Acoustomicrofluidic Concentration and Signal Enhancement of Fluorescent Nanodiamond Sensors.

Diamond nitrogen-vacancy (NV) centers constitute a promising class of quantum nanosensors owing to the unique magneto-optic properties associated with their spin states. The large surface area and photostability of diamond nanoparticles, together with their relatively low synthesis costs, make them a suitable platform for the detection of biologically relevant quantities such as paramagnetic ions and molecules in solution. Nevertheless, their sensing performance in solution is often hampered by poor signal-to-noise ratios and long acquisition times due to distribution inhomogeneities throughout the analyte sample. By concentrating the diamond nanoparticles through an intense microcentrifugation effect in an acoustomicrofluidic device, we show that the resultant dense NV ensembles within the diamond nanoparticles give rise to an order-of-magnitude improvement in the measured acquisition time. The ability to concentrate nanoparticles under surface acoustic wave (SAW) microcentrifugation in a sessile droplet is, in itself, surprising given the well-documented challenge of achieving such an effect for particles below 1 µm in dimension. In addition to a demonstration of their sensing performance, we thus reveal in this work that the reason why the diamond nanoparticles readily concentrate under the SAW-driven recirculatory flow can be attributed to their considerably higher density and hence larger acoustic contrast compared to those for typical particles and cells for which the SAW microcentrifugation flow has been shown to date.

Entanglement and control of single nuclear spins in isotopically engineered silicon carbide.

Nuclear spins in the solid state are both a cause of decoherence and a valuable resource for spin qubits. In this work, we demonstrate control of isolated 29Si nuclear spins in silicon carbide (SiC) to create an entangled state between an optically active divacancy spin and a strongly coupled nuclear register. We then show how isotopic engineering of SiC unlocks control of single weakly coupled nuclear spins and present an ab initio method to predict the optimal isotopic fraction that maximizes the number of usable nuclear memories. We bolster these results by reporting high-fidelity electron spin control (F = 99.984(1)%), alongside extended coherence times (Hahn-echo T2 = 2.3 ms, dynamical decoupling T2DD > 14.5 ms), and a >40-fold increase in Ramsey spin dephasing time (T2*) from isotopic purification. Overall, this work underlines the importance of controlling the nuclear environment in solid-state systems and links single photon emitters with nuclear registers in an industrially scalable material.

Gradiometer Using Separated Diamond Quantum Magnetometers.

The negatively charged nitrogen-vacancy (NV) center in diamonds is known as the spin defect and using its electron spin, magnetometry can be realized even at room temperature with extremely high sensitivity as well as a high dynamic range. However, a magnetically shielded enclosure is usually required to sense weak magnetic fields because environmental magnetic field noises can disturb high sensitivity measurements. Here, we fabricated a gradiometer with variable sensor length that works at room temperature using a pair of diamond samples containing negatively charged NV centers. Each diamond is attached to an optical fiber to enable free sensor placement. Without any magnetically shielding, our gradiometer realizes a magnetic noise spectrum comparable to that of a three-layer magnetically shielded enclosure, reducing the noises at the low-frequency range below 1 Hz as well as at the frequency of 50 Hz (power line frequency) and its harmonics. These results indicate the potential of highly sensitive magnetic sensing by the gradiometer using the NV center for applications in noisy environments such as outdoor and in vehicles.

Purcell Enhancement of a Single Silicon Carbide Color Center with Coherent Spin Control.

Silicon carbide has recently been developed as a platform for optically addressable spin defects. In particular, the neutral divacancy in the 4H polytype displays an optically addressable spin-1 ground state and near-infrared optical emission. Here, we present the Purcell enhancement of a single neutral divacancy coupled to a photonic crystal cavity. We utilize a combination of nanolithographic techniques and a dopant-selective photoelectrochemical etch to produce suspended cavities with quality factors exceeding 5000. Subsequent coupling to a single divacancy leads to a Purcell factor of ∼50, which manifests as increased photoluminescence into the zero-phonon line and a shortened excited-state lifetime. Additionally, we measure coherent control of the divacancy ground-state spin inside the cavity nanostructure and demonstrate extended coherence through dynamical decoupling. This spin-cavity system represents an advance toward scalable long-distance entanglement protocols using silicon carbide that require the interference of indistinguishable photons from spatially separated single qubits.

Electrical Charge State Manipulation of Single Silicon Vacancies in a Silicon Carbide Quantum Optoelectronic Device.

Color centers with long-lived spins are established platforms for quantum sensing and quantum information applications. Color centers exist in different charge states, each of them with distinct optical and spin properties. Application to quantum technology requires the capability to access and stabilize charge states for each specific task. Here, we investigate charge state manipulation of individual silicon vacancies in silicon carbide, a system which has recently shown a unique combination of long spin coherence time and ultrastable spin-selective optical transitions. In particular, we demonstrate charge state switching through the bias applied to the color center in an integrated silicon carbide optoelectronic device. We show that the electronic environment defined by the doping profile and the distribution of other defects in the device plays a key role for charge state control. Our experimental results and numerical modeling evidence that control of these complex interactions can, under certain conditions, enhance the photon emission rate. These findings open the way for deterministic control over the charge state of spin-active color centers for quantum technology and provide novel techniques for monitoring doping profiles and voltage sensing in microscopic devices.

Scalable Quantum Photonics with Single Color Centers in Silicon Carbide.

Silicon carbide is a promising platform for single photon sources, quantum bits (qubits), and nanoscale sensors based on individual color centers. Toward this goal, we develop a scalable array of nanopillars incorporating single silicon vacancy centers in 4H-SiC, readily available for efficient interfacing with free-space objective and lensed-fibers. A commercially obtained substrate is irradiated with 2 MeV electron beams to create vacancies. Subsequent lithographic process forms 800 nm tall nanopillars with 400-1400 nm diameters. We obtain high collection efficiency of up to 22 kcounts/s optical saturation rates from a single silicon vacancy center while preserving the single photon emission and the optically induced electron-spin polarization properties. Our study demonstrates silicon carbide as a readily available platform for scalable quantum photonics architecture relying on single photon sources and qubits.

Nanomechanical Sensing Using Spins in Diamond.

Nanomechanical sensors and quantum nanosensors are two rapidly developing technologies that have diverse interdisciplinary applications in biological and chemical analysis and microscopy. For example, nanomechanical sensors based upon nanoelectromechanical systems (NEMS) have demonstrated chip-scale mass spectrometry capable of detecting single macromolecules, such as proteins. Quantum nanosensors based upon electron spins of negatively charged nitrogen-vacancy (NV) centers in diamond have demonstrated diverse modes of nanometrology, including single molecule magnetic resonance spectroscopy. Here, we report the first step toward combining these two complementary technologies in the form of diamond nanomechanical structures containing NV centers. We establish the principles for nanomechanical sensing using such nanospin-mechanical sensors (NSMS) and assess their potential for mass spectrometry and force microscopy. We predict that NSMS are able to provide unprecedented AC force images of cellular biomechanics and to not only detect the mass of a single macromolecule but also image its distribution. When combined with the other nanometrology modes of the NV center, NSMS potentially offer unparalleled analytical power at the nanoscale.

Isolated electron spins in silicon carbide with millisecond coherence times.

The elimination of defects from SiC has facilitated its move to the forefront of the optoelectronics and power-electronics industries. Nonetheless, because certain SiC defects have electronic states with sharp optical and spin transitions, they are increasingly recognized as a platform for quantum information and nanoscale sensing. Here, we show that individual electron spins in high-purity monocrystalline 4H-SiC can be isolated and coherently controlled. Bound to neutral divacancy defects, these states exhibit exceptionally long ensemble Hahn-echo spin coherence times, exceeding 1 ms. Coherent control of single spins in a material amenable to advanced growth and microfabrication techniques is an exciting route towards wafer-scale quantum technologies.

Coherent control of single spins in silicon carbide at room temperature.

Spins in solids are cornerstone elements of quantum spintronics. Leading contenders such as defects in diamond or individual phosphorus dopants in silicon have shown spectacular progress, but either lack established nanotechnology or an efficient spin/photon interface. Silicon carbide (SiC) combines the strength of both systems: it has a large bandgap with deep defects and benefits from mature fabrication techniques. Here, we report the characterization of photoluminescence and optical spin polarization from single silicon vacancies in SiC, and demonstrate that single spins can be addressed at room temperature. We show coherent control of a single defect spin and find long spin coherence times under ambient conditions. Our study provides evidence that SiC is a promising system for atomic-scale spintronics and quantum technology.

Detection of atomic spin labels in a lipid bilayer using a single-spin nanodiamond probe.

Magnetic field fluctuations arising from fundamental spins are ubiquitous in nanoscale biology, and are a rich source of information about the processes that generate them. However, the ability to detect the few spins involved without averaging over large ensembles has remained elusive. Here, we demonstrate the detection of gadolinium spin labels in an artificial cell membrane under ambient conditions using a single-spin nanodiamond sensor. Changes in the spin relaxation time of the sensor located in the lipid bilayer were optically detected and found to be sensitive to near-individual (4 ± 2) proximal gadolinium atomic labels. The detection of such small numbers of spins in a model biological setting, with projected detection times of 1 s [corresponding to a sensitivity of ∼5 Gd spins per Hz(1/2)], opens a pathway for in situ nanoscale detection of dynamical processes in biology.

Investigation of the silicon vacancy color center for quantum key distribution.

Single photon sources (SPS) are crucial for quantum key distribution. Here we demonstrate a stable triggered SPS at 738 nm with linewidth less than 5 nm at room temperature based on a negatively charged single silicon vacancy color center. Thanks to the short photon duration of about 1.3-1.7 ns, by using high repetition pulsed excitation at 30 MHz, the triggered single photon source generates 16.6 kcounts/s. And we discuss the feasibility of this triggered SPS in the application of quantum key distribution.

Columnar excitation fluorescence microscope for accurate evaluation of quantum properties of color centers in bulk materials.

Color centers in wide band-gap semiconductors, which have superior quantum properties even at room temperature and atmospheric pressure, have been actively applied to quantum sensing devices. Characterizing the quantum properties of the color centers in the semiconductor materials and ensuring that these properties are uniform over a wide area are key issues for developing quantum sensing devices based on color centers. In this article, we have developed an optics design protocol optimized for evaluating the quantum properties of color centers and have used this design approach to develop a new microscopy system called columnar excitation fluorescence microscope (CEFM). The essence of this system is to maximize the amount of fluorescence detection of polarized color centers, which is achieved by large-volume and uniform laser excitation along the sample thickness with sufficient laser power density. This laser excitation technique prevents undesirable transitions to undesirable charge states and undesirable light, such as unpolarized color center fluorescence, while significantly increasing the color center fluorescence. This feature enables fast measurements with a high signal-to-noise ratio, making it possible to evaluate the spatial distribution of quantum properties across an entire mm-size sample without using a darkroom, which is difficult with typical confocal microscope systems.

Multi-species optically addressable spin defects in a van der Waals material.

Optically addressable spin defects hosted in two-dimensional van der Waals materials represent a new frontier for quantum technologies, promising to lead to a new class of ultrathin quantum sensors and simulators. Recently, hexagonal boron nitride (hBN) has been shown to host several types of optically addressable spin defects, thus offering a unique opportunity to simultaneously address and utilise various spin species in a single material. Here we demonstrate an interplay between two separate spin species within a single hBN crystal, namely S = 1 boron vacancy defects and carbon-related electron spins. We reveal the S = 1/2 character of the carbon-related defect and further demonstrate room temperature coherent control and optical readout of both S = 1 and S = 1/2 spin species. By tuning the two spin ensembles into resonance with each other, we observe cross-relaxation indicating strong inter-species dipolar coupling. We then demonstrate magnetic imaging using the S = 1/2 defects and leverage their lack of intrinsic quantization axis to probe the magnetic anisotropy of a test sample. Our results establish hBN as a versatile platform for quantum technologies in a van der Waals host at room temperature.

Highly Dispersed 3C Silicon Carbide Nanoparticles with a Polydopamine/Polyglycerol Shell for Versatile Functionalization.

Silicon carbide (SiC) nanoparticles containing lattice defects are attracting considerable attention as next-generation imaging probes and quantum sensors for visualizing and sensing life activities. However, SiC nanoparticles are not currently used in biomedical applications because of the lack of technology for controlling their physicochemical properties. Therefore, in this study, SiC nanoparticles are deaggregated, surface-coated, functionalized, and selectively labeled to biomolecules of interest. A thermal-oxidation chemical-etching method is developed for deaggregating and producing a high yield of dispersed metal-contaminant-free SiC nanoparticles. We further demonstrated a polydopamine coating with controllable thickness that can be used as a platform for decorating gold nanoparticles on the surface, enabling photothermal application. We also demonstrated a polyglycerol coating, which gives excellent dispersity to SiC nanoparticles. Furthermore, a single-pot method is developed to produce mono/multifunctional polyglycerol-modified SiC nanoparticles. Using this method, CD44 proteins on cell surfaces are selectively labeled through biotin-mediated immunostaining. The methods developed in this study are fundamental for applying SiC nanoparticles to biomedical applications and should considerably accelerate the development of various SiC nanoparticles to exploit their potential applications in bioimaging and biosensing.

Boron-Related Defects in N-Type 4H-SiC Schottky Barrier Diodes.

We report on boron-related defects in the low-doped n-type (nitrogen-doped) 4H-SiC semitransparent Schottky barrier diodes (SBDs) studied by minority carrier transient spectroscopy (MCTS). An unknown concentration of boron was introduced during chemical vapor deposition (CVD) crystal growth. Boron incorporation was found to lead to the appearance of at least two boron-related deep-level defects, namely, shallow (B) and deep boron (D-center), with concentrations as high as 1 × 1015 cm-3. Even though the boron concentration exceeded the nitrogen doping concentration by almost an order of magnitude, the steady-state electrical characteristics of the n-type 4H-SiC SBDs did not deteriorate.

Radiation Response of Large-Area 4H-SiC Schottky Barrier Diodes.

We report on the effects of large-area 4H-SiC Schottky barrier diodes on the radiation response to ionizing particles. Two different diode areas were compared: 1 mm × 1 mm and 5 mm × 5 mm. 6LiF and 10B4C films, which were placed on top of the diodes, were used as thermal neutron converters. We achieved a thermal neutron efficiency of 5.02% with a 6LiF thermal neutron converter, which is one of the highest efficiencies reported to date. In addition, a temperature-dependent radiation response to alpha particles was presented. Neutron irradiations were performed in a JSI TRIGA dry chamber and an Am-241 wide-area alpha source was used for testing the alpha response of the 4H-SiC Schottky barrier diodes.

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.

High-precision robust monitoring of charge/discharge current over a wide dynamic range for electric vehicle batteries using diamond quantum sensors.

Accurate prediction of the remaining driving range of electric vehicles is difficult because the state-of-the-art sensors for measuring battery current are not accurate enough to estimate the state of charge. This is because the battery current of EVs can reach a maximum of several hundred amperes while the average current is only approximately 10 A, and ordinary sensors do not have an accuracy of several tens of milliamperes while maintaining a dynamic range of several hundred amperes. Therefore, the state of charge has to be estimated with an ambiguity of approximately 10%, which makes the battery usage inefficient. This study resolves this limitation by developing a diamond quantum sensor with an inherently wide dynamic range and high sensitivity for measuring the battery current. The design uses the differential detection of two sensors to eliminate in-vehicle common-mode environmental noise, and a mixed analog-digital control to trace the magnetic resonance microwave frequencies of the quantum sensor without deviation over a wide dynamic range. The prototype battery monitor was fabricated and tested. The battery module current was measured up to 130 A covering WLTC driving pattern, and the accuracy of the current sensor to estimate battery state of charge was analyzed to be 10 mA, which will lead to 0.2% CO2 reduction emitted in the 2030 WW transportation field. Moreover, an operating temperature range of - 40 to + 85 °C and a maximum current dynamic range of ± 1000 A were confirmed.

Photon extraction enhancement of praseodymium ions in gallium nitride nanopillars.

Lanthanoid-doped Gallium Nitride (GaN) integrated into nanophotonic technologies is a promising candidate for room-temperature quantum photon sources for quantum technology applications. We manufactured praseodymium (Pr)-doped GaN nanopillars of varying size, and showed significantly enhanced room-temperature photon extraction efficiency compared to unstructured Pr-doped GaN. Implanted Pr ions in GaN show two main emission peaks at 650.3 nm and 651.8 nm which are attributed to 3P0-3F2 transition in the 4f-shell. The maximum observed enhancement ratio was 23.5 for 200 nm diameter circular pillars, which can be divided into the emitted photon extraction enhancement by a factor of 4.5 and the photon collection enhancement by a factor of 5.2. The enhancement mechanism is explained by the eigenmode resonance inside the nanopillar. Our study provides a pathway for Lanthanoid-doped GaN nano/micro-scale photon emitters and quantum technology applications.

Five-second coherence of a single spin with single-shot readout in silicon carbide.

An outstanding hurdle for defect spin qubits in silicon carbide (SiC) is single-shot readout, a deterministic measurement of the quantum state. Here, we demonstrate single-shot readout of single defects in SiC via spin-to-charge conversion, whereby the defect's spin state is mapped onto a long-lived charge state. With this technique, we achieve over 80% readout fidelity without pre- or postselection, resulting in a high signal-to-noise ratio that enables us to measure long spin coherence times. Combined with pulsed dynamical decoupling sequences in an isotopically purified host material, we report single-spin T2 > 5 seconds, over two orders of magnitude greater than previously reported in this system. The mapping of these coherent spin states onto single charges unlocks both single-shot readout for scalable quantum nodes and opportunities for electrical readout via integration with semiconductor devices.