Showing 1–42 of 42 results for author: Ji, W

A tunable feedback-controlled magnetic trap for a magnet in free fall

Authors: Changhao Xu, Alexander Heidt, Mohammadreza Nematollahi, Christoph Lotz, Ernst Maria Rasel, Yan Liu, Wei Ji, Dmitry Budker

Abstract: Ferromagnets in free space are predicted to exhibit pure Larmor precession at near-zero magnetic fields and provide exceptional sensitivity for magnetometry and gyroscopy. Notably, pure Larmor precession has not been observed in a macroscopic ferromagnetic particle, despite its fundamental importance and potential for probing relativistic effects and dark-matter interactions. Realizing such dynami… ▽ More Ferromagnets in free space are predicted to exhibit pure Larmor precession at near-zero magnetic fields and provide exceptional sensitivity for magnetometry and gyroscopy. Notably, pure Larmor precession has not been observed in a macroscopic ferromagnetic particle, despite its fundamental importance and potential for probing relativistic effects and dark-matter interactions. Realizing such dynamics requires true free fall to eliminate clamping losses and trap-induced systematics. A central challenge is designing a tunable trap that is weak enough to permit near-free evolution yet robust enough to withstand the disturbances of launch and release. Here, we propose and demonstrate a novel master proportional-integral-differential magnetic trap (MPIDMT) combining a PID-controlled coil system with a master control coil system. Implemented in the third-generation drop tower - Einstein-Elevator, during the microgravity phase the system stably levitates a ferromagnetic particle against shock accelerations up to 1.5 g and resolves its motion in both a low-field (0.4 g) configuration and in pure free fall. These results represent a key step toward free-fall ferromagnetic magnetometry, the long-sought direct observation of macroscopic Larmor precession, and future space-based experiments. △ Less

Submitted 31 May, 2026; originally announced June 2026.

G-type antiferromagnetic structure in Rb1-xV2Te2O

Authors: Wu Xie, Changchao Liu, Fayuan Zhang, Zhenhong Tan, Wenhai Ji, Nan Zhao, Lingxiang Bao, Dong Zhang, Feiran Shen, Lunhua He, Hao Wang, Rong Du, Guanghan Cao, Chaoyu Chen, Ping Miao

Abstract: Altermagnetism, known for its non-relativistic spin-split band structures with yet compensated moments, is being intensively investigated. Discovering new altermagnetic materials with characteristics suitable for practical use remains an important ongoing task. Recently a metallic room-temperature altermagnet candidate Rb1-xV2Te2O with a layered structure and d-wave spin symmetry has been reported… ▽ More Altermagnetism, known for its non-relativistic spin-split band structures with yet compensated moments, is being intensively investigated. Discovering new altermagnetic materials with characteristics suitable for practical use remains an important ongoing task. Recently a metallic room-temperature altermagnet candidate Rb1-xV2Te2O with a layered structure and d-wave spin symmetry has been reported based on experimental results from the spin-resolved photoemission spectroscopy and scanning tunnelling microscopy/spectroscopy (STM/STS) measurements. Here we report neutron powder diffraction (NPD) investigations on the magnetic structure of Rb1-xV2Te2O, which shows a G-type antiferromagnetic structure below the transition temperature of 337 K. The result is different from the original theoretical expectation, which might lead to new insights on the physics of this altermagnet candidate. △ Less

Submitted 22 April, 2026; v1 submitted 19 April, 2026; originally announced April 2026.

Comments: 6 pages, 4 figures

Quantum relaxometry for detecting biomolecular interactions with single NV centers

Authors: Min Li, Qi Zhang, Xi Kong, Sheng Zhao, Bin-Bin Pan, Ziting Sun, Pei Yu, Zhecheng Wang, Mengqi Wang, Wentao Ji, Fei Kong, Guanglei Cheng, Si Wu, Ya Wang, Sanyou Chen, Xun-Cheng Su, Fazhan Shi

Abstract: The investigation of biomolecular interactions at the single-molecule level has emerged as a pivotal research area in life science, particularly through optical, mechanical, and electrochemical approaches. Spins existing widely in biological systems, offer a unique degree of freedom for detecting such interactions. However, most previous studies have been largely confined to ensemble-level detecti… ▽ More The investigation of biomolecular interactions at the single-molecule level has emerged as a pivotal research area in life science, particularly through optical, mechanical, and electrochemical approaches. Spins existing widely in biological systems, offer a unique degree of freedom for detecting such interactions. However, most previous studies have been largely confined to ensemble-level detection in the spin degree. Here, we developed a molecular interaction analysis method approaching single-molecule level based on relaxometry using the quantum sensor, nitrogen-vacancy (NV) center in diamond. Experiments utilized an optimized diamond surface functionalized with a polyethylenimine nanogel layer, achieving $\sim$10 nm average protein distance and mitigating interfacial steric hindrance. Then we measured the strong interaction between streptavidin and spin-labeled biotin complexes, as well as the weak interaction between bovine serum albumin and biotin complexes, at both the micrometer scale and nanoscale. For the micrometer-scale measurements using ensemble NV centers, we re-examined the often-neglected fast relaxation component and proposed a relaxation rate evaluation method, substantially enhancing the measurement sensitivity. Furthermore, we achieved nanoscale detection approaching single-molecule level using single NV centers. This methodology holds promise for applications in molecular screening, identification and kinetic studies at the single-molecule level, offering critical insights into molecular function and activity mechanisms. △ Less

Submitted 10 December, 2025; originally announced December 2025.

Comments: 37 pages, 5 figures in maintext, 23 figures in SI

Journal ref: PNAS 122 (35) e2509102122 (2025)

Detecting gravitational waves with spin systems

Authors: Jiamin Liang, Mingqiu Li, Yu Gao, Wei Ji, Sichun Sun, Qi-Shu Yan

Abstract: The observation of gravitational waves has opened a new window into the Universe through gravitational-wave astronomy. However, high-frequency gravitational waves remain undetected. In this work, we propose that spin systems can be employed to detect gravitational waves in this unexplored frequency regime. We derive the spin's response to gravitational waves and identify three distinct effects: th… ▽ More The observation of gravitational waves has opened a new window into the Universe through gravitational-wave astronomy. However, high-frequency gravitational waves remain undetected. In this work, we propose that spin systems can be employed to detect gravitational waves in this unexplored frequency regime. We derive the spin's response to gravitational waves and identify three distinct effects: the well-known Gertsenshtein effect, a metric-induced interaction, and the gravitational spin Hall effect. We focus on nuclear spins and utilize nuclear magnetic resonance to enhance the gravitational response, leveraging the advantages of long coherence time, high polarization, and a small gyromagnetic ratio. The proposed experimental scheme is capable of probing gravitational waves in the kilohertz to gigahertz range, with projected sensitivities reaching $\sqrt{S_h}\approx10^{-20}~\mathrm{Hz}^{-1/2}$. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 8 pages, 3 figures

Anomaly-free symmetries with obstructions to gauging and onsiteability

Authors: Wilbur Shirley, Carolyn Zhang, Wenjie Ji, Michael Levin

Abstract: We present counterexamples to the lore that symmetries that cannot be gauged or made on-site are necessarily anomalous. Specifically, we construct unitary, internal symmetries of two-dimensional lattice models that cannot be consistently coupled to background or dynamical gauge fields or disentangled to a tensor product of on-site operators. These symmetries are nevertheless anomaly-free in the se… ▽ More We present counterexamples to the lore that symmetries that cannot be gauged or made on-site are necessarily anomalous. Specifically, we construct unitary, internal symmetries of two-dimensional lattice models that cannot be consistently coupled to background or dynamical gauge fields or disentangled to a tensor product of on-site operators. These symmetries are nevertheless anomaly-free in the sense that they admit symmetric, gapped Hamiltonians with unique, invertible ground states. We show that symmetries of this kind are characterized by an index $[ω]\in H^2(G,\mathbb{Q}_+)$, where $\mathbb{Q}_+$ is the multiplicative group of rational numbers labeling one-dimensional quantum cellular automata. △ Less

Submitted 28 July, 2025; originally announced July 2025.

Comments: 6+20 pages, 12 figures

Search for a parity-violating long-range spin-dependent interaction

Authors: Xing Heng, Zitong Xu, Xiaofei Huang, Dinghui Gong, Guoqing Tian, Wei Ji, Jiancheng Fang, Dmitry Budker, Kai Wei

Abstract: High-sensitivity quantum sensors are a promising tool for experimental searches for beyond-Standard-Model interactions. Here, we demonstrate an atomic comagnetometer operating under a resonantly-coupled hybrid spin-resonance (HSR) regime to probe P-odd, T-even interactions. The HSR regime enables robust nuclear-electron spin coupling, enhancing measurement bandwidth and stability without compromis… ▽ More High-sensitivity quantum sensors are a promising tool for experimental searches for beyond-Standard-Model interactions. Here, we demonstrate an atomic comagnetometer operating under a resonantly-coupled hybrid spin-resonance (HSR) regime to probe P-odd, T-even interactions. The HSR regime enables robust nuclear-electron spin coupling, enhancing measurement bandwidth and stability without compromising the high sensitivity of spin-exchange relaxation-free magnetometers. To minimize vibration noise from velocity-modulated sources, we implement a multistage vibration isolation system, achieving a vibration noise reduction exceeding 700-fold. We establish new constraints on vector-boson-mediated parity-violating interactions, improving experimental sensitivity by three orders of magnitude compared to previous limits. The new constraints complement existing astrophysical and laboratory studies of potential extensions to the Standard Model. △ Less

Submitted 1 May, 2025; originally announced May 2025.

Entanglement-Enhanced Nanoscale Single-Spin Sensing

Authors: Xu Zhou, Mengqi Wang, Xiangyu Ye, Haoyu Sun, Yuhang Guo, Han Shuo, Zihua Chai, Wentao Ji, Kangwei Xia, Fazhan Shi, Ya Wang, Jiangfeng Du

Abstract: Detecting individual spins--including stable and metastable states--represents a fundamental challenge in quantum sensing with broad applications across condensed matter physics, quantum chemistry, and single-molecule magnetic resonance imaging. While nitrogen-vacancy (NV) centers in diamond have emerged as powerful nanoscale sensors, their performance for single-spin detection remains constrained… ▽ More Detecting individual spins--including stable and metastable states--represents a fundamental challenge in quantum sensing with broad applications across condensed matter physics, quantum chemistry, and single-molecule magnetic resonance imaging. While nitrogen-vacancy (NV) centers in diamond have emerged as powerful nanoscale sensors, their performance for single-spin detection remains constrained by substantial environmental noise and restricted sensing volume. Here, we propose and demonstrate an entanglement-enhanced sensing protocol that overcomes these limitations through the strategic use of entangled NV pairs. Our approach achieves a 3.4-fold enhancement in sensitivity and a 1.6-fold reduction in spatial resolution relative to single NV centers under ambient conditions. The protocol employs carefully engineered entangled states that amplify target spin signals through quantum interference while suppressing environmental noise. Crucially, we extend these capabilities to resolve metastable single-spin dynamics, directly observing stochastic transitions between different spin states by identifying state-dependent coupling strengths. This dual functionality enables simultaneous detection of static and dynamic spin species for studying complex quantum systems. The achieved performance establishes entanglement-enhanced sensing as a viable pathway toward atomic-scale characterization of quantum materials and interface. △ Less

Submitted 30 April, 2025; originally announced April 2025.

Journal ref: Nature 647, 883-888 (2025)

Investigation of Rare-Earth Ion-Photon Interaction and Strong Coupling in Optical Microcavities

Authors: Quanshen Shen, Wentao Ji, Junyu Guan, Li Qian, Zihua Chai, ChangKui Duan, Ya Wang, Kangwei Xia

Abstract: The strong coupling between an emitter and a cavity is significant for advancing quantum networks. Due to their long optical and spin coherence times, rare-earth ions (REIs) represent a compelling platform for quantum networks. However, their inherently weak intra-4f optical transitions typically result in low coupling strength, thus restricting most current achievements to the weak coupling regim… ▽ More The strong coupling between an emitter and a cavity is significant for advancing quantum networks. Due to their long optical and spin coherence times, rare-earth ions (REIs) represent a compelling platform for quantum networks. However, their inherently weak intra-4f optical transitions typically result in low coupling strength, thus restricting most current achievements to the weak coupling regime. This work proposes a scheme to realize an on-chip quantum network by coupling REIs to high-quality whispering gallery mode (WGM) microcavities. Additionally, we provide numerical validation for a parametric amplification technique to enhance the emitter-cavity coupling strength. As an extension of this approach, the coupled system efficiently achieves the quantum entanglement of local and flying qubits. This study deepens the understanding of emitter-cavity interactions and contributes to realizing REIs-based photonic platforms, which are crucial to distributed quantum computing and developing robust quantum networks. △ Less

Submitted 14 April, 2025; originally announced April 2025.

Artificially creating emergent interfacial antiferromagnetism and its manipulation in a magnetic van-der-Waals heterostructure

Authors: Xiangqi Wang, Cong Wang, Yupeng Wang, Chunhui Ye, Azizur Rahman, Min Zhang, Suhan Son, Jun Tan, Zengming Zhang, Wei Ji, Je-Geun Park, Kai-Xuan Zhang

Abstract: Van der Waals (vdW) magnets, with their two-dimensional (2D) atomic structures, provide a unique platform for exploring magnetism at the nanoscale. Although there have been numerous reports on their diverse quantum properties, the emergent interfacial magnetism--artificially created at the interface between two layered magnets--remains largely unexplored. This work presents observations of such em… ▽ More Van der Waals (vdW) magnets, with their two-dimensional (2D) atomic structures, provide a unique platform for exploring magnetism at the nanoscale. Although there have been numerous reports on their diverse quantum properties, the emergent interfacial magnetism--artificially created at the interface between two layered magnets--remains largely unexplored. This work presents observations of such emergent interfacial magnetism at the ferromagnet/antiferromagnet interface in a vdW heterostructure. We report the discovery of an intermediate Hall resistance plateau in the anomalous Hall loop, indicative of emergent interfacial antiferromagnetism fostered by the heterointerface. This plateau can be stabilized and further manipulated under varying pressures but collapses under high pressures over 10 GPa. Our theoretical calculations reveal that charge transfer at the interface is pivotal in establishing the interlayer antiferromagnetic spin-exchange interaction. This work illuminates the previously unexplored emergent interfacial magnetism at a vdW interface comprised of a ferromagnetic metal and an antiferromagnetic insulator, and highlights its gradual evolution under increasing pressure. These findings enrich the portfolio of emergent interfacial magnetism and support further investigations on vdW magnetic interfaces and the development of next-generation spintronic devices. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Comments: Accepted by ACS Nano; 42 pages, 5 main figures, 8 supporting figures

Journal ref: ACS Nano 19, 8108 (2025)

Two-photon interference between mutually-detuned resonance fluorescence signals scattered off a semiconductor quantum dot

Authors: Guoqi Huang, Jian Wang, Ziqi Zeng, Hanqing Liu, Li Liu, Weijie Ji, Bang Wu, Haiqiao Ni, Zhichuan Niu, Rongzhen Jiao, Davide G. Marangon, Zhiliang Yuan

Abstract: The radiative linewidth of a two-level emitter (TLE) fundamentally limits the bandwidth available for quantum information processing. Despite its importance, no prior experiment has systematically examined how driving detuning affects the indistinguishability of photons scattered from a TLE - a parameter critical for photonic quantum computing. Here, we perform post-selective two-photon interferen… ▽ More The radiative linewidth of a two-level emitter (TLE) fundamentally limits the bandwidth available for quantum information processing. Despite its importance, no prior experiment has systematically examined how driving detuning affects the indistinguishability of photons scattered from a TLE - a parameter critical for photonic quantum computing. Here, we perform post-selective two-photon interference measurements between mutually detuned resonance fluorescence signals from an InAs quantum dot embedded in a micropillar cavity. At small mutual laser detunings (<=0.5GHz), the results are accurately described by the pure-state model [Nat. Commun. 16, 6453 (2025)], which treats all resonance-fluorescence photons as spontaneous emission. At larger detunings, we uncover an anomalous feature in the two-photon interference, where the normalised second-order correlation function under orthogonal polarisations yields g2_vert(0) < 0.5. △ Less

Submitted 6 November, 2025; v1 submitted 28 January, 2025; originally announced January 2025.

Comments: 13 pages, 11 figures

Journal ref: Phys. Rev. A 113, 023718 (2026)

New Constraints on Axion Mediated Dipole-Dipole Interactions

Authors: Zitong Xu, Xing Heng, Guoqing Tian, Di Gong, Lei Cong, Wei Ji, Dmitry Budker, Kai Wei

Abstract: The search for axions sits at the intersection of solving critical problems in fundamental physics, including the strong CP problem in QCD, uncovering the nature of dark matter, and understanding the origin of the universe's matter-antimatter asymmetry. The measurement of axion-mediated spin-dependent interactions offers a powerful approach for axion detection. However, it has long been restricted… ▽ More The search for axions sits at the intersection of solving critical problems in fundamental physics, including the strong CP problem in QCD, uncovering the nature of dark matter, and understanding the origin of the universe's matter-antimatter asymmetry. The measurement of axion-mediated spin-dependent interactions offers a powerful approach for axion detection. However, it has long been restricted to regions outside the 'axion window' due to a significant trade-off: the need to effectively suppress the magnetic leakage from highly polarized spin sources while simultaneously detecting sub-femtotesla level exotic physics signals at sub-decimeter-scale distances. In this work, we report new experimental results on axion-mediated exotic spin-spin interactions using an iron-shielded SmCo$_5$ spin source in combination with a specially designed self-compensation comagnetometer. Employing a composite shielding structure, we achieved a suppression of the magnetic field by up to $10^{11}$. This enabled us to establish new constraints on the coupling between electrons and neutrons, surpassing previous experimental limits by more than 10000 times within the axion window. Furthermore, we also set strongest constraints on the coupling between electrons and protons. The proposed method holds substantial potential not only for advancing the search for new physics beyond the Standard Model but also for enabling transformative applications in biological and chemical research. △ Less

Submitted 14 January, 2025; originally announced January 2025.

Spin-dependent exotic interactions

Authors: Lei Cong, Wei Ji, Pavel Fadeev, Filip Ficek, Min Jiang, Victor V. Flambaum, Haosen Guan, Derek F. Jackson Kimball, Mikhail G. Kozlov, Yevgeny V. Stadnik, Dmitry Budker

Abstract: Novel interactions beyond the four known fundamental forces in nature (electromagnetic, gravitational, strong and weak interactions), may arise due to "new physics" beyond the standard model, manifesting as a "fifth force". This review is focused on spin-dependent fifth forces mediated by exotic bosons such as spin-0 axions and axionlike particles and spin-1 Z' bosons, dark photons, or paraphotons… ▽ More Novel interactions beyond the four known fundamental forces in nature (electromagnetic, gravitational, strong and weak interactions), may arise due to "new physics" beyond the standard model, manifesting as a "fifth force". This review is focused on spin-dependent fifth forces mediated by exotic bosons such as spin-0 axions and axionlike particles and spin-1 Z' bosons, dark photons, or paraphotons. Many of these exotic bosons are candidates to explain the nature of dark matter and dark energy, and their interactions may violate fundamental symmetries. Spin-dependent interactions between fermions mediated by the exchange of exotic bosons have been investigated in a variety of experiments, particularly at the low-energy frontier. Experimental methods and tools used to search for exotic spin-dependent interactions, such as atomic comagnetometers, torsion balances, nitrogen-vacancy spin sensors, and precision atomic and molecular spectroscopy, are described. A complete set of interaction potentials, derived based on quantum field theory with minimal assumptions and characterized in terms of reduced coupling constants, are presented. A comprehensive summary of existing experimental and observational constraints on exotic spin-dependent interactions is given, illustrating the current research landscape and promising directions of further research. △ Less

Submitted 14 October, 2024; v1 submitted 28 August, 2024; originally announced August 2024.

Comments: 100 pages, 16 figures, Invited review submitted to Reviews of Modern Physics (RMP)

Journal ref: Rev. Mod. Phys. 97, 025005 (2025)

Ultralight dark matter detection with levitated ferromagnets

Authors: Saarik Kalia, Dmitry Budker, Derek F. Jackson Kimball, Wei Ji, Zhen Liu, Alexander O. Sushkov, Chris Timberlake, Hendrik Ulbricht, Andrea Vinante, Tao Wang

Abstract: Levitated ferromagnets act as ultraprecise magnetometers, which can exhibit high quality factors due to their excellent isolation from the environment. These instruments can be utilized in searches for ultralight dark matter candidates, such as axionlike dark matter or dark-photon dark matter. In addition to being sensitive to an axion-photon coupling or kinetic mixing, which produce physical magn… ▽ More Levitated ferromagnets act as ultraprecise magnetometers, which can exhibit high quality factors due to their excellent isolation from the environment. These instruments can be utilized in searches for ultralight dark matter candidates, such as axionlike dark matter or dark-photon dark matter. In addition to being sensitive to an axion-photon coupling or kinetic mixing, which produce physical magnetic fields, ferromagnets are also sensitive to the effective magnetic field (or "axion wind") produced by an axion-electron coupling. While the dynamics of a levitated ferromagnet in response to a DC magnetic field have been well studied, all of these couplings would produce AC fields. In this work, we study the response of a ferromagnet to an applied AC magnetic field and use these results to project their sensitivity to axion and dark-photon dark matter. We pay special attention to the direction of motion induced by an applied AC field, in particular, whether it precesses around the applied field (similar to an electron spin) or librates in the plane of the field (similar to a compass needle). We show that existing levitated ferromagnet setups can already have comparable sensitivity to an axion-electron coupling as comagnetometer or torsion balance experiments. In addition, future setups can become sensitive probes of axion-electron coupling, dark-photon kinetic mixing, and axion-photon coupling, for ultralight dark matter masses $m_\mathrm{DM}\lesssim\mathrm{feV}$. △ Less

Submitted 19 December, 2024; v1 submitted 27 August, 2024; originally announced August 2024.

Comments: 21 pages, 6 figures. Published version

Journal ref: Phys. Rev. D 110, 115029 (2024)

Topological defects of 2+1D systems from line excitations in 3+1D bulk

Authors: Wenjie Ji, Xie Chen

Abstract: The bulk-boundary correspondence of topological phases suggests strong connections between the topological features in a d+1-dimensional bulk and the potentially gapless theory on the (d-1)+1-dimensional boundary. In 2+1D topological phases, a direct correspondence can exist between anyonic excitations in the bulk and the topological point defects/primary fields in the boundary 1+1D conformal fiel… ▽ More The bulk-boundary correspondence of topological phases suggests strong connections between the topological features in a d+1-dimensional bulk and the potentially gapless theory on the (d-1)+1-dimensional boundary. In 2+1D topological phases, a direct correspondence can exist between anyonic excitations in the bulk and the topological point defects/primary fields in the boundary 1+1D conformal field theory. In this paper, we study how line excitations in 3+1D topological phases become line defects in the boundary 2+1D theory using the Topological Holography/Symmetry Topological Field Theory framework. We emphasize the importance of "descendent" line excitations and demonstrate in particular the effect of the Majorana chain defect: it leads to a distinct loop condensed gapped boundary state of the 3+1D fermionic Z2 topological order, and leaves signatures in the 2+1D Majorana-cone critical theory that describes the transition between the two types of loop condensed boundaries. Effects of non-invertible line excitations, such as Cheshire strings, are also discussed in bosonic 3+1D topological phases and the corresponding 2+1D critical points. △ Less

Submitted 2 July, 2024; originally announced July 2024.

Comments: 19 pages, 13 figures. Comments are welcome

Unusual charge density wave introduced by Janus structure in monolayer vanadium dichalcogenides

Authors: Ziqiang Xu, Yan Shao, Chun Huang, Genyu Hu, Shihao Hu, Zhi-Lin Li, Xiaoyu Hao, Yanhui Hou, Teng Zhang, Jin-An Shi, Chen Liu, Jia-Ou Wang, Wu Zhou, Jiadong Zhou, Wei Ji, Jingsi Qiao, Xu Wu, Hong-Jun Gao, Yeliang Wang

Abstract: As a fundamental structural feature, the symmetry of materials determines the exotic quantum properties in transition metal dichalcogenides (TMDs) with charge density wave (CDW). Breaking the inversion symmetry, the Janus structure, an artificially constructed lattice, provides an opportunity to tune the CDW states and the related properties. However, limited by the difficulties in atomic-level fa… ▽ More As a fundamental structural feature, the symmetry of materials determines the exotic quantum properties in transition metal dichalcogenides (TMDs) with charge density wave (CDW). Breaking the inversion symmetry, the Janus structure, an artificially constructed lattice, provides an opportunity to tune the CDW states and the related properties. However, limited by the difficulties in atomic-level fabrication and material stability, the experimental visualization of the CDW states in 2D TMDs with Janus structure is still rare. Here, using surface selenization of VTe2, we fabricated monolayer Janus VTeSe. With scanning tunneling microscopy, an unusual root13-root13 CDW state with threefold rotational symmetry breaking was observed and characterized. Combined with theoretical calculations, we find this CDW state can be attributed to the charge modulation in the Janus VTeSe, beyond the conventional electron-phonon coupling. Our findings provide a promising platform for studying the CDW states and artificially tuning the electronic properties toward the applications. △ Less

Submitted 17 June, 2024; originally announced June 2024.

Convert laser light into single photons via interference

Authors: Yanfeng Li, Manman Wang, Guoqi Huang, Li Liu, Wenyan Wang, Weijie Ji, Hanqing Liu, Xiangbin Su, Shulun Li, Deyan Dai, Xiangjun Shang, Haiqiao Ni, Zhichuan Niu, Chengyong Hu

Abstract: Laser light possesses perfect coherence, but cannot be attenuated to single photons via linear optics. An elegant route to convert laser light into single photons is based on photon blockade in a cavity with a single atom in the strong coupling regime. However, the single-photon purity achieved by this method remains relatively low. Here we propose an interference-based approach where laser light… ▽ More Laser light possesses perfect coherence, but cannot be attenuated to single photons via linear optics. An elegant route to convert laser light into single photons is based on photon blockade in a cavity with a single atom in the strong coupling regime. However, the single-photon purity achieved by this method remains relatively low. Here we propose an interference-based approach where laser light can be transformed into single photons by destructively interfering with a weak but super-bunched incoherent field emitted from a cavity coupling to a single quantum emitter. We demonstrate this idea by measuring the reflected light of a laser field which drives a double-sided optical microcavity containing a single artificial atom-quantum dot (QD) in the Purcell regime. The reflected light consists of a superposition of the driving field with the cavity output field. We achieve the second-order autocorrelation g2(0)=0.030+-0.002 and the two-photon interference visibility 94.3%+-0.2. By separating the coherent and incoherent fields in the reflected light, we observe that the incoherent field from the cavity exhibits super-bunching with g2(0)=41+-2 while the coherent field remains Poissonian statistics. By controlling the relative amplitude of coherent and incoherent fields, we verify that photon statistics of reflected light is tuneable from perfect anti-bunching to super-bunching in agreement with our predictions. Our results demonstrate photon statistics of light as a quantum interference phenomenon that a single QD can scatter two photons simultaneously at low driving fields in contrast to the common picture that a single two-level quantum emitter can only scatter (or absorb and emit) single photons. This work opens the door to tailoring photon statistics of laser light via cavity or waveguide quantum electrodynamics and interference. △ Less

Submitted 25 March, 2024; originally announced March 2024.

Comments: Comments are welcome

Journal ref: Optica 12, 1838-1844(2025)

Levitated ferromagnetic magnetometer with energy resolution well below $\hbar$

Authors: Felix Ahrens, Wei Ji, Dmitry Budker, Chris Timberlake, Hendrik Ulbricht, Andrea Vinante

Abstract: A quantum limit on the measurement of magnetic field has been recently pointed out, stating that the so-called Energy Resolution $E_\mathrm{R}$ is bounded to $E_\mathrm{R} \gtrsim \hbar$. This limit holds indeed true for the vast majority of existing quantum magnetometers, including SQUIDs, solid state spins and optically pumped atomic magnetometers. However, it can be surpassed by highly correlat… ▽ More A quantum limit on the measurement of magnetic field has been recently pointed out, stating that the so-called Energy Resolution $E_\mathrm{R}$ is bounded to $E_\mathrm{R} \gtrsim \hbar$. This limit holds indeed true for the vast majority of existing quantum magnetometers, including SQUIDs, solid state spins and optically pumped atomic magnetometers. However, it can be surpassed by highly correlated spin systems, as recently demonstrated with a single-domain spinor Bose-Einstein Condensate. Here we show that similar and potentially much better resolution can be achieved with a hard ferromagnet levitated above a superconductor at cryogenic temperature. We demonstrate $E_\mathrm{R}=\left( 0.064 \pm 0.010 \right) \, \hbar$ and anticipate that $E_\mathrm{R}<10^{-3} \, \hbar$ is within reach with near-future improvements. This finding opens the way to new applications in condensed matter, biophysics and fundamental science. In particular, we propose an experiment to search for axionlike dark matter and project a sensitivity orders of magnitude better than in previous searches. △ Less

Submitted 8 January, 2024; originally announced January 2024.

Comments: 26 pages, 8 figures (including Supplementary Information)

Correlated sensing with a solid-state quantum multi-sensor system for atomic-scale structural analysis

Authors: Wentao Ji, Zhaoxin Liu, Yuhang Guo, Zhihao Hu, Jingyang Zhou, Siheng Dai, Yu Chen, Pei Yu, Mengqi Wang, Kangwei Xia, Fazhan Shi, Ya Wang, Jiangfeng Du

Abstract: Developing superior quantum sensing strategies ranging from ultra-high precision measurement to complex structural analysis is at the heart of quantum technologies. While strategies using quantum resources, such as entanglement among sensors, to enhance the sensing precision have been abundantly demonstrated, the signal correlation among quantum sensors is rarely exploited. Here we develop a novel… ▽ More Developing superior quantum sensing strategies ranging from ultra-high precision measurement to complex structural analysis is at the heart of quantum technologies. While strategies using quantum resources, such as entanglement among sensors, to enhance the sensing precision have been abundantly demonstrated, the signal correlation among quantum sensors is rarely exploited. Here we develop a novel sensing paradigm exploiting the signal correlation among multiple quantum sensors to resolve overlapping signals from multiple targets that individual sensors can't resolve and complex structural construction struggles with. With three nitrogen-vacancy centers as a quantum electrometer system, we demonstrate this multi-sensor paradigm by resolving individual defects' fluctuating electric fields from ensemble signals. We image the three-dimensional distribution of 16 dark electronic point-defects in diamond with accuracy approaching 1.7 nm via a GPS-like localization method. Furthermore, we obtain the real-time charge dynamics of individual point defects and visualize how the dynamics induce the well-known optical spectral diffusion. The multi-sensor paradigm extends the quantum sensing toolbox and offers new possibilities for structural analysis. △ Less

Submitted 4 January, 2024; originally announced January 2024.

Coherence in resonance fluorescence

Authors: Xu-Jie Wang, Guoqi Huang, Ming-Yang Li, Yuan-Zhuo Wang, Li Liu, Bang Wu, Hanqing Liu, Haiqiao Ni, Zhichuan Niu, Weijie Ji, Rongzhen Jiao, Hua-Lei Yin, Zhiliang Yuan

Abstract: Resonance fluorescence of a two-level emitter displays persistently anti-bunching irrespective of the excitation intensity, but inherits the driving laser's linewidth under weak monochromatic excitation. These properties are commonly explained in terms of two disjoined pictures, i.e., the emitter's single photon saturation or passively scattering light. Here, we propose a unified model that treats… ▽ More Resonance fluorescence of a two-level emitter displays persistently anti-bunching irrespective of the excitation intensity, but inherits the driving laser's linewidth under weak monochromatic excitation. These properties are commonly explained in terms of two disjoined pictures, i.e., the emitter's single photon saturation or passively scattering light. Here, we propose a unified model that treats all fluorescence photons as spontaneous emission, one at a time, and can explain simultaneously both the spectral and correlation properties of the emission. We theoretically derive the excitation power dependencies, measurable at the single-photon incidence level, of the first-order coherence of the whole resonance fluorescence and super-bunching of the spectrally filtered, followed by experimental confirmation on a semiconductor quantum dot micro-pillar device. Furthermore, our model explains peculiar coincidence bunching observed in phase-dependent two-photon interference experiments. Our work provides an intuitive understanding of coherent light-matter interaction and may stimulate new applications. △ Less

Submitted 28 January, 2025; v1 submitted 21 December, 2023; originally announced December 2023.

Comments: Theoretical derivation of Eq. (1) is included

Journal ref: Nature Communications 16, 6453 (2025)

Constraining Ultralight Dark Matter through an Accelerated Resonant Search

Authors: Zitong Xu, Xiaolin Ma, Kai Wei, Yuxuan He, Xing Heng, Xiaofei Huang, Tengyu Ai, Jian Liao, Wei Ji, Jia Liu, Xiao-Ping Wang, Dmitry Budker

Abstract: Experiments aimed at detecting ultralight dark matter typically rely on resonant effects, which are sensitive to the dark matter mass that matches the resonance frequency. In this study, we investigate the nucleon couplings of ultralight axion dark matter using a magnetometer operating in a nuclear magnetic resonance (NMR) mode. Our approach involves the use of a $^{21}$Ne spin-based sensor, which… ▽ More Experiments aimed at detecting ultralight dark matter typically rely on resonant effects, which are sensitive to the dark matter mass that matches the resonance frequency. In this study, we investigate the nucleon couplings of ultralight axion dark matter using a magnetometer operating in a nuclear magnetic resonance (NMR) mode. Our approach involves the use of a $^{21}$Ne spin-based sensor, which features the lowest nuclear magnetic moment among noble-gas spins. This configuration allows us to achieve an ultrahigh sensitivity of 0.73 fT/Hz$^{1/2}$ at around 5 Hz, corresponding to energy resolution of approximately 1.5$\times 10^{-23}\,\rm{eV/Hz^{1/2}}$. Our analysis reveals that under certain conditions it is beneficial to scan the frequency with steps significantly larger than the resonance width. The analytical results are in agreement with experimental data and the scan strategy is potentially applicable to other resonant searches. Further, our study establishes stringent constraints on axion-like particles (ALP) in the 4.5--15.5 Hz Compton-frequency range coupling to neutrons and protons, improving on prior work by several-fold. Within a band around 4.6--6.6 Hz and around 7.5 Hz, our laboratory findings surpass astrophysical limits derived from neutron-star cooling. Hence, we demonstrate an accelerated resonance search for ultralight dark matter, achieving an approximately 30-fold increase in scanning step while maintaining competitive sensitivity. △ Less

Submitted 11 July, 2024; v1 submitted 28 September, 2023; originally announced September 2023.

Comments: 13 pages, 11 figures, accepted by Communications Physics

Journal ref: Commun.Phys. 7 (2024) 1, 226

Dark Matter Search with a Resonantly-Coupled Hybrid Spin System

Authors: Kai Wei, Zitong Xu, Yuxuan He, Xiaolin Ma, Xing Heng, Xiaofei Huang, Wei Quan, Wei Ji, Jia Liu, Xiao-Ping Wang, Dmitry Budker, Jiancheng Fang

Abstract: Recent advances in tabletop quantum sensor technology have enabled searches for nongravitational interactions of dark matter (DM). Traditional axion DM experiments rely on sharp resonance, resulting in extensive scanning time to cover a wide mass range. In this work, we present a broadband approach in an alkali-${}^{21}$Ne spin system. We identify two distinct hybrid spin-coupled regimes: a self-c… ▽ More Recent advances in tabletop quantum sensor technology have enabled searches for nongravitational interactions of dark matter (DM). Traditional axion DM experiments rely on sharp resonance, resulting in extensive scanning time to cover a wide mass range. In this work, we present a broadband approach in an alkali-${}^{21}$Ne spin system. We identify two distinct hybrid spin-coupled regimes: a self-compensation (SC) regime at low frequencies and a hybrid spin resonance (HSR) regime at higher frequencies. By utilizing these two distinct regimes, we significantly enhance the bandwidth of ${}^{21}$Ne nuclear spin compared to conventional nuclear magnetic resonance, while maintaining competitive sensitivity. We present a comprehensive broadband search for axion-like dark matter, covering 5 orders of magnitude of Compton frequencies range within $[10^{-2}, \, 10^3]$ Hz. We set new constraints on the axion dark matter interactions with neutrons and protons, accounting for the effects of DM stochasticity. For the axion-neutron coupling, our results reach a low value of $|g_{ann}|\le 3\times 10^{-10}$ in the frequency range $[2\times 10^{-2}, \, 4]$ Hz surpassing astrophysical limits and providing the strongest laboratory constraints in the $[10, \, 100]$ Hz range. For the axion-proton coupling, we offer the best terrestrial constraints for the frequency ranges $[2\times 10^{-2}, \, 5]$Hz and $[16, \, 7\times 10^{2}]$ Hz. △ Less

Submitted 9 April, 2025; v1 submitted 13 June, 2023; originally announced June 2023.

Comments: v2: matched to journal version, 16 pages, 7 figures

Journal ref: Rept.Prog.Phys. 88 (2025) 5, 057801

Ultra-narrowband interference circuits enable low-noise and high-rate photon counting for InGaAs/InP avalanche photodiodes

Authors: Yuanbin Fan, Tingting Shi, Weijie Ji, Lai Zhou, Yang Ji, Zhiliang Yuan

Abstract: Afterpulsing noise in InGaAs/InP single photon avalanche photodiodes (APDs) is caused by carrier trapping and can be suppressed successfully through limiting the avalanche charge via sub-nanosecond gating. Detection of faint avalanches requires an electronic circuit that is able to effectively remove the gate-induced capacitive response while keeping photon signals intact. Here we demonstrate a no… ▽ More Afterpulsing noise in InGaAs/InP single photon avalanche photodiodes (APDs) is caused by carrier trapping and can be suppressed successfully through limiting the avalanche charge via sub-nanosecond gating. Detection of faint avalanches requires an electronic circuit that is able to effectively remove the gate-induced capacitive response while keeping photon signals intact. Here we demonstrate a novel ultra-narrowband interference circuit (UNIC) that can reject the capacitive response by up to 80 dB per stage with little distortion to avalanche signals. Cascading two UNIC's in a readout circuit, we were able to enable high count rate of up to 700 MC/s and low afterpulsing of 0.5 % at a detection efficiency of 25.3 % for 1.25 GHz sinusoidally gated InGaAs/InP APDs. At -30 degree C, we measured 1 % afterpulsing at a detection efficiency of 21.2 %. △ Less

Submitted 14 February, 2023; v1 submitted 4 January, 2023; originally announced January 2023.

Journal ref: Optics Express 2023

Boundary states of Three Dimensional Topological Order and the Deconfined Quantum Critical Point

Authors: Wenjie Ji, Nathanan Tantivasadakarn, Cenke Xu

Abstract: We study the boundary states of the archetypal three-dimensional topological order, i.e. the three-dimensional $\mathbb{Z}_2$ toric code. There are three distinct elementary types of boundary states that we will consider in this work. In the phase diagram that includes the three elementary boundaries there may exist a multi-critical point, which is captured by the so-called deconfined quantum crit… ▽ More We study the boundary states of the archetypal three-dimensional topological order, i.e. the three-dimensional $\mathbb{Z}_2$ toric code. There are three distinct elementary types of boundary states that we will consider in this work. In the phase diagram that includes the three elementary boundaries there may exist a multi-critical point, which is captured by the so-called deconfined quantum critical point (DQCP) with an "easy-axis" anisotropy. Moreover, there is an emergent $\mathbb{Z}_{2,\text{d}}$ symmetry that swaps two of the boundary types, and it becomes part of the global symmetry of the DQCP. The emergent $\mathbb{Z}_{2,\text{d}}$ symmetry on the boundary is originated from a type of surface defect in the bulk. We further find a gapped boundary with a surface topological order that is invariant under the emergent symmetry. △ Less

Submitted 11 December, 2023; v1 submitted 19 December, 2022; originally announced December 2022.

Comments: 11 pages, 8 figures. Comments are welcome

Journal ref: SciPost Phys. 15, 231 (2023)

Ultrasensitive atomic comagnetometer with enhanced nuclear spin coherence

Authors: Kai Wei, Tian Zhao, Xiujie Fang, Zitong Xu, Chang Liu, Qian Cao, Arne Wickenbrock, Yanhui Hu, Wei Ji, Dmitry Budker

Abstract: Achieving high energy resolution in spin systems is important for fundamental physics research and precision measurements, with alkali-noble-gas comagnetometers being among the best available sensors. We found a new relaxation mechanism in such devices, the gradient of the Fermi-contact-interaction field that dominates the relaxation of hyperpolarized nuclear spins. We report on precise control ov… ▽ More Achieving high energy resolution in spin systems is important for fundamental physics research and precision measurements, with alkali-noble-gas comagnetometers being among the best available sensors. We found a new relaxation mechanism in such devices, the gradient of the Fermi-contact-interaction field that dominates the relaxation of hyperpolarized nuclear spins. We report on precise control over spin distribution, demonstrating a tenfold increase of nuclear spin hyperpolarization and transverse coherence time with optimal hybrid optical pumping. Operating in the self-compensation regime, our $^{21}$Ne-Rb-K comagnetometer achieves an ultrahigh inertial rotation sensitivity of $3\times10^{-8}$\,rad/s/Hz$^{1/2}$ in the frequency range from 0.2 to 1.0 Hz, which is equivalent to the energy resolution of $3.1\times 10^{-23}$\,eV/Hz$^{1/2}$. We propose to use this comagnetometer to search for exotic spin-dependent interactions involving proton and neutron spins. The projected sensitivity surpasses the previous experimental and astrophysical limits by more than four orders of magnitude. △ Less

Submitted 17 October, 2022; originally announced October 2022.

Decoding Measurement-Prepared Quantum Phases and Transitions: from Ising model to gauge theory, and beyond

Authors: Jong Yeon Lee, Wenjie Ji, Zhen Bi, Matthew P. A. Fisher

Abstract: Measurements allow efficient preparation of interesting quantum many-body states with long-range entanglement, conditioned on additional transformations based on measurement outcomes. Here, we demonstrate that the so-called conformal quantum critical points (CQCP) can be obtained by performing general single-site measurements in an appropriate basis on the cluster states in $d\geq2$. The equal-tim… ▽ More Measurements allow efficient preparation of interesting quantum many-body states with long-range entanglement, conditioned on additional transformations based on measurement outcomes. Here, we demonstrate that the so-called conformal quantum critical points (CQCP) can be obtained by performing general single-site measurements in an appropriate basis on the cluster states in $d\geq2$. The equal-time correlators of the said states are described by correlation functions of certain $d$-dimensional classical models at finite temperatures and feature spatial conformal invariance. This establishes an exact correspondence between the measurement-prepared critical states and conformal field theories of a range of critical spin models, including familiar Ising models and gauge theories. Furthermore, by mapping the long-range entanglement structure of measured quantum states into the correlations of the corresponding thermal spin model, we rigorously establish the stability condition of the long-range entanglement in the measurement-prepared quantum states deviating from the ideal setting. Most importantly, we describe protocols to decode the resulting quantum phases and transitions without post-selection, thus transferring the exponential measurement complexity to a polynomial classical computation. Therefore, our findings suggest a novel mechanism in which a quantum critical wavefunction emerges, providing new practical ways to study quantum phases and conformal quantum critical points. △ Less

Submitted 6 September, 2022; v1 submitted 24 August, 2022; originally announced August 2022.

Comments: 37 pages, 11 figures

Towards Non-Invertible Anomalies from Generalized Ising Models

Authors: Shang Liu, Wenjie Ji

Abstract: We present a general approach to the bulk-boundary correspondence of noninvertible topological phases, including both topological and fracton orders. This is achieved by a novel bulk construction protocol where solvable $(d+1)$-dimensional bulk models with noninvertible topology are constructed from the so-called generalized Ising (GI) models in $d$ dimensions. The GI models can then terminate on… ▽ More We present a general approach to the bulk-boundary correspondence of noninvertible topological phases, including both topological and fracton orders. This is achieved by a novel bulk construction protocol where solvable $(d+1)$-dimensional bulk models with noninvertible topology are constructed from the so-called generalized Ising (GI) models in $d$ dimensions. The GI models can then terminate on the boundaries of the bulk models. The construction generates abundant examples, including not only prototype ones such as $Z_2$ toric code models in any dimensions no less than two, and the X-cube fracton model, but also more diverse ones such as the $Z_2\times Z_2$ topological order, the 4d $Z_2$ topological order with pure-loop excitations, etc. The boundary of the solvable model is potentially anomalous and corresponds to precisely only sectors of the GI model that host certain total symmetry charges and/or satisfy certain boundary conditions. We derive a concrete condition for such bulk-boundary correspondence. The condition is violated only when the bulk model is either trivial or fracton ordered. A generalized notion of Kramers-Wannier duality plays an important role in the construction. Also, utilizing the duality, we find an example where a single anomalous theory can be realized on the boundaries of two distinct bulk fracton models, a phenomenon not expected in the case of topological orders. More generally, topological orders may also be generated starting with lattice models beyond the GI models, such as those with symmetry protected topological orders, through a variant bulk construction, which we provide in an appendix. △ Less

Submitted 30 January, 2024; v1 submitted 18 August, 2022; originally announced August 2022.

Comments: published version, 17+18 pages, 5 figures

Journal ref: SciPost Phys. 15, 150 (2023)

Constraints on Spin-Spin-Velocity-Dependent Interaction

Authors: Wei Ji, Weipeng Li, Pavel Fadeev, Filip Ficek, Jianan Qin, Kai Wei, Yong-Chun Liu, Dmitry Budker

Abstract: The existence of exotic spin-dependent forces may shine light on new physics beyond the Standard Model. We utilize two iron shielded SmCo$_5$ electron-spin sources and two optically pumped magnetometers to search for exotic long-range spin-spin-velocity-dependent force. The orientations of spin sources and magnetometers are optimized such that the exotic force is enhanced and common-mode noise is… ▽ More The existence of exotic spin-dependent forces may shine light on new physics beyond the Standard Model. We utilize two iron shielded SmCo$_5$ electron-spin sources and two optically pumped magnetometers to search for exotic long-range spin-spin-velocity-dependent force. The orientations of spin sources and magnetometers are optimized such that the exotic force is enhanced and common-mode noise is effectively subtracted. We set direct limit on proton-electron interaction in the force range from 1\,cm to 1\,km. Our experiment represents more than ten orders of magnitude improvement than previous works. △ Less

Submitted 21 November, 2022; v1 submitted 1 August, 2022; originally announced August 2022.

Layer-dependent interlayer antiferromagnetic spin reorientation in air-stable semiconductor CrSBr

Authors: Chen Ye, Cong Wang, Qiong Wu, Sheng Liu, Jiayuan Zhou, Guopeng Wang, Aljoscha Soll, Zdenek Sofer, Ming Yue, Xue Liu, Mingliang Tian, Qihua Xiong, Wei Ji, X. Renshaw Wang

Abstract: Magnetic van der Waals (vdW) materials offer a fantastic platform to investigate and exploit rich spin configurations stabilized in reduced dimensions. One tantalizing magnetic order is the interlayer antiferromagnetism in A-type vdW antiferromagnet, which may be effectively modified by the magnetic field, stacking order and thickness scaling. However, atomically revealing the interlayer spin orie… ▽ More Magnetic van der Waals (vdW) materials offer a fantastic platform to investigate and exploit rich spin configurations stabilized in reduced dimensions. One tantalizing magnetic order is the interlayer antiferromagnetism in A-type vdW antiferromagnet, which may be effectively modified by the magnetic field, stacking order and thickness scaling. However, atomically revealing the interlayer spin orientation in the vdW antiferromagnet is highly challenging, because most of the material candidates exhibit an insulating ground state or instability in ambient conditions. Here, we report the layer-dependent interlayer antiferromagnetic reorientation in air-stable semiconductor CrSBr using magnetotransport characterization and first-principles calculations. We reveal a pronounced odd-even layer effect of interlayer reorientation, which originates from the competitions among interlayer exchange, magnetic anisotropy energy and extra Zeeman energy of uncompensated magnetization. Furthermore, we quantitatively constructed the layer-dependent magnetic phase diagram with the help of a linear-chain model. Our work uncovers the layer-dependent interlayer antiferromagnetic reorientation engineered by magnetic field in the air-stable semiconductor, which could contribute to future vdW spintronic devices. △ Less

Submitted 20 May, 2022; v1 submitted 11 May, 2022; originally announced May 2022.

SAPPHIRE: Search for exotic parity-violation interactions with quantum spin amplifiers

Authors: Yuanhong Wang, Ying Huang, Chang Guo, Min Jiang, Xiang Kang, Haowen Su, Yushu Qin, Wei Ji, Dongdong Hu, Xinhua Peng, Dmitry Budker

Abstract: Quantum sensing provides sensitive tabletop tools to search for exotic spin-dependent interactions beyond the Standard Model, which has attracted great attention in theories and experiments. Here we develop a technique based on quantum Spin Amplifier for Particle PHysIcs REsearch (SAPPHIRE) to resonantly search for exotic interactions, specifically parity-odd spin-spin interactions. The present te… ▽ More Quantum sensing provides sensitive tabletop tools to search for exotic spin-dependent interactions beyond the Standard Model, which has attracted great attention in theories and experiments. Here we develop a technique based on quantum Spin Amplifier for Particle PHysIcs REsearch (SAPPHIRE) to resonantly search for exotic interactions, specifically parity-odd spin-spin interactions. The present technique effectively amplifies the pseudomagnetic field generated by exotic interactions by a factor of about 200 while being insensitive to spurious external magnetic fields. Our studies, using such a quantum amplification technique, open the doors to exploring the parity-violation interactions mediated by Z' bosons in the challenging parameter space (force range between 3 mm and 0.1 km) and set the most stringent constraints on Z'-mediated electron-neutron couplings, significantly improving previous limits by up to five orders of magnitude. Moreover, our bounds on Z'-mediated couplings between nucleons reaches into a hitherto unexplored parameter space (force range below 1 m), complementing the existing astrophysical and laboratory studies. △ Less

Submitted 15 May, 2022; originally announced May 2022.

Comments: 8 pages, 5 figures

Journal ref: Science advances. 2023 Jan 6;9(1):eade0353

Spin Quantum Heat Engine Quantified by Quantum Steering

Authors: Wentao Ji, Zihua Chai, Mengqi Wang, Yuhang Guo, Xing Rong, Fazhan Shi, Changliang Ren, Ya Wang, Jiangfeng Du

Abstract: Following the rising interest in quantum information science, the extension of a heat engine to the quantum regime by exploring microscopic quantum systems has seen a boom of interest in the last decade. Although quantum coherence in the quantum system of the working medium has been investigated to play a nontrivial role, a complete understanding of the intrinsic quantum advantage of quantum heat… ▽ More Following the rising interest in quantum information science, the extension of a heat engine to the quantum regime by exploring microscopic quantum systems has seen a boom of interest in the last decade. Although quantum coherence in the quantum system of the working medium has been investigated to play a nontrivial role, a complete understanding of the intrinsic quantum advantage of quantum heat engines remains elusive. We experimentally demonstrate that the quantum correlation between the working medium and the thermal bath is critical for the quantum advantage of a quantum Szilard engine, where quantum coherence in the working medium is naturally excluded. By quantifying the non-classical correlation through quantum steering, we reveal that the heat engine is quantum when the demon can truly steer the working medium. The average work obtained by taking different ways of work extraction on the working medium can be used to verify the real quantum Szilard engine. △ Less

Submitted 4 February, 2022; originally announced February 2022.

Non-Abelian Hybrid Fracton Orders

Authors: Nathanan Tantivasadakarn, Wenjie Ji, Sagar Vijay

Abstract: We introduce lattice gauge theories which describe three-dimensional, gapped quantum phases exhibiting the phenomenology of both conventional three-dimensional topological orders and fracton orders, starting from a finite group $G$, a choice of an Abelian normal subgroup $N$, and a choice of foliation structure. These hybrid fracton orders -- examples of which were introduced in arXiv:2102.09555 -… ▽ More We introduce lattice gauge theories which describe three-dimensional, gapped quantum phases exhibiting the phenomenology of both conventional three-dimensional topological orders and fracton orders, starting from a finite group $G$, a choice of an Abelian normal subgroup $N$, and a choice of foliation structure. These hybrid fracton orders -- examples of which were introduced in arXiv:2102.09555 -- can also host immobile, point-like excitations that are non-Abelian, and therefore give rise to a protected degeneracy. We construct solvable lattice models for these orders which interpolate between a conventional, three-dimensional $G$ gauge theory and a pure fracton order, by varying the choice of normal subgroup $N$. We demonstrate that certain universal data of the topological excitations and their mobilities are directly related to the choice of $G$ and $N$, and also present complementary perspectives on these orders: certain orders may be obtained by gauging a global symmetry which enriches a particular fracton order, by either fractionalizing on or permuting the excitations with restricted mobility, while certain hybrid orders can be obtained by condensing excitations in a stack of initially decoupled, two-dimensional topological orders. △ Less

Submitted 11 September, 2021; v1 submitted 7 June, 2021; originally announced June 2021.

Comments: 17+11 pages, 4 figures, 7 tables. Added discussion on planons supported in multiple layers. Published version

Journal ref: Phys. Rev. B 104, 115117 (2021)

Resonant Quantum Principal Component Analysis

Authors: Zhaokai Li, Zihua Chai, Yuhang Guo, Wentao Ji, Mengqi Wang, Fazhan Shi, Ya Wang, Seth Lloyd, Jiangfeng Du

Abstract: Principal component analysis has been widely adopted to reduce the dimension of data while preserving the information. The quantum version of PCA (qPCA) can be used to analyze an unknown low-rank density matrix by rapidly revealing the principal components of it, i.e. the eigenvectors of the density matrix with largest eigenvalues. However, due to the substantial resource requirement, its experime… ▽ More Principal component analysis has been widely adopted to reduce the dimension of data while preserving the information. The quantum version of PCA (qPCA) can be used to analyze an unknown low-rank density matrix by rapidly revealing the principal components of it, i.e. the eigenvectors of the density matrix with largest eigenvalues. However, due to the substantial resource requirement, its experimental implementation remains challenging. Here, we develop a resonant analysis algorithm with the minimal resource for ancillary qubits, in which only one frequency scanning probe qubit is required to extract the principal components. In the experiment, we demonstrate the distillation of the first principal component of a 4$\times$4 density matrix, with the efficiency of 86.0% and fidelity of 0.90. This work shows the speed-up ability of quantum algorithm in dimension reduction of data and thus could be used as part of quantum artificial intelligence algorithms in the future. △ Less

Submitted 25 January, 2022; v1 submitted 6 April, 2021; originally announced April 2021.

Comments: 10 pages, 7 figures

Journal ref: Science Advance 7, 34 abg2589 (2021)

Search for exotic spin-dependent interactions with a spin-based amplifier

Authors: Haowen Su, Yuanhong Wang, Min Jiang, Wei Ji, Pavel Fadeev, Dongdong Hu, Xinhua Peng, Dmitry Budker

Abstract: Development of new techniques to search for particles beyond the standard model is crucial for understanding the ultraviolet completion of particle physics. Several hypothetical particles are predicted to mediate exotic spin-dependent interactions between particles of the standard model that may be accessible to laboratory experiments. However, laboratory searches are mostly conducted for static s… ▽ More Development of new techniques to search for particles beyond the standard model is crucial for understanding the ultraviolet completion of particle physics. Several hypothetical particles are predicted to mediate exotic spin-dependent interactions between particles of the standard model that may be accessible to laboratory experiments. However, laboratory searches are mostly conducted for static spin-dependent interactions, with only a few experiments so far addressing spin- and velocity-dependent interactions. Here, we demonstrate a search for exotic spin- and velocity-dependent interactions with a spin-based amplifier. Our technique makes use of hyperpolarized nuclear spins as a pre-amplifier to enhance the effect of pseudo-magnetic field produced by exotic interactions by an amplification factor of > 100. Using such a spin-based amplifier, we establish constraints on the spin- and velocity-dependent interactions between polarized and unpolarized nucleons in the force range of 0.03-100 m. Our limits represent at least two orders of magnitude improvement compared to previous experiments. The established technique can be further extended to investigate other exotic spin-dependent interactions. △ Less

Submitted 28 March, 2021; originally announced March 2021.

Comments: 7 pages, 4 figures

Journal ref: Science advances. 2021 Nov 17;7(47):eabi9535

Hybrid Fracton Phases: Parent Orders for Liquid and Non-Liquid Quantum Phases

Authors: Nathanan Tantivasadakarn, Wenjie Ji, Sagar Vijay

Abstract: We introduce hybrid fracton orders: three-dimensional gapped quantum phases that exhibit the phenomenology of both conventional three-dimensional topological orders and fracton orders. Hybrid fracton orders host both (i) mobile topological quasiparticles and loop excitations, as well as (ii) point-like topological excitations with restricted mobility, with non-trivial fusion rules and mutual braid… ▽ More We introduce hybrid fracton orders: three-dimensional gapped quantum phases that exhibit the phenomenology of both conventional three-dimensional topological orders and fracton orders. Hybrid fracton orders host both (i) mobile topological quasiparticles and loop excitations, as well as (ii) point-like topological excitations with restricted mobility, with non-trivial fusion rules and mutual braiding statistics between the two sets of excitations. Furthermore, hybrid fracton phases can realize either conventional three-dimensional topological orders or fracton orders after undergoing a phase transition driven by the condensation of certain gapped excitations. Therefore, they serve as parent orders for both long-range-entangled quantum liquid and non-liquid phases. We study the detailed properties of hybrid fracton phases through exactly solvable models in which the resulting orders hybridize a three-dimensional $\mathbb Z_2$ topological order with (i) the X-Cube fracton order, or (ii) Haah's code. The hybrid orders presented here can also be understood as the deconfined phase of a gauge theory whose gauge group is given by an Abelian global symmetry $G$ and subsystem symmetries of a normal subgroup $N$ along lower-dimensional sub-regions. A further generalization of this construction to non-Abelian gauge groups is presented in arXiv:2106.03842. △ Less

Submitted 24 June, 2021; v1 submitted 18 February, 2021; originally announced February 2021.

Comments: 19+10 pages, 17 figures, 5 tables. Published version

Journal ref: Phys. Rev. B 103, 245136 (2021)

Parallel optically detected magnetic resonance spectrometer for dozens of single nitrogen-vacancy centers using laser-spot lattice

Authors: Mingcheng Cai, Zhongzhi Guo, Fazhan Shi, Chunxing Li, Mengqi Wang, Wei Ji, Pengfei Wang, Jiangfeng Du

Abstract: We develop a parallel optically detected magnetic resonance (PODMR) spectrometer to address, manipulate and read out an array of single nitrogen-vacancy (NV) centers in diamond in parallel. In this spectrometer, we use an array of micro-lens to generate 20 * 20 laser-spot lattice (LSL) on the objective focal plane, and then align the LSL with an array of single NV centers. The quantum states of NV… ▽ More We develop a parallel optically detected magnetic resonance (PODMR) spectrometer to address, manipulate and read out an array of single nitrogen-vacancy (NV) centers in diamond in parallel. In this spectrometer, we use an array of micro-lens to generate 20 * 20 laser-spot lattice (LSL) on the objective focal plane, and then align the LSL with an array of single NV centers. The quantum states of NV centers are manipulated by a uniform microwave field from a Ω-shape coplanar coil. As an experimental demonstration, we observe 80 NV centers in the field of view. Among them, magnetic resonance (MR) spectrums and Rabi oscillations of 18 NV centers along the external magnetic field are measured in parallel. These results can be directly used to realize parallel quantum sensing and multiple times speedup compared with the confocal technique. Regarding the nanoscale MR technique, PODMR will be crucial for high throughput single molecular MR spectrum and imaging. △ Less

Submitted 6 November, 2020; originally announced November 2020.

Comments: 15 pages, 9 figures

High-fidelity single-shot readout of single electron spin in diamond with spin-to-charge conversion

Authors: Qi Zhang, Yuhang Guo, Wentao Ji, Mengqi Wang, Jun Yin, Fei Kong, Yiheng Lin, Chunming Yin, Fazhan Shi, Ya Wang, Jiangfeng Du

Abstract: High fidelity single-shot readout of qubits is a crucial component for fault-tolerant quantum computing and scalable quantum networks. In recent years, the nitrogen-vacancy (NV) center in diamond has risen as a leading platform for the above applications. The current single-shot readout of the NV electron spin relies on resonance fluorescence method at cryogenic temperature. However, the the spin-… ▽ More High fidelity single-shot readout of qubits is a crucial component for fault-tolerant quantum computing and scalable quantum networks. In recent years, the nitrogen-vacancy (NV) center in diamond has risen as a leading platform for the above applications. The current single-shot readout of the NV electron spin relies on resonance fluorescence method at cryogenic temperature. However, the the spin-flip process interrupts the optical cycling transition, therefore, limits the readout fidelity. Here, we introduce a spin-to-charge conversion method assisted by near-infrared (NIR) light to suppress the spin-flip error. This method leverages high spin-selectivity of cryogenic resonance excitation and high flexibility of photonionization. We achieve an overall fidelity $>$ 95% for the single-shot readout of an NV center electron spin in the presence of high strain and fast spin-flip process. With further improvements, this technique has the potential to achieve spin readout fidelity exceeding the fault-tolerant threshold, and may also find applications on integrated optoelectronic devices. △ Less

Submitted 29 September, 2020; originally announced September 2020.

Journal ref: Nature Communications 12, 1529 (2021)

Quantum dynamical characterization and simulation of topological phases with high-order band inversion surfaces

Authors: Xiang-Long Yu, Wentao Ji, Lin Zhang, Ya Wang, Jiansheng Wu, Xiong-Jun Liu

Abstract: How to characterize topological quantum phases is a fundamental issue in the broad field of topological matter. From a dimension reduction approach, we propose the concept of high-order band inversion surfaces (BISs) which enable the optimal schemes to characterize equilibrium topological phases by far-from-equilibrium quantum dynamics, and further report the experimental simulation. We show that… ▽ More How to characterize topological quantum phases is a fundamental issue in the broad field of topological matter. From a dimension reduction approach, we propose the concept of high-order band inversion surfaces (BISs) which enable the optimal schemes to characterize equilibrium topological phases by far-from-equilibrium quantum dynamics, and further report the experimental simulation. We show that characterization of a d-dimensional (dD) topological phase can be reduced to lower-dimensional topological invariants in the high-order BISs, of which the nth-order BIS is a (d-n)D interface in momentum space. In quenching the system from trivial phase to topological regime, we unveil a high-order dynamical bulk-surface correspondence that the quantum dynamics exhibits nontrivial topological pattern in arbitrary nth-order BISs, which universally corresponds to and so characterizes the equilibrium topological phase of the post-quench Hamiltonian. This high-order dynamical bulk-surface correspondence provides new and optimal dynamical schemes with fundamental advantages to simulate and detect topological states, in which through the highest-order BISs that are of zero dimension, the detection of topological phase relies on only minimal measurements. We experimentally build up a quantum simulator with spin qubits to investigate a 3D chiral topological insulator through emulating each momentum one by one and measure the high-order dynamical bulk-surface correspondence, with the advantages of topological characterization via highest-order BISs being demonstrated. △ Less

Submitted 15 May, 2021; v1 submitted 30 April, 2020; originally announced April 2020.

Journal ref: PRX Quantum 2, 020320 (2021)

Quantum simulation for three-dimensional chiral topological insulator

Authors: Wentao Ji, Lin Zhang, Mengqi Wang, Long Zhang, Yuhang Guo, Zihua Chai, Xing Rong, Fazhan Shi, Xiong-Jun Liu, Ya Wang, Jiangfeng Du

Abstract: Quantum simulation, as a state-of-art technique, provides the powerful way to explore topological quantum phases beyond natural limits. Nevertheless, a previously-not-realized three-dimensional (3D) chiral topological insulator, and demonstrate by quantum quenches a complete study of both the bulk and surface topological physics. First, a dynamical bulk-surface correspondence in momentum space is… ▽ More Quantum simulation, as a state-of-art technique, provides the powerful way to explore topological quantum phases beyond natural limits. Nevertheless, a previously-not-realized three-dimensional (3D) chiral topological insulator, and demonstrate by quantum quenches a complete study of both the bulk and surface topological physics. First, a dynamical bulk-surface correspondence in momentum space is observed, showing that the bulk topology of the 3D phase uniquely corresponds to the nontrivial quench dynamics emerging on 2D momentum hypersurfaces called band inversion surfaces (BISs), equivalent to the bulk-boundary correspondence in real space. Further, the symmetry protection of the 3D chiral phase is uncovered by measuring dynamical spin textures on BISs, which exhibit perfect (broken) topology when the chiral symmetry is preserved (broken). Finally we measure the topological charges to characterize directly the bulk topology, and identify an emergent dynamical topological transition when varying the quenches from deep to shallow regimes. This work opens a new avenue of quantum simulation towards for the complete study of topological quantum phases. △ Less

Submitted 26 February, 2020; v1 submitted 26 February, 2020; originally announced February 2020.

Comments: 11 pages in total; 6 pages and 4 figures for main manuscript; 5 pages for supplementary materials

Journal ref: Phys. Rev. Lett. 125, 020504 (2020)

Categorical symmetry and non-invertible anomaly in symmetry-breaking and topological phase transitions

Authors: Wenjie Ji, Xiao-Gang Wen

Abstract: For a zero-temperature Landau symmetry breaking transition in $n$-dimensional space that completely breaks a finite symmetry $G$, the critical point at the transition has the symmetry $G$. In this paper, we show that the critical point also has a dual symmetry - a $(n-1)$-symmetry described by a higher group when $G$ is Abelian or an algebraic $(n-1)$-symmetry beyond higher group when $G$ is non-A… ▽ More For a zero-temperature Landau symmetry breaking transition in $n$-dimensional space that completely breaks a finite symmetry $G$, the critical point at the transition has the symmetry $G$. In this paper, we show that the critical point also has a dual symmetry - a $(n-1)$-symmetry described by a higher group when $G$ is Abelian or an algebraic $(n-1)$-symmetry beyond higher group when $G$ is non-Abelian. In fact, any $G$-symmetric system can be viewed as a boundary of $G$-gauge theory in one higher dimension. The conservation of gauge charge and gauge flux in the bulk $G$-gauge theory gives rise to the symmetry and the dual symmetry respectively. So any $G$-symmetric system actually has a larger symmetry called categorical symmetry, which is a combination of the symmetry and the dual symmetry. However, part (and only part) of the categorical symmetry must be spontaneously broken in any gapped phase of the system, but there exists a gapless state where the categorical symmetry is not spontaneously broken. Such a gapless state corresponds to the usual critical point of Landau symmetry breaking transition. The above results remain valid even if we expand the notion of symmetry to include higher symmetries and algebraic higher symmetries. Thus our result also applies to critical points for transitions between topological phases of matter. In particular, we show that there can be several critical points for the transition from the 3+1D $Z_2$ gauge theory to a trivial phase. The critical point from Higgs condensation has a categorical symmetry formed by a $Z_2$ 0-symmetry and its dual - a $Z_2$ 2-symmetry, while the critical point of the confinement transition has a categorical symmetry formed by a $Z_2$ 1-symmetry and its dual - another $Z_2$ 1-symmetry. △ Less

Submitted 17 September, 2020; v1 submitted 31 December, 2019; originally announced December 2019.

Comments: 27 pages, 7 figures, 1 table

Journal ref: Phys. Rev. Research 2, 033417 (2020)

Experimental observation of dynamical bulk-surface correspondence for topological phases

Authors: Ya Wang, Wentao Ji, Zihua Chai, Yuhang Guo, Mengqi Wang, Xiangyu Ye, Pei Yu, Long Zhang, Xi Qin, Pengfei Wang, Fazhan Shi, Xing Rong, Dawei Lu, Xiong-Jun Liu, Jiangfeng Du

Abstract: We experimentally demonstrate a dynamical classification approach for investigation of topological quantum phases using a solid-state spin system through nitrogen-vacancy (NV) center in diamond. Similar to the bulkboundary correspondence in real space at equilibrium, we observe a dynamical bulk-surface correspondence in the momentum space from a dynamical quench process. An emergent dynamical topo… ▽ More We experimentally demonstrate a dynamical classification approach for investigation of topological quantum phases using a solid-state spin system through nitrogen-vacancy (NV) center in diamond. Similar to the bulkboundary correspondence in real space at equilibrium, we observe a dynamical bulk-surface correspondence in the momentum space from a dynamical quench process. An emergent dynamical topological invariant is precisely measured in experiment by imaging the dynamical spin-textures on the recently defined band-inversion surfaces, with high topological numbers being implemented. Importantly, the dynamical classification approach is shown to be independent of quench ways and robust to the decoherence effects, offering a novel and practical strategy for dynamical topology characterization, especially for high dimensional gapped topological phases. △ Less

Submitted 18 April, 2019; originally announced April 2019.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. A 100, 052328 (2019)

Bloch oscillations of spin-orbit-coupled cold atoms in an optical lattice and spin current generation

Authors: Wei Ji, Keye Zhang, Weiping Zhang, Lu Zhou

Abstract: We study the Bloch oscillation dynamics of a spin-orbit-coupled cold atomic gas trapped inside a one-dimensioanl optical lattice. The eigenspectra of the system is identified as two interpenetrating Wannier-Stark ladder. Based on that, we carefully analyzed the Bloch oscillation dynamics and found out that intraladder coupling between neighboring rungs of Wannier-Stark ladder give rise to ordinary… ▽ More We study the Bloch oscillation dynamics of a spin-orbit-coupled cold atomic gas trapped inside a one-dimensioanl optical lattice. The eigenspectra of the system is identified as two interpenetrating Wannier-Stark ladder. Based on that, we carefully analyzed the Bloch oscillation dynamics and found out that intraladder coupling between neighboring rungs of Wannier-Stark ladder give rise to ordinary Bloch oscillation while interladder coupling lead to small amplitude high frequency oscillation superimposed on it. Specifically spin-orbit interaction breaks Galilean invariance, which can be reflected by out-of-phase oscillation of the two spin components in the accelerated frame. The possibility of generating spin current in this system are also explored. △ Less

Submitted 1 February, 2019; originally announced February 2019.

Comments: 8 pages, 6 figures

Journal ref: Phys. Rev. A 99, 023604 (2019)

Mesoscopic entanglement of atomic ensembles through non-resonant stimulated Raman scattering

Authors: Wenhai Ji, Chunbai Wu, S. J. van Enk, M. G. Raymer

Abstract: We propose a scheme of generating and verifying mesoscopic-level entanglement between two atomic ensembles using non-resonant stimulated Raman scattering. Entanglement can be generated by direct detection or balanced homodyne detection of the Stokes fields from the two cells, after they interfere on a beam splitter. The entanglement of the collective atomic fields can be transferred to the anti-… ▽ More We propose a scheme of generating and verifying mesoscopic-level entanglement between two atomic ensembles using non-resonant stimulated Raman scattering. Entanglement can be generated by direct detection or balanced homodyne detection of the Stokes fields from the two cells, after they interfere on a beam splitter. The entanglement of the collective atomic fields can be transferred to the anti-Stokes fields in a readout process. By measuring the operator moments of the anti-Stokes fields, we can verify the presence of entanglement. We model the effects of practical factors such as Stokes field detector quantum efficiency and additive thermal noise in the entanglement generating process, and anti-Stokes field losses in the entanglement verification process, and find achievable regimes in which entanglement can be verified at the levels of tens to hundreds of atomic excitations in the ensembles. △ Less

Submitted 21 March, 2007; v1 submitted 7 December, 2006; originally announced December 2006.

Comments: 35 papges, 6 figures and 1 table, accepted by Phys. Rev. A

Journal ref: Phys. Rev. A, 75, 052305 (2007)