arXiv subjects

Junichi Isoya

Publications and source records attributed to Junichi Isoya.

At least 19 recordsLinked to original sources

arXiv2025

Robust gigahertz-range ac magnetometry with an ensemble of NV centers in diamond using concatenated continuous dynamical decoupling

Sub-picotesla level magnetometry has been demonstrated using negatively-charged nitrogen-vacancy (NV) centers in diamond by increasing the number of spins simultaneously used for sensing in an NV ensemble. However, such scale-up often introduces spatial inhomogeneities in detuning and control field amplitudes, which degrade sensitivity. Although several techniques have been utilized to overcome these challenges, including pulsed dynamical decoupling or shaped pulses, these are not generally compatible with the current state-of-the-art techniques for GHz-range AC magnetometry with NV ensembles, which are typically based on Rabi oscillations. In this work we experimentally demonstrate GHz-range AC magnetometry using a large ensemble of NV centers under spatially inhomogeneous drive fields by employing concatenated continuous dynamical decoupling, which is designed for robustness against such imperfections. We compare its performance with the conventional direct Rabi method and show that the robust dressed states in our method extend significantly the measuring range to weaker signals in GHz-range AC magnetometry.

quant-ph↗

arXiv2025

A near-quantum-limited diamond maser amplifier operating at millikelvin temperatures

Microwave quantum technologies require amplification of weak signals with minimal added noise at millikelvin temperatures. This stringent demand has been met with superconducting parametric amplifiers. While masers offer another fundamental approach, their dependence on cryogenic operation has historically posed challenges for classical communication technologies -- a barrier that does not apply to microwave quantum technologies. In this work, we demonstrate an ultra-low-noise maser amplifier utilizing impurity spins in diamond at millikelvin temperatures. We achieve power gains exceeding $\mathrm{30\,dB}$, a minimum noise temperature of $\mathrm{0.86\,K}$ (corresponding to $2.2$ noise photons), and a maximum $\mathrm{1\,\text{dB}}$ output compression point of $\mathrm{-63\,dBm}$ at $\mathrm{6.595\,\text{GHz}}$. Our results establish masers as viable components of microwave quantum technologies.

quant-ph↗

arXiv2025

Efficient Detection of Statistical RF Fields at High Magnetic Field with a Quantum Sensor

Nuclear magnetic resonance (NMR) spectroscopy is widely used in fields ranging from chemistry, material science to neuroscience. Nanoscale NMR spectroscopy using Nitrogen-vacancy (NV) centers in diamond has emerged as a promising platform due to an unprecedented sensitivity down to the single spin level. At the nanoscale, high nuclear spin polarization through spin fluctuations (statistical polarization) far outweighs thermal polarization. However, until now efficient NMR detection using coherent averaging techniques could not be applied to the detection of statistical polarization, leading to long measurement times. Here we present two protocols to enable coherent averaging of statistical oscillating signals through rectification. We demonstrate these protocols on an artificial radiofrequency signal detected with a single NV center at 2.7 T. Through this, the signal-to-noise scaling with number of measurements $N$ is improved from $N^{0.5}$ to $N^1$, improving the measurement time significantly. The relevance of rectification for the detection of statistical polarization using NV ensembles is outlined, paving the way for efficient nanoscale NMR spectroscopy.

quant-ph↗

arXiv2024

Blueprint for Diamond Magnetometry: Unraveling Quantum Dephasing of Nitrogen-Vacancy Center Ensembles in Diamond

Diamonds with nitrogen-vacancy (NV) center ensembles are one of the most promising solid-state quantum platforms for various sensing applications. The combination of a long spin dephasing time ($T_2^*$) and a high NV center concentration is crucial for pushing the sensitivity limits. In this work, we propose a systematic measurement approach to quantify the electron spin dephasing in NV center ensembles and analyze the contributions of various sources to the dephasing time, including NV-NV interactions, strain and electric field distributions, $^{13}$C nuclear spins, and P1 electron spins. Our method is validated using a series of high-performance diamond samples, providing a comprehensive understanding of dephasing mechanisms and revealing correlations between NV concentration and different dephasing sources. Based on these insights, we further evaluate and propose strategies to improve the achievable sensitivity limits for DC magnetic field measurements.

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

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

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↗

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↗

arXiv2020

Efficient Conversion of Nitrogen to Nitrogen-Vacancy Centers in Diamond Particles with High-Temperature Electron Irradiation

Fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy (NV$^-$) centers are promising for a wide range of applications, such as for sensing, as fluorescence biomarkers, or to hyperpolarize nuclear spins. NV$^-$ centers are formed from substitutional nitrogen (P1 centers) defects and vacancies in the diamond lattice. Maximizing the concentration of NVs is most beneficial, which justifies the search for methods with a high yield of conversion from P1 to NV$^-$. We report here the characterization of surface cleaned fluorescent micro- and nanodiamonds, obtained by irradiation of commercial diamond powder with high-energy (10 MeV) electrons and simultaneous annealing at 800°C. Using this technique and increasing the irradiation dose, we demonstrate the creation of NV$^-$ with up to 25 % conversion yield. Finally, we monitor the creation of irradiation-induced spin-1 defects in microdiamond particles, which we associate with W16 and W33 centers, and investigate the effects of irradiation dose and particle size on the coherence time of NV$^-$.

cond-mat.mtrl-sci↗

arXiv2020

Photoluminescence at the ground state level anticrossing of the nitrogen-vacancy center in diamond

The nitrogen-vacancy center (NV center) in diamond at magnetic fields corresponding to the ground state level anticrossing (GSLAC) region gives rise to rich photoluminescence (PL) signals due to the vanishing energy gap between the electron spin states, which enables to have an effect on the NV center's luminescence for a broad variety of environmental couplings. In this article we report on the GSLAC photoluminescence signature of NV ensembles in different spin environments at various external fields. We investigate the effects of transverse electric and magnetic fields, P1 centers, NV centers, and the $^{13}$C nuclear spins, each of which gives rise to a unique PL signature at the GSLAC. The comprehensive analysis of the couplings and related optical signal at the GSLAC provides a solid ground for advancing various microwave-free applications at the GSLAC, including but not limited to magnetometry, spectroscopy, dynamic nuclear polarization (DNP), and nuclear magnetic resonance (NMR) detection. We demonstrate that not only the most abundant $^{14}$NV center but the $^{15}$NV can also be utilized in such applications and that nuclear spins coupled to P1 centers can be polarized directly by the NV center at the GSLAC, through a giant effective nuclear $g$-factor arising from the NV center-P1 center-nuclear spin coupling. We report on new alternative for measuring defect concentration in the vicinity of NV centers and on the optical signatures of interacting, mutually aligned NV centers.

cond-mat.mes-hall↗

arXiv2020

Microwave-free vector magnetometry with nitrogen-vacancy centers along a single axis in diamond

Sensing vector magnetic fields is critical to many applications in fundamental physics, bioimaging, and material science. Magnetic-field sensors exploiting nitrogen-vacancy (NV) centers are particularly compelling as they offer high sensitivity and spatial resolution even at nanoscale. Achieving vector magnetometry has, however, often required applying microwaves sequentially or simultaneously, limiting the sensors' applications under cryogenic temperature. Here we propose and demonstrate a microwave-free vector magnetometer that simultaneously measures all Cartesian components of a magnetic field using NV ensembles in diamond. In particular, the present magnetometer leverages the level anticrossing in the triplet ground state at 102.4 mT, allowing the measurement of both longitudinal and transverse fields with a wide bandwidth from zero to megahertz range. Full vector sensing capability is proffered by modulating fields along the preferential NV axis and in the transverse plane and subsequent demodulation of the signal. This sensor exhibits a root mean square noise floor of about 300 pT/Hz^(1/2) in all directions. The present technique is broadly applicable to both ensemble sensors and potentially also single-NV sensors, extending the vector capability to nanoscale measurement under ambient temperatures.

physics.app-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↗

arXiv2019

Zero-field magnetometry based on nitrogen-vacancy ensembles in diamond

Ensembles of nitrogen-vacancy (NV) centers in diamonds are widely utilized for magnetometry, magnetic-field imaging and magnetic-resonance detection. They have not been used for magnetometry at zero ambient field because Zeeman sublevels lose first-order sensitivity to magnetic fields as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a 12C-enriched HPHT crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of 250 pT/Hz^(1/2). This technique opens the door to practical applications of NV sensors for ZF magnetic sensing, such as ZF nuclear magnetic resonance, and investigation of magnetic fields in biological systems.

physics.app-ph↗

arXiv2019

Coherent electrical readout of defect spins in 4H-SiC by photo-ionization at ambient conditions

Quantum technology relies on proper hardware, enabling coherent quantum state control as well as efficient quantum state readout. In this regard, wide-bandgap semiconductors are an emerging material platform with scalable wafer fabrication methods, hosting several promising spin-active point defects. Conventional readout protocols for such defect spins rely on fluorescence detection and are limited by a low photon collection efficiency. Here, we demonstrate a photo-electrical detection technique for electron spins of silicon vacancy ensembles in the 4H polytype of silicon carbide (SiC). Further, we show coherent spin state control, proving that this electrical readout technique enables detection of coherent spin motion. Our readout works at ambient conditions, while other electrical readout approaches are often limited to low temperatures or high magnetic fields. Considering the excellent maturity of SiC electronics with the outstanding coherence properties of SiC defects the approach presented here holds promises for scalability of future SiC quantum devices.

cond-mat.mes-hall↗

arXiv2019

Robust and accurate electric field sensing with solid state spin ensembles

Electron spins in solids constitute remarkable quantum sensors. Individual defect centers in diamond were used to detect individual nuclear spins with nanometer scale resolution, and ensemble magnetometers rival SQUID and vapor cell magnetometers when taking into account room temperature operation and size. NV center spins can also detect electric field vectors, despite their weak coupling to electric fields. %even that of an isolated fundamental charge, despite their weak coupling to electric fields. Here, we employ ensembles of NV center spins to measure macroscopic AC electric vector fields with high precision. We utilize low strain, $^{12}$C enriched diamond to achieve maximum sensitivity and tailor the spin Hamiltonian via proper magnetic field adjustment to map out the AC electric field strength and polarization and arrive at refined electric field coupling constants. For high precision measurements we combine classical lock-in detection with aspects from quantum phase estimation for effective suppression of technical noise. Eventually, this enables $t^{-1/2}$ uncertainty scaling of the electric field strength over extended averaging periods, enabling us to reach a sensitivity down to $10^{-7}$ V/$μ$m.

cond-mat.mes-hall↗