Session S52: Quantum Sensing with Defect Spin Sensors I

Characterizing the Diamond Waveguide Platform with High Density of NV− Centers for Quantum Sensing Applications

The negatively charged nitrogen-vacancy center in diamond (NV) is an excellent candidate for nanoscale sensing and quantum information applications at room temperature. Efficient light-matter interaction is crucial for manipulating NV-center systems in diamond because it directly affects the contrast and, consequently, sensitivity. Laser-written waveguides enhance this coupling [1] by both guiding light and creating additional NV centers during the laser writing process. Thus, a comprehensive characterization of this platform is essential for its future use. In this study, we present an in-depth analysis of this platform using the ensemble of NV centers it contains, with insights into the diffusion of vacancies, waveguide translational symmetry, and spatial profiles of strain and changes in the refractive index.

The laser writing process is used to fabricate the waveguides, while simultaneously generating a high concentration of vacancies. Subsequently, the annealing process combines these vacancies with the high-density of native nitrogen impurities in the waveguide, producing a considerable number of NVs. We examine the vacancy diffusion profile to determine the diffusion constant. Next, we perform zero-field optically detected magnetic resonance (ODMR) measurements using a confocal microscope. The probed ODMR signal is sensitive to strain-driven changes to NV center spin eigenstates. We fit the ODMR data with a theoretical model that simulates the response from all NV center orientations under strain, assuming the waveguide's translational symmetry. As a result, we extract relevant strain components indirectly justifying this assumption. To gain further insights, we perform strain imaging within the structure. Based on this, we also determine the spatial variation of the refractive index, which directly affects the signal quality.   

Probing condensed matter systems with nanoscale covariance magnetometers based on diamond quantum sensors

Correlated phenomena play a central role in condensed matter physics, but in many cases, there are no tools available for measurements of correlations at the relevant length scales (nanometers - microns). We recently demonstrated that nitrogen vacancy (NV) centers in diamond can be used as point sensors for measuring two-point magnetic field correlators, in contrast to standard techniques where the magnetic field is measured by averaging sequential measurements of single NV centers, or by spatial averaging over ensembles of many centers. This novel quantum sensing platform allows us to measure new physical quantities that are otherwise inaccessible with current tools, particularly in condensed matter systems where two-point correlators can be used to characterize charge transport, magnetism, and non-equilibrium dynamics. Being able to measure correlations between two arbitrary centers that are close to a system of interest also opens the possibility to probe anisotropic phenomena such as variation in correlation parallel or perpendicular to the current flow that could not be probed with a single center otherwise. I will describe an NV center platform for measuring correlations in condensed matter targets, using excess quasiparticle noise near a superconducting phase transition as a model system. The platform features multiplexed sensing with low readout noise and high Rabi frequency microwave manipulation in a cryogenic environment, giving us access to a wide range of condensed matter phenomena.   

Correlated sensing of stochastic coulomb fields in interacting NV clusters

The NV center in diamond is a versatile spin qubit widely used for nanoscale magnetometry and electrometry. Here we demonstrate a new sensing modality, where correlating noise across centers in sub-diffraction NV clusters allows us to study charge dynamics at the nanoscale. We further show strong (classical) Coulomb interaction between neighboring NVs, a step towards NV-NV entanglement through electrical means.

Our experiments leverage low temperature (7K) resonant excitation of NV- to simultaneously read-out the electric field at four nearby NVs [1]. We first exploit NV-NV Coulomb interaction to map out the position of each NVs and then perform repetitive read-out of common stochastic Coulomb fields (spectral diffusion in the bulk). By correlating the electric noise across dimensions and NVs, we identify and localize discrete charge traps in 3D [2].

These results open intriguing opportunities for the microscopic characterization of photo-carrier dynamics and for the manipulation of nanoscale spin-qubit clusters connected via electric fields.

[1] R. Monge, T. Delord, C. Meriles, Nature Nanotechnology, 2023, in press.

[2] T. Delord, R. Monge, C. Meriles, in preparation.   

Theory of Optically Detected Magnetic Resonance of a Silicon Vacancy in SiC: a Quantum Sensor of Magnetic Fields

We simulate optically detected magnetic resonance (ODMR) of the negatively charged silicon vacancy in 6H-SiC (V_Si^–), using Lindblad master equations, and compare with experimental data [1]. An emerging candidate for quantum magnetometry, V_Si^– can be described as a synthetic atom with S=3/2 ground and excited manifolds, separated by 1.397 eV, and a metastable state in the energy gap in between [1-3]. Fitting our calculations to experimental results, we extract the zero field splittings and g-factors of the ground and excited spin manifolds: 2D_g = 124 MHz, 2D_e = 1.22 GHz, g_g = 2.003, g_e = 2.0045; microwave magnetic and electric field strengths: B₁ = 0.25 mT, E₁ = 750 V/cm; Stark coupling coefficients [4]: d_⊥/h = 20 Hz (V/cm)^-1; hyperfine couplings: A = 1.4 MHz; intersystem crossing rates from the excited manifold to the metastable state (metastable state to the ground manifold): 31.4 MHz and 27.8 MHz (1.8 MHz and 1.6 MHz); ground and excited coherence times: T_2,g = 29 μs and T_2,e = 16 ns; and lower bounds of ground and excited manifold spin relaxation times: T_1,g > 2 ms, T_1,e > 10 μs. Finally, we simulated ODMR of V_Si^–  in an isotopically purified sample and predict, in a simple picture, an order of magnitude improvement in magnetometer sensitivity.

[1] H. Kraus, V. Soltamov, D. Riedel, et al. Nature Phys 10, 157 (2014).

[2] H. Kraus, V. Soltamov, F. Fuchs, et al. Sci Rep 4, 5303 (2014).

[3] C. Cochrane, J. Blacksberg, M. Anders, et al. Sci Rep 6, 37077 (2016).

[4] A. Falk, P. Klimov, B. Buckley, et al. Phys. Rev. Lett. 112, 187601 (2014).   

Wavelet-enhanced Ramsey magnetometry of a nanodiamond single NV center

Nitrogen-vacancy (NV) centers in diamond constitute a solid-state nanosensing paradigm. Specifically for high precision magnetometry, the so-called Ramsey interferometry is the prevalent choice where the sensing signal is extracted from a time-resolved spin-state-dependent photoluminescence (PL) data. Its sensitivity is ultimately limited by the photon shot noise (PSN), which cannot be sufficiently suppressed by averaging or frequency filtering. Here, we propose Ramsey magnetometry of a single NV center enhanced by a wavelet-denoising scheme tailored against PSN. Without invoking any quantum resource, it operates as a post-processing applied over a collected PL time series. Our implementation is based on template margin thresholding which we computationally benchmark, and demonstrate its signal to noise ratio improvement over the standard quantum limit of up to an order of magnitude for the case of limited resources.   

Wide-field AC magnetic field imaging with nitrogen-vacancy centers in diamond

Imaging of AC magnetic fields is crucial for a wide range of diverse applications, from life sciences to microwave (MW) technologies to integrated circuit failure analysis and beyond. Magnetometry based on nitrogen-vacancy (NV) centers in diamond represents a powerful tool, as it allows micron-scale resolution, millimeter-scale field of view, high sensitivity, and non-invasive magnetic field imaging compatible with a large variety of samples. However, it has mostly been used for imaging static or low frequency magnetic fields. In this work, we used an ensemble of NV centers in diamond to achieve wide-field imaging of AC magnetic fields produced by a high frequency (GHz range) MW current running through a test device consisting of a fabricated 100-μm-wide copper wire as well as other microwave antennae. In our microscope, an omega loop is used to drive Rabi oscillations within the NV spin-states and the contribution of the MW field generated by the device-under-test was measured through local variations in the Rabi frequency. Our protocol allowed to obtain AC magnetic field maps with a measured magnetic field sensitivity of ~ 1.6 µT Hz^-1/2, over a field of view of 340x340 µm² with spatial resolution of ~ 1 µm, and a dynamic range of 25 dB.   

Precision measurement and spectroscopy with diamond NV centers

Precision measurements of the electron-spin precession of nitrogen-vacancy (NV) centers in diamond form the basis of numerous applications, ranging from imaging nanomagnetism to nuclear magnetic resonance (NMR) spectroscopy, gyroscopes, magnetometry, and searches for new spin physics. Solid-state spins offer advantages over alkali-vapor and SQUID sensors in that a high density of immobile spins form a tunable sensing voxel that can be tailored for a given application. However, the scaling up of NV sensors to large ensembles, as needed for the most sensitivity-demanding applications, has proved challenging. These experiments require the ability to measure changes in ~10 GHz electron-spin transitions at the sub-mHz level, i.e. a fractional resolution of better than 10^(-13). This is a regime that is only routinely realized in the field of atomic clocks. The ultimate limits in NV sensing precision are fundamental and cannot be avoided (e.g., spin projection noise), but some sources of noise are due to experimental imperfections and can be mitigated. I will discuss methods being developed to overcome challenges due to microwave phase noise and laser heating. I will present our latest progress towards implementing these methods in NV-detected NMR spectroscopy and femtotesla magnetometry [1].   

Noise analysis and sensitivity optimization in NV-diamond based magnetic field imaging systems

Magnetic field imaging technologies based on nitrogen-vacancy centers (NVCs) in diamond, the quantum diamond microscope (QDM), has emerged as a unique sensing modality in detecting spatiotemporally-varying magnetic fields for applications including the characterization of microelectronic circuits. For many impactful use cases, sub nT/√Hz sensitivities are required to resolve the dynamical behavior of small signals.  While these sensitivities have been achieved in bulk (3D) NVCs, the 2D NVC magnetometers employed for near-field, wide-field-of-view magnetic microscopy applications have only reached nT/√Hz sensitivities. 

 

Closing this sensitivity gap is a critical challenge in the field and requires a comprehensive understanding of how environmental noise diminishes sensitivity.  This can be accomplished with a quantitative mapping between noise sources across the system stack and the measured imaging sensitivities to understand how environmental noise diminishes sensitivity. We demonstrate a combined experimental and theoretical framework to map experimental noise sources from an imager to measured sensitivities. We present frequency-dependent noise contributions from the optical, microwave, and readout chains then demonstrate their effects on the coherence times and sensitivities for QDM systems operating with pulsed control and LED optical control. We then calibrate our framework against a theoretical noise model and suggest techniques for further improving sensitivities.                 

Integration of a near-field coupling device with scanning probes for Nitrogen-Vacancy magnetometry

We present the design and the implementation of a new type of scanning Nitrogen-Vacancy (NV) magnetic imaging probe with an integrated microwave (MW) near-field coupling device for optimized spin manipulation. The microwave coupling loop is directly integrated onto the attachment structure of the scanning probe eliminating the need for external MW delivery solutions. The diamond probe is in close proximity to the coupling loop and its rigid attachment results in a constant MW coupling to the NV center during scanning operation. The device is created through a subtractive manufacturing process followed by the evaporation of a conductive material on its top side to form the MW stripline and loop. This original approach is simple, highly reproducible, and more importantly enables large-scale production as it does not rely on lithography. The characterization and the proof-of-principle scanning NV magnetometry experiment demonstrate that this new devices match the performance of state-of-the-art MW delivery solutions, making it a compelling alternative. This holds particularly true for low-temperature experiments but is also anticipated to generally reduce the technical barriers for the broader adoption of NV magnetometry across a larger research community.   

Resonant versus non-resonant spin readout of a nitrogen-vacancy center in diamond under cryogenic conditions

The last decade has seen an explosive growth in the use of color centers for metrology applications, the paradigm example arguably being the nitrogen-vacancy (NV) center in diamond. Here, we focus on the regime of cryogenic temperatures, where the NV's zero-phonon line splits into a set of narrow, spin-selective optical transitions and investigate alternative (non-ionizing) spin read-out strategies. In the low power limit, narrow-band laser excitation resonant with a cycling transition has shown high fidelity, non-destructive spin read-out, at the cost of a strong overhead. Here, we use a similar pathway at higher power to demonstrate more than four-fold improvement in sensitivity compared to off-resonant green excitation, largely due to a boost in readout contrast and integrated photon count. We also leverage nuclear spin relaxation to polarize the Nitrogen-14 host, which we prove beneficial for spin magnetometry.   

Enhancing nitrogen-vacancy activation in diamond with phosphorus co-implantation

Diamond is a promising material for the creation of stable nitrogen-vacancy (NV) centers for quantum sensing. The sensitivity of an NV-ensemble sensor depends on the number of activated NVs in the substrate and their proximity to the sample under test.  However, their activation rate close to the diamond surface is drastically reduced from ~30% in the bulk to <1% at 10nm. The low activation rate near the surface hinders the use of NVs as a nano-scale material sensor. The addition of phosphorus (P) to nitrogen (N)-doped diamond has been shown to increase the creation yield percent of NV per implanted N, but specific combinations must be optimized.

We present a systematic co-implantation study of P and N near the surface of diamond to increase activation of NVs while maintaining ideal spin properties. We first implant the diamond with P in 100μm² patches (1–2700ppm), activating the P with a UHV high temperature anneal, followed by a blanket implant of N (20ppb or 10ppm). We compare photoluminescence (PL) intensity and spectra for each treatment. Then we characterize spin properties by measuring PL spin contrast, T2*, T2, and T1.  Initial results show that co-implantation of P at 100ppm can improve NV activation by nearly a factor of 2x with spin properties soon to be determined.   

Scanning Quantum Sensing of Quantum Materials

Emergent quantum systems with unconventional material properties have attracted immense interest in modern condensed matter physics research over the past decades. The discovery and development of new classes of quantum materials relies simultaneously on advances in theory, material synthesis, and development of sensitive metrology tools capable of evaluating the key material properties at the nanoscale. Nitrogen-vacancy (NV) centers, optically active atomic defects in diamond, are directly relevant in this context due to their unprecedented field and spatial sensitivity and remarkable functionality over a broad range of experimental conditions. In this talk, I will report our recent efforts on developing scanning NV magnetometry techniques and utilizing them to reveal some interesting spin behaviors in novel quantum material systems. Our results demonstrate the significant potential of scanning quantum sensing techniques on studying the exciting new physics in the cutting-edge quantum state of matter.