitrogen vacancy (NV) centers in diamond are atom-scale defects that can be used to sense magnetic fields with high sensitivity and spatial resolution. Typically, the magnetic field is measured by averaging sequential measurements of single NV centers, or by spatial averaging over ensembles of many NV centers, which provides mean values that contain no nonlocal information about the relationship between two points separated in space or time. Here, we propose and implement a sensing modality whereby two or more NV centers are measured simultaneously, and we extract temporal and spatial correlations in their signals that would otherwise be inaccessible. We demonstrate measurements of correlated applied noise using spin-to-charge readout of two NV centers and implement a spectral reconstruction protocol for disentangling local and nonlocal noise sources.

Single-Nitrogen-Vacancy NMR of Amine-Functionalized Diamond Surfaces.

Nuclear magnetic resonance (NMR) imaging with shallow nitrogen-vacancy (NV) centers in diamond offers an exciting route toward sensitive and localized chemical characterization at the nanoscale. Remarkable progress has been made to combat the degradation in coherence time and stability suffered by near-surface NV centers using suitable chemical surface termination. However, approaches that also enable robust control over adsorbed molecule density, orientation, and binding configuration are needed. We demonstrate a diamond surface preparation for mixed nitrogen- and oxygen-termination that simultaneously improves NV center coherence times for <10 nm-deep emitters and enables direct and recyclable chemical functionalization via amine-reactive cross-linking. Using this approach, we probe single NV centers embedded in nanopillar waveguides to perform 19F NMR sensing of covalently bound fluorinated molecules with detection on the order of 100 molecules. This work signifies an important step toward nuclear spin localization and structure interrogation at the single-molecule level.

Experimental Investigation of Quantum Correlations in a Two-Qutrit Spin System.

We report an experimental investigation of quantum correlations in a two-qutrit spin system in a single nitrogen-vacancy center in diamond at room temperatures. Quantum entanglement between two qutrits was observed at room temperature, and the existence of nonclassical correlations beyond entanglement in the qutrit case has been revealed. Our work demonstrates the potential of the NV centers as the multiqutrit system to execute quantum information tasks and provides a powerful experimental platform for studying the fundamental physics of high-dimensional quantum systems in the future.

All-Optical Modulation of Single Defects in Nanodiamonds: Revealing Rotational and Translational Motions in Cell Traction Force Fields.

Measuring the mechanical interplay between cells and their surrounding microenvironment is vital in cell biology and disease diagnosis. Most current methods can only capture the translational motion of fiduciary markers in the deformed matrix, but their rotational motions are normally ignored. Here, by utilizing single nitrogen-vacancy (NV) centers in nanodiamonds (NDs) as fluorescent markers, we propose a linear polarization modulation (LPM) method to monitor in-plane rotational and translational motions of the substrate caused by cell traction forces. Specifically, precise orientation measurement and localization with background suppression were achieved via optical polarization selective excitation of single NV centers with precisions of ∼0.5°/7.5 s and 2 nm/min, respectively. Additionally, we successfully applied this method to monitor the multidimensional movements of NDs attached to the vicinity of cell focal adhesions. The experimental results agreed well with our theoretical calculations, demonstrating the practicability of the NV-based LPM method in studying mechanobiology and cell-material interactions.

Robust magnetometry with single nitrogen-vacancy centers via two-step optimization

Shallow Nitrogen-Vacancy (NV) centers are promising candidates for high-precision sensing applications; these defects, when positioned a few nanometers below the surface, provide an atomic-scale resolution along with substantial sensitivity. However, the dangling bonds and impurities on the diamond surface result in a complex environment which reduces the sensitivity and is unique to each shallow NV center. To avoid the environment's detrimental effect, we apply feedback-based quantum optimal control. We first show how a direct search can improve the initialization/readout process. In a second step, we optimize microwave pulses for pulsed Optically Detected Magnetic Resonance (ODMR) and Ramsey measurements. Throughout the sensitivity optimizations, we focus on robustness against errors in the control field amplitude. This feature not only protects the protocols' sensitivity from drifts but also enlarges the sensing volume. The resulting ODMR measurements produce sensitivities below 1$\mu$T\,Hz$^{-\frac{1}{2}}$ for an 83\% decrease in control power, increasing the robustness by approximately one third. The optimized Ramsey measurements produce sensitivities below 100\,nT\,Hz$^{-\frac{1}{2}}$ giving a two-fold sensitivity improvement. Being on par with typical sensitivities obtained via single NV magnetometry, the complementing robustness of the presented optimization strategy may provide an advantage for other NV-based applications.

Scanning gradiometry with a single spin quantum magnetometer

Quantum sensors based on spin defects in diamond have recently enabled detailed imaging of nanoscale magnetic patterns, such as chiral spin textures, two-dimensional ferromagnets, or superconducting vortices, based on a measurement of the static magnetic stray field. Here, we demonstrate a gradiometry technique that significantly enhances the measurement sensitivity of such static fields, leading to new opportunities in the imaging of weakly magnetic systems. Our method relies on the mechanical oscillation of a single nitrogen-vacancy center at the tip of a scanning diamond probe, which up-converts the local spatial gradients into ac magnetic fields enabling the use of sensitive ac quantum protocols. We show that gradiometry provides important advantages over static field imaging: (i) an order-of-magnitude better sensitivity, (ii) a more localized and sharper image, and (iii) a strong suppression of field drifts. We demonstrate the capabilities of gradiometry by imaging the nanotesla fields appearing above topographic defects and atomic steps in an antiferromagnet, direct currents in a graphene device, and para- and diamagnetic metals.

Toward the Speed Limit of High-Fidelity Two-Qubit Gates.

Most implementations of quantum gate operations rely on external control fields to drive the evolution of the quantum system. Generating these control fields requires significant efforts to design the suitable control Hamiltonians. Furthermore, any error in the control fields reduces the fidelity of the implemented control operation with respect to the ideal target operation. Achieving sufficiently fast gate operations at low error rates remains therefore a huge challenge. In this Letter, we present a novel approach to overcome this challenge by eliminating, for specific gate operations, the time-dependent control fields entirely. This approach appears useful for maximizing the speed of the gate operation while simultaneously eliminating relevant sources of errors. We present an experimental demonstration of the concept in a single nitrogen-vacancy center in diamond at room temperature.

A diamond-confined open microcavity featuring a high quality-factor and a small mode-volume

With a highly coherent, optically addressable electron spin, the nitrogen-vacancy (NV) center in diamond is a promising candidate for a node in a quantum network. A resonant microcavity can boost the flux of coherent photons emerging from single NV centers. Here, we present an open Fabry–Pérot microcavity geometry containing a single-crystal diamond membrane, which operates in a regime where the vacuum electric field is strongly confined to the diamond membrane. There is a field anti-node at the diamond–air interface. Despite the presence of surface losses, a finesse of F=11500 was observed. The quality (Q) factor for the lowest mode number is 120000; the mode volume V is estimated to be 3.9λ03, where λ0 is the free-space wavelength. We investigate the interplay between different loss mechanisms and the impact these loss channels have on the performance of the cavity. This analysis suggests that the surface waviness (roughness with a spatial frequency comparable to that of the microcavity mode) is the mechanism preventing the Q/V ratio from reaching even higher values. Finally, we apply the extracted cavity parameters to the NV center and calculate a predicted Purcell factor exceeding 150.

Optimization of a quantum control sequence for initializing a nitrogen-vacancy spin register

Implementation of many quantum information protocols requires an efficient initialization of the quantum register. In the present paper, we optimize a population trapping protocol for initializing a hybrid spin register associated with a single nitrogen-vacancy (NV) center in diamond. We initialize the quantum register by polarizing the electronic and nuclear spins of the NV with a sequence of microwave, radio-frequency, and optical pulses. We use a rate equation model to explain the distribution of population under the effect of the optical pulses. The model is compared to the experimental data obtained by performing partial quantum state tomography. To further increase the spin polarization, we propose a recursive protocol with optimized optical pulses. We also discuss the role of the relative values of the nuclear- and electronic-spin pumping rate in achieving the maximum degree of spin polarization.

Charge stability of shallow single nitrogen-vacancy centers in lightly boron-doped diamond

Nitrogen vacancy (NV) centers in diamond must be in a stable negatively charged state for their application to quantum sensing and quantum information processing. In this study, we investigated the charge-state stability of single NV centers in lightly boron-doped diamond ([B] ≈ 1 × 1015 cm−3). Photoluminescence and optically detected magnetic resonance measurements indicated that single NV centers near the diamond surface are negatively charged despite the presence of boron acceptors. This unique phenomenon can be explained by charge transfer between the NV centers and their local environment under laser excitation. This study provides a new perspective on the charge stability of single NV centers in lightly doped diamond and insights for further development of high-performance diamond quantum devices.

Stacked metasurfaces for enhancing the emission and extraction rate of single nitrogen-vacancy centers in nanodiamond

Dielectric metasurfaces with unique possibilities of manipulating light–matter interaction lead to new insights in exploring spontaneous emission control using single quantum emitters. Here, we study the stacked metasurfaces in one- (1D) and two-dimensions (2D) to enhance the emission rate of a single quantum emitter using the associated optical resonances. The 1D structures with stacked bilayers are investigated to exhibit Tamm plasmon resonance optimized at the zero phonon line (ZPL) of the negative nitrogen-vacancy (NV−) center. The 2D stacked metasurface comprising of two-slots silicon nano-disks is studied for the Kerker condition at ZPL wavelength. The far-field radiation plots for the 1D and 2D stacked metasurfaces show an increased extraction efficiency rate for the NV− center at ZPL wavelength that reciprocates the localized electric field intensity. The modified local density of optical states results in large Purcell enhancement of 3.8 times and 25 times for the single NV− center integrated with 1D and 2D stacked metasurface, respectively. These results have implications in exploring stacked metasurfaces for applications such as single photon generation and CMOS compatible light sources for on-demand chip integration.

Quantum calibrated magnetic force microscopy

We report the quantum calibration of a magnetic force microscope (MFM) by measuring the two-dimensional magnetic stray field distribution of the MFM tip using a single nitrogen vacancy (NV) center in diamond. From the measured stray field distribution and the mechanical properties of the cantilever a calibration function is derived allowing to convert MFM images to quantum calibrated stray field maps. This novel approach overcomes limitations of prior MFM calibration schemes and allows quantum calibrated nanoscale stray field measurements in a field range inaccessible to scanning NV magnetometry. Quantum calibrated measurements of a stray field reference sample allow its use as a transfer standard, opening the road towards fast and easily accessible quantum traceable calibrations of virtually any MFM.

Coupling a single NV center to a superconducting flux qubit via a nanomechanical resonator

We investigate the hybrid quantum system of a superconducting flux qubit, a single electronic spin associated with a negatively charged nitrogen vacancy (NV) center in diamond, and a high-quality magnetized nanomechanical resonator (NAMR). It is shown that the quantized motion of the NAMR can strongly couple to both the NV spin and the flux qubit. We estimate that the coupling strength of the NAMR to both the NV spin and the flux qubit can be about 2 π × 100 kHz, which is one order of magnitude larger than direct spin–qubit coupling. This enables the quantum state transfer to be faster and more robust against decoherence. The present work may provide an appealing way for quantum information processing based on hybrid solid-state quantum systems.

Imaging oersted field around current flowing wire based on a diamond scanning magnetometer

We demonstrate imaging current flow in a transport device by measuring the current-induced Oersted field. A diamond scanning magnetometer that hosts a single nitrogen-vacancy (NV) center at the tip apex is used to obtain the image. We fabricate a U-shape of Pt wire and study evolution of the Oersted field as the current continuously changes its direction along the wire. We find a good agreement between the obtained results and the simulated images based on the Biot-Savart law. In order to show the capability of imaging different field components, we perform the same experiment but with two different axes of the NV center. This work provides a novel imaging method of the current profile and can be applied to various transport experiments.

Magnetometry based on the excited-state lifetimes of a single nitrogen-vacancy center in diamond

So far most well-established quantum sensing techniques based on the negatively charged nitrogen-vacancy (NV) center in diamond utilize the recorded fluorescence intensity to detect the electronic spin states. However, the fluorescence intensity of a NV center is not only dependent on its spin state, but also affected by measurement issues, such as fluctuations of the excitation laser power and charge state transformation of the NV center. Instabilities in terms of output power or polarization changes in the laser source as well as sample drifts during a measurement are common factors that weaken the precision. Here, we demonstrate proof-of-principle of a sensing method making use of the excited-state lifetimes of a NV center for magnetometry experiments.

Ground-state microwave-stimulated Raman transitions and adiabatic spin transfer in the N15 nitrogen vacancy center

Microwave pulse sequences are the basis of coherent manipulation of the electronic spin ground state in nitrogen vacancy (NV) centers. In this work we demonstrate stimulated Raman transitions (SRTs) and stimulated Raman adiabatic passage (STIRAP), two ways to drive the dipole-forbidden transition between two spin sublevels in the electronic triplet ground state of the NV center. This driving is achieved by a two-photon Raman microwave pulse which simultaneously drives two detuned transitions via a virtual level for SRTs or via two adiabatic and partially overlapping resonant microwave pulses for STIRAP. We lay the theoretical framework of SRT and STIRAP dynamics and verify experimentally the theoretical predictions of population inversion by observing the dipole-forbidden transition in the ground state of a single NV center. A comparison of the two schemes showed better robustness and success of the spin swap for STIRAP compared to SRT.