arXiv subjects

Hanfeng Wang

Publications and source records attributed to Hanfeng Wang.

9 recordsLinked to original sources

arXiv2026

Dispersive readout of cavity-coupled solid-state sensor with near-unity readout fidelity

Solid-state quantum sensors based on ensembles of nitrogen-vacancy (NV) centers in diamond have emerged as powerful platforms for high-precision metrology. Coupling the NV ensemble to a microwave cavity mode in a cavity quantum electrodynamics (cQED) configuration enables spin readout that surpasses the limitations of conventional optical detection, achieving sub-picotesla magnetic sensitivities. However, existing continuous-wave cQED approaches remain far from the intrinsic spin-projection-noise limit due to spin saturation and power broadening. Here, we introduce a dispersive cQED readout technique to overcome these fundamental limitations in NV ensemble sensing. We develop a comprehensive theoretical framework describing the dispersive interaction and analyze the time-domain dynamics of a strongly-coupled NV-cavity system. Our results indicate near-unity inverse readout fidelity and femtotesla-level sensitivity using a commercially available diamond NV ensemble. Importantly, the dispersive readout exhibits a distinct sensitivity scaling that improves as 1/N with increasing number of spins N, providing a practical pathway toward approaching the standard quantum limit for solid-state spin-ensemble sensors.

quant-ph↗

arXiv2025

Wake dynamics of finite-aspect-ratio rotating circular cylinders at low Reynolds number

We perform direct numerical simulations of flows over finite-aspect-ratio rotating circular cylinders at a Reynolds number of 150 over a range of aspect ratios ($AR=2-12$) and rotation rates ($\alpha=0-5$), aiming at revealing the free-end effects on the wake dynamics and aerodynamic performance. As a direct consequence of lift generation, a pair of counter-rotating tip vortices is formed at the free ends. At low rotation rates, the finite rotating cylinder behaves like a typical bluff body that generates unsteady vortex shedding with three-dimensional modal structures. Such unsteady flows can be stabilized not only by increasing rotation rate that weakens the free shear layer, but also by decreasing aspect ratio which enhances the tip-vortex-induced downwash. A further increase of $\alpha$ triggers the onset of unsteadiness in the tip vortices. At still higher rotation rates, the C-shaped Taylor-like vortices bounded on the cylinder surface emerge from the free ends and propel towards the midspan due to the self-induced velocity by vortex-wall interaction. With increasing $\alpha$, the free-end effects penetrate to the inboard span, leading to reduced lift and elevated drag compared to the two-dimensional flows. The three-dimensional end effects can be effectively suppressed by the addition of end plates, which position the tip vortices away from the cylinder, thereby significantly improving the aerodynamic performance. This study reveals the mechanisms for the formation of three-dimensional wakes under the influence of the free ends of finite rotating cylinders. The insights obtained here can serve as a stepping stone for understanding the complex high-$Re$ flows that are more relevant to industrial applications.

physics.flu-dyn↗

arXiv2025

Exceptional sensitivity near the bistable transition point of a hybrid quantum system

Phase transitions can dramatically alter system dynamics, unlocking new behavior and improving performance. Exceptional points (EPs), where the eigenvalues and corresponding eigenvectors of a coupled linear system coalesce, are particularly relevant for sensing applications as they can increase sensor response to external perturbations to a range of phenomena from optical phase shifts to gravitational waves. However, the coalescence of eigenstates at linear EPs amplifies noise, negating the signal-to-noise ratio (SNR) enhancement. Here, we overcome this limitation using nonlinearity, which exhibits exceptional SNR around a bistable transition point (BP). We couple a state-of-the-art diamond quantum sensor to a nonlinear Van der Pol oscillator, forming a self-oscillating hybrid system that exhibits both a single-valued and bistable phase. The boundaries between these phases are marked by both adiabatic and deterministic non-adiabatic transitions that enable chiral state switching and state coalescence at the BP. Crucially, NV magnetometry performed near the BP exhibits a 17x enhancement in SNR, achieving a record sensitivity of 170 fT/\sqrt{Hz}. This result surpasses the sensitivity limit of an ideal, thermally-limited electron magnetometer and resolves a long-standing debate regarding EP-like physics in advanced quantum sensing.

quant-ph↗

arXiv2025

A Bayesian Approach towards Atomically-precise Localization in Fluorescence Microscopy

Super-resolution microscopy has revolutionized the imaging of complex physical and biological systems by surpassing the Abbe diffraction limit. Recent advancements, particularly in single-molecule localization microscopy (SMLM), have pushed localization below nanometer precision, by applying prior knowledge of correlated fluorescence emission from single emitters. However, achieving a refinement from 1 nm to 1 Angstrom demands a hundred-fold increase in collected photon signal. This quadratic resource scaling imposes a fundamental barrier in SMLM, where the intense photon collection is challenged by photo-bleaching, prolonged integration times, and inherent practical constraints. Here, we break this limit by harnessing the periodic nature of the atomic lattice structure. Applying this discrete grid imaging technique (DIGIT) in a quantum emitter system, we observe an exponential collapse of localization uncertainty once surpassing the host crystal's atomic lattice constant. We further applied DIGIT to a large-scale quantum emitter array, enabling parallel positioning of each emitter through wide-field imaging. Collectively, these advancements establish DIGIT as a competitive tool for achieving unprecedented, precise measurements, ultimately paving the way to direct optical resolution of crystal and atomic features within quantum and biological systems.

physics.optics↗

arXiv2025

Cavity-enhanced solid-state nuclear spin gyroscope

Solid-state quantum sensors based on ensembles of nitrogen-vacancy (NV) centers in diamond have emerged as powerful tools for precise sensing applications. Nuclear spin sensors are particularly well-suited for applications requiring long coherence times, such as inertial sensing, but remain underexplored due to control complexity and limited optical readout efficiency. In this work, we propose cooperative cavity quantum electrodynamic (cQED) coupling to achieve efficient nuclear spin readout. Unlike previous cQED methods used to enhance electron spin readout, here we employ two-field interference in the NV hyperfine subspace to directly probe the nuclear spin transitions. We model the nuclear spin NV-cQED system (nNV-cQED) and observe several distinct regimes, including electromagnetically induced transparency, masing without inversion, and oscillatory behavior. We then evaluate the nNV-cQED system as an inertial sensor, indicating a rotation sensitivity improved by three orders of magnitude compared to previous solid-state spin demonstrations. Furthermore, we show that the NV electron spin can be simultaneously used as a comagnetometer, and the four crystallographic axes of NVs can be employed for vector resolution in a single nNV-cQED system. These results showcase the applications of two-field interference using the nNV-cQED platform, providing critical insights into the manipulation and control of quantum states in hybrid NV systems and unlocking new possibilities for high-performance quantum sensing.

quant-ph↗

arXiv2024

Effects of Thom disk on alleviating ground effects of a wall-mounted rotating cylinder

This study investigates the effects of Thom disks on alleviating ground effects by wall-mounted rotating cylinders, also known as Flettner rotors, which utilize wind energy for ship propulsion. Through three-dimensional direct numerical simulations, our findings reveal that introducing a secondary Thom disk near the ground significantly reduces the three-dimensional flow pattern induced by the ground, leading to a more uniform pressure distribution along the rotor's surface. We also explore how the vertical placement of a secondary Thom disk influences wake dynamics and aerodynamic forces. Optimal placement of the secondary disk is found at the ground, which maximizes the lift-to-drag ratio.

physics.flu-dyn↗

arXiv2024

A spin-refrigerated cavity quantum electrodynamic sensor

Quantum sensors based on solid-state defects, in particular nitrogen-vacancy (NV) centers in diamond, enable precise measurement of magnetic fields, temperature, rotation, and electric fields. However, the sensitivity of leading NV spin ensemble sensors remains far from the intrinsic spin-projection noise limit. Here we move towards this quantum limit of performance by introducing (i) a cavity quantum electrodynamic (cQED) hybrid system operating in the strong coupling regime, which enables high readout fidelity of an NV ensemble using microwave homodyne detection; (ii) a comprehensive nonlinear model of the cQED sensor operation, including NV ensemble inhomogeneity and optical polarization; and (iii) ``spin refrigeration'' where the optically-polarized spin ensemble sharply reduces the ambient-temperature microwave thermal noise, resulting in enhanced sensitivity. Applying these advances to magnetometry, we demonstrate a broadband sensitivity of 580 fT/$\sqrt{\mathrm{Hz}}$ around 15 kHz in ambient conditions. We then discuss the implications of this model for design of future magnetometers, including devices approaching 12 fT/$\sqrt{\mathrm{Hz}}$ sensitivity. Applications of these techniques extend to the fields of gyroscope and clock technologies.

quant-ph↗

arXiv2022

Electric-Field Programmable Spin Arrays for Scalable Quantum Repeaters

Large scale control over thousands of quantum emitters desired by quantum network technology is limited by power consumption and cross-talk inherent in current microwave techniques. Here we propose a quantum repeater architecture based on densely-packed diamond color centers (CCs) in a programmable electrode array. This 'electric-field programmable spin array' (eFPSA) enables high-speed spin control of individual CCs with low cross-talk and power dissipation. Integrated in a slow-light waveguide for efficient optical coupling, the eFPSA serves as a quantum interface for optically-mediated entanglement. We evaluate the performance of the eFPSA architecture in comparison to a routing tree design and show increased entanglement generation rate into thousands of qubits regime. Our results enable high fidelity control of dense quantum emitter arrays for scalable networking.

quant-ph↗

arXiv2020

Electrical Control of Coherent Spin Rotation of a Single-Spin Qubit

Nitrogen vacancy (NV) centers, optically-active atomic defects in diamond, have attracted tremendous interest for quantum sensing, network, and computing applications due to their excellent quantum coherence and remarkable versatility in a real, ambient environment. One of the critical challenges to develop NV-based quantum operation platforms results from the difficulty to locally address the quantum spin states of individual NV spins in a scalable, energy-efficient manner. Here, we report electrical control of the coherent spin rotation rate of a single-spin qubit in NV-magnet based hybrid quantum systems. By utilizing electrically generated spin currents, we are able to achieve efficient tuning of magnetic damping and the amplitude of the dipole fields generated by a micrometer-sized resonant magnet, enabling electrical control of the Rabi oscillation frequency of NV spins. Our results highlight the potential of NV centers in designing functional hybrid solid-state systems for next-generation quantum-information technologies. The demonstrated coupling between the NV centers and the propagating spin waves harbored by a magnetic insulator further points to the possibility to establish macroscale entanglement between distant spin qubits.

cond-mat.mes-hall↗