Showing 1–50 of 93 results for author: Shen, J

Quantum Kernels and the Cross-Section of Stock Returns: Anatomy of a Vanishing Advantage

Authors: Junchi Shen

Abstract: Do quantum kernels improve cross-sectional stock return prediction? We run a controlled horse race on the Chinese A-share market in which a quantum fidelity kernel, a projected quantum kernel, and a classical RBF control share identical training subsamples, solver, and tuning budgets, so that only the kernel is exchanged. On the main evaluation -- a point-in-time universe and 170 walk-forward wind… ▽ More Do quantum kernels improve cross-sectional stock return prediction? We run a controlled horse race on the Chinese A-share market in which a quantum fidelity kernel, a projected quantum kernel, and a classical RBF control share identical training subsamples, solver, and tuning budgets, so that only the kernel is exchanged. On the main evaluation -- a point-in-time universe and 170 walk-forward windows (2012-2025) -- no quantum advantage exists: the fidelity kernel is indistinguishable from its RBF control ($Δ$IC $=+0.005$, $p=0.42$), and a $2\times2$ design crossing kernel type with training budget (a Nystrom extension to the full ~38,000-observation windows) shows quantum kernels matching, but never beating, equal-budget linear models; after family-wise correction no pairwise difference among eleven models is significant, with point estimates favoring penalized linear regressions throughout. We then document how the opposite conclusion arises: a 60-window evaluation on a universe screened with full-sample information makes the same quantum kernel appear dominant on stability criteria and significantly better than neural baselines. Interaction characteristics from the anomalies literature help nothing, quantum or classical; a widened bandwidth grid reveals an interior optimum rather than the near-classical endpoint a coarse grid suggests; and the geometric difference, while large throughout ($g \gg 1$), does not predict out-of-sample gains ($ρ=-0.20$). We propose protocol standards -- kernel-swap controls, budget-equalized comparisons, point-in-time universes, and multiplicity-robust inference -- for empirical claims of quantum advantage in finance. △ Less

Submitted 22 July, 2026; originally announced July 2026.

Comments: 16 pages, 2 figures, 5 tables. Code and data pipeline available on request

Routing Techniques for Error-Corrected Silicon Spin Qubit Quantum Architectures

Authors: Julian Shen, Ludwig Schmid, Robert Wille

Abstract: Silicon spin qubits have emerged as a promising qubit technology due to their favorable scaling and fabrication properties. However, efficiently compiling quantum circuits onto spin qubit platforms remains challenging, particularly when accounting for hardware constraints and the high sensitivity to static defects. Existing compilation approaches for spin qubits either largely ignore error correct… ▽ More Silicon spin qubits have emerged as a promising qubit technology due to their favorable scaling and fabrication properties. However, efficiently compiling quantum circuits onto spin qubit platforms remains challenging, particularly when accounting for hardware constraints and the high sensitivity to static defects. Existing compilation approaches for spin qubits either largely ignore error correction, despite its critical role for large-scale quantum computation, or focus on low-level schedule constructions, missing a high-level compilation and routing for logical, error-corrected algorithms. To address this gap, we introduce a compilation framework for spin qubits based on the recent snakes on a plane model, which utilizes a 2D surface code and qubit teleportation to mitigate errors. Building on this model, we propose shortest-path and rotation-based algorithms as two novel classes of qubit-routing techniques, along with additional defect-handling and initial-mapping strategies. We evaluate both algorithms across diverse architectural settings and problem sizes, demonstrating that shortest-path methods excel in sparse, low-defect scenarios, while rotation-based approaches perform better in high-density environments. An open-source implementation of our framework is publicly available on GitHub as part of the Munich Quantum Toolkit (MQT) at https://github.com/munich-quantum-toolkit/spin-qubit-routing. △ Less

Submitted 25 July, 2026; v1 submitted 8 July, 2026; originally announced July 2026.

Comments: 11 pages, 14 figures, 1 table, minor changes, accepted at the IEEE International Conference on Quantum Computing and Engineering (QCE), 2026

Quantum-Informed Portfolio Selection: An End-to-End Pipeline Validated on Trapped-Ion Hardware with Real Market Data

Authors: Romina Yalovetzky, Martin J. A. Schuetz, Zichang He, Jiayu Shen, Yue Sun, Rudy Raymond, Shauna Sahay, Kishore Perla, Ruben S. Andrist, Grant Salton, Helmut G. Katzgraber, Roger Bongiovanni, Niraj Kumar, Rob Otter

Abstract: Portfolio diversification - a cornerstone of modern investment management - can be formulated as a Maximum Independent Set (MIS) problem on asset correlation graphs. Solving this problem at scale is computationally challenging, motivating the exploration of quantum algorithms for practical financial optimization. We propose an end-to-end pipeline leveraging qReduMIS, a recursive hybrid quantum-cla… ▽ More Portfolio diversification - a cornerstone of modern investment management - can be formulated as a Maximum Independent Set (MIS) problem on asset correlation graphs. Solving this problem at scale is computationally challenging, motivating the exploration of quantum algorithms for practical financial optimization. We propose an end-to-end pipeline leveraging qReduMIS, a recursive hybrid quantum-classical algorithm. Rather than using quantum optimization to directly produce a final solution, qReduMIS leverages independent set measurements from the Quantum Approximate Optimization Algorithm (QAOA) to identify frozen nodes - vertices likely to belong to optimal solutions - thereby guiding and unblocking subsequent (provably optimal) classical reductions on the remaining graph. We benchmark qReduMIS on real financial data from four major market indices with up to 225 assets, executing experiments on Quantinuum's 98-qubit trapped-ion Helios system, with QAOA circuits acting on kernels of up to 78 qubits and 1016 two-qubit gates. While standalone QAOA fails to find the optimal solution for two of the largest indices (S&P 100 and Nikkei 225), qReduMIS achieves success probabilities of $0.40$ and $0.95$, respectively, with average approximation ratios $\geq 0.96$ across all four indices. We perform a systematic benchmark on the Quantinuum H2-1 noisy emulator over 73 asset correlation graphs of varying size showing that, for $p=2$ QAOA layers, the optimal time-to-solution scaling exponent of qReduMIS is $3.2$ times smaller than that of standalone QAOA. △ Less

Submitted 13 July, 2026; v1 submitted 1 July, 2026; originally announced July 2026.

A superconducting surface-code processor with lattice-surgery logical operations

Authors: Yanzhe Wang, Fanhao Shen, Haipeng Xie, Aosai Zhang, Yu Gao, Chuanyu Zhang, Xuhao Zhu, Feitong Jin, Yiren Zou, Ning Wang, Zhengyi Cui, Zehang Bao, Zitian Zhu, Jiarun Zhong, Gongyu Liu, Jia-Nan Yang, Yihang Han, Yiyang He, Jiayuan Shen, Han Wang, Jiahua Huang, Xinrong Zhang, Sailang Zhou, Hang Dong, Jinfeng Deng , et al. (9 additional authors not shown)

Abstract: Fault-tolerant logical operations are fundamental for scalable quantum computation. Here, we report the experimental realization of lattice-surgery operations between a pair of distance-three surface-code logical qubits on a planar superconducting processor. During repeated syndrome extraction cycles, the logical qubits exhibit per-cycle error rates of $0.0365(2)$ and $0.0282(1)$, respectively, af… ▽ More Fault-tolerant logical operations are fundamental for scalable quantum computation. Here, we report the experimental realization of lattice-surgery operations between a pair of distance-three surface-code logical qubits on a planar superconducting processor. During repeated syndrome extraction cycles, the logical qubits exhibit per-cycle error rates of $0.0365(2)$ and $0.0282(1)$, respectively, after leakage events are rejected. By leveraging joint initialization and lattice splitting, we deterministically prepare a logical Bell state, confirming genuine bipartite entanglement via the error-corrected logical state fidelity. We further execute a two-qubit Deutsch-Jozsa algorithm at the logical level to demonstrate algorithmic utility in a fault-tolerant framework. Finally, to achieve universal control, we implement magic-state injection and gate teleportation to realize continuous non-Clifford rotations about the logical $X$ axis. For the logical $R_{X}(π/4)$ gate, we achieve a logical gate fidelity of $0.943_{-9}^{+10}$ conditioned on the absence of detected errors. These results establish lattice surgery as a practical and versatile paradigm for logical computation in near-term surface-code architectures, representing a critical milestone toward scalable fault-tolerant quantum advantage in superconducting circuits. △ Less

Submitted 4 June, 2026; originally announced June 2026.

Teleportation transition of surface codes on a superconducting quantum processor

Authors: Yiren Zou, Hong-Kuan Xia, Aosai Zhang, Xuhao Zhu, Feitong Jin, Qingyuan Wang, Yu Gao, Chuanyu Zhang, Ning Wang, Zhengyi Cui, Fanhao Shen, Zehang Bao, Zitian Zhu, Jiarun Zhong, Gongyu Liu, Jia-Nan Yang, Yihang Han, Yiyang He, Jiayuan Shen, Han Wang, Yanzhe Wang, Jiahua Huang, Xinrong Zhang, Sailang Zhou, Hang Dong , et al. (10 additional authors not shown)

Abstract: The topological surface code is a leading candidate for harnessing long-range entanglement to protect logical quantum information against errors, and teleportation of logical states is desirable for robust quantum information processing. Nevertheless, scaling up the surface code in quantum teleportation poses a formidable challenge to experiment. Here on a superconducting quantum processor with 12… ▽ More The topological surface code is a leading candidate for harnessing long-range entanglement to protect logical quantum information against errors, and teleportation of logical states is desirable for robust quantum information processing. Nevertheless, scaling up the surface code in quantum teleportation poses a formidable challenge to experiment. Here on a superconducting quantum processor with 125 qubits, we demonstrate the robust teleportation of topological rotated surface code prepared by a linear-depth unitary circuit, with code distances up to 7. We obtain the teleportation phase diagram by tuning the local entangling gates uniformly across a finite threshold. Furthermore, we show that the entangling threshold can be boosted by coherent qubit rotations that inject magic resources beyond the Clifford regime, restoring the duality symmetry of the topological phase, which serves as a guiding principle to minimize the entanglement resource. Our results shed light on simulating and leveraging topological quantum matter on quantum devices, and pave the way to the ultimate goal of distributed fault tolerant quantum computation. △ Less

Submitted 24 February, 2026; originally announced February 2026.

Millimeter-scale rigid diamond probe for high sensitivity endoscopic-magnetometry applications

Authors: Jihongbo Shen, Heng Yuan, Hongyu Tao, Zekun Niu, Haoming Xu, Chentao Zhang, Chen Su, Zhuo Wang, Chen Zhang

Abstract: Magnetometry based on diamond nitrogen-vacancy (NV) centers has been extensively studied for applications requiring diverse capabilities, spanning from nanometer spatial resolution to subpicotesla sensitivity. Among various applications, diamond magnetometers can demonstrate high sensitivity magnetic sensing within millimeter-scale size for endoscopic applications. However, the trade-off between s… ▽ More Magnetometry based on diamond nitrogen-vacancy (NV) centers has been extensively studied for applications requiring diverse capabilities, spanning from nanometer spatial resolution to subpicotesla sensitivity. Among various applications, diamond magnetometers can demonstrate high sensitivity magnetic sensing within millimeter-scale size for endoscopic applications. However, the trade-off between sensitivity and spatial resolution of diamond magnetometry makes it difficult to achieve such a probe. In this study, we present a millimeter-scale rigid diamond magnetometer probe with enhanced sensitivity via optimizing the optical design. By coupling the frustum diamond with the miniaturized compound parabolic concentrator (CPC) lens, we enhance the fluorescence collection efficiency by 37% within 4 mm diameter, and the achieved sensitivity is 200 pT/Hz1/2 based on the sample with the resonance linewidth of ~8 MHz. With this verified structure, endoscopes with mm-size probe and picotesla sensitivity can be projected for surgical and industrial applications in the future. △ Less

Submitted 3 February, 2026; originally announced February 2026.

Comments: 27 pages, 12 figures

UNIQ: Communication-Efficient Distributed Quantum Computing via Unified Nonlinear Integer Programming

Authors: Hui Zhong, Jiachen Shen, Lei Fan, Xinyue Zhang, Hao Wang, Miao Pan, Zhu Han

Abstract: Distributed quantum computing (DQC) is widely regarded as a promising approach to overcome quantum hardware limitations. A major challenge in DQC lies in reducing the communication cost introduced by remote CNOT gates, which are significantly slower and more resource-consuming than local operations. Existing DQC approaches treat the three essential components (qubit allocation, entanglement manage… ▽ More Distributed quantum computing (DQC) is widely regarded as a promising approach to overcome quantum hardware limitations. A major challenge in DQC lies in reducing the communication cost introduced by remote CNOT gates, which are significantly slower and more resource-consuming than local operations. Existing DQC approaches treat the three essential components (qubit allocation, entanglement management, and network scheduling) as independent stages, optimizing each in isolation. However, we observe that these components are inherently interdependent, and therefore adopting a unified optimization strategy can be more efficient to achieve the global optimal solutions. Consequently, we propose UNIQ, a novel DQC optimization framework that integrates all three components into a non-linear integer programming (NIP) model. UNIQ aims to reduce the circuit runtime by maximizing parallel Einstein-Podolsky-Rosen (EPR) pair generation through the use of idle communication qubits, while simultaneously minimizing the communication cost of remote gates. To solve this NP-hard formulated problem, we adopt two key strategies: a greedy algorithm for efficiently mapping logical qubits to different QPUs, and a JIT (Just-In-Time) approach that builds EPR pairs in parallel within each time slot. Extensive simulation results demonstrate that our approach is widely applicable to diverse quantum circuits and QPU topologies, while substantially reducing communication cost and runtime over existing methods. △ Less

Submitted 29 November, 2025; originally announced December 2025.

Exchange operation of Majorana zero modes in topological insulator-based Josephson trijunctions

Authors: Yunxiao Zhang, Zhaozheng Lyu, Xiang Wang, Yukun Shi, Duolin Wang, Xiaozhou Yang, Enna Zhuo, Bing Li, Yuyang Huang, Zenan Shi, Anqi Wang, Heng Zhang, Fucong Fei, Xiaohui Song, Peiling Li, Bingbing Tong, Ziwei Dou, Jie Shen, Guangtong Liu, Fanming Qu, Fengqi Song, Li Lu

Abstract: Majorana zero modes are anyons obeying non-Abelian exchange statistics distinct from fermions or bosons. While significant progresses have been achieved in the past two decades in searching for these exotic excitations in solid-state systems, their non-Abelian nature remains unverified, as definitive proof requires braiding operations. Here, we report preliminarily experimental advances in creatin… ▽ More Majorana zero modes are anyons obeying non-Abelian exchange statistics distinct from fermions or bosons. While significant progresses have been achieved in the past two decades in searching for these exotic excitations in solid-state systems, their non-Abelian nature remains unverified, as definitive proof requires braiding operations. Here, we report preliminarily experimental advances in creating, manipulating, and exchanging the presumed Majorana zero modes in an envelope-shaped Josephson device composed of multiple trijunctions on a topological insulator surface. We observed the signatures of in-gap states migration consistent with the expectations of the Fu-Kane model, supporting the realization of an exchange operation. This work would establish a critical pathway toward ultimately braiding Majorana zero modes in the Fu-Kane scheme of topological quantum computation. △ Less

Submitted 2 November, 2025; originally announced November 2025.

Comments: 4 pages, 4 figures

Fock space prethermalization and time-crystalline order on a quantum processor

Authors: Zehang Bao, Zitian Zhu, Yang-Ren Liu, Zixuan Song, Feitong Jin, Xuhao Zhu, Yu Gao, Chuanyu Zhang, Ning Wang, Yiren Zou, Ziqi Tan, Aosai Zhang, Zhengyi Cui, Fanhao Shen, Jiarun Zhong, Yiyang He, Han Wang, Jia-Nan Yang, Yanzhe Wang, Jiayuan Shen, Gongyu Liu, Yihang Han, Yaozu Wu, Jinfeng Deng, Hang Dong , et al. (9 additional authors not shown)

Abstract: Periodically driven quantum many-body systems exhibit a wide variety of exotic nonequilibrium phenomena and provide a promising pathway for quantum applications. A fundamental challenge for stabilizing and harnessing these highly entangled states of matter is system heating by energy absorption from the drive. Here, we propose and demonstrate a disorder-free mechanism, dubbed Fock space prethermal… ▽ More Periodically driven quantum many-body systems exhibit a wide variety of exotic nonequilibrium phenomena and provide a promising pathway for quantum applications. A fundamental challenge for stabilizing and harnessing these highly entangled states of matter is system heating by energy absorption from the drive. Here, we propose and demonstrate a disorder-free mechanism, dubbed Fock space prethermalization (FSP), to suppress heating. This mechanism divides the Fock-space network into linearly many sparse sub-networks, thereby prolonging the thermalization timescale even for initial states at high energy densities. Using 72 superconducting qubits, we observe an FSP-based time-crystalline order that persists over 120 cycles for generic initial Fock states. The underlying kinetic constraint of approximately conserved domain wall (DW) numbers is identified by measuring site-resolved correlators. Further, we perform finite-size scaling analysis for DW and Fock-space dynamics by varying system sizes, which reveals size-independent regimes for FSP-thermalization crossover and links the dynamical behaviors to the eigenstructure of the Floquet unitary. Our work establishes FSP as a robust mechanism for breaking ergodicity, and paves the way for exploring novel nonequilibrium quantum matter and its applications. △ Less

Submitted 18 March, 2026; v1 submitted 28 October, 2025; originally announced October 2025.

Comments: 8 pages, 4 figures + supplementary information

Journal ref: Phys. Rev. Lett. 137, 050407 (2026)

Broad nonlocal spectrum in the Pb-InSb hybrid three terminals for potential realization of Kitaev chains

Authors: Guoan Li, Xiaofan Shi, Ruixuan Zhang, Yuxiao Song, Marco Rossi, Ghada Badawy, Zhiyuan Zhang, Anqi Wang, Xingchen Guo, Xiao Deng, Xiao Chen, Liangqian Xu, Bingbing Tong, Peiling Li, Xiaohui Song, Zhaozheng Lyu, Guangtong Liu, Fanming Qu, Michał P. Nowak, Paweł Wójcik, Ziwei Dou, Erik P. A. M. Bakkers, Li Lu, Jie Shen

Abstract: Hybrid superconductor-semiconductor(SC-SM) nanowires remain one of the foremost platforms for engineering topological superconductivity and Majorana zero modes(MZMs) towards fault-tolerant topological qubits, especially with the rapid development of artificial Kitaev chains. In contrast to the widely used aluminum(Al)-based hybrids, lead(Pb) offers a bulk superconducting gap of ~1.4meV and a criti… ▽ More Hybrid superconductor-semiconductor(SC-SM) nanowires remain one of the foremost platforms for engineering topological superconductivity and Majorana zero modes(MZMs) towards fault-tolerant topological qubits, especially with the rapid development of artificial Kitaev chains. In contrast to the widely used aluminum(Al)-based hybrids, lead(Pb) offers a bulk superconducting gap of ~1.4meV and a critical temperature of ~7.2K, giving rise to a proximity-induced gap that is roughly five times larger than that obtained with Al. Here we present the first three-terminal Pb-hybrid devices and perform nonlocal differential-conductance spectroscopy on this platform. The nonlocal measurement simultaneously resolves a dual-gap feature of the parent Pb gap and the large, hard, gate-tunable induced superconducting gap, distinguished by a switch between electron- and hole-like dissipation processes. Within the induced gap we observe several types of Andreev bound states(ABSs) that undergo singlet-doublet transitions. Moreover, by tuning gate voltages we achieve gate-controlled resonating sign reversals of the nonlocal conductance, identifying three distinct regimes that correspond to different configurations of quantum-dot(QD) resonances(single-resonance, double-resonance, and series-resonance). Finally, the coupling between ABSs and QDs also present and can be modulated from the weak- to strong-coupling limit, indicating the feasibility of realizing the artificial Kitaev chains. Crucially, the robust nonlocal signatures persist up to temperatures(~1K) far above the operating temperature of Al-based devices thanks to the unusually large induced gap, thereby widening the accessible parameter space greatly and underscoring the suitability of Pb-based hybrids for implementing warm temperature artificial Kitaev chains and the topological quantum devices protected by a substantially larger topological gap. △ Less

Submitted 11 October, 2025; originally announced October 2025.

Mechanisms for Quantum Advantage in Global Optimization of Nonconvex Functions

Authors: Dylan Herman, Guneykan Ozgul, Anuj Apte, Junhyung Lyle Kim, Anupam Prakash, Jiayu Shen, Shouvanik Chakrabarti

Abstract: We present new theoretical mechanisms for quantum speedup in the global optimization of nonconvex functions, expanding the scope of quantum advantage beyond traditional tunneling-based explanations. As our main building-block, we demonstrate a rigorous correspondence between the spectral properties of Schrödinger operators and the mixing times of classical Langevin diffusion. This correspondence m… ▽ More We present new theoretical mechanisms for quantum speedup in the global optimization of nonconvex functions, expanding the scope of quantum advantage beyond traditional tunneling-based explanations. As our main building-block, we demonstrate a rigorous correspondence between the spectral properties of Schrödinger operators and the mixing times of classical Langevin diffusion. This correspondence motivates a mechanism for separation on functions with unique global minimum: while quantum algorithms operate on the original potential, classical diffusions correspond to a Schrödinger operators with a WKB potential having nearly degenerate global minima. We formalize these ideas by proving that a real-space adiabatic quantum algorithm (RsAA) achieves provably polynomial-time optimization for broad families of nonconvex functions. First, for block-separable functions, we show that RsAA maintains polynomial runtime while known off-the-shelf algorithms require exponential time and structure-aware algorithms exhibit arbitrarily large polynomial runtimes. These results leverage novel non-asymptotic results in semiclassical analysis. Second, we use recent advances in the theory of intrinsic hypercontractivity to demonstrate polynomial runtimes for RsAA on appropriately perturbed strongly convex functions that lack global structure, while off-the-shelf algorithms remain exponentially bottlenecked. In contrast to prior works based on quantum tunneling, these separations do not depend on the geometry of barriers between local minima. Our theoretical claims about classical algorithm runtimes are supported by rigorous analysis and comprehensive numerical benchmarking. These findings establish a rigorous theoretical foundation for quantum advantage in continuous optimization and open new research directions connecting quantum algorithms, stochastic processes, and semiclassical analysis. △ Less

Submitted 3 October, 2025; originally announced October 2025.

Combinatorial optimization enhanced by shallow quantum circuits with 104 superconducting qubits

Authors: Xuhao Zhu, Zuoheng Zou, Feitong Jin, Pavel Mosharev, Maolin Luo, Yaozu Wu, Jiachen Chen, Chuanyu Zhang, Yu Gao, Ning Wang, Yiren Zou, Aosai Zhang, Fanhao Shen, Zehang Bao, Zitian Zhu, Jiarun Zhong, Zhengyi Cui, Yihang Han, Yiyang He, Han Wang, Jia-Nan Yang, Yanzhe Wang, Jiayuan Shen, Gongyu Liu, Zixuan Song , et al. (9 additional authors not shown)

Abstract: A pivotal task for quantum computing is to speed up solving problems that are both classically intractable and practically valuable. Among these, combinatorial optimization problems have attracted tremendous attention due to their broad applicability and natural fitness to Ising Hamiltonians. Here we propose a quantum sampling strategy, based on which we design an algorithm for accelerating solvin… ▽ More A pivotal task for quantum computing is to speed up solving problems that are both classically intractable and practically valuable. Among these, combinatorial optimization problems have attracted tremendous attention due to their broad applicability and natural fitness to Ising Hamiltonians. Here we propose a quantum sampling strategy, based on which we design an algorithm for accelerating solving the ground states of Ising model, a class of NP-hard problems in combinatorial optimization. The algorithm employs a hybrid quantum-classical workflow, with a shallow-circuit quantum sampling subroutine dedicated to navigating the energy landscape. Using up to 104 superconducting qubits, we demonstrate that this algorithm outputs favorable solutions against even a highly-optimized classical simulated annealing (SA) algorithm. Furthermore, we illustrate the path toward quantum speedup based on the time-to-solution metric against SA running on a single-core CPU with just 100 qubits. Our results indicate a promising alternative to classical heuristics for combinatorial optimization, a paradigm where quantum advantage might become possible on near-term superconducting quantum processors with thousands of qubits and without the assistance of error correction. △ Less

Submitted 14 September, 2025; originally announced September 2025.

Density of States (Gate) - Controlled Andreev Molecule and Sensor

Authors: Xiaofan Shi, Ziwei Dou, Guoan Li, Dong Pan, Yuxiao Song, Anqi Wang, Zhiyuan Zhang, Xingchen Guo, Xiao Deng, Ruixuan Zhang, Liangqian Xu, Xiao Chen, Yupeng Li, Bingbing Tong, Xiaohui Song, Zhaozheng Lyu, Peiling Li, Fanming Qu, Guangtong Liu, Jianhua Zhao, Li Lu, Jie Shen

Abstract: Topological quantum computing typically relies on topological Andreev bound states (ABSs) engineered in hybrid superconductor-semiconductor devices, where gate control offers key advantages. While strong Zeeman fields can induce such states, an alternative approach emerges through Andreev molecules -- closely spaced, coupled ABSs, also key building-block for Kitaev chain -- that enable topological… ▽ More Topological quantum computing typically relies on topological Andreev bound states (ABSs) engineered in hybrid superconductor-semiconductor devices, where gate control offers key advantages. While strong Zeeman fields can induce such states, an alternative approach emerges through Andreev molecules -- closely spaced, coupled ABSs, also key building-block for Kitaev chain -- that enable topological behavior without high magnetic fields. However, existing Andreev molecules are controlled via magnetic flux in superconducting loops, limiting scalability. Here, we introduce a gate-controlled Andreev molecule, where electrostatic tuning of the density of states in one site nonlocally enhances the critical current of another. This eliminates superconducting loops, offering superior tunability, scalability, and sensitivity. We further extend such an Andreev molecule to a multi-site Kitaev chain, and a noninvasive sensor resolving single-Cooper-pair charge for parity readout. This platform bridges the gap between scalable ABS engineering and high-sensitivity quantum sensing, advancing the development for constructing and parity-readout in topological ABSs and long Kitaev chains towards topological qubits. △ Less

Submitted 6 August, 2025; originally announced August 2025.

Enhanced and modulable induced superconducting gap and effective Landé g-factor in Pb-InSb hybrid devices

Authors: Guoan Li, Xiaofan Shi, Ziwei Dou, Guang Yang, Jiayu Shi, Marco Rossi, Ghada Badawy, Yuxiao Song, Ruixuan Zhang, Yupeng Li, Zhiyuan Zhang, Anqi Wang, Xingchen Guo, Xiao Deng, Bingbing Tong, Peiling Li, Zhaozheng Lyu, Guangtong Liu, Fanming Qu, Erik P. A. M. Bakkers, Michał P. Nowak, Paweł Wójcik, Li Lu, Jie Shen

Abstract: The hybrid system of a conventional superconductor (SC) on a semiconductor (SM) nanowire with strong spin-orbit coupling (SOC) represents a promising platform for achieving topological superconductivity and Majorana zero modes (MZMs) towards topological quantum computation. While aluminum (Al)-based hybrid nanowire devices have been widely utilized, their limited superconducting gap and intrinsic… ▽ More The hybrid system of a conventional superconductor (SC) on a semiconductor (SM) nanowire with strong spin-orbit coupling (SOC) represents a promising platform for achieving topological superconductivity and Majorana zero modes (MZMs) towards topological quantum computation. While aluminum (Al)-based hybrid nanowire devices have been widely utilized, their limited superconducting gap and intrinsic weak SOC as well as small Landé g-factor may hinder future experimental advancements. In contrast, we demonstrate that lead (Pb)-based hybrid quantum devices exhibit a remarkably large and hard proximity-induced superconducting gap, exceeding that of Al by an order of magnitude. By exploiting electrostatic gating to modulate wavefunction distribution and SC-SM interfacial coupling, this gap can be continuously tuned from its maximum value (~1.4 meV, matching the bulk Pb gap) down to nearly zero while maintaining the hardness. Furthermore, magnetic-field-dependent measurements reveal a radial evolution of the gap structure with anti-crossing feature, indicative of strong SOC and huge effective g-factors up to 76. These findings underscore the superior functionality of Pb-based hybrid systems, significantly advancing their potential for realizing and stabilizing MZMs and the further scalable topological quantum architectures. △ Less

Submitted 4 June, 2025; originally announced June 2025.

Circuit-level-configurable Zero-field Superconducting Diodes: A Universal Platform Beyond Intrinsic Symmetry Breaking

Authors: Xiaofan Shi, Ziwei Dou, Dong Pan, Guoan Li, Yupeng Li, Anqi Wang, Zhiyuan Zhang, Xingchen Guo, Xiao Deng, Bingbing Tong, Zhaozheng Lyu, Peiling Li, Fanming Qu, Guangtong Liu, Jianhua Zhao, Jiangping Hu, Li Lu, Jie Shen

Abstract: Modern industry seeks next-generation microelectronics with ultra-low dissipation and noise beyond semiconducting systems, where the superconducting electronics offer promise. Its physical foundation is the superconducting diode effect (SDE) with nonreciprocal supercurrent. SDE has hitherto mainly relied on material-specific intrinsic symmetry breaking in superconductors, suffering from low yield,… ▽ More Modern industry seeks next-generation microelectronics with ultra-low dissipation and noise beyond semiconducting systems, where the superconducting electronics offer promise. Its physical foundation is the superconducting diode effect (SDE) with nonreciprocal supercurrent. SDE has hitherto mainly relied on material-specific intrinsic symmetry breaking in superconductors, suffering from low yield, controllability, and compatibility with further functional extension - an undesirable aspect for applications. Here, we demonstrated a field-free SDE due to the chemical potential shift from external circuit line resistance, which is generic and challenges the previous interpretations of the intrinsic symmetry breaking in superconductivity for zero-field SDE. Moreover, this SDE is circuit-level configurable since it can be electrically switched on/off with its polarity and efficiency precisely modulated via gate voltage and circuit reconfiguration, facilitating functional extension. Such a generic, controllable and extensible SDE addresses critical challenges in dissipationless circuit towards application, and thus establishes a robust platform for scalable superconducting electronics. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Procedure of tuning up a three-site artificial Kitaev chain based on transmon measurements

Authors: Xiaozhou Yang, Zhaozheng Lyu, Xiang Wang, Enna Zhuo, Yunxiao Zhang, Duolin Wang, Yukun Shi, Yuyang Huang, Bing Li, Xiaohui Song, Peiling Li, Bingbing Tong, Ziwei Dou, Jie Shen, Guangtong Liu, Fanming Qu, Li Lu

Abstract: Artificial Kitaev chains (AKCs), formed of quantum dot-superconductor linear arrays, provide a promising platform for hosting Majorana bound states (MBSs) and implementing topological quantum computing. The main challenges along this research direction would include the tuning up of AKCs for hosting MBSs and the readout of the parity of the chains. In this work, we present a step-by-step procedure… ▽ More Artificial Kitaev chains (AKCs), formed of quantum dot-superconductor linear arrays, provide a promising platform for hosting Majorana bound states (MBSs) and implementing topological quantum computing. The main challenges along this research direction would include the tuning up of AKCs for hosting MBSs and the readout of the parity of the chains. In this work, we present a step-by-step procedure for tuning up a three-site AKC to its sweet spots based on the spectra of a transmon circuit which is integrated with the chain for the purpose of reading out the parity of the chain. The signatures of the transmon's plasma modes in each step, particular those related to the appearance of MBSs in the chain, will be given. We find that the sweet spots in a three-site AKC can be classified into three types based on the relative strengths of elastic cotunneling (ECT) and crossed Andreev reflection (CAR): ECT-dominated sweet spots, genuine sweet spots and CAR-dominated sweet spots. We show that the ECT-dominated and CAR-dominated sweet spots can be more conveniently accessed and utilized in transmon-based measurements. △ Less

Submitted 21 May, 2025; originally announced May 2025.

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.

Journal ref: Nature Physics (2026)

Controllable creation of topological boundary states in topological-insulator-based Josephson corner junctions

Authors: Xiang Wang, Duolin Wang, Yunxiao Zhang, Xiaozhou Yang, Yukun Shi, Bing Li, Enna Zhuo, Yuyang Huang, Anqi Wang, Zhaozheng Lyu, Xiaohui Song, Peiling Li, Bingbing Tong, Ziwei Dou, Jie Shen, Guangtong Liu, Fanming Qu, Li Lu

Abstract: Majorana zero modes (MZMs) in condensed matter systems have attracted great attention in the past two decades, due to their interesting physics and potential application in topological quantum computing (TQC). However, the topologically protected nature of MZMs still need more experimental verifications. In this study, we have realized controllable creation of a topological boundary state at the c… ▽ More Majorana zero modes (MZMs) in condensed matter systems have attracted great attention in the past two decades, due to their interesting physics and potential application in topological quantum computing (TQC). However, the topologically protected nature of MZMs still need more experimental verifications. In this study, we have realized controllable creation of a topological boundary state at the corner of topological insulator (TI)-based Josephson corner junctions. This state demonstrates protected existence across a broad region in parametric space, and exhibits a non-2π-period but 4π-period-compatible energy-phase relation. Our study suggests that TI-based Josephson junctions, as proposed in the Fu-Kane scheme of TQC, may provide a promising platform for hosting and braiding MZMs. △ Less

Submitted 13 May, 2025; originally announced May 2025.

Comments: 5 pages, 4 figures

Quantum Pattern Detection: Accurate State- and Circuit-based Analyses

Authors: Julian Shen, Joshua Ammermann, Christoph König, Ina Schaefer

Abstract: Quantum computers have the potential to solve certain problems faster than classical computers by exploiting quantum mechanical effects such as superposition. However, building high-quality quantum software is challenging due to the fundamental differences between quantum and traditional programming and the lack of abstraction mechanisms. To mitigate this challenge, researchers have introduced qua… ▽ More Quantum computers have the potential to solve certain problems faster than classical computers by exploiting quantum mechanical effects such as superposition. However, building high-quality quantum software is challenging due to the fundamental differences between quantum and traditional programming and the lack of abstraction mechanisms. To mitigate this challenge, researchers have introduced quantum patterns to capture common high-level design solutions to recurring problems in quantum software engineering. In order to utilize patterns as an abstraction level for implementation, a mapping between the theoretical patterns and the source code is required, which has only been addressed to a limited extent. To close this gap, we propose a framework for the automatic detection of quantum patterns using state- and circuit-based code analysis. Furthermore, we contribute a dataset for benchmarking quantum pattern detection approaches. In an empirical evaluation, we show that our framework is able to detect quantum patterns very accurately and that it outperforms existing quantum pattern detection approaches in terms of detection accuracy. △ Less

Submitted 27 January, 2025; originally announced January 2025.

Comments: 10 pages, accepted at Q-SE 2025 (ICSE workshop) and to be published in ICSE-Companion, this is a preprint version before submission

Measurement of parity-dependent energy-phase relation of the low-energy states in a potential artificial Kitaev chain utilizing a transmon qubit

Authors: Enna Zhuo, Xiaozhou Yang, Yuyang Huang, Zhaozheng Lyu, Ang Li, Bing Li, Yunxiao Zhang, Xiang Wang, Duolin Wang, Yukun Shi, Anqi Wang, E. P. A. M. Bakkers, Xiaodong Han, Xiaohui Song, Peiling Li, Bingbing Tong, Ziwei Dou, Guangtong Liu, Fanming Qu, Jie Shen, Li Lu

Abstract: Artificial Kitaev chains have emerged as a promising platform for realizing topological quantum computing. Once the chains are formed and the Majorana zero modes are braided/fused, reading out the parity of the chains is essential for further verifying the non-Abelian property of the Majorana zero modes. Here we demonstrate the feasibility of using a superconducting transmon qubit, which incorpora… ▽ More Artificial Kitaev chains have emerged as a promising platform for realizing topological quantum computing. Once the chains are formed and the Majorana zero modes are braided/fused, reading out the parity of the chains is essential for further verifying the non-Abelian property of the Majorana zero modes. Here we demonstrate the feasibility of using a superconducting transmon qubit, which incorporates an end of a four-site quantum dot-superconductor chain based on a Ge/Si nanowire, to directly detect the singlet/doublet state, and thus the parity of the entire chain. We also demonstrate that for multiple-dot chains there are two types of 0-π transitions between different charging states: the parity-flip 0-π transition and the parity-preserved 0-π transition. Furthermore, we show that the inter-dot coupling, hence the strengths of cross Andreev reflection and elastic cotunneling of electrons, can be adjusted by local electrostatic gating in chains fabricated on Ge/Si core-shell nanowires. Our exploration would be helpful for the ultimate realization of topological quantum computing based on artificial Kitaev chains. △ Less

Submitted 20 May, 2025; v1 submitted 22 January, 2025; originally announced January 2025.

Experimentally validated quantum-secure federated learning over a multi-user quantum network

Authors: Zhi-Ping Liu, Xiao-Yu Cao, Hao-Wen Liu, Xiao-Ran Sun, Yu Bao, Jian-Yu Shen, Yu-Shuo Lu, Hua-Lei Yin, Zeng-Bing Chen

Abstract: Federated learning enables decentralized, privacy-preserving training but remains vulnerable to privacy leakage in the quantum era. Quantum federated learning (QFL) offers a promising path towards enhanced security and efficiency. However, a practical and experimentally validated QFL protocol utilizing near-term quantum techniques to address data privacy has been lacking. Here we present QuNetQFL,… ▽ More Federated learning enables decentralized, privacy-preserving training but remains vulnerable to privacy leakage in the quantum era. Quantum federated learning (QFL) offers a promising path towards enhanced security and efficiency. However, a practical and experimentally validated QFL protocol utilizing near-term quantum techniques to address data privacy has been lacking. Here we present QuNetQFL, a QFL protocol implemented on quantum networks, in which local model updates are masked with distributed quantum secret keys, offering information-theoretic security during aggregation. We experimentally validate the protocol on a four-client quantum network and benchmark its performance using the generated keys on quantum and real-world datasets. Adding a single quantum client significantly improves global accuracy for classifying multipartite entangled and non-stabilizer quantum datasets. For language tasks, we apply QuNetQFL to sentiment analysis by federated fine-tuning of a hybrid classical-quantum language model, achieving comparable and robust performance in simulation and on real quantum hardware. Large-scale simulations further demonstrate scalability to 200 clients for handwritten-digit recognition, with rapid convergence and a $75\%$ reduction in communication cost via model compression. Our work establishes a practical and scalable route to quantum-secure federated learning for the emerging quantum internet. △ Less

Submitted 16 May, 2026; v1 submitted 22 January, 2025; originally announced January 2025.

Comments: 25 pages, 7 figures, 7 tables, Accepted by Research

Journal ref: Research 9, 1299 (2026)

Demonstrating quantum error mitigation on logical qubits

Authors: Aosai Zhang, Haipeng Xie, Yu Gao, Jia-Nan Yang, Zehang Bao, Zitian Zhu, Jiachen Chen, Ning Wang, Chuanyu Zhang, Jiarun Zhong, Shibo Xu, Ke Wang, Yaozu Wu, Feitong Jin, Xuhao Zhu, Yiren Zou, Ziqi Tan, Zhengyi Cui, Fanhao Shen, Tingting Li, Yihang Han, Yiyang He, Gongyu Liu, Jiayuan Shen, Han Wang , et al. (10 additional authors not shown)

Abstract: A long-standing challenge in quantum computing is developing technologies to overcome the inevitable noise in qubits. To enable meaningful applications in the early stages of fault-tolerant quantum computing, devising methods to suppress post-correction logical failures is becoming increasingly crucial. In this work, we propose and experimentally demonstrate the application of zero-noise extrapola… ▽ More A long-standing challenge in quantum computing is developing technologies to overcome the inevitable noise in qubits. To enable meaningful applications in the early stages of fault-tolerant quantum computing, devising methods to suppress post-correction logical failures is becoming increasingly crucial. In this work, we propose and experimentally demonstrate the application of zero-noise extrapolation, a practical quantum error mitigation technique, to error correction circuits on state-of-the-art superconducting processors. By amplifying the noise on physical qubits, the circuits yield outcomes that exhibit a predictable dependence on noise strength, following a polynomial function determined by the code distance. This property enables the effective application of polynomial extrapolation to mitigate logical errors. Our experiments demonstrate a universal reduction in logical errors across various quantum circuits, including fault-tolerant circuits of repetition and surface codes. We observe a favorable performance in multi-round error correction circuits, indicating that this method remains effective when the circuit depth increases. These results advance the frontier of quantum error suppression technologies, opening a practical way to achieve reliable quantum computing in the early fault-tolerant era. △ Less

Submitted 15 January, 2025; originally announced January 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.

Exploring nontrivial topology at quantum criticality in a superconducting processor

Authors: Ziqi Tan, Ke Wang, Sheng Yang, Fanhao Shen, Feitong Jin, Xuhao Zhu, Yujie Ji, Shibo Xu, Jiachen Chen, Yaozu Wu, Chuanyu Zhang, Yu Gao, Ning Wang, Yiren Zou, Aosai Zhang, Tingting Li, Zehang Bao, Zitian Zhu, Jiarun Zhong, Zhengyi Cui, Yihang Han, Yiyang He, Han Wang, Jianan Yang, Yanzhe Wang , et al. (15 additional authors not shown)

Abstract: The discovery of nontrivial topology in quantum critical states has introduced a new paradigm for classifying quantum phase transitions and challenges the conventional belief that topological phases are typically associated with a bulk energy gap. However, realizing and characterizing such topologically nontrivial quantum critical states with large particle numbers remains an outstanding experimen… ▽ More The discovery of nontrivial topology in quantum critical states has introduced a new paradigm for classifying quantum phase transitions and challenges the conventional belief that topological phases are typically associated with a bulk energy gap. However, realizing and characterizing such topologically nontrivial quantum critical states with large particle numbers remains an outstanding experimental challenge in statistical and condensed matter physics. Programmable quantum processors can directly prepare and manipulate exotic quantum many-body states, offering a powerful path for exploring the physics behind these states. Here, we present an experimental exploration of the critical cluster Ising model by preparing its low-lying critical states on a superconducting processor with up to $100$ qubits. We develop an efficient method to probe the boundary $g$-function based on prepared low-energy states, which allows us to uniquely identify the nontrivial topology of the critical systems under study. Furthermore, by adapting the entanglement Hamiltonian tomography technique, we recognize two-fold topological degeneracy in the entanglement spectrum under periodic boundary condition, experimentally verifying the universal bulk-boundary correspondence in topological critical systems. Our results demonstrate the low-lying critical states as useful quantum resources for investigating the interplay between topology and quantum criticality. △ Less

Submitted 8 January, 2025; originally announced January 2025.

Journal ref: Communications Physics 9, 136 (2026)

Differential Privacy Preserving Distributed Quantum Computing

Authors: Hui Zhong, Keyi Ju, Jiachen Shen, Xinyue Zhang, Xiaoqi Qin, Tomoaki Ohtsuki, Miao Pan, Zhu Han

Abstract: Existing quantum computers can only operate with hundreds of qubits in the Noisy Intermediate-Scale Quantum (NISQ) state, while quantum distributed computing (QDC) is regarded as a reliable way to address this limitation, allowing quantum computers to achieve their full computational potential. However, similar to classical distributed computing, QDC also faces the problem of privacy leakage. Exis… ▽ More Existing quantum computers can only operate with hundreds of qubits in the Noisy Intermediate-Scale Quantum (NISQ) state, while quantum distributed computing (QDC) is regarded as a reliable way to address this limitation, allowing quantum computers to achieve their full computational potential. However, similar to classical distributed computing, QDC also faces the problem of privacy leakage. Existing research has introduced quantum differential privacy (QDP) for privacy protection in central quantum computing, but there is no dedicated privacy protection mechanisms for QDC. To fill this research gap, our paper introduces a novel concept called quantum Rényi differential privacy (QRDP), which incorporates the advantages of classical Rényi DP and is applicable in the QDC domain. Based on the new quantum Rényi divergence, QRDP provides delicate and flexible privacy protection by introducing parameter $α$. In particular, the QRDP composition is well suited for QDC, since it allows for more precise control of the total privacy budget in scenarios requiring multiple quantum operations. We analyze a variety of noise mechanisms that can implement QRDP, and derive the lowest privacy budget provided by these mechanisms. Finally, we investigate the impact of different quantum parameters on QRDP. Through our simulations, we also find that adding noise will make the data less usable, but increase the level of privacy protection. △ Less

Submitted 6 January, 2025; v1 submitted 16 December, 2024; originally announced December 2024.

Disentanglement in dephasing channel with machine learning

Authors: Qihang Liu, Anran Qiao, Jung-Tsung Shen

Abstract: Quantum state classification and entanglement quantification are of significant importance in the fundamental research of quantum information science and various quantum applications. Traditional methods, such as quantum state tomography, face exponential measurement demands with increasing numbers of qubits, necessitating more efficient approaches. Recent work has shown promise in using artificia… ▽ More Quantum state classification and entanglement quantification are of significant importance in the fundamental research of quantum information science and various quantum applications. Traditional methods, such as quantum state tomography, face exponential measurement demands with increasing numbers of qubits, necessitating more efficient approaches. Recent work has shown promise in using artificial neural networks (ANNs) for quantum state analysis. However, existing ANNs may falter when confronted with states affected by dephasing noise, especially with limited data and computational resources. In this study, we employ a machine-learning approach to investigate the disentanglement process in two-qubit systems in the presence of dephasing noise. Our findings highlight the limitations of general state-trained ANNs in classifying states under dephasing noise. Specialized ANN algorithms, tailored for classifying states and quantifying entanglement in such noisy environments, demonstrate excellent performance using only a subset of tomographic features. △ Less

Submitted 5 February, 2025; v1 submitted 28 October, 2024; originally announced October 2024.

Evidence of P-wave Pairing in K$_2$Cr$_3$As$_3$ Superconductors from Phase-sensitive Measurement

Authors: Zhiyuan Zhang, Ziwei Dou, Anqi Wang, Cuiwei Zhang, Yu Hong, Xincheng Lei, Yue Pan, Zhongchen Xu, Zhipeng Xu, Yupeng Li, Guoan Li, Xiaofan Shi, Xingchen Guo, Xiao Deng, Zhaozheng Lyu, Peiling Li, Faming Qu, Guangtong Liu, Dong Su, Kun Jiang, Youguo Shi, Li Lu, Jie Shen, Jiangping Hu

Abstract: P-wave superconductors hold immense promise for both fundamental physics and practical applications due to their unusual pairing symmetry and potential topological superconductivity. However, the exploration of the p-wave superconductors has proved to be a complex endeavor. Not only are they rare in nature but also the identification of p-wave superconductors has been an arduous task in history. F… ▽ More P-wave superconductors hold immense promise for both fundamental physics and practical applications due to their unusual pairing symmetry and potential topological superconductivity. However, the exploration of the p-wave superconductors has proved to be a complex endeavor. Not only are they rare in nature but also the identification of p-wave superconductors has been an arduous task in history. For example, phase-sensitive measurement, an experimental technique which can provide conclusive evidence for unconventional pairing, has not been implemented successfully to identify p-wave superconductors. Here, we study a recently discovered family of superconductors, A$_2$Cr$_3$As$_3$ (A = K, Rb, Cs), which were proposed theoretically to be a candidate of p-wave superconductors. We fabricate superconducting quantum interference devices (SQUIDs) on exfoliated K$_2$Cr$_3$As$_3$, and perform the phase-sensitive measurement. We observe that such SQUIDs exhibit a pronounced second-order harmonic component sin(2$π$) in the current-phase relation, suggesting the admixture of 0- and $π$-phase. By carefully examining the magnetic field dependence of the oscillation patterns of critical current and Shapiro steps under microwave irradiation, we reveal a crossover from 0- to $π$-dominating phase state and conclude that the existence of the $π$-phase is in favor of the p-wave pairing symmetry in K$_2$Cr$_3$As$_3$. △ Less

Submitted 5 February, 2025; v1 submitted 14 August, 2024; originally announced August 2024.

Spin-Energy Entanglement of a Time-Focused Neutron

Authors: J. C. Leiner, S. J. Kuhn, S. McKay, J. K. Jochum, F. Li, A. A. M. Irfan, F. Funama, D. Mettus, L. Beddrich, C. Franz, J. Shen, S. R. Parnell, R. M. Dalgliesh, M. Loyd, N. Geerits, G. Ortiz, C. Pfleiderer, R. Pynn

Abstract: Intra-particle entanglement of individual particles such as neutrons could enable another class of scattering probes that are sensitive to entanglement in quantum systems and materials. In this work, we present experimental results demonstrating quantum contextuality as a result of entanglement between the spin and energy modes (i.e., degrees of freedom) of single neutrons in a beam using a pair o… ▽ More Intra-particle entanglement of individual particles such as neutrons could enable another class of scattering probes that are sensitive to entanglement in quantum systems and materials. In this work, we present experimental results demonstrating quantum contextuality as a result of entanglement between the spin and energy modes (i.e., degrees of freedom) of single neutrons in a beam using a pair of resonant radio-frequency neutron spin flippers in the MIEZE configuration (Modulated IntEnsity with Zero Effort). We verified the mode-entanglement by measuring a Clauser-Horne-Shimony-Holt (CHSH) contextuality witness $S$ defined in the spin and energy subsystems, observing a clear breach of the classical bound of $|S| \leq 2$, obtaining $S = 2.40 \pm 0.02$. These entangled beams could enable alternative approaches for directly probing dynamics and entanglement in quantum materials whose low-energy excitation scales match those of the incident entangled neutron. △ Less

Submitted 30 September, 2024; v1 submitted 11 April, 2024; originally announced April 2024.

Comments: 11 pages, 7 figures

Journal ref: Physical Review Applied 22, L031005 (2024)

Quantum counterdiabatic driving with local control

Authors: Changhao Li, Jiayu Shen, Ruslan Shaydulin, Marco Pistoia

Abstract: Suppression of diabatic transitions in quantum adiabatic evolution stands as a significant challenge for ground state preparations. Counterdiabatic driving has been proposed to compensate for diabatic losses and achieve shortcut to adiabaticity. However, its implementation necessitates the generation of adiabatic gauge potential, which requires knowledge of the spectral gap of instantaneous Hamilt… ▽ More Suppression of diabatic transitions in quantum adiabatic evolution stands as a significant challenge for ground state preparations. Counterdiabatic driving has been proposed to compensate for diabatic losses and achieve shortcut to adiabaticity. However, its implementation necessitates the generation of adiabatic gauge potential, which requires knowledge of the spectral gap of instantaneous Hamiltonians and involves highly non-local drivings in many-body systems. In this work, we consider local counterdiabatic (LCD) driving with approximate adiabatic gauge potential. Using transverse-field Ising model as an example, we present an in-depth study of the performance and optimization of LCD protocols. We then propose a novel two-step protocol based on LCD and simple local single-body control to further improve the performance. The optimization of these LCD-based protocols does not require knowledge of instantaneous Hamiltonians, and only additional local driving is involved. To benchmark the performance of LCD and the proposed local control-enhanced LCD technique, we experimentally implement digitized adiabatic quantum evolution in a trapped-ion system. We characterize the quality of the prepared states and explore the scaling behavior with system size up to 14 qubits. Our demonstration of quantum shortcut to adiabaticity opens a path towards preparing ground states of complex systems with accessible local controls. △ Less

Submitted 4 March, 2024; originally announced March 2024.

Comments: 28 pages, 13 figures

Influence of initial states on memory effects: A study of early-time superradiance

Authors: S. C. Hou, G. Q. Shuai, X. Y. Zhang, J. Shen, X. X. Yi

Abstract: The initial state of a quantum system can significantly influence its future dynamics, especially in non-Markovain quantum processes due to the environmental memory effects. Based on a previous work of ours, we propose a method to quantify the memory effects of a non-Markovian quantum process conditioned on a particular system initial state. We apply our method to study the early-time dynamics of… ▽ More The initial state of a quantum system can significantly influence its future dynamics, especially in non-Markovain quantum processes due to the environmental memory effects. Based on a previous work of ours, we propose a method to quantify the memory effects of a non-Markovian quantum process conditioned on a particular system initial state. We apply our method to study the early-time dynamics of a superradiance model where $N$ atoms (the system) interacting with a single-mode vacuum cavity (the environment) with several types of initial states. We find that the value of the memory effects in the early-time regime is half the environmental photon number for the (dephased) Dicke states. Besides, the memory effects, the environmental photon number and the degree of superradiance can be simultaneously enhanced by the coherence or entanglement of some initial states. In our study, the transitions from non-superradiant initial states to superradiant ones are always accompanied by the enhancement of memory effects, showing the importance of memory effects in superradiance. △ Less

Submitted 21 May, 2024; v1 submitted 18 December, 2023; originally announced December 2023.

Comments: 17 pages, 12 figures

Journal ref: Phys. Rev. A 109, 053708 (2024)

Quantum Multiphoton Rabi Oscillations in Waveguide QED

Authors: Debsuvra Mukhopadhyay, Jung-Tsung Shen

Abstract: The future of quantum information processing hinges on chip-scale nanophotonics, specifically cavity QED and waveguide QED. One of the foremost processes underpinning quantum photonic technologies is the phenomenon of Rabi oscillations, which manifests when a qubit is irradiated by an intense laser source. Departing from the conventional semiclassical framework, we expound on the more general, qua… ▽ More The future of quantum information processing hinges on chip-scale nanophotonics, specifically cavity QED and waveguide QED. One of the foremost processes underpinning quantum photonic technologies is the phenomenon of Rabi oscillations, which manifests when a qubit is irradiated by an intense laser source. Departing from the conventional semiclassical framework, we expound on the more general, quantum-theoretic case where the optical excitation takes the form of a multiphoton Fock state, and the qubit couples to a continuum of radiation modes. By employing the real-space formalism, we analytically explore the scattering dynamics of the photonic Fock state as it interfaces with a two-level emitter. The resulting amplitude for atomic excitation features a linear superposition of various independent scattering events that are triggered by the potential of sequential photon absorptions and emissions. The lowest-order excitation event, initiated by the stochastic scattering of one of the several photons, aptly characterizes the dynamics in a weak-field environment. This is complemented by a multitude of higher-order scattering events ensuing from repeated atom-photon interactions. The temporal evolution of the qubit excitation in our configuration closely mirrors the semiclassical predictions, particularly in the strong-pumping limit where Rabi oscillations unfold. Notably, this compatibility with the semiclassical paradigm applies both to the weak-driving and large-detuning limits. Our analysis, therefore, extends the existing results on quantum Rabi oscillations pertinent to single-mode cavity QED, to the multimode, waveguide-QED configurations wherein flying photons are the information carriers. Finally, we explore the scattering dynamics of pulsed wave packets, highlighting the potential to substantially enhance excitation efficiency, even in scenarios involving just a few photons. △ Less

Submitted 23 October, 2023; originally announced October 2023.

Journal ref: New Journal of Physics, Volume 26, October 2024

An optimized quantum minimum searching algorithm with sure-success probability and its experiment simulation with Cirq

Authors: Wenjie Liu, Qingshan Wu, Jiahao Shen, Jiaojiao Zhao, Mohammed Zidan, Lian Tong

Abstract: Finding a minimum is an essential part of mathematical models, and it plays an important role in some optimization problems. Durr and Hoyer proposed a quantum searching algorithm (DHA), with a certain probability of success, to achieve quadratic speed than classical ones. In this paper, we propose an optimized quantum minimum searching algorithm with sure-success probability, which utilizes Grover… ▽ More Finding a minimum is an essential part of mathematical models, and it plays an important role in some optimization problems. Durr and Hoyer proposed a quantum searching algorithm (DHA), with a certain probability of success, to achieve quadratic speed than classical ones. In this paper, we propose an optimized quantum minimum searching algorithm with sure-success probability, which utilizes Grover-Long searching to implement the optimal exact searching, and the dynamic strategy to reduce the iterations of our algorithm. Besides, we optimize the oracle circuit to reduce the number of gates by the simplified rules. The performance evaluation including the theoretical success rate and computational complexity shows that our algorithm has higher accuracy and efficiency than DHA algorithm. Finally, a simulation experiment based on Cirq is performed to verify its feasibility. △ Less

Submitted 25 September, 2023; originally announced September 2023.

Comments: 15 pages, 8 figures. arXiv admin note: text overlap with arXiv:1908.07943 by other authors

Journal ref: Journal of Ambient Intelligence and Humanized Computing,2021.12(11): p.10425-10434

The QUATRO Application Suite: Quantum Computing for Models of Human Cognition

Authors: Raghavendra Pradyumna Pothukuchi, Leon Lufkin, Yu Jun Shen, Alejandro Simon, Rome Thorstenson, Bernardo Eilert Trevisan, Michael Tu, Mudi Yang, Ben Foxman, Viswanatha Srinivas Pothukuchi, Gunnar Epping, Thi Ha Kyaw, Bryant J Jongkees, Yongshan Ding, Jerome R Busemeyer, Jonathan D Cohen, Abhishek Bhattacharjee

Abstract: Research progress in quantum computing has, thus far, focused on a narrow set of application domains. Expanding the suite of quantum application domains is vital for the discovery of new software toolchains and architectural abstractions. In this work, we unlock a new class of applications ripe for quantum computing research -- computational cognitive modeling. Cognitive models are critical to und… ▽ More Research progress in quantum computing has, thus far, focused on a narrow set of application domains. Expanding the suite of quantum application domains is vital for the discovery of new software toolchains and architectural abstractions. In this work, we unlock a new class of applications ripe for quantum computing research -- computational cognitive modeling. Cognitive models are critical to understanding and replicating human intelligence. Our work connects computational cognitive models to quantum computer architectures for the first time. We release QUATRO, a collection of quantum computing applications from cognitive models. The development and execution of QUATRO shed light on gaps in the quantum computing stack that need to be closed to ease programming and drive performance. Among several contributions, we propose and study ideas pertaining to quantum cloud scheduling (using data from gate- and annealing-based quantum computers), parallelization, and more. In the long run, we expect our research to lay the groundwork for more versatile quantum computer systems in the future. △ Less

Submitted 8 December, 2023; v1 submitted 1 September, 2023; originally announced September 2023.

Qualitative quantum simulation of resonant tunneling and localization with the shallow quantum circuits

Authors: J. L. Shen, P. Wang

Abstract: In a circuit-based quantum computer, the computing is performed via the discrete-time evolution driven by quantum gates. Accurate simulation of continuoustime evolution requires a large number of quantum gates and therefore suffers from more noise. In this paper, we find that shallow quantum circuits are sufficient to qualitatively observe some typical quantum phenomena in the continuous-time evol… ▽ More In a circuit-based quantum computer, the computing is performed via the discrete-time evolution driven by quantum gates. Accurate simulation of continuoustime evolution requires a large number of quantum gates and therefore suffers from more noise. In this paper, we find that shallow quantum circuits are sufficient to qualitatively observe some typical quantum phenomena in the continuous-time evolution limit, such as resonant tunneling and localization phenomena. We study the propagation of a spin excitation in Trotter circuits with a large step size. The circuits are formed of two types of two-qubit gates, i.e. XY gates and controlled- Rx gates, and single-qubit Rz gates. The configuration of the Rz gates determines the distribution of the spin excitation at the end of evolution. We demonstrate the resonant tunneling with up to four steps and the localization phenomenon with dozens of steps in Trotter circuits. Our results show that the circuit depth required for qualitative observation of some significant quantum phenomena is much smaller than that required for quantitative computation, suggesting that it is feasible to apply qualitative observations to near-term quantum computers. We also provide a way to use the physics laws to understand the error propagation in quantum circuits. △ Less

Submitted 8 May, 2026; v1 submitted 6 February, 2023; originally announced February 2023.

Comments: 16 pages,4 figures

Simulating the Femtouniverse on a Quantum Computer

Authors: Nouman Butt, Patrick Draper, Jiayu Shen

Abstract: We compute the low-lying spectrum of 4D SU(2) Yang-Mills in a finite volume using quantum simulations. In contrast to small-volume lattice truncations of the Hilbert space, we employ toroidal dimensional reduction to the ``femtouniverse" matrix quantum mechanics model. In this limit the theory is equivalent to the quantum mechanics of three interacting particles moving inside a 3-ball with certain… ▽ More We compute the low-lying spectrum of 4D SU(2) Yang-Mills in a finite volume using quantum simulations. In contrast to small-volume lattice truncations of the Hilbert space, we employ toroidal dimensional reduction to the ``femtouniverse" matrix quantum mechanics model. In this limit the theory is equivalent to the quantum mechanics of three interacting particles moving inside a 3-ball with certain boundary conditions. We use the variational quantum eigensolver and quantum subspace expansion techniques to compute the string tension to glueball mass ratio near the small/large-volume transition point, finding qualitatively good agreement with large volume Euclidean lattice simulations. △ Less

Submitted 20 November, 2022; originally announced November 2022.

Quasiparticle poisoning rate in a superconducting transmon qubit involving Majorana zero modes

Authors: Xiaopei Sun, Zhaozheng Lyu, Enna Zhuo, Bing Li, Zhongqing Ji, Jie Fan, Xiaohui Song, Fanning Qu, Guangtong Liu, Jie Shen, Li Lu

Abstract: Majorana zero modes have been attracting considerable attention because of their prospective applications in fault-tolerant topological quantum computing. In recent years, some schemes have been proposed to detect and manipulate Majorana zero modes using superconducting qubits. However, manipulating and reading the Majorana zero modes must be kept in the time window of quasiparticle poisoning. In… ▽ More Majorana zero modes have been attracting considerable attention because of their prospective applications in fault-tolerant topological quantum computing. In recent years, some schemes have been proposed to detect and manipulate Majorana zero modes using superconducting qubits. However, manipulating and reading the Majorana zero modes must be kept in the time window of quasiparticle poisoning. In this work, we study the problem of quasiparticle poisoning in a split transmon qubit containing hybrid Josephson junctions involving Majorana zero modes. We show that Majorana coupling will cause parity mixing and 4π Josephson effect. In addition, we obtained the expression of qubit parameter-dependent parity switching rate and demonstrated that quasiparticle poisoning can be greatly suppressed by reducing E_J/E_C via qubit design. △ Less

Submitted 15 November, 2022; originally announced November 2022.

A Reorder Trick for Decision Diagram Based Quantum Circuit Simulation

Authors: Jingcheng Shen, Linbo Long, Masao Okita, Fumihiko Ino

Abstract: Quantum computing is a hotspot technology for its potential to accelerate specific applications by exploiting quantum parallelism. However, current physical quantum computers are limited to a relatively small scale, simulators based on conventional machines are significantly relied on to perform quantum computing research. The straightforward array-based simulators require a tremendous amount of m… ▽ More Quantum computing is a hotspot technology for its potential to accelerate specific applications by exploiting quantum parallelism. However, current physical quantum computers are limited to a relatively small scale, simulators based on conventional machines are significantly relied on to perform quantum computing research. The straightforward array-based simulators require a tremendous amount of memory that increases exponentially with respect to the number of qubits. To mitigate such computing resource concerns, decision diagram based simulators were proposed that can efficiently exploit data redundancies in quantum states and operations. In this paper, we study two classes of quantum circuits on which the state-of-the-art decision diagram based simulators failed to perform well in terms of simulation time. We also propose a simple and powerful reorder trick to boost the simulation of such quantum circuits. Preliminary evaluation results demonstrate the usefulness of the proposed trick. Especially, for the Quantum Phase Estimation circuits, the proposed trick achieved speedups up to 313.6x compared to a state-of-the-art approach that relies on an auxiliary tool to optimize simulation order. △ Less

Submitted 13 November, 2022; originally announced November 2022.

Comments: 4 pages, 1 figure

QuACK: Accelerating Gradient-Based Quantum Optimization with Koopman Operator Learning

Authors: Di Luo, Jiayu Shen, Rumen Dangovski, Marin Soljačić

Abstract: Quantum optimization, a key application of quantum computing, has traditionally been stymied by the linearly increasing complexity of gradient calculations with an increasing number of parameters. This work bridges the gap between Koopman operator theory, which has found utility in applications because it allows for a linear representation of nonlinear dynamical systems, and natural gradient metho… ▽ More Quantum optimization, a key application of quantum computing, has traditionally been stymied by the linearly increasing complexity of gradient calculations with an increasing number of parameters. This work bridges the gap between Koopman operator theory, which has found utility in applications because it allows for a linear representation of nonlinear dynamical systems, and natural gradient methods in quantum optimization, leading to a significant acceleration of gradient-based quantum optimization. We present Quantum-circuit Alternating Controlled Koopman learning (QuACK), a novel framework that leverages an alternating algorithm for efficient prediction of gradient dynamics on quantum computers. We demonstrate QuACK's remarkable ability to accelerate gradient-based optimization across a range of applications in quantum optimization and machine learning. In fact, our empirical studies, spanning quantum chemistry, quantum condensed matter, quantum machine learning, and noisy environments, have shown accelerations of more than 200x speedup in the overparameterized regime, 10x speedup in the smooth regime, and 3x speedup in the non-smooth regime. With QuACK, we offer a robust advancement that harnesses the advantage of gradient-based quantum optimization for practical benefits. △ Less

Submitted 4 May, 2024; v1 submitted 2 November, 2022; originally announced November 2022.

Comments: Advances in Neural Information Processing Systems 36 (NeurIPS 2023) spotlight

Report number: MIT-CTP/5488

Quantum mechanical modeling of the multi-stage Stern$\unicode{x2013}$Gerlach experiment conducted by Frisch and Segrè

Authors: S. Süleyman Kahraman, Kelvin Titimbo, Zhe He, Jung-Tsung Shen, Lihong V. Wang

Abstract: The multi-stage Stern$\unicode{x2013}$Gerlach experiment conducted by Frisch and Segrè includes two cascaded quantum measurements with a nonadiabatic flipper in between. The Frisch and Segrè experiment has been modeled analytically by Majorana without the nuclear effect and subsequently revised by Rabi with the hyperfine interaction. However, the theoretical predictions do not match the experiment… ▽ More The multi-stage Stern$\unicode{x2013}$Gerlach experiment conducted by Frisch and Segrè includes two cascaded quantum measurements with a nonadiabatic flipper in between. The Frisch and Segrè experiment has been modeled analytically by Majorana without the nuclear effect and subsequently revised by Rabi with the hyperfine interaction. However, the theoretical predictions do not match the experimental observation accurately. Here, we numerically solve the standard quantum mechanical model, via the von Neumann equation, including the hyperfine interaction for the time evolution of the spin. Thus far, the coefficients of determination from the standard quantum mechanical model without using free parameters are still low, indicating a mismatch between the theory and the experiment. Non-standard variants that improve the match are explored for discussion. △ Less

Submitted 26 August, 2024; v1 submitted 20 October, 2022; originally announced October 2022.

Comments: 11 pages, 6 figures

Asymmetric Fraunhofer pattern in Josephson junctions from heterodimensional superlattice V$_5$S$_8$

Authors: Juewen Fan, Bingyan Jiang, Jiaji Zhao, Ran Bi, Jiadong Zhou, Zheng Liu, Guang Yang, Jie Shen, Fanming Qu, Li Lu, Ning Kang, Xiaosong Wu

Abstract: Introduction of spin-orbit coupling (SOC) in a Josephson junction (JJ) gives rise to unusual Josephson effects. We investigate JJs based on a newly discovered heterodimensional superlattice V$_5$S$_8$ with a special form of SOC. The unique homointerface of our JJs enables elimination of extrinsic effects due to interfaces and disorder. We observe asymmetric Fraunhofer patterns with respect to both… ▽ More Introduction of spin-orbit coupling (SOC) in a Josephson junction (JJ) gives rise to unusual Josephson effects. We investigate JJs based on a newly discovered heterodimensional superlattice V$_5$S$_8$ with a special form of SOC. The unique homointerface of our JJs enables elimination of extrinsic effects due to interfaces and disorder. We observe asymmetric Fraunhofer patterns with respect to both the perpendicular magnetic field and the current. The asymmetry is influenced by an in-plane magnetic field. Analysis of the pattern points to a nontrivial spatial distribution of the Josephson current that is intrinsic to the SOC in V$_5$S$_8$. △ Less

Submitted 7 June, 2022; v1 submitted 31 March, 2022; originally announced March 2022.

Comments: 15 pages,5 figures

Journal ref: Chin. Phys. B, 31, 057402 (2022)

Optimized nonadiabatic holonomic quantum computation based on Förster resonance in Rydberg atoms

Authors: Shuai Liu, Jun-Hui Shen, Ri-Hua Zheng, Yi-Hao Kang, Zhi-Cheng Shi, Jie Song, Yan Xia

Abstract: In this paper, we propose a scheme for implementing the nonadiabatic holonomic quantum computation (NHQC+) of two Rydberg atoms by using invariant-based reverse engineering (IBRE). The scheme is based on Förster resonance induced by strong dipole-dipole interaction between two Rydberg atoms, which provides a selective coupling mechanism to simply the dynamics of system. Moreover, for improving the… ▽ More In this paper, we propose a scheme for implementing the nonadiabatic holonomic quantum computation (NHQC+) of two Rydberg atoms by using invariant-based reverse engineering (IBRE). The scheme is based on Förster resonance induced by strong dipole-dipole interaction between two Rydberg atoms, which provides a selective coupling mechanism to simply the dynamics of system. Moreover, for improving the fidelity of the scheme, the optimal control method is introduced to enhance the gate robustness against systematic errors. Numerical simulations show the scheme is robust against the random noise in control fields, the deviation of dipole-dipole interaction, the Förster defect, and the spontaneous emission of atoms. Therefore, the scheme may provide some useful perspectives for the realization of quantum computation with Rydberg atoms. △ Less

Submitted 30 July, 2021; originally announced July 2021.

Supercurrent parity-meter in a nanowire Cooper-pair transistor

Authors: Ji-Yin Wang, Constantin Schrade, Vukan Levajac, David van Driel, Kongyi Li, Sasa Gazibegovic, Ghada Badawy, Roy L. M. Op het Veld, Joon Sue Lee, Mihir Pendharkar, Connor P. Dempsey, Chris J. Palmstrøm, Erik P. A. M. Bakkers, Liang Fu, Leo P. Kouwenhoven, Jie Shen

Abstract: We study a Cooper-pair transistor realized by two Josephson weak links that enclose a superconducting island in an InSb-Al hybrid nanowire. When the nanowire is subject to a magnetic field, isolated subgap levels arise in the superconducting island and, due to the Coulomb blockade,mediate a supercurrent by coherent co-tunneling of Cooper pairs. We show that the supercurrent resulting from such co-… ▽ More We study a Cooper-pair transistor realized by two Josephson weak links that enclose a superconducting island in an InSb-Al hybrid nanowire. When the nanowire is subject to a magnetic field, isolated subgap levels arise in the superconducting island and, due to the Coulomb blockade,mediate a supercurrent by coherent co-tunneling of Cooper pairs. We show that the supercurrent resulting from such co-tunneling events exhibits, for low to moderate magnetic fields, a phase offset that discriminates even and odd charge ground states on the superconducting island. Notably,this phase offset persists when a subgap state approaches zero energy and, based on theoretical considerations, permits parity measurements of subgap states by supercurrent interferometry. Such supercurrent parity measurements could, in a new series of experiments, provide an alternative approach for manipulating and protecting quantum information stored in the isolated subgap levels of superconducting islands. △ Less

Submitted 18 July, 2021; originally announced July 2021.

Journal ref: Science Advances 8, eabm9896 (2022)

Simulating Quantum Mechanics with a $θ$-term and an 't Hooft Anomaly on a Synthetic Dimension

Authors: Jiayu Shen, Di Luo, Chenxi Huang, Bryan K. Clark, Aida X. El-Khadra, Bryce Gadway, Patrick Draper

Abstract: A topological $θ$-term in gauge theories, including quantum chromodynamics in 3+1 dimensions, gives rise to a sign problem that makes classical Monte Carlo simulations impractical. Quantum simulations are not subject to such sign problems and are a promising approach to studying these theories in the future. In the near term, it is interesting to study simpler models that retain some of the physic… ▽ More A topological $θ$-term in gauge theories, including quantum chromodynamics in 3+1 dimensions, gives rise to a sign problem that makes classical Monte Carlo simulations impractical. Quantum simulations are not subject to such sign problems and are a promising approach to studying these theories in the future. In the near term, it is interesting to study simpler models that retain some of the physical phenomena of interest and their implementation on quantum hardware. For example, dimensionally-reducing gauge theories on small spatial tori produces quantum mechanical models which, despite being relatively simple to solve, retain interesting vacuum and symmetry structures from the parent gauge theories. Here we consider quantum mechanical particle-on-a-circle models, related by dimensional reduction to the 1+1d Schwinger model, that possess a $θ$-term and realize an 't Hooft anomaly or global inconsistency at $θ= π$. These models also exhibit the related phenomena of spontaneous symmetry breaking and instanton-anti-instanton interference in real time. We propose an experimental scheme for the real-time simulation of a particle on a circle with a $θ$-term and a $\mathbb{Z}_n$ potential using a synthetic dimension encoded in a Rydberg atom. Simulating the Rydberg atom with realistic experimental parameters, we demonstrate that the essential physics can be well-captured by the experiment, with expected behavior in the tunneling rate as a function of $θ$. Similar phenomena and observables can also arise in more complex quantum mechanical models connected to higher-dimensional nonabelian gauge theories by dimensional reduction. △ Less

Submitted 6 May, 2022; v1 submitted 16 July, 2021; originally announced July 2021.

Comments: 25 pages, 11 figures; minor changes and refs added, version accepted for publication in PRD

Journal ref: Phys. Rev. D 105, 074505 (2022)

Neutron State Entanglement with Overlapping Paths

Authors: S. J. Kuhn, S. McKay, J. Shen, N. Geerits, R. M. Dalgliesh, E. Dees, A. A. M. Irfan, F. Li, S. Lu, V. Vangelista, D. V. Baxter, G. Ortiz, S. R. Parnell, W. M. Snow, R. Pynn

Abstract: The development of direct probes of entanglement is integral to the rapidly expanding field of complex quantum materials. Here we test the robustness of entangled neutrons as a quantum probe by measuring the Clauser-Horne-Shimony-Holt contextuality witness while varying the beam properties. Specifically, we prove that the entanglement of the spin and path subsystems of individual neutrons prepared… ▽ More The development of direct probes of entanglement is integral to the rapidly expanding field of complex quantum materials. Here we test the robustness of entangled neutrons as a quantum probe by measuring the Clauser-Horne-Shimony-Holt contextuality witness while varying the beam properties. Specifically, we prove that the entanglement of the spin and path subsystems of individual neutrons prepared in two different experiments using two different apparatuses persists even after varying the entanglement length, coherence length, and neutron energy difference of the paths. The two independent apparatuses acting as entangler-disentangler pairs are static-field magnetic Wollaston prisms and resonance-field radio frequency flippers. Our results show that the spatial and energy properties of the neutron beam may be significantly altered without reducing the contextuality witness value below the Tsirelson bound, meaning that maximum entanglement is preserved. We also show that two paths may be considered distinguishable even when separated by less than the neutron coherence length. This work is the key step in the realization of the new modular, robust technique of entangled neutron scattering. △ Less

Submitted 21 December, 2020; originally announced December 2020.

Journal ref: Phys. Rev. Research 3, 023227 (2021)

Accurate phonon blockade detector composed of a quadratically coupled optomechanical system

Authors: Ye-Xiong Zeng, Tesfay Gebremariam, Jian Shen, Biao Xiong, Chong Li

Abstract: The observation of phonon blockade in a nanomechanical oscillator is clear evidence of its quantum nature. However, it is still a severe challenge to measure the strong phonon blockade in an optomechanical system with effective nonlinear coupling. In this paper, we propose a theoretical proposal for detecting the phonon blockade effect in a quadratically coupled optomechanical system by exploiting… ▽ More The observation of phonon blockade in a nanomechanical oscillator is clear evidence of its quantum nature. However, it is still a severe challenge to measure the strong phonon blockade in an optomechanical system with effective nonlinear coupling. In this paper, we propose a theoretical proposal for detecting the phonon blockade effect in a quadratically coupled optomechanical system by exploiting supervised machine learning. The detected optical signals are injected into the neural network as the input, while the output is the mechanical equal-time second-order correlation. Our results show our scheme performs superior performance on detecting phonon blockade. Specifically, it is efficient for nonlinear coupling systems; it performs a high precision for strong photon blockade; it is robust against the small disturbance of system parameters. Our work opens a promising way to build a phonon blockade detector. △ Less

Submitted 7 November, 2020; originally announced November 2020.

Comments: 7 figures

Report number: Appl. Phys. Lett. 118, 164003 (2021)

Journal ref: Appl. Phys. Lett. 118, 164003 (2021)

(1+1)-d U(1) Quantum link models from effective Hamiltonians of dipolar molecules

Authors: Jiayu Shen, Di Luo, Michael Highman, Bryan K. Clark, Brian DeMarco, Aida X. El-Khadra, Bryce Gadway

Abstract: We study the promising idea of using dipolar molecular systems as analog quantum simulators for quantum link models, which are discrete versions of lattice gauge theories. In a quantum link model the link variables have a finite number of degrees of freedom and discrete values. We construct the effective Hamiltonian of a system of dipolar molecules with electric dipole-dipole interactions, where w… ▽ More We study the promising idea of using dipolar molecular systems as analog quantum simulators for quantum link models, which are discrete versions of lattice gauge theories. In a quantum link model the link variables have a finite number of degrees of freedom and discrete values. We construct the effective Hamiltonian of a system of dipolar molecules with electric dipole-dipole interactions, where we use the tunable parameters of the system to match it to the target Hamiltonian describing a U(1) quantum link model in 1+1 dimensions. △ Less

Submitted 27 January, 2020; originally announced January 2020.

Comments: Proceedings of the 37th International Symposium on Lattice Field Theory - Lattice 2019, Wuhan (China)

Journal ref: PoS(LATTICE2019)125

Framework for simulating gauge theories with dipolar spin systems

Authors: Di Luo, Jiayu Shen, Michael Highman, Bryan K. Clark, Brian DeMarco, Aida X. El-Khadra, Bryce Gadway

Abstract: Gauge theories appear broadly in physics, ranging from the standard model of particle physics to long-wavelength descriptions of topological systems in condensed matter. However, systems with sign problems are largely inaccessible to classical computations and also beyond the current limitations of digital quantum hardware. In this work, we develop an analog approach to simulating gauge theories w… ▽ More Gauge theories appear broadly in physics, ranging from the standard model of particle physics to long-wavelength descriptions of topological systems in condensed matter. However, systems with sign problems are largely inaccessible to classical computations and also beyond the current limitations of digital quantum hardware. In this work, we develop an analog approach to simulating gauge theories with an experimental setup that employs dipolar spins (molecules or Rydberg atoms). We consider molecules fixed in space and interacting through dipole-dipole interactions, avoiding the need for itinerant degrees of freedom. Each molecule represents either a site or gauge degree of freedom, and Gauss law is preserved by a direct and programmatic tuning of positions and internal state energies. This approach can be regarded as a form of analog systems programming and charts a path forward for near-term quantum simulation. As a first step, we numerically validate this scheme in a small-system study of U(1) quantum link models in (1+1) dimensions with link spin S = 1/2 and S = 1 and illustrate how dynamical phenomena such as string inversion and string breaking could be observed in near-term experiments. Our work brings together methods from atomic and molecular physics, condensed matter physics, high-energy physics, and quantum information science for the study of nonperturbative processes in gauge theories. △ Less

Submitted 22 September, 2020; v1 submitted 24 December, 2019; originally announced December 2019.

Journal ref: Phys. Rev. A 102, 032617 (2020)

An Operator Analysis of Contextuality Witness Measurements for Multimode-Entangled Single Neutron Interferometry

Authors: Shufan Lu, Abu Ashik Md. Irfan, Jiazhou Shen, Steve J. Kuhn, W. Michael Snow, David V. Baxter, Roger Pynn, Gerardo Ortiz

Abstract: We develop an operator-based description of two types of multimode-entangled single-neutron quantum optical devices: Wollaston prisms and radio-frequency spin flippers in inclined magnetic field gradients. This treatment is similar to the approach used in quantum optics, and is convenient for the analysis of quantum contextuality measurements in certain types of neutron interferometers. We describ… ▽ More We develop an operator-based description of two types of multimode-entangled single-neutron quantum optical devices: Wollaston prisms and radio-frequency spin flippers in inclined magnetic field gradients. This treatment is similar to the approach used in quantum optics, and is convenient for the analysis of quantum contextuality measurements in certain types of neutron interferometers. We describe operationally the way multimode-entangled single-neutron states evolve in these devices, and provide expressions for the associated operators describing the dynamics, in the limit in which the neutron state space is approximated by a finite tensor product of distinguishable subsystems. We design entangled-neutron interferometers to measure entanglement witnesses for the Clauser, Horne, Shimony and Holt, and Mermin inequalities, and compare the theoretical predictions with recent experimental results. We present the generalization of these expressions to $n$ entangled distinguishable subsystems, which could become relevant in the future if it becomes possible to add neutron orbital angular momentum to the experimentally-accessible list of entangled modes. We view this work as a necessary first step towards a theoretical description of entangled neutron scattering from strongly entangled matter, and we explain why it should be possible to formulate a useful generalization of the usual Van Hove linear response theory for this case. We also briefly describe some other scientific extensions and applications which can benefit from interferometric measurements using the types of single-neutron multimode entanglement described by this analysis. △ Less

Submitted 21 December, 2019; originally announced December 2019.

Comments: 18 pages, 11 figures

Journal ref: Phys. Rev. A 101, 042318 (2020)

Unveiling contextual realities by microscopically entangling a neutron

Authors: J. Shen, S. J. Kuhn, R. M. Dalgliesh, V. O. de Haan, N. Geerits, A. A. M. Irfan, F. Li, S. Lu, S. R. Parnell, J. Plomp, A. A. van Well, A. Washington, D. V. Baxter, G. Ortiz, W. M. Snow, R. Pynn

Abstract: The development of qualitatively new measurement capabilities is often a prerequisite for critical scientific and technological advances. The dramatic progress made by modern probe techniques to uncover the microscopic structure of matter is fundamentally rooted in our control of two defining traits of quantum mechanics: discreteness of physical properties and interference phenomena. Magnetic Reso… ▽ More The development of qualitatively new measurement capabilities is often a prerequisite for critical scientific and technological advances. The dramatic progress made by modern probe techniques to uncover the microscopic structure of matter is fundamentally rooted in our control of two defining traits of quantum mechanics: discreteness of physical properties and interference phenomena. Magnetic Resonance Imaging, for instance, exploits the fact that protons have spin and can absorb photons at frequencies that depend on the medium to image the anatomy and physiology of living systems. Scattering techniques, in which photons, electrons, protons or neutrons are used as probes, make use of quantum interference to directly image the spatial position of individual atoms, their magnetic structure, or even unveil their concomitant dynamical correlations. None of these probes have so far exploited a unique characteristic of the quantum world: entanglement. Here we introduce a fundamentally new quantum probe, an entangled neutron beam, where individual neutrons can be entangled in spin, trajectory and energy. Its tunable entanglement length from nanometers to microns and energy differences from peV to neV will enable new investigations of microscopic magnetic correlations in systems with strongly entangled phases, such as those believed to emerge in unconventional superconductors. We develop an interferometer to prove entanglement of these distinguishable properties of the neutron beam by observing clear violations of both Clauser-Horne-Shimony-Holt and Mermin contextuality inequalities in the same experimental setup. Our work opens a pathway to a future era of entangled neutron scattering in matter. △ Less

Submitted 26 August, 2019; originally announced August 2019.

Comments: 11 pages, 4 figures

Hard superconducting gap and diffusion-induced superconductors in Ge-Si nanowires

Authors: Joost Ridderbos, Matthias Brauns, Jie Shen, Folkert K. de Vries, Ang Li, Sebastian Kölling, Marcel A. Verheijen, Alexander Brinkman, Wilfred G. van der Wiel, Erik P. A. M. Bakkers, Floris A. Zwanenburg

Abstract: We show a hard induced superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of $250$ mT, an important step towards creating and detecting Majorana zero modes in this system. A hard induced gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at… ▽ More We show a hard induced superconducting gap in a Ge-Si nanowire Josephson transistor up to in-plane magnetic fields of $250$ mT, an important step towards creating and detecting Majorana zero modes in this system. A hard induced gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. This is realized by annealing devices at $180$ $^\circ$C during which aluminium inter-diffuses and replaces the germanium in a section of the nanowire. Next to Al, we find a superconductor with lower critical temperature ($T_\mathrm{C}=0.9$ K) and a higher critical field ($B_\mathrm{C}=0.9-1.2$ T). We can therefore selectively switch either superconductor to the normal state by tuning the temperature and the magnetic field and observe that the additional superconductor induces a proximity supercurrent in the semiconducting part of the nanowire even when the Al is in the normal state. In another device where the diffusion of Al rendered the nanowire completely metallic, a superconductor with a much higher critical temperature ($T_\mathrm{C}=2.9$ K) and critical field ($B_\mathrm{C}=3.4$ T) is found. The small size of diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction. △ Less

Submitted 20 December, 2019; v1 submitted 11 July, 2019; originally announced July 2019.