arXiv subjects

Xing Rong

Publications and source records attributed to Xing Rong.

At least 19 recordsLinked to original sources

arXiv2026

Experimental Investigation of Tunable-Order Hilbert-Space Ergodicity

Hilbert-space ergodicity (HSE) provides a new framework for studying thermalization in driven quantum systems, complementing the eigenstate thermalization hypothesis, which is restricted to static systems. This ergodicity is hierarchical: by quantifying how randomly the dynamics explores the Hilbert space, one obtains a family of levels termed $k$-HSE. While HSE has been observed at the lowest and highest levels, finite-order HSE dynamics remains largely unexplored due to the difficulty of constructing such drives. Here, we explore this intermediate regime and uncover its distinctive physics. We first propose and prove that a family of $m$-tone drives on qubits realizes $k$-HSE up to $k = 2m{-}3$, with drive parameters determined at $O(k)$ cost. Using a single nitrogen-vacancy center in diamond, we verify this design by showing that a 3-tone drive realizes 3-HSE, with fourth-order statistics depending on the initial state. Further in-depth theoretical analysis shows that this initial-state dependence is generic across drives, demonstrating the possibility of recovering the initial state from higher-order statistics even when the dynamics is ergodic. Our work broadens the study of quantum ergodicity and reveals intriguing physics within its hierarchy.

quant-ph↗

arXiv2025

Exotic-interaction searches on small scales via near-threshold enhancement

Exotic interactions between fermions mediated by new bosons beyond the Standard Model may hold the key to several fundamental conundrums on the frontier of physics.However, laboratory searching for exotic interactions on small length scales is fundamentally held back by the short force range.Here we propose that the force range of exotic interactions tends to infinity when the oscillation frequency of exotic interactions approaches the mass of the new bosons, i.e., the new bosons become nearly on-shell. This is named as the near-threshold enhancement. Through the near-threshold enhancement, even fermions at distances larger than the original force range can make considerable contributions to exotic interactions. Therefore, the size of the experimental apparatus can break the limitation of the force range, and thus both the signal of exotic interactions and the sensitivity of sensors can be greatly enhanced. We also propose a method to search for the exotic interactions in the mass range between 80 $\mu$eV and 800 $\mu$eV taking advantage of the near-threshold enhancement. For the coupling $g_{\rm A}^{\rm e}g_{\rm A}^{\rm e}$, we expect an improvement on its upper bounds of 10 orders of magnitude at 800 $\mu$eV. This method can be further extended to enhance the search for other types of exotic interactions and boost the study of new physics beyond the Standard Model.

hep-ph↗

arXiv2025

Observation of hierarchy of Hilbert space ergodicities in the quantum dynamics of a single spin

Ergodicity, the property that all allowed configurations are explored over time, plays a pivotal role in explaining the equilibrium behavior of classical dynamical systems. Yet, such a property is typically precluded in quantum systems owing to the presence of energy eigenstates, which are stationary states in dynamics. However, recent theoretical works have argued that ergodic explorations of the Hilbert space, occurring at varying levels as measured by statistical pseudorandomness of the time-evolved quantum states, may be exhibited for quantum systems driven by Hamiltonians with aperiodic time dependencies, which do not face such obstacles. Here, we experimentally investigate the hierarchy of Hilbert-space ergodicities (HSE) achievable in the dynamics of a single quantum spin realized by a solid-state defect in diamond, upon subjecting it to various time-dependent modulations. Through continuous monitoring of spin trajectories with full state tomography, different degrees of HSE were observed, ranging from no HSE in a time-periodic (Floquet) drive, to partial HSE in a smoothly kicked time-quasiperiodic drive, to complete HSE in a drive composed of a sequence of kicks generated by the Fibonacci word. We formulate a theoretical understanding of the increasing levels of HSE observed by attributing them to increasing levels of complexities associated with the drive sequences, whose notions we elucidate. Our work constitutes the first unambiguous experimental evidence of Hilbert space ergodicity and promotes deeper investigations into the mechanisms and fine-grained levels with which closed quantum systems reach equilibrium.

quant-ph↗

arXiv2025

Haloscope Searching for Dark Photons at Q-band with a Novel Coupling Tuning Structure

Laboratory searching for dark matter is crucial for understanding several fundamental conundrums in physics and cosmology. Most cavity-based haloscope searches focus on the frequency range below 10 GHz, while the parameter space with higher frequency remains rarely explored, due to the challenges lying in the fabrication of microwave cavities. Here we report the first Q-band haloscope searching for dark photons with a 33.141 GHz cavity. A novel coupling tuning structure separated from the cavity was designed so as not to degrade the quality factor of the cavity. We have established the most stringent constraints $\chi<2.5\times10^{-12}$ at a confidence level of 90$\%$ in the frequency range from 33.139 GHz to 33.143 GHz, corresponding to the mass of dark photons ranging from 137.05 $\mu$eV to 137.07 $\mu$eV. The results surpass the previous astronomical constraints by nearly three orders of magnitude. This work has demonstrated the feasibility of dark matter haloscopes at Q band. In the future, the constraints can be further improved by more than one order of magnitude through low-temperature experiments, and the setup can be extended to search for axions, axion-like particles, and high-frequency gravitational waves.

astro-ph.CO↗

arXiv2025

New Constraints on Dark Photon Dark Matter with a Millimeter-Wave Dielectric Haloscope

Dark matter remains one of the most profound and unresolved mysteries in modern physics. To unravel its nature, numerous haloscope experiments have been implemented across various mass ranges. However, very few haloscope experiments conducted within millimeter-wave frequency range, which is in the favored mass region for well-motivated dark matter candidates. Here we designed and constructed a millimeter-wave dielectric haloscope featuring a dark matter detector composed of dielectric disks and a mirror. Using this setup, we conducted a search for randomly polarized dark photon dark matter and found no evidence for its existence. Our results established new constraints on the kinetic mixing parameter in the mass range from $387.72$ to $391.03$ $\mu eV$, improving the existing limits by two orders of magnitude. With future enhancements, our system has the potential to explore new parameter space for dark photon as well as axion dark matter within the millimeter-wave frequency range.

astro-ph.CO↗

arXiv2025

Scalable architecture for dark photon searches: Superconducting-qubit proof of principle

The dark photon is a well-motivated candidate of dark matter due to its potential to open the window of new physics beyond the Standard Model. A fundamental mass-range-sensitivity dilemma is always haunting the dark photon searching experiments: The resonant haloscopes have excellent sensitivity but are narrowband, and vice versa for the non-resonant ones. A scalable architecture integrating numerous resonant haloscopes will be a desirable solution to this dilemma. However, even the concept of scalable searching remains rarely explored, due to the size limitation of conventional haloscopes imposed by the dark photon wavelength. Here we propose and demonstrate a novel architecture using superconducting qubits as sub-wavelength haloscope units. By virtue of the scalability of superconducting qubits, it is possible to integrate multiple qubits with different frequencies on a chip-scale device. Furthermore, the frequencies of the qubits can be tuned to extend the searching mass range. Thus, our architectures allow for searching for dark photons in a broad mass range with high sensitivity. As a proof-of-principle experiment, we designed and fabricated a three-qubit chip and successfully demonstrated a scalable dark-photon searching. Our work established constraints on dark photons in the mass range of 15.632 $\mu$eV$\sim$15.638 $\mu$eV, 15.838 $\mu$eV$\sim$15.845 $\mu$eV, and 16.463 $\mu$eV$\sim$16.468 $\mu$eV, simultaneously, and the constraints are much more stringent than the cosmology constraints. Our work can be scaled up in the future to boost the scrutiny of new physics and extended to search for more dark matter candidates, including dark photons, axions and axion-like particles.

hep-ph↗

arXiv2025

Non-Hermitian non-Abelian topological transition in the S=1 electron spin system of a nitrogen vacancy centre in diamond

Topological phases and transitions are of fundamental importance in physics, which provide a deep insight into the understanding of materials. Recently, non-Abelian topological transitions have been investigated in Hermitian systems, revealing important topological features. With non-Hermiticity introduced, non-Hermitian non-Abelian topological transitions bring about more intriguing topological features, yet has not been experimentally explored. In this work, we report the observation of the non-Hermitian non-Abelian topological transition at the atomic scale utilizing a nitrogenvacancy center in diamond. While the well-established topological numbers, failed to recognize this transition, we successfully characterized such a transition with the measurement of the complex eigenvalue braids. We obtained the braid invariants from the measured relative phases between eigenvalues. The observed change in braid invariants provides a clear signature of the non-Abelian topological transition. Furthermore, we experimentally revealed an intriguing consequence of this transition, which is the creation of a third-order exceptional point through the collision of two second-order exceptional points with opposite charges. Our experimental findings shed light on the abundant non-Abelian topological phenomena involving non-Hermiticity, and provide insights into manipulating the spectral topology in atomic scale systems to achieve exotic functionalities arising from non-Abelian band braiding.

quant-ph↗

arXiv2025

Experimental observation of Dirac exceptional point

The energy level degeneracies, also known as exceptional points (EPs), are crucial for comprehending emerging phenomena in materials and enabling innovative functionalities for devices. Since EPs were proposed over half a century age, only two types of EPs have been experimentally discovered, revealing intriguing phases of materials such as Dirac and Weyl semimetals. These discoveries have showcased numerous exotic topological properties and novel applications, such as unidirectional energy transfer. Here we report the observation of a novel type of EP, named the Dirac EP, utilizing a nitrogen-vacancy center in diamond. Two of the eigenvalues are measured to be degenerate at the Dirac EP and remain real in its vicinity. This exotic band topology associated with the Dirac EP enables the preservation of the symmetry when passing through, and makes it possible to achieve adiabatic evolution in non-Hermitian systems. We examined the degeneracy between the two eigenstates by quantum state tomography, confirming that the degenerate point is a Dirac EP rather than a Hermitian degeneracy. Our research of the distinct type of EP contributes a fresh perspective on dynamics in non-Hermitian systems and is potentially valuable for applications in quantum control in non-Hermitian systems and the study of the topological properties of EP.

quant-ph↗

arXiv2024

Programmable simulation of high-order exceptional point with a trapped ion

The nontrivial degeneracies in non-Hermitian systems, exceptional points (EPs), have attracted extensive attention due to intriguing phenomena. Compared with commonly observed second-order EPs, high-order EPs show rich physics due to their extended dimension and parameter space, ranging from the coalescence of EPs into higher order to potential applications in topological properties. However, these features also pose challenges in controlling multiple coherent and dissipative elements in a scaled system. Here we experimentally demonstrate a native programmable control to simulate a high-order non-Hermitian Hamiltonian in a multi-dimensional trapped ion system. We simulate a series of non-Hermitian systems with varied parameters and observe the coalescence of second-order EPs into a fourth-order EP. Our results pave the way for scalable quantum simulation of high-dimensional dissipative systems and can be beneficial for the application of high-order EPs in quantum sensing and quantum control.

quant-ph↗

arXiv2024

Searches for exotic spin-dependent interactions with spin sensors

Numerous theories have postulated the existence of exotic spin-dependent interactions beyond the Standard Model of particle physics. Spin-based quantum sensors, which utilize the quantum properties of spins to enhance measurement precision, emerge as powerful tools for probing these exotic interactions. These sensors encompass a wide range of technologies, such as optically pumped magnetometers, atomic comagnetometers, spin masers, nuclear magnetic resonance, spin amplifiers, and nitrogen-vacancy centers. These technologies stand out for their ultrahigh sensitivity, compact tabletop design, and cost-effectiveness, offering complementary approaches to the large-scale particle colliders and astrophysical observations. This article reviews the underlying physical principles of various spin sensors and highlights the recent theoretical and experimental progress in the searches for exotic spin-dependent interactions with these quantum sensors. Investigations covered include the exotic interactions of spins with ultralight dark matter, exotic spin-dependent forces, electric dipole moment, spin-gravity interactions, and among others. Ongoing and forthcoming experiments using advanced spin-based sensors to investigate exotic spin-dependent interactions are discussed.

quant-ph↗

arXiv2024

Observation of freezing phenomenon in high-dimensional quantum correlation dynamics

Quantum information processing (QIP) based on high-dimensional quantum systems provides unique advantages and new potentials where high-dimensional quantum correlations (QCs) play vital roles. Exploring the resistance of QCs against noises is crucial as QCs are fragile due to complex and unavoidable system-environment interactions. In this study, we investigate the performance of high-dimensional QCs under local dephasing noise using a single nitrogen-vacancy center in diamond. A freezing phenomenon in the high-dimensional quantum discord dynamics was observed, showing discord is robust against local dephasing noise. Utilizing a robustness metric known as freezing index, we found that the discord of qutrits outperforms their qubits counterpart when confronted with dephasing noise. Furthermore, we developed a geometric picture to explain this intriguing freezing phenomenon phenomenon. Our findings highlight the potential of utilizing discord as a physical resource for advancing QIP in high-dimensional quantum settings.

quant-ph↗

arXiv2024

Experimental investigation of coherent ergotropy in a single spin system

Ergotropy is defined as the maximum amount of work that can be extracted through a unitary cyclic evolution. It plays a crucial role in assessing the work capacity of a quantum system. Recently, the significance of quantum coherence in work extraction has been theoretically identified, revealing that quantum states with more coherence possess more ergotropy compared to their dephased counterparts. However, an experimental study of the coherent ergotropy remains absent. Here, we report an experimental investigation of the coherent ergotropy in a single spin system. Based on the method of measuring ergotropy with an ancilla qubit, both the coherent and incoherent components of the ergotropy for the non-equilibrium state were successfully extracted. The increase in ergotropy induced by the increase in the coherence of the system was observed by varying the coherence of the state. Our work reveals the interplay between quantum thermodynamics and quantum information theory, future investigations could further explore the role other quantum attributes play in thermodynamic protocols.

quant-ph↗

arXiv2024

Non-invasive magnetocardiography of living rat based on diamond quantum sensor

Magnetocardiography (MCG) has emerged as a sensitive and precise method to diagnose cardiovascular diseases, providing more diagnostic information than traditional technology. However, the sensor limitations of conventional MCG systems, such as large size and cryogenic requirement, have hindered the widespread application and in-depth understanding of this technology. In this study, we present a high-sensitivity, room-temperature MCG system based on the negatively charged Nitrogen-Vacancy (NV) centers in diamond. The magnetic cardiac signal of a living rat, characterized by an approximately 20 pT amplitude in the R-wave, is successfully captured through non-invasive measurement using this innovative solid-state spin sensor. To detect these extremely weak biomagnetic signals, we utilize sensitivity-enhancing techniques such as magnetic flux concentration. These approaches have enabled us to simultaneously achieve a magnetometry sensitivity of 9 $\text{pT}\cdot \text{Hz}^{-1/2}$ and a sensor scale of 5 $\text{mm}$. By extending the sensing scale of the NV centers from cellular and molecular level to macroscopic level of living creatures, we have opened the future of solid-state quantum sensing technologies in clinical environments.

physics.med-ph↗

arXiv2024

Near-Quantum-limited Haloscope Detection of Dark Photon Dark Matter Enhanced by a High-Q Superconducting Cavit

We report new experimental results on the search for dark photons based on a near-quantum-limited haloscope equipped with a superconducting cavity. The loaded quality factor of the superconducting cavity is $6\times10^{5}$, so that the expected signal from dark photon dark matter can be enhanced by more than one order compared to a copper cavity. A Josephson parametric amplifier with a near-quantum-limited noise temperature has been utilized to minimize the noise during the search. Furthermore, a digital acquisition card based on field programmable gate arrays has been utilized to maximize data collection efficiency with a duty cycle being 100$\%$. This work has established the most stringent constraints on dark photons at around 26.965 $μ$eV. In the future, our apparatus can be extended to search for other dark matter candidates, such as axions and axion-like particles, and scrutinize new physics beyond the Standard Model.

hep-ex↗

arXiv2024

New Constraints on Exotic Spin-Spin-Velocity-Dependent Interactions with Solid-State Quantum Sensors

We report new experimental results on exotic spin-spin-velocity-dependent interactions between electron spins. We designed an elaborate setup that is equipped with two nitrogen-vacancy (NV) ensembles in diamonds. One of the NV ensembles serves as the spin source, while the other functions as the spin sensor. By coherently manipulating the quantum states of two NV ensembles and their relative velocity at the micrometer scale, we are able to scrutinize exotic spin-spin-velocity-dependent interactions at short force ranges. For a T-violating interaction, $V_6$, new limits on the corresponding coupling coefficient, $f_6$, have been established for the force range shorter than 1 cm. For a P,T-violating interaction, $V_{14}$, new constraints on the corresponding coupling coefficient, $f_{14}$, have been obtained for the force range shorter than 1 km.

hep-ex↗

arXiv2024

Searching for Exotic Spin-Dependent Interactions with Diamond-Based Vector Magnetometer

We propose a new method to search for exotic spin-spin interactions between electrons and nucleons using a diamond-based vector magnetometer. The vector magnetometer can be constructed from ensembles of nitrogen-vacancy centers along different axes in a diamond. The $^{14}\mathrm{N} $ nuclear spins of nitrogen-vacancy centers in the same diamond can be polarized through the dynamic nuclear polarization method to serve as spin sources. With the vector magnetometer, the sought-after exotic interactions can be distinguished from the magnetic dipole-dipole interaction. For the axion-mediated interaction $ V_{PP} $, new upper bounds of the coupling $ |g_{P}^{e} g_{P}^{N}| $ are expected within the force range from 10 nm to 100 $ μ$m. For the $ Z' $-mediated interaction $ V_{AA} $, new upper bounds of the coupling $ |g_{A}^{e} g_{A}^{N}| $ are expected within the force range from 10 nm to 1 cm. The new upper bounds for $ V_{PP} $ and $ V_{AA} $ are both expected to be more than 5 orders of magnitude more stringent than existing constraints at the force range of 1 $ μ$m with the total measurement time of one day.

hep-ex↗

arXiv2024

Experimental test of the Crooks fluctuation theorem in a single nuclear spin

We experimentally test the Crooks fluctuation theorem in a quantum spin system. Our results show that the Crooks fluctuation theorem is valid for different speeds of the nonequilibrium processes and under various effective temperatures. Work is not an observable in quantum systems, which makes tests of quantum thermodynamic theorems challenging. In this work, we developed high-fidelity single-shot readouts of a single nuclear spin in diamond and implemented the two-point work measurement protocol, enabling a direct experimental test of the Crooks fluctuation theorem. Our results provide a quantum insight into fluctuations and the methods we developed can be utilized to study other quantum thermodynamic theorems.

quant-ph↗

arXiv2024

Third-order exceptional line in a nitrogen-vacancy spin system

The exceptional points (EPs) aroused from the non-Hermiticity bring rich phenomena, such as exceptional nodal topologies, unidirectional invisibility, single-mode lasing, sensitivity enhancement and energy harvesting. Isolated high-order EPs have been observed to exhibit richer topological characteristics and better performance in sensing over 2nd-order EPs. Recently, high-order EP geometries, such as lines or rings formed entirely by high order EPs, are predicted to provide richer phenomena and advantages over stand-alone high-order EPs. However, experimental exploration of high-order EP geometries is hitherto beyond reach due to the demand of more degrees of freedom in the Hamiltonian's parameter space or a higher level of symmetries. Here we report the observation of the third-order exceptional line (EL) at the atomic scale. By introducing multiple symmetries, the emergence of the third-order EL has been successfully realized with a single electron spin of nitrogen-vacancy center in diamond. Furthermore, the behaviors of the EP structure under different symmetries are systematically investigated. The symmetries are shown to play essential roles in the occurrence of high-order EPs and the related EP geometries. Our work opens a new avenue to explore high-order EP-related topological physics at the atomic scale and to the potential applications of high-order EPs in quantum technologies.

quant-ph↗