Showing 1–50 of 130 results for author: Sun, Z

Universal Structure of Nonlocal Operators for Deterministic Navigation and Geometric Locking

Authors: Jia Bao, Bin Guo, Shu Qu, Fanqin Xu, Zhaoyu Sun

Abstract: We establish a universal geometric framework that transforms the search for optimal nonlocal operators from a combinatorial black box into a deterministic predict-verify operation. We discover that the principal eigenvalue governing nonlocality is rigorously dictated by a low-dimensional manifold parameterized by merely two fundamental angular variables, $θ$ and $φ$, whose symmetry leads to furthe… ▽ More We establish a universal geometric framework that transforms the search for optimal nonlocal operators from a combinatorial black box into a deterministic predict-verify operation. We discover that the principal eigenvalue governing nonlocality is rigorously dictated by a low-dimensional manifold parameterized by merely two fundamental angular variables, $θ$ and $φ$, whose symmetry leads to further simplification. This geometric distillation establishes a precise mapping connecting external control parameters directly to optimal measurement configurations. Crucially, a comparative analysis of the geometric angles against the principal eigenvalue spectrum, including its magnitude, susceptibility, and nonlocal gap, reveals a fundamental dichotomy in quantum criticality. While transitions involving symmetry sector rotation manifest as geometric criticality with drastic operator reorientation, transitions dominated by strong anisotropy exhibit geometric locking, where the optimal basis remains robust despite clear signatures of phase transitions in the spectral indicators. This distinction offers a novel structural classification of quantum phase transitions and provides a precision navigation chart for Bell experiments. △ Less

Submitted 16 December, 2025; originally announced December 2025.

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)

Higher Josephson harmonics in a tunable double-junction transmon qubit

Authors: Ksenia Shagalov, David Feldstein-Bofill, Leo Uhre Jakobsen, Zhenhai Sun, Casper Wied, Amalie T. J. Paulsen, Johann Bock Severin, Malthe A. Marciniak, Clinton A. Potts, Anders Kringhøj, Jacob Hastrup, Karsten Flensberg, Svend Krøjer, Morten Kjaergaard

Abstract: Tunable Josephson harmonics open new avenues for qubit design. We demonstrate a superconducting circuit element consisting of a tunnel junction in series with a SQUID loop, yielding a Josephson potential whose harmonic content is strongly tunable by magnetic flux. Through spectroscopy of the first four qubit transitions, together with an effective single-mode model renormalized by the internal mod… ▽ More Tunable Josephson harmonics open new avenues for qubit design. We demonstrate a superconducting circuit element consisting of a tunnel junction in series with a SQUID loop, yielding a Josephson potential whose harmonic content is strongly tunable by magnetic flux. Through spectroscopy of the first four qubit transitions, together with an effective single-mode model renormalized by the internal mode, we resolve a second harmonic with an amplitude up to $\sim10\%$ of the fundamental. We identify a flux sweet spot where the dispersive shift vanishes, achieved by balancing the dispersive couplings to the internal and qubit modes. This highly tunable element provides a route toward protected qubits and customizable nonlinear microwave devices. △ Less

Submitted 11 December, 2025; v1 submitted 9 December, 2025; originally announced December 2025.

Unified Bulk-Entanglement Correspondence in Non-Hermitian Systems

Authors: Xudong Zhang, Zhaoyu Sun, Bin Guo

Abstract: The non-Hermitian skin effect (NHSE) fundamentally invalidates the conventional bulk-boundary correspondence (BBC), leading topological diagnostics into a crisis. While the non-Bloch polarization $P_β$ defined on the generalized Brillouin zone restores momentum-space topology, a direct, robust real-space bulk probe has remained elusive. We resolve this by establishing a universal correspondence be… ▽ More The non-Hermitian skin effect (NHSE) fundamentally invalidates the conventional bulk-boundary correspondence (BBC), leading topological diagnostics into a crisis. While the non-Bloch polarization $P_β$ defined on the generalized Brillouin zone restores momentum-space topology, a direct, robust real-space bulk probe has remained elusive. We resolve this by establishing a universal correspondence between $P_β$ and the entanglement polarization $χ$ of the biorthogonal ground state. Introducing a quasi-reciprocal Hamiltonian $\tilde{H}$ that removes the NHSE while preserving bulk topology, we rigorously prove the fundamental identity $P_β \equiv χ(\tilde{H})\pmod 1$ in the thermodynamic limit under the quasi-locality assumption. Crucially, we demonstrate that this equivalence transcends the locality constraints that limit traditional topological invariants. While the conventional Resta polarization fails when $\tilde{H}$ becomes non-local due to the divergence of position variance, we reveal that $χ(\tilde{H})$ remains robustly quantized, protected by the Fredholm index of Toeplitz operators. Our work thus identifies entanglement as the unique real-space diagnostic capable of capturing non-Bloch topology beyond the breakdown of locality, successfully restoring the BBC across diverse non-Hermitian systems such as line-gap, point-gap, and gapless phases, thereby unifying the geometric and entanglement paradigms in non-Hermitian physics. △ Less

Submitted 10 December, 2025; v1 submitted 21 November, 2025; originally announced November 2025.

Scalable quantum error mitigation with phase-cycled dynamical decoupling

Authors: Weibin Ni, Zhijie Li, Guanyu Qu, Zhecheng Sun, Jiale Dai, Fazhan Shi, Lei Sun

Abstract: The realization of quantum technologies in the Noisy Intermediate-Scale Quantum era is severely constrained by qubit decoherence and control errors, presenting fundamental challenges to achieving quantum advantages. Dynamical decoupling is a widely used, powerful technique for decoherence error suppression. However, it is susceptible to control errors, making non-robust sequences like UDD impracti… ▽ More The realization of quantum technologies in the Noisy Intermediate-Scale Quantum era is severely constrained by qubit decoherence and control errors, presenting fundamental challenges to achieving quantum advantages. Dynamical decoupling is a widely used, powerful technique for decoherence error suppression. However, it is susceptible to control errors, making non-robust sequences like UDD impractical to implement and robust ones like CPMG to significantly overestimate decoherence times. This overestimation issue remains largely unexplored in the past few decades, leading to many reports of exceptionally long yet plausible decoherence times across various qubit platforms. Here, we construct Hadamard phase cycling as a non-Markovian quantum error mitigation method for dynamical decoupling. This method exploits group structure to design phase configurations of equivalent ensemble quantum circuits, effectively eliminates circuit outputs generated from erroneous dynamics, and scales linearly with circuit depth. Harnessing its error mitigation capability for ensemble solid-state electron spin qubits embedded in paramagnetic molecules and nitrogen-vacancy centers in diamond enables accurate acquisition of decoherence times. Applying Hadamard phase cycling on single trapped ion and superconducting transmon qubits effectively preserves their state fidelity during dynamical decoupling. The integration of scalable quantum error mitigation and suppression would facilitate the development of quantum technologies with noisy qubits and control hardware. △ Less

Submitted 15 November, 2025; originally announced November 2025.

Comments: 48 pages, 11 figures

QRTlib: A Library for Fast Quantum Real Transforms

Authors: Armin Ahmadkhaniha, Lu Chen, Jake Doliskani, Zhifu Sun

Abstract: Real-valued transforms such as the discrete cosine, sine, and Hartley transforms play a central role in classical computing, complementing the Fourier transform in applications from signal and image processing to data compression. However, their quantum counterparts have not evolved in parallel, and no unified framework exists for implementing them efficiently on quantum hardware. This article add… ▽ More Real-valued transforms such as the discrete cosine, sine, and Hartley transforms play a central role in classical computing, complementing the Fourier transform in applications from signal and image processing to data compression. However, their quantum counterparts have not evolved in parallel, and no unified framework exists for implementing them efficiently on quantum hardware. This article addresses this gap by introducing QRTlib, a library for fast and practical implementations of quantum real transforms, including the quantum Hartley, cosine, and sine transforms of various types. We develop new algorithms and circuit optimizations that make these transforms efficient and suitable for near-term devices. In particular, we present a quantum Hartley transform based on the linear combination of unitaries (LCU) technique, achieving a fourfold reduction in circuit size compared to prior methods, and an improved quantum sine transform of Type I that removes large multi-controlled operations. We also introduce circuit-level optimizations, including two's-complement and or-tree constructions. QRTlib provides the first complete implementations of these quantum real transforms in Qiskit. △ Less

Submitted 18 October, 2025; originally announced October 2025.

Efficient and accurate tensor network algorithm for Anderson impurity problems

Authors: Zhijie Sun, Zhenyu Li, Chu Guo

Abstract: The Anderson impurity model (AIM) is of fundamental importance in condensed matter physics to study strongly correlated effects. However, accurately solving its long-time dynamics still remains a great numerical challenge. An emergent and rapidly developing numerical strategy to solve the AIM is to represent the Feynman-Vernon influence functional (IF), which encodes all the bath effects on the im… ▽ More The Anderson impurity model (AIM) is of fundamental importance in condensed matter physics to study strongly correlated effects. However, accurately solving its long-time dynamics still remains a great numerical challenge. An emergent and rapidly developing numerical strategy to solve the AIM is to represent the Feynman-Vernon influence functional (IF), which encodes all the bath effects on the impurity dynamics, as a matrix product state (MPS) in the temporal domain. The computational cost of this strategy is basically determined by the bond dimension $χ$ of the temporal MPS. In this work, we propose an efficient and accurate method which, when the hybridization function in the IF can be approximated as the summation of $n$ exponential functions, can systematically build the IF as a MPS by multiplying $O(n)$ small MPSs, each with bond dimension $2$. Our method gives a worst case scaling of $χ$ as $2^{8n}$ and $2^{2n}$ for real- and imaginary-time evolution respectively. We demonstrate the performance of our method for two commonly used bath spectral functions, where we show that the actually required $χ$s are much smaller than the worst case. △ Less

Submitted 13 October, 2025; originally announced October 2025.

Comments: 11 pages, 4 figures

Accurate and Scalable Simulation of Cavity-Based Networks in Modular Quantum Architectures

Authors: Sahar Ben Rached, Zezhou Sun, Guilu Long, Santiago Rodrigo, Carmen G. Almudéver, Eduard Alarcón, Sergi Abadal

Abstract: Cavity-mediated interconnects are a promising platform for scaling modular quantum computers by enabling high-fidelity inter-chip quantum state transmission and entanglement generation. In this work, we first model the dynamics of deterministic inter-chip quantum state transfer using the Stimulated Raman Adiabatic Passage (STIRAP) protocol, analyzing fidelity loss mechanisms under experimentally a… ▽ More Cavity-mediated interconnects are a promising platform for scaling modular quantum computers by enabling high-fidelity inter-chip quantum state transmission and entanglement generation. In this work, we first model the dynamics of deterministic inter-chip quantum state transfer using the Stimulated Raman Adiabatic Passage (STIRAP) protocol, analyzing fidelity loss mechanisms under experimentally achievable qubit-cavity coupling and decoherence parameters. We then extend the NetSquid simulator, typically used for simulating long-range quantum communication networks, to support cavity-based communication channels for mediating inter-chip state transfer and entanglement generation. We model cavities as amplitude damping channels parameterized by physical system characteristics; cavity decay rate k and qubit-cavity coupling strength g, and analyze the impact of intrinsic qubit decoherence factors dictated by T1 and T2 times. Our simulations accurately represent the system's dynamics in both strong and weak coupling regimes, and identify critical trade-offs between fidelity, latency, and noise factors. The proposed framework supports faithful modeling and scalable simulation of modular architectures, and provides insights into design optimization for practical quantum network implementations. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: Accepted at IEEE International Conference on Quantum Computing and Engineering (QCE) 2025. 8 pages, 9 figures

Space and Time Cost of Continuous Rotations in Surface Codes

Authors: Zhu Sun, Balint Koczor

Abstract: While Clifford operations are relatively easy to implement in fault-tolerant quantum computers,continuous rotation gates remain a significant bottleneck in typical quantum algorithms. In this work, we ask the question: "What is the most efficient approach for implementing continuous rotations in a surface code architecture?" Several techniques have been developed to reduce the T-count or T-depth o… ▽ More While Clifford operations are relatively easy to implement in fault-tolerant quantum computers,continuous rotation gates remain a significant bottleneck in typical quantum algorithms. In this work, we ask the question: "What is the most efficient approach for implementing continuous rotations in a surface code architecture?" Several techniques have been developed to reduce the T-count or T-depth of rotations, such as Hamming weight phasing and catalyst towers. However, these methods often require additional a number of ancilla qubits, and thus the ultimate cost function one needs to optimise against should rather be the total runtime or the total space required for performing a rotation. We explicitly construct surface code layouts for catalyst towers in two practical application examples in the context of option pricing: (a) implementing a phase oracle circuit, which is a ubiquitous subroutine in many quantum algorithms, and (b) state preparation using a variational quantum circuit. Our analysis shows that, at small and medium code distances, catalyst towers not only reduce the runtime but can also decrease the total spacetime volume of rotations. However, at large code distances, conventional Clifford+T synthesis may prove more efficient. Additionally, we note that our conclusions are sensitive to specific application scenarios and the choices of various parameters. Nevertheless, catalyst towers may be particularly advantageous for early fault-tolerant quantum applications, where low and medium code distances are assumed and a spacetime tradeoff is needed to reduce the runtime of individual circuit runs, such as in scenarios involving high circuit repetition counts. △ Less

Submitted 8 August, 2025; originally announced August 2025.

Comments: 12 pages, 8 figures

Many-body delocalization with a two-dimensional 70-qubit superconducting quantum simulator

Authors: Tian-Ming Li, Zheng-Hang Sun, Yun-Hao Shi, Zhen-Ting Bao, Yong-Yi Wang, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Yi-Han Yu, Zheng-He Liu, Chi-Tong Chen, Li Li, Hao Li, Hao-Tian Liu, Si-Yun Zhou, Zhen-Yu Peng, Yan-Jun Liu, Ziting Wang, Yue-Shan Xu, Kui Zhao, Yang He, Da'er Feng, Jia-Cheng Song, Cai-Ping Fang, Junrui Deng , et al. (13 additional authors not shown)

Abstract: Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D… ▽ More Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D) superconducting quantum simulator, we experimentally explore the robustness of the MBL regime in controlled finite-size 2D systems. We observe that the decay of imbalance becomes more pronounced with increasing system sizes, scaling up from 21, 42 to 70 qubits, with a relatively large disorder strength, and for the first time, provide an evidence for the many-body delocalization in 2D disordered systems. Our experimental results are consistent with the avalanche theory that predicts the instability of MBL regime beyond one spatial dimension. This work establishes a scalable platform for probing high-dimensional non-equilibrium phases of matter and their finite-size effects using superconducting quantum circuits. △ Less

Submitted 22 July, 2025; originally announced July 2025.

Comments: main text: 7 pages, 3 figures; supplementary information: 19 pages, 17 figures

Scalable tensor network algorithm for quantum impurity problems

Authors: Zhijie Sun, Ruofan Chen, Zhenyu Li, Chu Guo

Abstract: The Grassmann time-evolving matrix product operator method has shown great potential as a general-purpose quantum impurity solver, as its numerical errors can be well-controlled and it is flexible to be applied on both the imaginary- and real-time axis. However, a major limitation of it is that its computational cost grows exponentially with the number of impurity flavors. In this work, we propose… ▽ More The Grassmann time-evolving matrix product operator method has shown great potential as a general-purpose quantum impurity solver, as its numerical errors can be well-controlled and it is flexible to be applied on both the imaginary- and real-time axis. However, a major limitation of it is that its computational cost grows exponentially with the number of impurity flavors. In this work, we propose a multi-flavor extension of it to overcome this limitation. The key insight is that to calculate multi-time correlation functions on one or a few impurity flavors, one could integrate out the degrees of freedom of the rest flavors before hand, which could greatly simplify the calculation. The idea is particularly effective for quantum impurity problems with diagonal hybridization function, i.e., each impurity flavor is coupled to an independent bath, a setting which is commonly used in the field. We demonstrate the accuracy and scalability of our method for the imaginary time evolution of impurity problems with up to three impurity orbitals, i.e., 6 flavors, and benchmark our results against continuous-time quantum Monte Carlo calculations. Our method paves the way of scaling up tensor network algorithms to solve large-scale quantum impurity problems. △ Less

Submitted 7 October, 2025; v1 submitted 16 July, 2025; originally announced July 2025.

Comments: 12 pages, 6 figures

Journal ref: Phys. Rev. B 112, 155115 (2025)

Ultralong Room-Temperature Qubit Lifetimes of Covalent Organic Frameworks

Authors: Zhecheng Sun, Weibin Ni, Denan Li, Xiya Du, Shi Liu, Lei Sun

Abstract: Molecular electron spin qubits offer atomic-level tunability and room-temperature quantum coherence. Their integration into engineered solid-state matrices can enhance performance towards ambient quantum information technologies. Herein, we demonstrate covalent organic frameworks (COFs) as programmable matrices of stable organic radical qubits allowing strategic optimization of spin-phonon and spi… ▽ More Molecular electron spin qubits offer atomic-level tunability and room-temperature quantum coherence. Their integration into engineered solid-state matrices can enhance performance towards ambient quantum information technologies. Herein, we demonstrate covalent organic frameworks (COFs) as programmable matrices of stable organic radical qubits allowing strategic optimization of spin-phonon and spin-spin interactions. Using two classic boronate-ester frameworks, COF-5 and COF-108, to host semiquinone-like radical qubits, we achieve ultralong spin relaxation time (T1 > 300 μs) at 298 K, which outperforms most molecular qubits and rivals inorganic spin defects. The suppression of spin relaxation is attributed to rigid and neutral structures as well as carbon-centered spin distributions that effectively weaken spin-phonon coupling. Employing dynamical decoupling methods to both COFs improves their quantum coherence and enables room-temperature detection of nuclear spins including 1H, 11B, and 13C. Our work establishes COFs as designer quantum materials, opening new avenues for quantum sensing of nuclear spins at room temperature. △ Less

Submitted 3 June, 2025; originally announced June 2025.

Comments: 21 pages, 5 figures

Demonstration of low-overhead quantum error correction codes

Authors: Ke Wang, Zhide Lu, Chuanyu Zhang, Gongyu Liu, Jiachen Chen, Yanzhe Wang, Yaozu Wu, Shibo Xu, Xuhao Zhu, Feitong Jin, Yu Gao, Ziqi Tan, Zhengyi Cui, Ning Wang, Yiren Zou, Aosai Zhang, Tingting Li, Fanhao Shen, Jiarun Zhong, Zehang Bao, Zitian Zhu, Yihang Han, Yiyang He, Jiayuan Shen, Han Wang , et al. (17 additional authors not shown)

Abstract: Quantum computers hold the potential to surpass classical computers in solving complex computational problems. However, the fragility of quantum information and the error-prone nature of quantum operations make building large-scale, fault-tolerant quantum computers a prominent challenge. To combat errors, pioneering experiments have demonstrated a variety of quantum error correction codes. Yet, mo… ▽ More Quantum computers hold the potential to surpass classical computers in solving complex computational problems. However, the fragility of quantum information and the error-prone nature of quantum operations make building large-scale, fault-tolerant quantum computers a prominent challenge. To combat errors, pioneering experiments have demonstrated a variety of quantum error correction codes. Yet, most of these codes suffer from low encoding efficiency, and their scalability is hindered by prohibitively high resource overheads. Here, we report the demonstration of two low-overhead quantum low-density parity-check (qLDPC) codes, a distance-4 bivariate bicycle code and a distance-3 qLDPC code, on our latest superconducting processor, Kunlun, featuring 32 long-range-coupled transmon qubits. Utilizing a two-dimensional architecture with overlapping long-range couplers, we demonstrate simultaneous measurements of all nonlocal weight-6 stabilizers via the periodic execution of an efficient syndrome extraction circuit. We achieve a logical error rate per logical qubit per cycle of $(8.91 \pm 0.17)\%$ for the distance-4 bivariate bicycle code with four logical qubits and $(7.77 \pm 0.12)\%$ for the distance-3 qLDPC code with six logical qubits. Our results establish the feasibility of implementing various qLDPC codes with long-range coupled superconducting processors, marking a crucial step towards large-scale low-overhead quantum error correction. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Modeling Quantum Links for the Exploration of Distributed Quantum Computing Systems

Authors: Sahar Ben Rached, Zezhou Sun, Junaid Khan, Guilu Long, Santiago Rodrigo, Carmen G. Almudéver, Eduard Alarcón, Sergi Abadal

Abstract: Quantum computing offers the potential to solve certain complex problems, yet, scaling monolithic processors remains a major challenge. Modular and distributed architectures are proposed to build large-scale quantum systems while bringing the security advantages of quantum communication. At present, this requires accurate and computationally efficient models of quantum links across different scale… ▽ More Quantum computing offers the potential to solve certain complex problems, yet, scaling monolithic processors remains a major challenge. Modular and distributed architectures are proposed to build large-scale quantum systems while bringing the security advantages of quantum communication. At present, this requires accurate and computationally efficient models of quantum links across different scales to advance system design and guide experimental prototyping. In this work, we review protocols and models for estimating latency, losses, and fidelity in quantum communication primitives relying on quantum state distribution via microwave photons. We also propose a scalable simulation framework to support the design and evaluation of future distributed quantum computing systems. △ Less

Submitted 13 May, 2025; originally announced May 2025.

Comments: Invited Paper for ICTON 2025

Interaction effects on electronic Floquet spectra: Excitonic effects

Authors: Teng Xiao, Tsan Huang, Changhua Bao, Zhiyuan Sun

Abstract: Floquet engineering of electronic states by light is a central topic in modern experiments. However, the impact of many-body interactions on the single-electron properties remains unclear in this non-equilibrium situation. We propose that interaction effects could be reasonably understood by performing perturbative expansion in both the pump field and the electron-electron interaction when computi… ▽ More Floquet engineering of electronic states by light is a central topic in modern experiments. However, the impact of many-body interactions on the single-electron properties remains unclear in this non-equilibrium situation. We propose that interaction effects could be reasonably understood by performing perturbative expansion in both the pump field and the electron-electron interaction when computing physical quantities. As an example, we apply this approach to semiconductors and show analytically that excitonic effects, i.e., effects of electron-hole interaction, lead to dramatic corrections to the single-electron Floquet spectra even when the excitons are only virtually excited by the pump light. We compute these effects in phosphorene and monolayer MoS$_2$ for time- and angle-resolved photoemission spectroscopy and ultrafast optical experiments. △ Less

Submitted 5 October, 2025; v1 submitted 12 May, 2025; originally announced May 2025.

Journal ref: Phys. Rev. B 112, L161106(2025)

Dynamical multipartite entanglement in a generalized Tavis-Cummings model with XY spin interaction

Authors: Yuguo Su, Zhijie Sun, Yiying Yan, Hengyan Wang, Junyan Luo, Tiantian Ying, Hongbin Liang, Yi-Xiao Huang

Abstract: Multipartite entanglement is a long-term pursuit in the resource theory, offering a potential resource for quantum metrology. Here, we present the dynamical multipartite entanglement, which is in terms of the quantum Fisher information, of a generalized Tavis-Cummings (TC) model introducing the XY spin interaction. Since our model cannot be solved exactly, we theoretically derive and numerically e… ▽ More Multipartite entanglement is a long-term pursuit in the resource theory, offering a potential resource for quantum metrology. Here, we present the dynamical multipartite entanglement, which is in terms of the quantum Fisher information, of a generalized Tavis-Cummings (TC) model introducing the XY spin interaction. Since our model cannot be solved exactly, we theoretically derive and numerically examine the effective description of our model. By the Holstein-Primakoff transformation, we show the bridge from the generalized TC model to the central spin model. Furthermore, the reduced density matrix of the central spins is presented, which is the prerequisite for calculating multipartite entanglement. We also discuss the effect of the temperature, the coupling constant, and the magnetic field on the dynamical multipartite entanglement in the central spin model, where the central spin is initially unentangled. Strong coupling and low temperature are necessary conditions for a genuine multipartite entanglement in the XY model, and together with the magnetic field, they govern the modulation of both the entanglement period and amplitude. Our results unveil the deep link between the TC model and the central spin model, allowing for a better comprehension of their dynamical multipartite entanglement. △ Less

Submitted 9 May, 2025; originally announced May 2025.

Comments: arXiv admin note: text overlap with arXiv:2307.16166

Journal ref: Phys. Rev. A 111, 052415 (2025)

Towards a Quantum Information Theory of Hadronization: Dihadron Fragmentation and Neutral Polarization in Heavy Baryons

Authors: Rebecca von Kuk, Kyle Lee, Johannes K. L. Michel, Zhiquan Sun

Abstract: We pioneer the application of quantum information theory to experimentally distinguish between classes of hadronization models. We adapt the CHSH inequality to the fragmentation of a single parton to hadron pairs, a violation of which would rule out classical dynamics of hadronization altogether. Furthermore, we apply and extend the theory of quantum contextuality and local quantum systems to the… ▽ More We pioneer the application of quantum information theory to experimentally distinguish between classes of hadronization models. We adapt the CHSH inequality to the fragmentation of a single parton to hadron pairs, a violation of which would rule out classical dynamics of hadronization altogether. Furthermore, we apply and extend the theory of quantum contextuality and local quantum systems to the neutral polarization of a single spin-1 hadronic system, namely the light constituents of excited Sigma baryons $Σ^{*}_{c,b}$ formed in the fragmentation of heavy quarks. △ Less

Submitted 25 June, 2025; v1 submitted 28 March, 2025; originally announced March 2025.

Comments: 5 pages + supplemental material, 3 figures; v2: added references, comparison to data from 2506.05499, additional detail in Supp. B

Report number: MIT-CTP 5818, Nikhef 2024-020, DESY-25-034

Probing prethermal nonergodicity through measurement outcomes of monitored quantum dynamics

Authors: Zheng-Hang Sun, Fabian Ballar Trigueros, Qicheng Tang, Markus Heyl

Abstract: Projective measurements are a key element in quantum physics and enable rich phenomena in monitored quantum dynamics. Here, we show that the measurement outcomes, recorded during monitored dynamics, can provide crucial information about the properties of the monitored dynamical system itself. We demonstrate this for a Floquet model of many-body localization, where we find that the prethermal many-… ▽ More Projective measurements are a key element in quantum physics and enable rich phenomena in monitored quantum dynamics. Here, we show that the measurement outcomes, recorded during monitored dynamics, can provide crucial information about the properties of the monitored dynamical system itself. We demonstrate this for a Floquet model of many-body localization, where we find that the prethermal many-body localized regime becomes unstable against rare measurements, yielding an unusual enhancement of quantum entanglement. Through an unsupervised learning and mutual information analysis on the classical dataset of measurement outcomes, we find that the information loss in the system, reflected by the increased entanglement, is compensated by an emergent structure in this classical dataset. Our findings highlight the crucial role of measurements and corresponding classical outcomes in capturing prethermal nonergodicity, offering a promising perspective for applications to other monitored quantum dynamics. △ Less

Submitted 14 March, 2025; originally announced March 2025.

Journal ref: Phys. Rev. B 112, L180306 (2025)

Probing quantum many-body dynamics using subsystem Loschmidt echos

Authors: Simon Karch, Souvik Bandyopadhyay, Zheng-Hang Sun, Alexander Impertro, SeungJung Huh, Irene Prieto Rodríguez, Julian F. Wienand, Wolfgang Ketterle, Markus Heyl, Anatoli Polkovnikov, Immanuel Bloch, Monika Aidelsburger

Abstract: The Loschmidt echo - the probability of a quantum many-body system to return to its initial state following a dynamical evolution - generally contains key information about a quantum system, relevant across various scientific fields including quantum chaos, quantum many-body physics, or high-energy physics. However, it is typically exponentially small in system size, posing an outstanding challeng… ▽ More The Loschmidt echo - the probability of a quantum many-body system to return to its initial state following a dynamical evolution - generally contains key information about a quantum system, relevant across various scientific fields including quantum chaos, quantum many-body physics, or high-energy physics. However, it is typically exponentially small in system size, posing an outstanding challenge for experiments. Here, we experimentally investigate the subsystem Loschmidt echo, a quasi-local observable that captures key features of the Loschmidt echo while being readily accessible experimentally. Utilizing quantum gas microscopy, we study its short- and long-time dynamics. In the short-time regime, we observe a dynamical quantum phase transition arising from genuine higher-order correlations. In the long-time regime, the subsystem Loschmidt echo allows us to quantitatively determine the effective dimension and structure of the accessible Hilbert space in the thermodynamic limit. Performing these measurements in the ergodic regime and in the presence of emergent kinetic constraints, we provide direct experimental evidence for ergodicity breaking due to fragmentation of the Hilbert space. Our results establish the subsystem Loschmidt echo as a novel and powerful tool that allows paradigmatic studies of both non-equilibrium dynamics and equilibrium thermodynamics of quantum many-body systems, applicable to a broad range of quantum simulation and computing platforms. △ Less

Submitted 28 January, 2025; originally announced January 2025.

Andreev spin relaxation time in a shadow-evaporated InAs weak link

Authors: Haoran Lu, David F. Bofill, Zhenhai Sun, Thomas Kanne, Jesper Nygård, Morten Kjaergaard, Valla Fatemi

Abstract: Andreev spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an InAs nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link s… ▽ More Andreev spin qubits are a new qubit platform that merges superconductivity with semiconductor physics. The mechanisms dominating observed energy relaxation remain unidentified. We report here on three steps taken to address these questions in an InAs nanowire weak link. First, we designed a microwave readout circuit tuned to be directly sensitive to the spin-dependent inductance of the weak link so that higher orbital states are not necessary for readout -- this resulted in larger windows in parameter space in which the spin state properties can be probed. Second, we implemented a successful gap-engineering strategy to mitigate quasiparticle poisoning. Third, the weak link was fabricated by \textit{in situ} shadow evaporation, which has been shown to improve atomic-scale disorder. We show how our design allows characterization of the spin stability and coherence over the full range of magnetic flux and gate voltage of an odd parity bias point. The spin relaxation and dephasing rates are comparable with the best devices previously reported, suggestive that surface atomic-scale disorder and QP poisoning are not linked to spin relaxation in InAs nanowires. Our design strategies are transferrable to novel materials platforms for Andreev qubits such as germanium and carbon. △ Less

Submitted 20 January, 2025; originally announced January 2025.

Journal ref: Phys. Rev. Applied 24, 024046 (2025)

Observation of topological prethermal strong zero modes

Authors: Feitong Jin, Si Jiang, Xuhao Zhu, Zehang Bao, Fanhao Shen, Ke Wang, Zitian Zhu, Shibo Xu, Zixuan Song, Jiachen Chen, Ziqi Tan, Yaozu Wu, Chuanyu Zhang, Yu Gao, Ning Wang, Yiren Zou, Aosai Zhang, Tingting Li, Jiarun Zhong, Zhengyi Cui, Yihang Han, Yiyang He, Han Wang, Jianan Yang, Yanzhe Wang , et al. (20 additional authors not shown)

Abstract: Symmetry-protected topological phases cannot be described by any local order parameter and are beyond the conventional symmetry-breaking paradigm for understanding quantum matter. They are characterized by topological boundary states robust against perturbations that respect the protecting symmetry. In a clean system without disorder, these edge modes typically only occur for the ground states of… ▽ More Symmetry-protected topological phases cannot be described by any local order parameter and are beyond the conventional symmetry-breaking paradigm for understanding quantum matter. They are characterized by topological boundary states robust against perturbations that respect the protecting symmetry. In a clean system without disorder, these edge modes typically only occur for the ground states of systems with a bulk energy gap and would not survive at finite temperatures due to mobile thermal excitations. Here, we report the observation of a distinct type of topological edge modes, which are protected by emergent symmetries and persist even up to infinite temperature, with an array of 100 programmable superconducting qubits. In particular, through digital quantum simulation of the dynamics of a one-dimensional disorder-free "cluster" Hamiltonian, we observe robust long-lived topological edge modes over up to 30 cycles at a wide range of temperatures. By monitoring the propagation of thermal excitations, we show that despite the free mobility of these excitations, their interactions with the edge modes are substantially suppressed in the dimerized regime due to an emergent U(1)$\times$U(1) symmetry, resulting in an unusually prolonged lifetime of the topological edge modes even at infinite temperature. In addition, we exploit these topological edge modes as logical qubits and prepare a logical Bell state, which exhibits persistent coherence in the dimerized and off-resonant regime, despite the system being disorder-free and far from its ground state. Our results establish a viable digital simulation approach to experimentally exploring a variety of finite-temperature topological phases and demonstrate a potential route to construct long-lived robust boundary qubits that survive to infinite temperature in disorder-free systems. △ Less

Submitted 8 January, 2025; originally announced January 2025.

Gatemon Qubit Revisited for Improved Reliability and Stability

Authors: David Feldstein-Bofill, Zhenhai Sun, Casper Wied, Shikhar Singh, Brian D. Isakov, Svend Krøjer, Jacob Hastrup, András Gyenis, Morten Kjaergaard

Abstract: The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building novel superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit. However, gatemons typically suffer from unstable and hysteretic qubit frequen… ▽ More The development of quantum circuits based on hybrid superconductor-semiconductor Josephson junctions holds promise for exploring their mesoscopic physics and for building novel superconducting devices. The gate-tunable superconducting transmon qubit (gatemon) is the paradigmatic example of such a superconducting circuit. However, gatemons typically suffer from unstable and hysteretic qubit frequencies with respect to the applied gate voltage and reduced coherence times. Here we develop methods for characterizing these challenges in gatemons and deploy these methods to compare the impact of shunt capacitor designs on gatemon performance. Our results indicate a strong frequency- and design-dependent behavior of the qubit stability, hysteresis, and dephasing times. Moreover, we achieve highly reliable tuning of the qubit frequency with 1 MHz precision over a range of several GHz, along with improved stability in grounded gatemons compared to gatemons with a floating capacitor design. △ Less

Submitted 16 December, 2024; originally announced December 2024.

Comments: 13 pages, 12 figures

Report number: NBI QDEV 2024

Efficacious qubit mappings for quantum simulations of the $^{12}$C rotational band

Authors: Darin C. Mumma, Zhonghao Sun, Alexis Mercenne, Kristina D. Launey, Soorya Rethinasamy, James A. Sauls

Abstract: Solving atomic nuclei from first principles places enormous demands on computational resources, which grow exponentially with increasing number of particles and the size of the space they occupy. We present first quantum simulations based on the variational quantum eigensolver for the low-lying structure of the $^{12}$C nucleus that provide acceptable bound-state energies even in the presence of n… ▽ More Solving atomic nuclei from first principles places enormous demands on computational resources, which grow exponentially with increasing number of particles and the size of the space they occupy. We present first quantum simulations based on the variational quantum eigensolver for the low-lying structure of the $^{12}$C nucleus that provide acceptable bound-state energies even in the presence of noise. We achieve this by taking advantage of two critical developments. First, we utilize an almost perfect symmetry of atomic nuclei that, in a complete symmetry-adapted basis, drastically reduces the size of the model space. Second, we use the efficacious Gray encoding, for which it has been recently shown that it is resource efficient, especially when coupled with a near band-diagonal structure of the nuclear Hamiltonian. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Comments: 5 pages, 4 figures

Journal ref: 2024 IEEE Computer Society Annual Symposium on VLSI (ISVLSI)

Infinite Grassmann time-evolving matrix product operators for quantum impurity problems after a quench

Authors: Zhijie Sun, Ruofan Chen, Zhenyu Li, Chu Guo

Abstract: An emergent numerical approach to solve quantum impurity problems is to encode the impurity path integral as a matrix product state. For time-dependent problems, the cost of this approach generally scales with the evolution time. Here we consider a common non-equilibrium scenario where an impurity, initially in equilibrium with a thermal bath, is driven out of equilibrium by a sudden quench of the… ▽ More An emergent numerical approach to solve quantum impurity problems is to encode the impurity path integral as a matrix product state. For time-dependent problems, the cost of this approach generally scales with the evolution time. Here we consider a common non-equilibrium scenario where an impurity, initially in equilibrium with a thermal bath, is driven out of equilibrium by a sudden quench of the impurity Hamiltonian. Despite that there is no time-translational invariance in the problem, we show that we could still make full use of the infinite matrix product state technique, resulting in a method whose cost is essentially independent of the evolution time. We demonstrate the effectiveness of this method in the integrable case against exact diagonalization, and against existing calculations on the L-shaped Kadanoff-Baym contour in the general case. Our method could be a very competitive method for studying long-time non-equilibrium quantum dynamics, and be potentially used as an efficient impurity solver in the non-equilibrium dynamical mean field theory. △ Less

Submitted 25 August, 2025; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: 13 pages, 10 figures

Journal ref: Phys. Rev. B, 112, 125145 (2025)

Certain BCS wavefunctions are quantum many-body scars

Authors: Kiryl Pakrouski, Zimo Sun

Abstract: We provide a method for constructing many-body scar states in fermionic lattice models that incorporate a given type of correlations with one of the states maximizing them over the full Hilbert space. Therefore this state may always be made the ground state by adding such correlations as a "pairing potential" $δH_0$ to any Hamiltonian $H=H_0+OT$ supporting group-invariant scars [arXiv:2007.00845].… ▽ More We provide a method for constructing many-body scar states in fermionic lattice models that incorporate a given type of correlations with one of the states maximizing them over the full Hilbert space. Therefore this state may always be made the ground state by adding such correlations as a "pairing potential" $δH_0$ to any Hamiltonian $H=H_0+OT$ supporting group-invariant scars [arXiv:2007.00845]. In case of single-flavour spin-full fermions the ground state is a special case of the BCS wavefunction written in real space and invariant under any site index relabelling. For multi-orbital fermions this state also resembles BCS but includes higher order terms corresponding to "pairing" of more than two fermions. The broad class of eligible Hamiltonians $H$ is well documented [arXiv:2007.00845],[arXiv:2106.10300] and includes many conventional condensed matter interactions. The part of the Hamiltonian $(H_0+δH_0)$ that governs the exact dynamics of the scar subspace coincides with the BCS mean-field Hamiltonian. We therefore show that its BCS ground state and the excitations above it are many-body scars that are dynamically decoupled from the rest of the Hilbert space and thereby protected from thermalization. These states are insensitive to a variety of $OT$ Hamiltonian terms that among others include interactions and (spin-orbit) hoppings. Our results point out a connection between the fields of superconductivity and weak ergodicity breaking (many-body scars) and will hopefully encourage further investigations. They also provide the first practical protocol to initialize a fermionic system to a scar state in (a quantum simulator) experiment. △ Less

Submitted 20 November, 2024; originally announced November 2024.

Quantum state tomography with muons

Authors: Leyun Gao, Alim Ruzi, Qite Li, Chen Zhou, Liangwen Chen, Xueheng Zhang, Zhiyu Sun, Qiang Li

Abstract: Entanglement is a fundamental pillar of quantum mechanics. Probing quantum entanglement and testing Bell inequality with muons can be a significant leap forward, as muon is arguably the only massive elementary particle that can be manipulated and detected over a wide range of energies, e.g., from approximately 0.3 to $10^2$ GeV, corresponding to velocities from 0.94 to nearly the speed of light. I… ▽ More Entanglement is a fundamental pillar of quantum mechanics. Probing quantum entanglement and testing Bell inequality with muons can be a significant leap forward, as muon is arguably the only massive elementary particle that can be manipulated and detected over a wide range of energies, e.g., from approximately 0.3 to $10^2$ GeV, corresponding to velocities from 0.94 to nearly the speed of light. In this work, we present a realistic proposal and a comprehensive study of quantum entanglement in a state composed of different-flavor fermions in muon-electron scattering. The polarization density matrix for the muon-electron system is derived using a kinematic approach within the relativistic quantum field theory framework. Entanglement in the resulting muon-electron qubit system and the violation of Bell inequalities can be observed with a high event rate. This paves the way for performing quantum tomography with muons. △ Less

Submitted 19 November, 2024; originally announced November 2024.

Comments: 6 pages, 3 figures; Probing and Knocking with Muon (PKMu) Experiment Proposal Series 3 for Quantum

Journal ref: J. Phys. G: Nucl. Part. Phys. 52 075002 (2025)

Optical signatures of dynamical excitonic condensates

Authors: Alexander Osterkorn, Yuta Murakami, Tatsuya Kaneko, Zhiyuan Sun, Andrew J. Millis, Denis Golež

Abstract: We theoretically study dynamical excitonic condensates occurring in bilayers with an imposed chemical potential difference and in photodoped semiconductors. We show that optical spectroscopy can experimentally identify phase-trapped and phase-delocalized dynamical regimes of condensation. In the weak-bias regime, the trapped dynamics of the order parameter's phase lead to an in-gap absorption line… ▽ More We theoretically study dynamical excitonic condensates occurring in bilayers with an imposed chemical potential difference and in photodoped semiconductors. We show that optical spectroscopy can experimentally identify phase-trapped and phase-delocalized dynamical regimes of condensation. In the weak-bias regime, the trapped dynamics of the order parameter's phase lead to an in-gap absorption line at a frequency almost independent of the bias voltage, while for larger biases, the frequency of the spectral feature increases approximately linearly with bias. In both cases there is a pronounced second harmonic response. Close to the transition between the trapped and freely oscillating states, we find a strong response upon application of a weak electric probe field and compare the results to those found in a minimal model description for the dynamics of the order parameter's phase and analyze the limitations of the latter. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Quantum continual learning on a programmable superconducting processor

Authors: Chuanyu Zhang, Zhide Lu, Liangtian Zhao, Shibo Xu, Weikang Li, Ke Wang, Jiachen Chen, Yaozu Wu, Feitong Jin, Xuhao Zhu, Yu Gao, Ziqi Tan, Zhengyi Cui, Aosai Zhang, Ning Wang, Yiren Zou, Tingting Li, Fanhao Shen, Jiarun Zhong, Zehang Bao, Zitian Zhu, Zixuan Song, Jinfeng Deng, Hang Dong, Pengfei Zhang , et al. (10 additional authors not shown)

Abstract: Quantum computers may outperform classical computers on machine learning tasks. In recent years, a variety of quantum algorithms promising unparalleled potential to enhance, speed up, or innovate machine learning have been proposed. Yet, quantum learning systems, similar to their classical counterparts, may likewise suffer from the catastrophic forgetting problem, where training a model with new t… ▽ More Quantum computers may outperform classical computers on machine learning tasks. In recent years, a variety of quantum algorithms promising unparalleled potential to enhance, speed up, or innovate machine learning have been proposed. Yet, quantum learning systems, similar to their classical counterparts, may likewise suffer from the catastrophic forgetting problem, where training a model with new tasks would result in a dramatic performance drop for the previously learned ones. This problem is widely believed to be a crucial obstacle to achieving continual learning of multiple sequential tasks. Here, we report an experimental demonstration of quantum continual learning on a fully programmable superconducting processor. In particular, we sequentially train a quantum classifier with three tasks, two about identifying real-life images and the other on classifying quantum states, and demonstrate its catastrophic forgetting through experimentally observed rapid performance drops for prior tasks. To overcome this dilemma, we exploit the elastic weight consolidation strategy and show that the quantum classifier can incrementally learn and retain knowledge across the three distinct tasks, with an average prediction accuracy exceeding 92.3%. In addition, for sequential tasks involving quantum-engineered data, we demonstrate that the quantum classifier can achieve a better continual learning performance than a commonly used classical feedforward network with a comparable number of variational parameters. Our results establish a viable strategy for empowering quantum learning systems with desirable adaptability to multiple sequential tasks, marking an important primary experimental step towards the long-term goal of achieving quantum artificial general intelligence. △ Less

Submitted 15 September, 2024; originally announced September 2024.

Comments: 21 pages, 14 figures

Low Depth Phase Oracle Using a Parallel Piecewise Circuit

Authors: Zhu Sun, Gregory Boyd, Zhenyu Cai, Hamza Jnane, Balint Koczor, Richard Meister, Romy Minko, Benjamin Pring, Simon C. Benjamin, Nikitas Stamatopoulos

Abstract: We explore the important task of applying a phase $\exp(i\,f(x))$ to a computational basis state $\left| x \right>$. The closely related task of rotating a target qubit by an angle depending on $f(x)$ is also studied. Such operations are key in many quantum subroutines, and frequently $f(x)$ can be well-approximated by a piecewise function; examples range from the application of diagonal Hamiltoni… ▽ More We explore the important task of applying a phase $\exp(i\,f(x))$ to a computational basis state $\left| x \right>$. The closely related task of rotating a target qubit by an angle depending on $f(x)$ is also studied. Such operations are key in many quantum subroutines, and frequently $f(x)$ can be well-approximated by a piecewise function; examples range from the application of diagonal Hamiltonian terms (such as the Coulomb interaction) in grid-based many-body simulation, to derivative pricing algorithms. Here we exploit a parallelisation of the piecewise approach so that all constituent elementary rotations are performed simultaneously, that is, we achieve a total rotation depth of one. Moreover, we explore the use of recursive catalyst `towers' to implement these elementary rotations efficiently. We find that strategies prioritising execution speed can achieve circuit depth as low as $O(\log{n}{+}\log{S})$ for a register of $n$ qubits and a piecewise approximation of $S$ sections (presuming prior preparation of enabling resource states), albeit total qubit count then scales with $S$. In the limit of multiple repetitions of the oracle, we find that catalyst tower approaches have an $O(S\cdot n)$ T-count. △ Less

Submitted 27 July, 2025; v1 submitted 6 September, 2024; originally announced September 2024.

Comments: 16 pages, table I updated

Journal ref: Phys. Rev. A 111, 062420 (2025); Erratum: Phys. Rev. A 112, 019903 (2025)

Kibble-Zurek Behavior in the Boundary-obstructed Phase Transitions

Authors: Menghua Deng, Zhoujian Sun, Fuxiang Li

Abstract: We study the nonadiabatic dynamics of a two-dimensional higher-order topological insulator when the system is slowly quenched across the boundary-obstructed phase transition, which is characterized by edge band gap closing. We find that the number of excitations produced after the quench exhibits power-law scaling behaviors with the quench rate. Boundary conditions can drastically modify the sca… ▽ More We study the nonadiabatic dynamics of a two-dimensional higher-order topological insulator when the system is slowly quenched across the boundary-obstructed phase transition, which is characterized by edge band gap closing. We find that the number of excitations produced after the quench exhibits power-law scaling behaviors with the quench rate. Boundary conditions can drastically modify the scaling behaviors: The scaling exponent is found to be $α=1/2$ for hybridized and fully open boundary conditions, and $α=2$ for periodic boundary condition. We argue that the exponent $α=1/2$ cannot be explained by the Kibble-Zurek mechanism unless we adopt an effective dimension $d^{\rm eff}=1$ instead of the real dimension $d=2$. For comparison, we also investigate the slow quench dynamics across the bulk-obstructed phase transitions and a single multicritical point, which obeys the Kibble-Zurek mechanism with dimension $d=2$. △ Less

Submitted 11 July, 2024; originally announced July 2024.

Comments: 6+10 pages, 3=7 figures

Probing many-body Bell correlation depth with superconducting qubits

Authors: Ke Wang, Weikang Li, Shibo Xu, Mengyao Hu, Jiachen Chen, Yaozu Wu, Chuanyu Zhang, Feitong Jin, Xuhao Zhu, Yu Gao, Ziqi Tan, Aosai Zhang, Ning Wang, Yiren Zou, Tingting Li, Fanhao Shen, Jiarun Zhong, Zehang Bao, Zitian Zhu, Zixuan Song, Jinfeng Deng, Hang Dong, Xu Zhang, Pengfei Zhang, Wenjie Jiang , et al. (10 additional authors not shown)

Abstract: Quantum nonlocality describes a stronger form of quantum correlation than that of entanglement. It refutes Einstein's belief of local realism and is among the most distinctive and enigmatic features of quantum mechanics. It is a crucial resource for achieving quantum advantages in a variety of practical applications, ranging from cryptography and certified random number generation via self-testing… ▽ More Quantum nonlocality describes a stronger form of quantum correlation than that of entanglement. It refutes Einstein's belief of local realism and is among the most distinctive and enigmatic features of quantum mechanics. It is a crucial resource for achieving quantum advantages in a variety of practical applications, ranging from cryptography and certified random number generation via self-testing to machine learning. Nevertheless, the detection of nonlocality, especially in quantum many-body systems, is notoriously challenging. Here, we report an experimental certification of genuine multipartite Bell correlations, which signal nonlocality in quantum many-body systems, up to 24 qubits with a fully programmable superconducting quantum processor. In particular, we employ energy as a Bell correlation witness and variationally decrease the energy of a many-body system across a hierarchy of thresholds, below which an increasing Bell correlation depth can be certified from experimental data. As an illustrating example, we variationally prepare the low-energy state of a two-dimensional honeycomb model with 73 qubits and certify its Bell correlations by measuring an energy that surpasses the corresponding classical bound with up to 48 standard deviations. In addition, we variationally prepare a sequence of low-energy states and certify their genuine multipartite Bell correlations up to 24 qubits via energies measured efficiently by parity oscillation and multiple quantum coherence techniques. Our results establish a viable approach for preparing and certifying multipartite Bell correlations, which provide not only a finer benchmark beyond entanglement for quantum devices, but also a valuable guide towards exploiting multipartite Bell correlation in a wide spectrum of practical applications. △ Less

Submitted 25 June, 2024; originally announced June 2024.

Comments: 11 pages,6 figures + 14 pages, 6 figures

Journal ref: Physical Review X 15, 021024 (2025)

Circuit-Efficient Qubit-Excitation-based Variational Quantum Eigensolver

Authors: Zhijie Sun, Jie Liu, Zhenyu Li, Jinlong Yang

Abstract: The wave function Ansatze are crucial in the context of the Variational Quantum Eigensolver (VQE). In the Noisy Intermediate-Scale Quantum era, the design of low-depth wave function Ansatze is of great importance for executing quantum simulations of electronic structure on noisy quantum devices. In this work, we present a circuit-efficient implementation of two-body Qubit-Excitation-Based (QEB) op… ▽ More The wave function Ansatze are crucial in the context of the Variational Quantum Eigensolver (VQE). In the Noisy Intermediate-Scale Quantum era, the design of low-depth wave function Ansatze is of great importance for executing quantum simulations of electronic structure on noisy quantum devices. In this work, we present a circuit-efficient implementation of two-body Qubit-Excitation-Based (QEB) operator for building shallow-circuit wave function Ansatze within the framework of Adaptive Derivative-Assembled Pseudo-Trotter (ADAPT) VQE. This new algorithm is applied to study ground- and excited-sate problems for small molecules, demonstrating significant reduction of circuit depths compared to fermionic excitation-based and QEB ADAPT-VQE algorithms. This circuit-efficient algorithm shows great promise for quantum simulations of electronic structures, leading to improved performance on current quantum hardware. △ Less

Submitted 17 June, 2024; originally announced June 2024.

Characterizing dynamical criticality of many-body localization transitions from the Fock-space perspective

Authors: Zheng-Hang Sun, Yong-Yi Wang, Jian Cui, Heng Fan, Markus Heyl

Abstract: Characterizing the nature of many-body localization transitions (MBLTs) and their potential critical behaviors has remained a challenging problem. In this work, we study the dynamics of the displacement, quantifying the spread of the radial probability distribution in the Fock space, for three systems with MBLTs, i.e., the Hamiltonian models with quasiperiodic and random fields, as well as a rando… ▽ More Characterizing the nature of many-body localization transitions (MBLTs) and their potential critical behaviors has remained a challenging problem. In this work, we study the dynamics of the displacement, quantifying the spread of the radial probability distribution in the Fock space, for three systems with MBLTs, i.e., the Hamiltonian models with quasiperiodic and random fields, as well as a random-circuit Floquet model of a MBLT. We then perform a finite-size scaling analysis of the long-time averaged displacement by considering two types of ansatz for MBLTs, i.e., continuous and BKT transitions. The data collapse based on the assumption of a continuous phase transition with power-law correlation length reveals that the scaling exponent of the MBLT induced by random field is close to that of the Floquet model, but significantly differes from the quasiperiodic model. Additionally, we find that the BKT-type scaling provides a more accurate description of the MBLTs in the random model and the Floquet model, yielding larger (finite-size) critical points compared to those obtained from power-law scaling. Our work highlights that the displacement is a valuable tool for studying MBLTs, as relevant to ongoing experimental efforts. △ Less

Submitted 28 March, 2025; v1 submitted 28 May, 2024; originally announced May 2024.

Journal ref: Phys. Rev. B 111,094210(2025)

Realizing triality and $p$-ality by lattice twisted gauging in (1+1)d quantum spin systems

Authors: Da-Chuan Lu, Zhengdi Sun, Yi-Zhuang You

Abstract: In this paper, we study the twisted gauging on the (1+1)d lattice and construct various non-local mappings on the lattice operators. To be specific, we define the twisted Gauss law operator and implement the twisted gauging of the finite group on the lattice motivated by the orbifolding procedure in the conformal field theory, which involves the data of non-trivial element in the second cohomology… ▽ More In this paper, we study the twisted gauging on the (1+1)d lattice and construct various non-local mappings on the lattice operators. To be specific, we define the twisted Gauss law operator and implement the twisted gauging of the finite group on the lattice motivated by the orbifolding procedure in the conformal field theory, which involves the data of non-trivial element in the second cohomology group of the gauge group. We show the twisted gauging is equivalent to the two-step procedure of first applying the SPT entangler and then untwisted gauging. We use the twisted gauging to construct the triality (order 3) and $p$-ality (order $p$) mapping on the $\mathbb{Z}_p\times \mathbb{Z}_p$ symmetric Hamiltonians, where $p$ is a prime. Such novel non-local mappings generalize Kramers-Wannier duality and they preserve the locality of symmetric operators but map charged operators to non-local ones. We further construct quantum process to realize these non-local mappings and analyze the induced mappings on the phase diagrams. For theories that are invariant under these non-local mappings, they admit the corresponding non-invertible symmetries. The non-invertible symmetry will constrain the theory at the multicritical point between the gapped phases. We further give the condition when the non-invertible symmetry can have symmetric gapped phase with a unique ground state. △ Less

Submitted 25 June, 2024; v1 submitted 23 May, 2024; originally announced May 2024.

Comments: 49 pages, 12 figures, 3 tables. v2 updates references and minor changes. Comments are welcome

Journal ref: SciPost Phys. 17, 136 (2024)

Calibration of the Cryogenic Measurement System of a Resonant Haloscope Cavity

Authors: Dong He, Jie Fan, Xin Gao, Yu Gao, Nick Houston, Zhongqing Ji, Yirong Jin, Chuang Li, Jinmian Li, Tianjun Li, Shi-hang Liu, Jia-Shu Niu, Zhihui Peng, Liang Sun, Zheng Sun, Jia Wang, Puxian Wei, Lina Wu, Zhongchen Xiang, Qiaoli Yang, Chi Zhang, Wenxing Zhang, Xin Zhang, Dongning Zheng, Ruifeng Zheng , et al. (1 additional authors not shown)

Abstract: Possible light bosonic dark matter interactions with the Standard Model photon have been searched by microwave resonant cavities. In this paper, we demonstrate the cryogenic readout system calibration of a 7.138 GHz copper cavity with a loaded quality factor $Q_l=10^4$, operated at 22 mK temperature based on a dilution refrigerator. Our readout system consists of High Electron Mobility Transistors… ▽ More Possible light bosonic dark matter interactions with the Standard Model photon have been searched by microwave resonant cavities. In this paper, we demonstrate the cryogenic readout system calibration of a 7.138 GHz copper cavity with a loaded quality factor $Q_l=10^4$, operated at 22 mK temperature based on a dilution refrigerator. Our readout system consists of High Electron Mobility Transistors as cryogenic amplifiers at 4 K, plus room-temperature amplifiers and a spectrum analyzer for signal power detection. We test the system with a superconducting two-level system as a single-photon source in the microwave frequency regime and report an overall 95.6 dB system gain and -71.4 dB attenuation in the cavity's input channel. The effective noise temperature of the measurement system is 7.5 K. △ Less

Submitted 15 April, 2024; originally announced April 2024.

Comments: 7 pages, 5 figures, version to appear in CPC

Quantum integrated sensing and communication via entanglement

Authors: Yu-Chen Liu, Yuan-Bin Cheng, Xing-Bo Pan, Ze-Zhou Sun, Dong Pan, Gui-Lu Long

Abstract: Quantum communication and quantum metrology are widely compelling applications in the field of quantum information science, and quantum remote sensing is an intersection of both. Despite their differences, there are notable commonalities between quantum communication and quantum remote sensing, as they achieve their functionalities through the transmission of quantum states. Here we propose a nove… ▽ More Quantum communication and quantum metrology are widely compelling applications in the field of quantum information science, and quantum remote sensing is an intersection of both. Despite their differences, there are notable commonalities between quantum communication and quantum remote sensing, as they achieve their functionalities through the transmission of quantum states. Here we propose a novel quantum integrated sensing and communication (QISAC) protocol, which achieves quantum sensing under the Heisenberg limit while simultaneously enabling quantum secure communication through the transmission of entanglements. We have theoretically proven its security against eavesdroppers. The security of QISAC is characterized by the secrecy capacity for information bit as well as asymmetric Fisher information gain for sensing. Through simulations conducted under the constraints of limited entanglement resources, we illustrate that QISAC maintains high accuracy in the estimation of phase. Hence our QISAC offers a fresh perspective for the applications of future quantum networks. △ Less

Submitted 26 September, 2024; v1 submitted 12 April, 2024; originally announced April 2024.

Journal ref: Physical Review Applied 22 (3), 034051 (2024)

Non-Abelian braiding of Fibonacci anyons with a superconducting processor

Authors: Shibo Xu, Zheng-Zhi Sun, Ke Wang, Hekang Li, Zitian Zhu, Hang Dong, Jinfeng Deng, Xu Zhang, Jiachen Chen, Yaozu Wu, Chuanyu Zhang, Feitong Jin, Xuhao Zhu, Yu Gao, Aosai Zhang, Ning Wang, Yiren Zou, Ziqi Tan, Fanhao Shen, Jiarun Zhong, Zehang Bao, Weikang Li, Wenjie Jiang, Li-Wei Yu, Zixuan Song , et al. (7 additional authors not shown)

Abstract: Non-Abelian topological orders offer an intriguing path towards fault-tolerant quantum computation, where information can be encoded and manipulated in a topologically protected manner immune to arbitrary local noises and perturbations. However, realizing non-Abelian topologically ordered states is notoriously challenging in both condensed matter and programmable quantum systems, and it was not un… ▽ More Non-Abelian topological orders offer an intriguing path towards fault-tolerant quantum computation, where information can be encoded and manipulated in a topologically protected manner immune to arbitrary local noises and perturbations. However, realizing non-Abelian topologically ordered states is notoriously challenging in both condensed matter and programmable quantum systems, and it was not until recently that signatures of non-Abelian statistics were observed through digital quantum simulation approaches. Despite these exciting progresses, none of them has demonstrated the appropriate type of topological orders and associated non-Abelian anyons whose braidings alone support universal quantum computation. Here, we report the realization of non-Abelian topologically ordered states of the Fibonacci string-net model and demonstrate braidings of Fibonacci anyons featuring universal computational power, with a superconducting quantum processor. We exploit efficient quantum circuits to prepare the desired states and verify their nontrivial topological nature by measuring the topological entanglement entropy. In addition, we create two pairs of Fibonacci anyons and demonstrate their fusion rule and non-Abelian braiding statistics by applying unitary gates on the underlying physical qubits. Our results establish a versatile digital approach to exploring exotic non-Abelian topological states and their associated braiding statistics with current noisy intermediate-scale quantum processors. △ Less

Submitted 29 March, 2024; originally announced April 2024.

Exploring Hilbert-Space Fragmentation on a Superconducting Processor

Authors: Yong-Yi Wang, Yun-Hao Shi, Zheng-Hang Sun, Chi-Tong Chen, Zheng-An Wang, Kui Zhao, Hao-Tian Liu, Wei-Guo Ma, Ziting Wang, Hao Li, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Tian-Ming Li, Yang He, Zheng-He Liu, Zhen-Yu Peng, Xiaohui Song, Guangming Xue, Haifeng Yu, Kaixuan Huang, Zhongcheng Xiang, Dongning Zheng, Kai Xu, Heng Fan

Abstract: Isolated interacting quantum systems generally thermalize, yet there are several counterexamples for the breakdown of ergodicity, such as many-body localization and quantum scars. Recently, ergodicity breaking has been observed in systems subjected to linear potentials, termed Stark many-body localization. This phenomenon is closely associated with Hilbert-space fragmentation, characterized by a s… ▽ More Isolated interacting quantum systems generally thermalize, yet there are several counterexamples for the breakdown of ergodicity, such as many-body localization and quantum scars. Recently, ergodicity breaking has been observed in systems subjected to linear potentials, termed Stark many-body localization. This phenomenon is closely associated with Hilbert-space fragmentation, characterized by a strong dependence of dynamics on initial conditions. Here, we experimentally explore initial-state dependent dynamics using a ladder-type superconducting processor with up to 24 qubits, which enables precise control of the qubit frequency and initial state preparation. In systems with linear potentials, we observe distinct non-equilibrium dynamics for initial states with the same quantum numbers and energy, but with varying domain wall numbers. This distinction becomes increasingly pronounced as the system size grows, in contrast with disordered interacting systems. Our results provide convincing experimental evidence of the fragmentation in Stark systems, enriching our understanding of the weak breakdown of ergodicity. △ Less

Submitted 14 March, 2024; originally announced March 2024.

Comments: main text: 7 pages, 4 figures; supplementary: 13 pages, 14 figures

Journal ref: PRX Quantum 6, 010325 (2025)

Quantum Generative Diffusion Model: A Fully Quantum-Mechanical Model for Generating Quantum State Ensemble

Authors: Chuangtao Chen, Qinglin Zhao, MengChu Zhou, Zhimin He, Zhili Sun, Haozhen Situ

Abstract: Classical diffusion models have shown superior generative results. Exploring them in the quantum domain can advance the field of quantum generative learning. This work introduces Quantum Generative Diffusion Model (QGDM) as their simple and elegant quantum counterpart. Through a non-unitary forward process, any target quantum state can be transformed into a completely mixed state that has the high… ▽ More Classical diffusion models have shown superior generative results. Exploring them in the quantum domain can advance the field of quantum generative learning. This work introduces Quantum Generative Diffusion Model (QGDM) as their simple and elegant quantum counterpart. Through a non-unitary forward process, any target quantum state can be transformed into a completely mixed state that has the highest entropy and maximum uncertainty about the system. A trainable backward process is used to recover the former from the latter. The design requirements for its backward process includes non-unitarity and small parameter count. We introduce partial trace operations to enforce non-unitary and reduce the number of trainable parameters by using a parameter-sharing strategy and incorporating temporal information as an input in the backward process. We present QGDM's resource-efficient version to reduce auxiliary qubits while preserving generative capabilities. QGDM exhibits faster convergence than Quantum Generative Adversarial Network (QGAN) because its adopted convex-based optimization can result in better convergence. The results of comparing it with QGAN demonstrate its effectiveness in generating both pure and mixed quantum states. It can achieve 53.02% higher fidelity in mixed-state generation than QGAN. The results highlight its great potential to tackle challenging quantum generation tasks. △ Less

Submitted 3 August, 2024; v1 submitted 13 January, 2024; originally announced January 2024.

Comments: Comments are welcome

Long-lived topological time-crystalline order on a quantum processor

Authors: Liang Xiang, Wenjie Jiang, Zehang Bao, Zixuan Song, Shibo Xu, Ke Wang, Jiachen Chen, Feitong Jin, Xuhao Zhu, Zitian Zhu, Fanhao Shen, Ning Wang, Chuanyu Zhang, Yaozu Wu, Yiren Zou, Jiarun Zhong, Zhengyi Cui, Aosai Zhang, Ziqi Tan, Tingting Li, Yu Gao, Jinfeng Deng, Xu Zhang, Hang Dong, Pengfei Zhang , et al. (16 additional authors not shown)

Abstract: Topologically ordered phases of matter elude Landau's symmetry-breaking theory, featuring a variety of intriguing properties such as long-range entanglement and intrinsic robustness against local perturbations. Their extension to periodically driven systems gives rise to exotic new phenomena that are forbidden in thermal equilibrium. Here, we report the observation of signatures of such a phenomen… ▽ More Topologically ordered phases of matter elude Landau's symmetry-breaking theory, featuring a variety of intriguing properties such as long-range entanglement and intrinsic robustness against local perturbations. Their extension to periodically driven systems gives rise to exotic new phenomena that are forbidden in thermal equilibrium. Here, we report the observation of signatures of such a phenomenon -- a prethermal topologically ordered time crystal -- with programmable superconducting qubits arranged on a square lattice. By periodically driving the superconducting qubits with a surface-code Hamiltonian, we observe discrete time-translation symmetry breaking dynamics that is only manifested in the subharmonic temporal response of nonlocal logical operators. We further connect the observed dynamics to the underlying topological order by measuring a nonzero topological entanglement entropy and studying its subsequent dynamics. Our results demonstrate the potential to explore exotic topologically ordered nonequilibrium phases of matter with noisy intermediate-scale quantum processors. △ Less

Submitted 8 January, 2024; originally announced January 2024.

Comments: 8 pages (main text), 16 pages (supplementary information)

Floquet engineering of many-body states by the ponderomotive potential

Authors: Zhiyuan Sun

Abstract: The ponderomotive force is an effective static force that a particle feels in an oscillating field, whose static potential may be called the ponderomotive potential. We generalize this notion to periodically driven quantum many-body systems, and propose it as a convenient tool to engineer their non-equilibrium steady states beyond the single particle level. Applied to materials driven by light, th… ▽ More The ponderomotive force is an effective static force that a particle feels in an oscillating field, whose static potential may be called the ponderomotive potential. We generalize this notion to periodically driven quantum many-body systems, and propose it as a convenient tool to engineer their non-equilibrium steady states beyond the single particle level. Applied to materials driven by light, the ponderomotive potential is intimately related to the equilibrium optical conductivity, which is enhanced close to resonances. We show that the ponderomotive potential from the incident light may be used to induce exciton condensates in semiconductors, to generate attractive interactions leading to superconductivity in certain electron-phonon systems, and to create additional free energy minima in systems with charge/spin/excitonic orders. These effects are presented with experimentally relevant parameters. △ Less

Submitted 6 September, 2024; v1 submitted 8 December, 2023; originally announced December 2023.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 110, 104301 (2024)

Universal scalefree non-Hermitian skin effect near the Bloch point

Authors: Wei Li, Zhoujian Sun, Ze Yang, Fuxiang Li

Abstract: The scalefree non-Hermitian skin effect (NHSE) refers to the phenomenon that the localization length of skin modes scales proportionally with system size in non-Hermitian systems. Authors of recent studies have demonstrated that the scalefree NHSE can be induced through various mechanisms, including the critical NHSE, local non-Hermiticity, and the boundary impurity effect. Nevertheless, these met… ▽ More The scalefree non-Hermitian skin effect (NHSE) refers to the phenomenon that the localization length of skin modes scales proportionally with system size in non-Hermitian systems. Authors of recent studies have demonstrated that the scalefree NHSE can be induced through various mechanisms, including the critical NHSE, local non-Hermiticity, and the boundary impurity effect. Nevertheless, these methods require careful modeling and precise parameter tuning. In contrast, in this paper, we suggest that the scalefree NHSE is a universal phenomenon, observable in extensive systems if these systems can be described by non-Bloch band theory and host Bloch points on the energy spectrum in the thermodynamic limit. Crucially, we discover that the geometry of the generalized Brillouin zone determines the scaling rule of the localization length, which can scale either linearly or quadratically with the system size. In this paper, we enriches the phenomenon of the scalefree NHSE. △ Less

Submitted 13 January, 2024; v1 submitted 24 November, 2023; originally announced November 2023.

Journal ref: Phys. Rev. B 109 (2024) 035119

Probing spin hydrodynamics on a superconducting quantum simulator

Authors: Yun-Hao Shi, Zheng-Hang Sun, Yong-Yi Wang, Zheng-An Wang, Yu-Ran Zhang, Wei-Guo Ma, Hao-Tian Liu, Kui Zhao, Jia-Cheng Song, Gui-Han Liang, Zheng-Yang Mei, Jia-Chi Zhang, Hao Li, Chi-Tong Chen, Xiaohui Song, Jieci Wang, Guangming Xue, Haifeng Yu, Kaixuan Huang, Zhongcheng Xiang, Kai Xu, Dongning Zheng, Heng Fan

Abstract: Characterizing the nature of hydrodynamical transport properties in quantum dynamics provides valuable insights into the fundamental understanding of exotic non-equilibrium phases of matter. Experimentally simulating infinite-temperature transport on large-scale complex quantum systems is of considerable interest. Here, using a controllable and coherent superconducting quantum simulator, we experi… ▽ More Characterizing the nature of hydrodynamical transport properties in quantum dynamics provides valuable insights into the fundamental understanding of exotic non-equilibrium phases of matter. Experimentally simulating infinite-temperature transport on large-scale complex quantum systems is of considerable interest. Here, using a controllable and coherent superconducting quantum simulator, we experimentally realize the analog quantum circuit, which can efficiently prepare the Haar-random states, and probe spin transport at infinite temperature. We observe diffusive spin transport during the unitary evolution of the ladder-type quantum simulator with ergodic dynamics. Moreover, we explore the transport properties of the systems subjected to strong disorder or a tilted potential, revealing signatures of anomalous subdiffusion in accompany with the breakdown of thermalization. Our work demonstrates a scalable method of probing infinite-temperature spin transport on analog quantum simulators, which paves the way to study other intriguing out-of-equilibrium phenomena from the perspective of transport. △ Less

Submitted 1 September, 2024; v1 submitted 10 October, 2023; originally announced October 2023.

Comments: Main text: 13 pages, 5 figures; Supplementary: 17 pages, 16 figures, 1 table

Journal ref: Nat. Commun. 15, 7573 (2024)

Learning Informative Latent Representation for Quantum State Tomography

Authors: Hailan Ma, Zhenhong Sun, Daoyi Dong, Dong Gong

Abstract: Quantum state tomography (QST) is the process of reconstructing the complete state of a quantum system (mathematically described as a density matrix) through a series of different measurements. These measurements are performed on a number of identical copies of the quantum system, with outcomes gathered as frequencies. QST aims to recover the density matrix or the properties of the quantum state f… ▽ More Quantum state tomography (QST) is the process of reconstructing the complete state of a quantum system (mathematically described as a density matrix) through a series of different measurements. These measurements are performed on a number of identical copies of the quantum system, with outcomes gathered as frequencies. QST aims to recover the density matrix or the properties of the quantum state from the measured frequencies. Although an informationally complete set of measurements can specify the quantum state accurately in an ideal scenario with a large number of identical copies, both the measurements and identical copies are restricted and imperfect in practical scenarios, making QST highly ill-posed. The conventional QST methods usually assume accurate measured frequencies or rely on manually designed regularizers to handle the ill-posed reconstruction problem, suffering from limited applications in realistic scenarios. Recent advances in deep neural networks (DNN) led to the emergence of deep learning in QST. However, existing DL-based QST approaches often employ generic DNN models that are not optimized for imperfect conditions of QST. In this paper, we propose a transformer-based autoencoder architecture tailored for QST with imperfect measurement data. Our method leverages a transformer-based encoder to extract an informative latent representation (ILR) from imperfect measurement data and employs a decoder to predict the quantum states based on the ILR. We anticipate that the high-dimensional ILR will capture more comprehensive information about the quantum states. To achieve this, we conduct pre-training of the encoder using a pretext task that involves reconstructing high-quality frequencies from measured frequencies. Extensive simulations and experiments demonstrate the remarkable ability of the informative latent representation to deal with imperfect measurement data in QST. △ Less

Submitted 7 January, 2025; v1 submitted 30 September, 2023; originally announced October 2023.

Investigations of 2D ion crystals in a hybrid optical cavity trap for quantum information processing

Authors: Zewen Sun, Yi Hong Teoh, Fereshteh Rajabi, Rajibul Islam

Abstract: We numerically investigate a hybrid trapping architecture for 2D ion crystals using static electrode voltages and optical cavity fields for in-plane and out-of-plane confinements, respectively. By studying the stability of 2D crystals against 2D-3D structural phase transitions, we identify the necessary trapping parameters for ytterbium ions. Multiple equilibrium configurations for 2D crystals are… ▽ More We numerically investigate a hybrid trapping architecture for 2D ion crystals using static electrode voltages and optical cavity fields for in-plane and out-of-plane confinements, respectively. By studying the stability of 2D crystals against 2D-3D structural phase transitions, we identify the necessary trapping parameters for ytterbium ions. Multiple equilibrium configurations for 2D crystals are possible, and we analyze their stability by estimating potential barriers between them. We find that scattering to anti-trapping states limits the trapping lifetime, which is consistent with recent experiments employing other optical trapping architectures. These 2D ion crystals offer an excellent platform for quantum simulation of frustrated spin systems, benefiting from their 2D triangular lattice structure and phonon-mediated spin-spin interactions. Quantum information processing with tens of ions is feasible in this scheme with current technologies. △ Less

Submitted 17 August, 2023; originally announced August 2023.

Comments: 10 pages, 7 figures, 1 table

Variational generation of spin squeezing on one-dimensional quantum devices with nearest-neighbor interactions

Authors: Zheng-Hang Sun, Yong-Yi Wang, Yu-Ran Zhang, Franco Nori, Heng Fan

Abstract: Efficient preparation of spin-squeezed states is important for quantum-enhanced metrology. Current protocols for generating strong spin squeezing rely on either high dimensionality or long-range interactions. A key challenge is how to generate considerable spin squeezing in one-dimensional systems with only nearest-neighbor interactions. Here, we develop variational spin-squeezing algorithms to so… ▽ More Efficient preparation of spin-squeezed states is important for quantum-enhanced metrology. Current protocols for generating strong spin squeezing rely on either high dimensionality or long-range interactions. A key challenge is how to generate considerable spin squeezing in one-dimensional systems with only nearest-neighbor interactions. Here, we develop variational spin-squeezing algorithms to solve this problem. We consider both digital and analog quantum circuits for these variational algorithms. After the closed optimization loop of the variational spin-squeezing algorithms, the generated squeezing can be comparable to the strongest squeezing created from two-axis twisting. By analyzing the experimental imperfections, the variational spin-squeezing algorithms proposed in this work are feasible in recent developed noisy intermediate-scale quantum computers. △ Less

Submitted 26 December, 2023; v1 submitted 28 June, 2023; originally announced June 2023.

Journal ref: Phys. Rev. Research 5, 043285 (2023)

Entanglement generation and detection in split exciton-polariton condensates

Authors: Jingyan Feng, Hui Li, Zheng Sun, Tim Byrnes

Abstract: We propose a method of generating and detecting entanglement in two spatially separated excitonpolariton Bose-Einstein condensates (BECs) at steady-state. In our scheme we first create a spinor polariton BEC, such that steady-state squeezing is obtained under a one-axis twisting interaction. Then the condensate is split either physically or virtually, which results in entanglement generated betwee… ▽ More We propose a method of generating and detecting entanglement in two spatially separated excitonpolariton Bose-Einstein condensates (BECs) at steady-state. In our scheme we first create a spinor polariton BEC, such that steady-state squeezing is obtained under a one-axis twisting interaction. Then the condensate is split either physically or virtually, which results in entanglement generated between the two parts. A virtual split means that the condensate is not physically split, but its near-field image is divided into two parts and the spin correlations are deduced from polarization measurements in each half. We theoretically model and examine logarithmic negativity criterion and several correlation-based criteria to show that entanglement exists under experimentally achievable parameters. △ Less

Submitted 19 May, 2023; originally announced May 2023.

Comments: 10 pages, 5 figures

Tomography of Quantum States from Structured Measurements via quantum-aware transformer

Authors: Hailan Ma, Zhenhong Sun, Daoyi Dong, Chunlin Chen, Herschel Rabitz

Abstract: Quantum state tomography (QST) is the process of reconstructing the state of a quantum system (mathematically described as a density matrix) through a series of different measurements, which can be solved by learning a parameterized function to translate experimentally measured statistics into physical density matrices. However, the specific structure of quantum measurements for characterizing a q… ▽ More Quantum state tomography (QST) is the process of reconstructing the state of a quantum system (mathematically described as a density matrix) through a series of different measurements, which can be solved by learning a parameterized function to translate experimentally measured statistics into physical density matrices. However, the specific structure of quantum measurements for characterizing a quantum state has been neglected in previous work. In this paper, we explore the similarity between highly structured sentences in natural language and intrinsically structured measurements in QST. To fully leverage the intrinsic quantum characteristics involved in QST, we design a quantum-aware transformer (QAT) model to capture the complex relationship between measured frequencies and density matrices. In particular, we query quantum operators in the architecture to facilitate informative representations of quantum data and integrate the Bures distance into the loss function to evaluate quantum state fidelity, thereby enabling the reconstruction of quantum states from measured data with high fidelity. Extensive simulations and experiments (on IBM quantum computers) demonstrate the superiority of the QAT in reconstructing quantum states with favorable robustness against experimental noise. △ Less

Submitted 28 March, 2025; v1 submitted 9 May, 2023; originally announced May 2023.

Entanglement entropy of nuclear systems

Authors: Chenyi Gu, Z. H. Sun, G. Hagen, T. Papenbrock

Abstract: We study entanglement entropies between the single-particle states of the hole space and its complement in nuclear systems. Analytical results based on the coupled-cluster method show that entanglement entropies are proportional to the particle number fluctuation and the depletion number of the hole space for sufficiently weak interactions. General arguments also suggest that the entanglement entr… ▽ More We study entanglement entropies between the single-particle states of the hole space and its complement in nuclear systems. Analytical results based on the coupled-cluster method show that entanglement entropies are proportional to the particle number fluctuation and the depletion number of the hole space for sufficiently weak interactions. General arguments also suggest that the entanglement entropy in nuclear systems fulfills a volume instead of an area law. We test and confirm these results by computing entanglement entropies of the pairing model and neutron matter, and the depletion number of finite nuclei. △ Less

Submitted 8 March, 2023; originally announced March 2023.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. C 108, 054309 (2023)

The effect of static disorder on the center line slope in 2D electronic spectroscopy

Authors: Zong-Hao Sun, Yi-Xuan Yao, Qing Ai, Yuan-Chung Cheng

Abstract: Two-dimensional electronic spectroscopy (2DES) is a powerful tool for investigating the dynamics of complex systems. However, analyzing the resulting spectra can be challenging, and thus may require the use of theoretical modeling techniques. The center line slope (CLS) method is one of such approaches, which aims to extract the time correlation function (TCF) from 2DES with minimal error. Since s… ▽ More Two-dimensional electronic spectroscopy (2DES) is a powerful tool for investigating the dynamics of complex systems. However, analyzing the resulting spectra can be challenging, and thus may require the use of theoretical modeling techniques. The center line slope (CLS) method is one of such approaches, which aims to extract the time correlation function (TCF) from 2DES with minimal error. Since static disorder is widely observed in complex systems, it may be interesting to ask whether the CLS approach still work in the presence of the static disorder. In this paper, the effect of the static disorder on the TCF obtained through the CLS method is investigated. It is found that the steady-state value of the CLS increases monotonically with respect to the static disorder, which suggests that the amplitude of the static disorder can be determined using the CLS in the long-time limit. Additionally, as the static disorder rises, the decay rate of the CLS first decreases to a certain value and remains at this value until the static disorder is sufficiently large. Afterward, the CLS begins to fluctuate significantly and thus results in obtaining the decay rate through the CLS method unreliable. Based on these discoveries, we propose a method to fix the error and obtain the TCF. Our findings may pave the way for obtaining reliable system-bath information by analyzing 2DES in the practical situations. △ Less

Submitted 20 February, 2023; originally announced February 2023.

Comments: 10 pages, 6 figures

Journal ref: Adv. Quantum Tech. 6, 2300163 (2023)