arXiv subjects

Shinobu Onoda

Publications and source records attributed to Shinobu Onoda.

At least 37 records · Page 2Linked to original sources

arXiv2024

Charge state transition of spectrally stabilized tin-vacancy centers in diamond

Solid-state quantum emitters are an important platform for quantum information processing. The fabrication of the emitters with stable photon frequency and narrow linewidth is a fundamental issue, and it is essential to understand optical conditions under which the emitter keeps a bright charge state or transitions to a dark state. For these purposes, in this study, we investigate the spectral stability and charge state transition of tin-vacancy (SnV) centers in diamond. The photoluminescence excitation spectra of multiple SnV centers are basically stable over time with nearly transform-limited linewidths under resonant excitation, while simultaneous irradiation of resonant and non-resonant lasers makes spectra from the SnV centers unstable. We find that the instability occurs due to the charge state transition to a dark state. The charge state transition rates are quantitatively investigated depending on the laser powers. Lastly, with first-principle calculations, we model the charge state transition of the SnV center under the laser irradiation.

quant-ph↗

arXiv2024

Quantum Orbital-State Control of a Neutral Nitrogen-Vacancy Center at Millikelvin Temperatures

A neutral nitrogen-vacancy center (NV$^0$) is promising for realizing strong coupling with a single microwave photon due to its large electric field sensitivity, although it is susceptible to environmental phonon noise at 5 K. Decreasing the temperature to 15 mK results in a tenfold increase in orbital relaxation time compared to that at 5 K. Dynamical decoupling pulses significantly increase the orbital coherence time to around 1.8 $μ$s, representing a 30-fold improvement compared to that without decoupling pulses. Based on these results, a single NV$^0$ can reach the strong coupling regime when coupled with a high-impedance microwave resonator, thus opening up the possibility of microwave quantum electrodynamics using a single optically-active defect center in diamond.

quant-ph↗

arXiv2024

Surface transfer doping of hydrogen-terminated diamond probed by shallow nitrogen-vacancy centers

The surface conductivity of hydrogen-terminated diamond is a topic of great interest from both scientific and technological perspectives. This is primarily due to the fact that the conductivity is exceptionally high without the need for substitutional doping, thus enabling a wide range of electronic applications. Although the conductivity is commonly explained by surface transfer doping due to air-borne surface acceptors, there remains uncertainty regarding the main determining factors that govern the degree of band bending and hole density, which are crucial for the design of electronic devices. Here, we elucidate the dominant factor influencing band bending by creating shallow nitrogen-vacancy (NV) centers beneath the hydrogen-terminated diamond surface through nitrogen ion implantation at varying fluences. We measured the photoluminescence and optically detected magnetic resonance (ODMR) of the NV centers, as well as the surface conductivity, as a function of the nitrogen implantation fluence. The disappearance of the conductivity with increasing nitrogen implantation fluence coincides with the appearance of photoluminescence and ODMR signals from negatively charged NV centers. This finding indicates that band bending is not exclusively determined by the work-function difference between diamond and the surface acceptor material, but by the finite density of surface acceptors. This work emphasizes the importance of distinguishing work-function-difference-limited band bending and surface-acceptor-density-limited band bending when modeling the surface transfer doping, and provides useful insights for the development of devices based on hydrogen-terminated diamond.

cond-mat.mtrl-sci↗

arXiv2023

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

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.

quant-ph↗

arXiv2023

Transform-Limited Photon Emission From a Lead-Vacancy Center in Diamond Above 10 K

Transform-limited photon emission from quantum emitters is essential for high-fidelity entanglement generation. In this study, we report the coherent optical property of a single negatively-charged lead-vacancy (PbV) center in diamond. Photoluminescence excitation measurements reveal stable fluorescence with a linewidth of 39 MHz at 6 K, close to the transform-limit estimated from the lifetime measurement. We observe four orders of magnitude different linewidths of the two zero-phonon-lines, and find that that the phonon-induced relaxation in the ground state contributes to this huge difference in the linewidth. Due to the suppressed phonon absorption in the PbV center, we observe nearly transform-limited photon emission up to 16 K, demonstrating its high temperature robustness compared to other color centers in diamond.

quant-ph↗

arXiv2023

All-optical determination of one or two emitters using quantum polarization with nitrogen-vacancy centers in diamond

Qubit technologies using nitrogen-vacancy color centers in diamonds require precise knowledge of the centers, including the number of emitters within a diffraction-limited spot and their orientations. However, the number of emitters is challenging to determine when there is finite background, which affects the precision of resulting quantum protocols. Here we show the photoluminescence (PL) intensity and quantum correlation (Hanbury Brown and Twiss) measurements as a function of polarization for one- and two-emitter systems. The sample was made by implanting low concentrations of adenine (C5H5N5) into a low nitrogen chemical vapor deposition diamond. This approach yielded well-spaced regions with few nitrogen-vacancy centers. By mapping the PL intensity and quantum correlation as a function of polarization, we can distinguish two emitter systems from single emitters with background, providing a method to quantify the background signal at implanted sites, which might be different from off-site background levels. This approach also provides a valuable new all-optical mechanism for the determination of one or two emitter systems useful for quantum sensing, communication, and computation tasks.

quant-ph↗

arXiv2023

The Transition from Quantum to Classical in weak measurements and reconstruction of Quantum Correlation

The ability to follow the dynamics of a quantum system in a quantitative manner is of key importance for quantum technology. Despite its central role, justifiable deduction of the quantum dynamics of a single quantum system in terms of a macroscopical observable remains a challenge. Here we show that the relation between the readout signal of a single electron spin and the quantum dynamics of the single nuclear spin is given by a parameter related to the measurement strength. We determine this measurement strength in independent experiments and use this value to compare our analysis of the quantum dynamics with experimental results. We prove the validity of our approach by measuring violations of the Leggett-Garg inequality.

quant-ph↗

arXiv2022

Identical Photons from Multiple Tin-Vacancy Centers in Diamond

We report the narrow inhomogeneous distribution of the zero-phonon line from tin-vacancy (SnV) centers in diamond and the overlap of spectra from multiple separated SnV centers. Photoluminescence excitation spectroscopy measurements at a cryogenic temperature showed that SnV centers exhibit stable fluorescence and linewidths close to the Fourier transform-limited linewidth. The inhomogeneous distribution was as low as ~4 GHz, which enabled the observation of Sn isotope-dependent resonant frequencies. Owing to the narrow inhomogeneous distribution, we observed multiple SnV centers showing identical photons with almost the same wavelength and linewidth. Identical SnV centers were also observed even in different diamond samples, confirming the reliable fabrication of the high-quality SnV centers.

quant-ph↗

arXiv2022

Quantum Heterodyne Sensing of Nuclear Spins via Double Resonance

Nanoscale nuclear magnetic resonance (NMR) signals can be measured through hyperfine interaction to paramagnetic electron sensor spins. A heterodyne approach is widely used to overcome the electron spin lifetime limit in spectral resolution. It uses a series of modified Hahn echo pulse sequences applied coherently with precession signal resulting in a subsampled NMR signal. Due to challenges with applying high electron Rabi frequencies its application is limited to low fields, thus the full potential of the method is not yet exploited at high magnetic fields, beneficial for NMR. Here we present heterodyne detection utilizing a series of phase coherent electron nuclear double resonance sensing blocks which extends nanoscale NMR protocols to arbitrary magnetic fields. We demonstrate this principle on a single NV center, both with an intrinsic $^{14}$N and a weekly coupled $^{13}$C nuclear spin in the bath surrounding single NV centres. We compare our protocol to existing heterodyne protocols and discuss its prospects. This work paves the way towards high field nanoscale heterodyne NMR protocols with NV centres which is crucial for reducing sample volumes and improving chemical resolution.

quant-ph↗

arXiv2021

Proximal nitrogen reduces the fluorescence quantum yield of nitrogen-vacancy centres in diamond

The nitrogen-vacancy colour centre in diamond is emerging as one of the most important solid-state quantum systems. It has applications to fields including high-precision sensing, quantum computing, single photon communication, metrology, nanoscale magnetic imaging and biosensing. For all of these applications, a high quantum yield of emitted photons is desirable. However, diamond samples engineered to have high densities of nitrogen-vacancy centres show levels of brightness varying significantly within single batches, or even within the same sample. Here we show that nearby nitrogen impurities quench emission of nitrogen-vacancy centres via non-radiative transitions, resulting in a reduced fluorescence quantum yield. We monitored the emission properties of nitrogen-vacancy centre ensembles from synthetic diamond samples with different concentrations of nitrogen impurities. While at low nitrogen densities of 1.81 ppm we measured a lifetime of 13.9 ns, we observed a strong reduction in lifetime with increasing nitrogen density. We measure a lifetime as low as 4.4 ns at a nitrogen density of 380 ppm. The change in lifetime matches a reduction in relative fluorescence quantum yield from 77.4 % to 32 % with an increase in nitrogen density from 88 ppm to 380 ppm, respectively. These results will inform the conditions required to optimise the properties of diamond crystals devices based on the fluorescence of nitrogen-vacancy centres. Furthermore, this work provides insights into the origin of inhomogeneities observed in high-density nitrogen-vacancy ensembles within diamonds and nanodiamonds.

quant-ph↗

arXiv2021

The Anomalous Formation of Irradiation Induced Nitrogen-Vacancy Centers in 5-Nanometer-Sized Detonation Nanodiamonds

Nanodiamonds containing negatively charged nitrogen-vacancy (NV$^-$) centers are versatile room-temperature quantum sensors in a growing field of research. Yet, knowledge regarding the NV-formation mechanism in very small particles is still limited. This study focuses on the formation of the smallest NV$^-$-containing diamonds, 5 nm detonation nanodiamonds (DNDs). As a reliable method to quantify NV$^-$ centers in nanodiamonds, half-field signals in electron paramagnetic resonance (EPR) spectroscopy are recorded. By comparing the NV$^-$ concentration with a series of nanodiamonds from high-pressure high-temperature (HPHT) synthesis (10 - 100 nm), it is shown that the formation process in 5 nm DNDs is unique in several aspects. NV$^-$ centers in DNDs are already formed at the stage of electron irradiation, without the need for high-temperature annealing. The effect is explained in terms of "self-annealing", where size and type dependent effects enable vacancy migration close to room temperature. Although our experiments show that NV$^-$ concentration generally increases with particle size, remarkably, the NV$^-$ concentration in 5 nm DNDs surpasses that of 20 nm-sized nanodiamonds. Using Monte Carlo simulations, we show that the ten times higher substitutional nitrogen concentration in DNDs compensates the vacancy loss induced by the large relative particle surface. Upon electron irradiation at a fluence of $1.5 \times 10 ^{19}$ e$^-$/cm$^2$, DNDs show a 12.5-fold increment in the NV$^-$ concentration with no sign of saturation. These findings can be of interest for the creation of defects in other very small semiconductor nanoparticles beyond NV-nanodiamonds as quantum sensors.

cond-mat.mtrl-sci↗

arXiv2021

Quantum-assisted Distortion-free audio signal sensing

Quantum sensors are keeping the cutting-edge sensitivities in metrology. However, for high-sensitive measurements of arbitrary signals, limitations in linear dynamic range could introduce distortions when sensing the frequency, magnitude and phase of unknown signals. Here, we overcome these limitations with advanced sensing protocol that combines quantum phase-sensitive detection with heterodyne readout. We present theoretical and experimental investigations using nitrogen-vacancy centers in diamond, showing the ability to sense radio signals with a 98 dB linear dynamic range, a 31 pT/Hz$^{1/2}$ sensitivity, and arbitrary frequency resolution. Further, we perform the quantum-assisted distortion-free audio signal (melody, speech) sensing with high fidelity. The methods developed here could broaden the horizon for quantum sensors towards applications in telecommunication, where high-fidelity and low-distortion at multiple frequency bands within small sensing volumes are required.

quant-ph↗

arXiv2021

Quantum nonlinear spectroscopy of single nuclear spins

Nonlinear spectroscopy is widely used for studying physical systems. Conventional nonlinear optical spectroscopy and magnetic resonance spectroscopy, which use classical probes such as electromagnetic waves, can only access certain types of correlations in a quantum system. The idea of quantum nonlinear spectroscopy was recently proposed to use quantum probes such as entangled photons to achieve sensitivities and resolutions beyond the classical limits. It is shown that quantum sensing can extract arbitrary types and orders of correlations in a quantum system by first quantum-entangling a sensor and the object and then measuring the sensor. Quantum sensing has been applied to achieve nuclear magnetic resonance (NMR) of single atoms and the second-order correlation spectroscopy has been adopted to enhance the spectral resolution. However, quantum nonlinear spectroscopy (i.e., the measurement of higher-order correlations) of single nuclear spins is still elusive. Here we demonstrate the extraction of fourth-order correlations of single nuclear spins that cannot be measured in conventional nonlinear spectroscopy, using sequential weak measurement via an atomic quantum sensor, namely, a nitrogen-vacancy center in diamond. We show that the quantum nonlinear spectroscopy provides fingerprint features to identify different types of objects, such as Gaussian noises, random-phased AC fields, and quantum spins, which would be indistinguishable in second-order correlations. The measured fourth-order correlation unambiguously differentiates a single nuclear spin and a random-phased AC field. This work constitutes an initial step toward the application of higher-order correlations to quantum sensing, to examining the quantum foundation (by, e.g., higher-order Leggett-Garg inequality), and to studying quantum many-body physics.

quant-ph↗

arXiv2021

Diamond magnetometry and gradiometry towards subpicotesla DC field measurement

Nitrogen vacancy (NV) centers in diamond have developed into a powerful solid-state platform for compact quantum sensors. However, high sensitivity measurements usually come with additional constraints on the pumping intensity of the laser and the pulse control applied. Here, we demonstrate high sensitivity NV ensemble based magnetic field measurements with low-intensity optical excitation. DC magnetometry methods like, e.g., continuous-wave optically detected magnetic resonance and continuously excited Ramsey measurements combined with lock-in detection, are compared to get an optimization. Gradiometry is also investigated as a step towards unshielded measurements of unknown gradients. The magnetometer demonstrates a minimum detectable field of 0.3-0.7 pT in a 73 s measurement by further applying a flux guide with a sensing dimension of 2 mm, corresponding to a magnetic field sensitivity of 2.6-6 pT/Hz^0.5. Combined with our previous efforts on the diamond AC magnetometry, the diamond magnetometer is promising to perform wide bandwidth magnetometry with picotesla sensitivity and a cubic-millimeter sensing volume under ambient conditions.

quant-ph↗

arXiv2021

Millimetre-scale magnetocardiography of living rats using a solid-state quantum sensor

A key challenge in cardiology is the non-invasive imaging of electric current propagation occurring in the cardiovascular system at an intra-cardiac scale. A promising approach for directly mapping the current dynamics is to monitor the associated stray magnetic field. However, in this magnetic field approach, the spatial resolution deteriorates significantly as the standoff distance between the target and the sensor increases. Existing sensors usually remain relatively far from the target and provide only centimetre-scale resolution because their operating temperature is not biocompatible. Here we demonstrate millimetre-scale magnetocardiography of living rats using a solid-state quantum sensor based on nitrogen-vacancy centres in diamond. The essence of the method is a millimetre proximity from the sensor to heart surface, which enhances the cardiac magnetic field to greater than nanoteslas and allows the mapping of these signals with intra-cardiac resolution. From the acquired magnetic images, we also estimate the source electric current vector, flowing from the right atria base via the Purkinje fibre bundle to the left ventricular apex. Our results establish the solid-state quantum sensor's capability to probe cardiac magnetic signals from mammalian animals and reveal their intra-cardiac electrodynamics. This technique will enable the study of the origin and progression of myriad cardiac arrhythmias including flutter, fibrillation, and tachycardia.

physics.med-ph↗

arXiv2020

Integrated and portable magnetometer based on nitrogen-vacancy ensembles in diamond

Magnetic field sensors that exploit quantum effects have shown that they can outperform classical sensors in terms of sensitivity enabling a range of novel applications in future, such as a brain machine interface. Negatively charged nitrogen-vacancy (NV) centers in diamond have emerged as a promising high sensitivity platform for measuring magnetic fields at room temperature. Transferring this technology from laboratory setups into products and applications, the total size of the sensor, the overall power consumption, and the costs need to be reduced and optimized. Here, we demonstrate a fiber-based NV magnetometer featuring a complete integration of all functional components without using any bulky laboratory equipment. This integrated prototype allows portable measurement of magnetic fields with a sensitivity of 344 pT/ SqrtHz.

quant-ph↗

arXiv2020

Heterodyne Sensing of Microwaves with a Quantum Sensor

Diamond quantum sensors are sensitive to weak microwave magnetic fields resonant to the spin transitions. However the spectral resolution in such protocols is limited ultimately by sensor lifetime. Here we demonstrate a heterodyne detection method for microwaves (MW) leading to a lifetime independent spectral resolution in the GHz range. We reference the MW-signal to a local oscillator by generating the initial superposition state from a coherent source. Experimentally we achieve a spectral resolution below $1\ \rm{Hz}$ for a $4\ \rm{GHz}$ signal far below the sensor lifetime limit of kilohertz. Furthermore we show control over the interaction of the MW-field with the two level system by applying dressing fields, pulsed Mollow absorption and Floquet dynamics under strong longitudinal radio frequency drive. While pulsed Mollow absorption leads to highest sensitivity, the Floquet dynamics allows robust control independent from the systems resonance frequency. Our work is important for future studies in sensing weak microwave signals in wide frequency range with high spectral resolution.

quant-ph↗

arXiv2019

Quantum Metrology with Strongly Interacting Spin Systems

Quantum metrology makes use of coherent superpositions to detect weak signals. While in principle the sensitivity can be improved by increasing the density of sensing particles, in practice this improvement is severely hindered by interactions between them. Using a dense ensemble of interacting electronic spins in diamond, we demonstrate a novel approach to quantum metrology. It is based on a new method of robust quantum control, which allows us to simultaneously eliminate the undesired effects associated with spin-spin interactions, disorder and control imperfections, enabling a five-fold enhancement in coherence time compared to conventional control sequences. Combined with optimal initialization and readout protocols, this allows us to break the limit for AC magnetic field sensing imposed by interactions, opening a promising avenue for the development of solid-state ensemble magnetometers with unprecedented sensitivity.

quant-ph↗