Spin Defects Research Articles

Manipulating solid-state spin concentration through charge transport

Solid-state defects are attractive platforms for quantum sensing and simulation, e.g., in exploring many-body physics and quantum hydrodynamics. However, many interesting properties can be revealed only upon changes in the density of defects, which instead is usually fixed in material systems. Increasing the interaction strength by creating denser defect ensembles also brings more decoherence. Ideally one would like to control the spin concentration at will while keeping fixed decoherence effects. Here, we show that by exploiting charge transport, we can take some steps in this direction, while at the same time characterizing charge transport and its capture by defects. By exploiting the cycling process of ionization and recombination of NV centers in diamond, we pump electrons from the valence band to the conduction band. These charges are then transported to modulate the spin concentration by changing the charge state of material defects. By developing a wide-field imaging setup integrated with a fast single photon detector array, we achieve a direct and efficient characterization of the charge redistribution process by measuring the complete spectrum of the spin bath with micrometer-scale spatial resolution. We demonstrate a two-fold concentration increase of the dominant spin defects while keeping the T2 of the NV center relatively unchanged, which also provides a potential experimental demonstration of the suppression of spin flip-flops via hyperfine interactions. Our work paves the way to studying many-body dynamics with temporally and spatially tunable interaction strengths in hybrid charge-spin systems.

Characterizing Temperature and Strain Variations with Qubit Ensembles for Their Robust Coherence Protection.

Solid-state spin defects, especially nuclear spins with potentially achievable long coherence times, are compelling candidates for quantum memories and sensors. However, their current performances are still limited by dephasing due to variations of their intrinsic quadrupole and hyperfine interactions. We propose an unbalanced echo to overcome this challenge by using a second spin to refocus variations of these interactions while preserving the quantum information stored in the nuclear spin free evolution. The unbalanced echo can be used to probe the temperature and strain distribution in materials. We develop first-principles methods to predict variations of these interactions and reveal their correlation over large temperature and strain ranges. Experiments performed in an ensemble of ∼10^{10} nuclear spins in diamond demonstrate a 20-fold dephasing time increase, limited by other noise sources. We further numerically show that our method can refocus even stronger noise variations than present in our experiments.

Coupling spin defects in hexagonal boron nitride to a microwave cavity

Optically addressable spin defects in hexagonal boron nitride (hBN) have become a promising platform for quantum sensing. While sensitivity of these defects is limited by their interactions with the spin environment in hBN, inefficient microwave delivery can further reduce their sensitivity. Here, we design and fabricate a microwave double arc resonator for efficient transferring of the microwave field at 3.8 GHz. The spin transitions in the ground state of VB− are coupled to the frequency of the microwave cavity, which result in enhanced optically detected magnetic resonance (ODMR) contrast. In addition, the linewidth of the ODMR signal further reduces, achieving a magnetic field sensitivity as low as 42.4 μT/√Hz. Our robust and scalable device engineering is promising for future employment of spin defects in hBN for quantum sensing.

Radiation Pressure Backaction on a Hexagonal Boron Nitride Nanomechanical Resonator.

Hexagonal boron nitride (hBN) is a van der Waals material with excellent mechanical properties hosting quantum emitters and optically active spin defects, with several of them being sensitive to strain. Establishing optomechanical control of hBN will enable hybrid quantum devices that combine the spin degree of freedom with the cavity optomechanical toolbox. In this Letter, we report the first observation of radiation pressure backaction at telecom wavelengths with a hBN drum-head mechanical resonator. The thermomechanical motion of the resonator is coupled to the optical mode of a high finesse fiber-based Fabry-Pérot microcavity in a membrane-in-the-middle configuration. We are able to resolve the optical spring effect and optomechanical damping with a single photon coupling strength of g0/2π = 1200 Hz. Our results pave the way for tailoring the mechanical properties of hBN resonators with light.

Magnetic Field Sensitivity Optimization of Negatively Charged Boron Vacancy Defects in hBN.

AbstractOptically active spin defects in hexagonal boron nitride (hBN) have recently emerged as compelling quantum sensors hosted by a two dimensional (2D) material. The photodynamics and sensitivity of spin defects are governed by their level structure and associated transition rates. These are, however, poorly understood for spin defects in hBN. Here, optical and microwave pump‐probe measurements are used to characterize the relaxation dynamics of the negatively charged boron vacancy (VB−)—the most widely‐studied spin defect in hBN. A 5‐level model is used to deduce transition rates that give rise to spin‐dependent VB− photoluminescence, and the lifetime of the VB− intersystem crossing metastable state. The obtained rates are used to simulate the magnetic field sensitivity of VB− defects and demonstrate high resolution imaging of the magnetic field generated by a single magnetic particle using optimal sensing parameters predicted by the model. The results reveal the rates that underpin VB− photodynamics, which is important for both a fundamental understanding of the VB− as a spin‐photon interface and for achieving optimal sensitivity in quantum sensing applications.

Detection of Paramagnetic Spins with an Ultrathin van der Waals Quantum Sensor.

Detecting magnetic noise from small quantities of paramagnetic spins is a powerful capability for chemical, biochemical, and medical analysis. Quantum sensors based on optically addressable spin defects in bulk semiconductors are typically employed for such purposes, but the 3D crystal structure of the sensor inhibits sensitivity by limiting the proximity of the defects to the target spins. Here we demonstrate the detection of paramagnetic spins using spin defects hosted in hexagonal boron nitride (hBN), a van der Waals material that can be exfoliated into the 2D regime. We first create negatively charged boron vacancy (VB-) defects in a powder of ultrathin hBN nanoflakes (<10 atomic monolayers thick on average) and measure the longitudinal spin relaxation time (T1) of this system. We then decorate the dry hBN nanopowder with paramagnetic Gd3+ ions and observe a clear T1 quenching under ambient conditions, consistent with the added magnetic noise. Finally, we demonstrate the possibility of performing spin measurements, including T1 relaxometry using solution-suspended hBN nanopowder. Our results highlight the potential and versatility of the hBN quantum sensor for a range of sensing applications and make steps toward the realization of a truly 2D, ultrasensitive quantum sensor.

Fast Wide‐Field Quantum Sensor Based on Solid‐State Spins Integrated with a SPAD Array

AbstractAchieving fast, sensitive, and parallel measurement of a large number of quantum particles is an essential task in building large‐scale quantum platforms for different quantum information processing applications such as sensing, computation, simulation, and communication. Current quantum platforms in experimental atomic and optical physics based on CMOS sensors and charged coupled device cameras are limited by either low sensitivity or slow operational speed. Here an array of single‐photon avalanche diodes is integrated with solid‐state spin defects in diamond to build a fast wide‐field quantum sensor, achieving a frame rate up to 100 kHz. The design of the experimental setup to perform spatially resolved imaging of quantum systems is presented. A few exemplary applications, including sensing DC and AC magnetic fields, temperature, strain, local spin density, and charge dynamics, are experimentally demonstrated using a nitrogen‐vacancy ensemble diamond sample. The developed photon detection array is broadly applicable to other platforms such as atom arrays trapped in optical tweezers, optical lattices, donors in silicon, and rare earth ions in solids.

Deterministic nanoscale quantum spin-defect implantation and diffraction strain imaging

Local crystallographic features negatively affect quantum spin defects by changing the local electrostatic environment, often resulting in degraded or varied qubit optical and coherence properties. Few tools exist that enable the deterministic synthesis and study of such intricate systems on the nano-scale, making defect-to-defect strain environment quantification difficult. In this paper, we highlight state-of-the-art capabilities from the U.S. Department of Energy’s Nanoscale Science Research Centers that directly address these shortcomings. Specifically, we demonstrate how complementary capabilities of nano-implantation and nano-diffraction can be used to demonstrate the quantum relevant, spatially deterministic creation of neutral divacancy centers in 4H silicon carbide, while investigating and characterizing these systems on the scale with strain sensitivities on the order of relevant to defect formation dynamics. This work lays the foundation for ongoing studies into the dynamics and deterministic formation of low strain homogeneous quantum relevant spin defects in the solid state.

DC Magnetic Field Sensitivity Optimization of Spin Defects in Hexagonal Boron Nitride.

Spin defects existing in van der Waals materials attract wide attention thanks to their natural advantages for in situ quantum sensing, especially the negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (h-BN). Here we systematically investigate the laser and microwave power broadening in continuous-wave optically detected magnetic resonance (ODMR) of the VB- ensemble in h-BN, by revealing the behaviors of ODMR contrast and line width as a function of the laser and microwave powers. The experimental results are well explained by employing a two-level simplified model of ODMR dynamics. Furthermore, with optimized power, the DC magnetic field sensitivity of VB- ensemble is significantly improved up to 2.87 ± 0.07 μT/. Our results provide important suggestions for further applications of VB- centers in quantum information processing and ODMR-based quantum sensing.

Manipulating the Charge State of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancies (VB-) in hexagonal boron nitride (hBN) have recently gained interest as spin defects for quantum information processing and quantum sensing by a layered material. However, the boron vacancy can exist in a number of charge states in the hBN lattice, but only the -1 state has spin-dependent photoluminescence and acts as a spin-photon interface. Here, we investigate the charge state switching of VB defects under laser and electron beam excitation. We demonstrate deterministic, reversible switching between the -1 and 0 states (VB- ⇌ VB0 + e-), occurring at rates controlled by excess electrons or holes injected into hBN by a layered heterostructure device. Our work provides a means to monitor and manipulate the VB charge state, and to stabilize the -1 state which is a prerequisite for spin manipulation and optical readout of the defect.

Coherent dynamics of strongly interacting electronic spin defects in hexagonal boron nitride

Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ({{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples. Crucially, we identify that the many-body interaction within the {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} ensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}. We find that at high ion implantation dosage, only a small portion of the created boron vacancy defects are in the desired negatively charged state. Finally, we investigate the spin response of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} to the local charged defects induced electric field signals, and estimate its ground state transverse electric field susceptibility. Our results provide new insights on the spin and charge properties of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}, which are important for future use of defects in hBN as quantum sensors and simulators.

Enhanced magnetometry with an electrically detected spin defect ensemble in silicon carbide

Spin defects in solid-state sensors are a highly promising platform for quantum sensing, a field with far-reaching applications in a variety of industries. Here, we investigate the magnetic sensitivity of a spin defect ensemble detected electrically in a silicon carbide pn-junction diode utilizing the hyperfine-induced spin-mixing effect observed in the vicinity of zero magnetic field. To enhance the baseline sensitivity, we employ above bandgap optical excitation to generate additional electron-hole pairs as well as a balanced detection scheme to reject common-mode noise, with an ultimate sensitivity of 30 nT/Hz achieved. Both techniques are demonstrated to greatly enhance the magnetic sensitivity of the device by a total factor of ∼24, paving the way toward sub-nanotesla magnetic field sensitivities with electrical detection.

Physico-chemical properties of γ-irradiated International Simple Glass (ISG) nuclear waste simulants with low levels of iron impurities

The effect of γ-irradiation on physico-chemical properties of International Simple Glass (ISG) nuclear waste glass simulants with low iron impurities was investigated. The study was performed on glasses produced by Corning Inc. (named ISG-1, ISG-2) following a 0.95 MGy dose. Fourier-transform infrared and Raman spectroscopy, chemical durability tests, dilatometry, differential scanning calorimetry, electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy appraisals were done in comparison with pristine glasses. The data indicated that structural, chemical, and thermal properties were not significantly altered following γ-ray exposure. EPR spectroscopy showed radiogenic signatures from spin defects for the γ-irradiated glasses which were absent in the pristine. Optical absorption also changed significantly for the γ-irradiated glasses which exhibited defects-related features around 3.45 and 2.25 eV. The optical band gap and Urbach energies were then determined and compared with those obtained for the γ-irradiated original ISG produced by Mo-SCI Corporation. PL spectroscopy revealed distinct changes after irradiation showing enhanced high-energy emission, contrasting with the γ-irradiated original ISG. The effects of thermal treatment on optical properties of γ-irradiated glasses were finally evaluated. Potential causes at the origin of the contrasting results are discussed based on the effects of different impurities.

The Role of Electrolytes in the Relaxation of Near-Surface Spin Defects in Diamond.

Quantum sensing with spin defects in diamond, such as the nitrogen vacancy (NV) center, enables the detection of various chemical species on the nanoscale. Molecules or ions with unpaired electronic spins are typically probed by their influence on the NV center's spin relaxation. Whereas it is well-known that paramagnetic ions reduce the NV center's relaxation time (T1), here we report on the opposite effect for diamagnetic ions. We demonstrate that millimolar concentrations of aqueous diamagnetic electrolyte solutions increase the T1 time of near-surface NV center ensembles compared to pure water. To elucidate the underlying mechanism of this surprising effect, single and double quantum NV experiments are performed, which indicate a reduction of magnetic and electric noise in the presence of diamagnetic electrolytes. In combination with ab initio simulations, we propose that a change in the interfacial band bending due to the formation of an electric double layer leads to a stabilization of fluctuating charges at the interface of an oxidized diamond. This work not only helps to understand noise sources in quantum systems but could also broaden the application space of quantum sensors toward electrolyte sensing in cell biology, neuroscience, and electrochemistry.

Coherent control of an ultrabright single spin in hexagonal boron nitride at room temperature

Hexagonal boron nitride (hBN) is a remarkable two-dimensional (2D) material that hosts solid-state spins and has great potential to be used in quantum information applications, including quantum networks. However, in this application, both the optical and spin properties are crucial for single spins but have not yet been discovered simultaneously for hBN spins. Here, we realize an efficient method for arraying and isolating the single defects of hBN and use this method to discover a new spin defect with a high probability of 85%. This single defect exhibits outstanding optical properties and an optically controllable spin, as indicated by the observed significant Rabi oscillation and Hahn echo experiments at room temperature. First principles calculations indicate that complexes of carbon and oxygen dopants may be the origin of the single spin defects. This provides a possibility for further addressing spins that can be optically controlled.

Spin Defects in hBN assisted by Metallic Nanotrenches for Quantum Sensing.

The omnipresence of hexagonal boron nitride (hBN) in devices embedding two-dimensional materials has prompted it as the most sought after platform to implement quantum sensing due to its testing while operating capability. The negatively charged boron vacancy (VB-) in hBN plays a prominent role, as it can be easily generated while its spin population can be initialized and read out by optical means at room-temperature. But the lower quantum yield hinders its widespread use as an integrated quantum sensor. Here, we demonstrate an emission enhancement amounting to 400 by nanotrench arrays compatible with coplanar waveguide (CPW) electrodes employed for spin-state detection. By monitoring the reflectance spectrum of the resonators as additional layers of hBN are transferred, we have optimized the overall hBN/nanotrench optical response, maximizing thereby the luminescence enhancement. Based on these finely tuned heterostructures, we achieved an enhanced DC magnetic field sensitivity as high as 6 × 10-5 T/Hz1/2.

Pure spin squeezing of h -BN spins coupled to superconducting resonator

The negatively charged boron vacancy (${\text{VB}}^{\ensuremath{-}}$) spin defect in two-dimensional (2D) hexagonal boron nitride ($h$-BN) has attracted much attention for potential applications in quantum photonics recently. Its inherent van der Waals force mechanism guarantees convenient heterostructures for quantum sensing. By virtue of such materials, researchers not only can fabricate enough thin spin film naturally close to the sensing target but also can prepare an almost perfect spin ensemble with a uniform orientation. We here propose a setup with an ensemble of ${\text{VB}}^{\ensuremath{-}}$ spins strongly coupled to the superconducting coplanar waveguide resonator through the magnetic-dipolar interaction. The collective coupling strength is predicted to be $G/2\ensuremath{\pi}\ensuremath{\sim}15$ MHz, which corresponds to the strong coupling region. This collective spin-photon interaction can mimic the one-axis twisting Lipkin-Meshkov-Glick model effectively and therefore guarantee the dynamic generation of the spin-squeezed state. In addition, we show the influence of inhomogeneous coupling caused by sample thickness on squeezing, which proves the validity of the homogeneity assumption in our scheme. This attempt not only explores the possibility and superiority of 2D ${\text{VB}}^{\ensuremath{-}}$ spins but also opens another avenue for quantum hybridization.

Plasmonic-Enhanced Bright Single Spin Defects in Silicon Carbide Membranes.

Optically addressable spin defects in silicon carbide (SiC) have emerged as attractable platforms for various quantum technologies. However, the low photon count rate significantly limits their applications. We strongly enhanced the brightness by 7 times and spin-control strength by 14 times of single divacancy defects in 4H-SiC membranes using a surface plasmon generated by gold film coplanar waveguides. The mechanism of the plasmonic-enhanced effect is further studied by tuning the distance between single defects and the surface of the gold film. A three-energy-level model is used to determine the corresponding transition rates consistent with the enhanced brightness of single defects. Lifetime measurements also verified the coupling between defects and surface plasmons. Our scheme is low-cost, without complicated microfabrication and delicate structures, which is applicable for other spin defects in different materials. This work would promote developing spin-defect-based quantum applications in mature SiC materials.

Laser writing of spin defects in nanophotonic cavities.

High-yield engineering and characterization of cavity-emitter coupling is an outstanding challenge in developing scalable quantum network nodes. Ex situ defect formation systems prevent real-time analysis, and previous in situ methods are limited to bulk substrates or require further processing to improve the emitter properties1-6. Here we demonstrate the direct laser writing of cavity-integrated spin defects using a nanosecond pulsed above-bandgap laser. Photonic crystal cavities in 4H-silicon carbide serve as a nanoscope monitoring silicon-monovacancy defect formation within the approximately 200 nm3 cavity-mode volume. We observe spin resonance, cavity-integrated photoluminescence and excited-state lifetimes consistent with conventional defect formation methods, without the need for post-irradiation thermal annealing. We further find an exponential reduction in excited-state lifetime at fluences approaching the cavity amorphization threshold and show the single-shot annealing of intrinsic background defects at silicon-monovacancy formation sites. This real-time in situ method of localized defect formation, paired with cavity-integrated defect spins, is necessary towards engineering cavity-emitter coupling for quantum networking.

Magnetometry Based on Silicon-Vacancy Centers in Isotopically Purified 4H - SiC

Point defects in solid-state materials are promising systems in the fields of quantum information, communications, and sensing. Particular to sensing, defects with coherent spin states are used for high-sensitivity room-temperature sensing under ambient conditions. Applications involving magnetic sensing with point defects are mostly dominated by the nitrogen-vacancy center in diamond, which possesses an attractive combination of spin coherence at room temperature as well as spin initialization and read-out. However, spin defects in other materials are explored as alternatives, especially in industrially mature materials, such as silicon carbide. Here, we report on the improved sensitivity of a magnetic sensor system utilizing an ensemble of silicon vacancies in silicon carbide due to isotopic purification of the host crystal. A maximum sensitivity of 4.0 nT/\ensuremath{\surd}Hz is reported, representing an order of magnitude improvement from the best previously reported sensitivity. Additional sensing modalities, such as angle-resolved magnetic imaging and highly broadband ac field sensing, are also demonstrated.