Showing 1–50 of 56 results for author: Guan, J

Formal Verification of Quantum Ancilla Safety

Authors: Jiqi Li, Jingyi Mei, Wang Fang, Ji Guan

Abstract: Ensuring ancilla safety is a critical correctness requirement for quantum compilation, since ancilla qubits are routinely introduced to implement complex operations with fewer gates and reduced depth. However, formally verifying this property is computationally hard due to state-space explosion in the number of qubits, particularly for dirty ancillae, which carry unknown initial states and must be… ▽ More Ensuring ancilla safety is a critical correctness requirement for quantum compilation, since ancilla qubits are routinely introduced to implement complex operations with fewer gates and reduced depth. However, formally verifying this property is computationally hard due to state-space explosion in the number of qubits, particularly for dirty ancillae, which carry unknown initial states and must be restored after use. We propose an end-to-end verification-and-repair framework that rigorously addresses both clean and dirty ancilla safety. Our core contribution is a two-step reduction strategy: we first prove that verifying an $m$-qubit dirty ancilla register decomposes into $2m$ independent clean ancilla safety checks; subsequently, we reduce each clean ancilla safety instance to an algebraic commutativity check against Pauli-$Z$ and Pauli-$X$ operators. This approach yields an efficient and naturally parallel verifier and enables actionable diagnosis by classifying violations into logic errors and phase errors. Leveraging this diagnosis, we further design lightweight repair routines that append local single-qubit rotations to eliminate a broad class of local ancilla faults. We implement the full pipeline in a prototype tool using a dual-backend architecture combining decision diagrams and weighted model counting, and validate it on diverse circuits ranging from arithmetic benchmarks to Grover's algorithm. Our experiments demonstrate scalability to thousands of qubits and show that the proposed repairs effectively improve ancilla safety while preserving circuit functionality. △ Less

Submitted 13 August, 2026; originally announced August 2026.

Parallelizable Exact Synthesis of Quantum Circuits via Semi-Tensor Product

Authors: Chenjian Li, Dingchao Gao, Xiangzhen Zhou, Ji Guan, Pengcheng Zhu, Zhufei Chu

Abstract: Exact synthesis is a key infrastructure in quantum circuit synthesis and optimization, which provides optimal implementations of small circuit shards and is widely used as a circuit re-synthesis optimization kernel. However, existing quantum exact synthesis methods suffer from encoding overhead, memory bottlenecks, and poor parallel scalability. In this work, we introduce a parallel exact synthesi… ▽ More Exact synthesis is a key infrastructure in quantum circuit synthesis and optimization, which provides optimal implementations of small circuit shards and is widely used as a circuit re-synthesis optimization kernel. However, existing quantum exact synthesis methods suffer from encoding overhead, memory bottlenecks, and poor parallel scalability. In this work, we introduce a parallel exact synthesis framework for CNOT and phase polynomial circuits based on the semi-tensor product (STP) theory of matrices that avoids these issues. The algorithm contains two stages: it first enumerates candidate circuit topologies, and then instantiates each topology by determining the control and target qubit of its partial gates via a STP-based circuit solver. In the second stage, circuit topologies are encoded as canonical STP expressions, and the CNOT gates are synthesized through right-to-left STP matrix factorization that progressively eliminates infeasible gate decisions. In the framework, topology enumeration and the subsequent solving process are independent across different topologies, and can be naturally parallelized. Despite the NP-hardness of the problem, our algorithm yields up to $12.8\times$ parallel speedup with 32 workers, whereas the parallel speedups of existing SAT-based methods remain below $5\times$ with the same worker budget. On randomly generated synthesis targets, the proposed algorithm is typically $100$-$1000\times$ faster than the SAT-based approach on small and moderately difficult instances, and remains competitive for more difficult instances. When integrated in a real-world circuit optimization workflow, our algorithm achieves a median speedup of $3.41\times$ on the QASMBench benchmark. △ Less

Submitted 16 August, 2026; v1 submitted 27 July, 2026; originally announced July 2026.

Hamiltonian-Aware ADAPT Variational Quantum Eigensolver for Molecular Ground-State Simulation

Authors: Runhong He, Chao Liu, Xin Hong, Qiaozhen Chai, Junyuan Zhou, Ji Guan, Guolong Cui, Shenggang Ying

Abstract: Designing compact ansätze in Variational Quantum Eigensolver (VQE) is crucial for solving energetic problems of practical molecules on near-term quantum devices. However, existing Adaptive Derivative-Assembled Pseudo-Trotter (ADAPT) ansätze face two challenges: improper operator selection and accumulation of degraded operators. In this paper, we propose the Hamiltonian-Aware (HA) ADAPT-VQE algorit… ▽ More Designing compact ansätze in Variational Quantum Eigensolver (VQE) is crucial for solving energetic problems of practical molecules on near-term quantum devices. However, existing Adaptive Derivative-Assembled Pseudo-Trotter (ADAPT) ansätze face two challenges: improper operator selection and accumulation of degraded operators. In this paper, we propose the Hamiltonian-Aware (HA) ADAPT-VQE algorithm to address these issues. First, we establish a novel excitation operator selection criterion. It breaks the local constraint of existing criteria by incorporating Hamiltonian information, prioritizes physically meaningful excitation operators, and incurs no extra classical or quantum computational overhead. Furthermore, we develop a problem-adaptive method for discriminating and pruning redundant excitation operators stemming from improper selection and inevitable degradation. This method balances redundant operator pruning and convergence guarantee, and is applicable to ansätze with arbitrary scales. Systematic numerical experiments on typical strongly correlated molecular systems demonstrate that our HA-ADAPT-VQE avoids energy plateaus and outperforms baseline algorithms in terms of energy error, ansatz size, and measurement cost. This work offers an efficient, robust ansatz construction paradigm, facilitating the development and practical deployment of large-scale VQE in quantum chemistry. △ Less

Submitted 11 June, 2026; originally announced June 2026.

Comments: no comments

Verifying Adversarial Robustness in Quantum Machine Learning: from theory to physical validation via a software tool

Authors: Ji Guan, Mingsheng Ying

Abstract: As with classical neural networks, quantum machine learning (QML) models are vulnerable to small input perturbations that can significantly alter output predictions. Certifying the robustness of QML models, particularly on NISQ hardware, is therefore a fundamental step toward trustworthy quantum AI. This chapter reviews our recently developed comprehensive formal framework for verifying adversaria… ▽ More As with classical neural networks, quantum machine learning (QML) models are vulnerable to small input perturbations that can significantly alter output predictions. Certifying the robustness of QML models, particularly on NISQ hardware, is therefore a fundamental step toward trustworthy quantum AI. This chapter reviews our recently developed comprehensive formal framework for verifying adversarial robustness in QML. The core of this framework is a fidelity-based robustness lower bound computable directly from the measurement outcome distribution, which enables both formal verification and empirical estimation on real quantum devices. Additionally, the optimal bound can be computed via semidefinite programming (SDP) with full knowledge of the quantum machine learning models. We incorporate these results into: (1) an efficient formal verification framework; (2) VeriQR, the first dedicated QML robustness verification tool; and (3) the first experimental benchmark of quantum adversarial robustness on a 20-qubit superconducting processor. Together, these systematic advances enable scalable, physically grounded robustness evaluation of QML models. △ Less

Submitted 28 May, 2026; originally announced May 2026.

Journal ref: Quantum Robustness in Artificial Intelligence, Quantum Science and Technology, Springer Nature Switzerland, 2026

QSeqSim: A Symbolic Simulator for Qiskit While Loops Using Sequential Quantum Circuits

Authors: Zihao Li, Ji Guan, Mingsheng Ying

Abstract: We present a tool QSeqSim, a Qiskit-integrated symbolic backend that fills the current gap of having no Qiskit-native support for simulating while-loop quantum programs and their induced sequential quantum circuits. QSeqSim takes Qiskit QuantumCircuit objects, translates them into OpenQASM 3 code, and organises the resulting program into a combination of combinational, dynamic, and sequential circ… ▽ More We present a tool QSeqSim, a Qiskit-integrated symbolic backend that fills the current gap of having no Qiskit-native support for simulating while-loop quantum programs and their induced sequential quantum circuits. QSeqSim takes Qiskit QuantumCircuit objects, translates them into OpenQASM 3 code, and organises the resulting program into a combination of combinational, dynamic, and sequential circuits, thereby assigning while-loops a precise sequential circuit semantics with explicit internal and external qubits. Building on this semantics, QSeqSim adopts a Binary Decision Diagram (BDD)-based symbolic representation and integrates weighted model counting to compute measurement probabilities efficiently by exploiting sharing in structured and sparse BDDs. On top of this Boolean backbone, it introduces dedicated symbolic operators for state composition and state retention, thereby enabling efficient symbolic execution of sequential quantum circuits. Our experiments demonstrate that QSeqSim scales to substantial while-induced sequential circuits; in particular, in the quantum random walk benchmark we successfully simulate circuits with over 1000 qubits for more than 10 loop iterations. QSeqSim is available at https://github.com/Veri-Q/QSeqSim. △ Less

Submitted 14 May, 2026; originally announced May 2026.

Comments: This is the full arXiv version of the paper accepted at FM 2026. The paper has 26 pages and 4 figures. Proceedings version: FM 2026, LNCS 16556, Springer, 2026

MSC Class: 81P68; 68Q12; 03B70 ACM Class: F.3.1; F.1.1; D.2.4

Journal ref: FM 2026, LNCS 16556, Springer, 2026

Model Checking Matrix Product States against Linear Chain Logic

Authors: Ming Xu, Yihao Chen, Ji Guan

Abstract: Matrix product states (MPS) are a standard tensor-network representation for ground states of one-dimensional quantum many-body systems, and they underpin widely used simulation tools such as DMRG. However, while quantum model checking has been developed mainly for quantum programs and communication protocols (with properties expressed along a time axis), there is still no comparable framework for… ▽ More Matrix product states (MPS) are a standard tensor-network representation for ground states of one-dimensional quantum many-body systems, and they underpin widely used simulation tools such as DMRG. However, while quantum model checking has been developed mainly for quantum programs and communication protocols (with properties expressed along a time axis), there is still no comparable framework for systematically verifying \emph{spatial} and \emph{size-dependent} properties of physical many-body states, where the key parameter is the system size. This paper takes a step toward bridging the gap. We propose \emph{Linear Chain Logic} (LCL), a spatial logic designed to specify physically meaningful properties of periodic MPS families as the system size grows, such as nontriviality on rings and large-size asymptotic patterns. Our approach builds on a simple but powerful connection: every periodic MPS naturally induces a completely positive map (a quantum operation) on its virtual space, so many quantitative features of the MPS can be analysed through the repeated application of the operation. Using this perspective, we derive an effective procedure to compute the inner products of an MPS at a given size and to support richer LCL specifications, without relying on brute-force state expansion. We then develop approximate model-checking algorithms that combine sound bounding with asymptotic structural analysis, enabling scalable reasoning about large system sizes. Experiments on representative MPS families illustrate that our method can automatically verify nontriviality and detect asymptotic spatial regimes in a way that complements traditional numerical techniques. △ Less

Submitted 14 May, 2026; originally announced May 2026.

Constructing Compact ADAPT Unitary Coupled-Cluster Ansatz with Parameter-Based Criterion

Authors: Runhong He, Xin Hong, Qiaozhen Chai, Ji Guan, Junyuan Zhou, Arapat Ablimit, Guolong Cui, Shenggang Ying

Abstract: The adaptive derivative-assembled pseudo-trotter variational quantum eigensolver (ADAPT-VQE) is a promising hybrid quantum-classical algorithm for molecular ground state energy calculation, yet its practical scalability is hampered by redundant excitation operators and excessive measurement costs. To address these challenges, we propose Param-ADAPT-VQE, a novel improved algorithm that selects exci… ▽ More The adaptive derivative-assembled pseudo-trotter variational quantum eigensolver (ADAPT-VQE) is a promising hybrid quantum-classical algorithm for molecular ground state energy calculation, yet its practical scalability is hampered by redundant excitation operators and excessive measurement costs. To address these challenges, we propose Param-ADAPT-VQE, a novel improved algorithm that selects excitation operators based on a parameter-based criterion instead of the traditional gradient-based metric. This strategy effectively eludes redundant operators. We further develop a sub-Hamiltonian technique and integrate a hot-start VQE optimization strategy, achieving a significant reduction in measurement costs. Numerical experiments on typical molecular systems demonstrate that Param-ADAPT-VQE outperforms the original ADAPT-VQE in computational accuracy, ansatz size, and measurement costs. Furthermore, our scheme retains the fundamental framework of ADAPT-VQE and is thus fully compatible with its various modified versions, enabling further performance improvements in specific aspects. This work presents an efficient and scalable enhancement to ADAPT-VQE, mitigating the core obstacles that impede its practical implementation in the field of molecular quantum chemistry. △ Less

Submitted 4 February, 2026; originally announced February 2026.

Comments: 12 pages, 8 figures

Hamiltonian-Informed Point Group Symmetry-Respecting Ansatz for Variational Quantum Eigensolver

Authors: Runhong He, Arapat Ablimit, Xin Hong, Qiaozhen Chai, Junyuan Zhou, Ji Guan, Guolong Cui, Shenggang Ying

Abstract: Solving molecular energy levels via the Variational Quantum Eigensolver (VQE) algorithm represents one of the most promising applications for demonstrating practically meaningful quantum advantage in the noisy intermediate-scale quantum (NISQ) era. To strike a balance between ansatz complexity and computational stability in VQE calculations, we propose the HiUCCSD, a novel symmetry-respecting ansa… ▽ More Solving molecular energy levels via the Variational Quantum Eigensolver (VQE) algorithm represents one of the most promising applications for demonstrating practically meaningful quantum advantage in the noisy intermediate-scale quantum (NISQ) era. To strike a balance between ansatz complexity and computational stability in VQE calculations, we propose the HiUCCSD, a novel symmetry-respecting ansatz engineered from the intrinsic information of the Hamiltonian. We theoretically prove the effectiveness of HiUCCSD within the scope of Abelian point groups. Furthermore, we compare the performance of HiUCCSD and the established SymUCCSD via VQE and Adaptive Derivative-Assembled Pseudo-Trotter (ADAPT)-VQE numerical experiments on ten molecules with distinct point groups. The results show that HiUCCSD achieves equivalent performance to SymUCCSD for Abelian point group molecules, while avoiding the potential performance failure of SymUCCSD in the case of non-Abelian point group molecules. Across the studied molecular systems, HiUCCSD cuts the parameter count by 18%-83% for VQE and reduces the excitation operator pool size by 27%-84% for ADAPT-VQE, as compared with the UCCSD ansatz. With enhanced robustness and broader applicability, HiUCCSD offers a new ansatz option for advancing large-scale molecular VQE implementation. △ Less

Submitted 24 December, 2025; originally announced December 2025.

Comments: 12 pages

Gradient-Based Excitation Filter for Molecular Ground-State Simulation

Authors: Runhong He, Qiaozhen Chai, Xin Hong, Ji Guan, Guolong Cui, Shengbin Wang, Shenggang Ying

Abstract: Molecular ground-state simulation is one of the most promising fields for demonstrating practical quantum advantage on near-term quantum computers. However, the Variational Quantum Eigensolver (VQE), a leading algorithm for this task, still faces significant challenges due to excessive circuit depth. This paper introduces a method to efficiently simplify the Unitary Coupled-Cluster with Single and… ▽ More Molecular ground-state simulation is one of the most promising fields for demonstrating practical quantum advantage on near-term quantum computers. However, the Variational Quantum Eigensolver (VQE), a leading algorithm for this task, still faces significant challenges due to excessive circuit depth. This paper introduces a method to efficiently simplify the Unitary Coupled-Cluster with Single and Double Excitations (UCCSD) ansatz on classical computers. We propose to estimate the correlation energy contributions of excitations using their gradients at Hartree-Fock state, supported by a theoretical proof. For molecular systems with $K$ orbitals, these gradients can be obtained with complexity only $O(K^8)$, which can be efficiently implemented on classical computers, especially in parallel. By sorting and truncating the excitations based on these gradients, the simplified ansatz can be obtained immediately, avoiding the challenging task of optimizing ansatz structure on a quantum computer. Furthermore, we introduce a strategy to indirectly identify critical excitations through spin-adapted constraints, reducing gradient computations by $60\%$. Numerical experiments on prototype molecular systems (H${_4}$, HF, H${_2}$O, BeH${_2}$ and NH$_3$) demonstrate that our approach achieves up to $46\%$ parameter decrease, $60\%$ circuit depth reduction and $678\times$ runtime speedup compared to the state-of-the-art ADAPT-VQE algorithm, enabling significantly more compact quantum circuits with enhanced near-term feasibility. △ Less

Submitted 6 July, 2025; v1 submitted 25 June, 2025; originally announced June 2025.

Comments: 16 pages

Experimental robustness benchmarking of quantum neural networks on a superconducting quantum processor

Authors: Hai-Feng Zhang, Zhao-Yun Chen, Peng Wang, Liang-Liang Guo, Tian-Le Wang, Xiao-Yan Yang, Ren-Ze Zhao, Ze-An Zhao, Sheng Zhang, Lei Du, Hao-Ran Tao, Zhi-Long Jia, Wei-Cheng Kong, Huan-Yu Liu, Athanasios V. Vasilakos, Yang Yang, Yu-Chun Wu, Ji Guan, Peng Duan, Guo-Ping Guo

Abstract: Quantum machine learning (QML) models, like their classical counterparts, are vulnerable to adversarial attacks, hindering their secure deployment. Here, we report the first systematic experimental robustness benchmark for 20-qubit quantum neural network (QNN) classifiers executed on a superconducting processor. Our benchmarking framework features an efficient adversarial attack algorithm designed… ▽ More Quantum machine learning (QML) models, like their classical counterparts, are vulnerable to adversarial attacks, hindering their secure deployment. Here, we report the first systematic experimental robustness benchmark for 20-qubit quantum neural network (QNN) classifiers executed on a superconducting processor. Our benchmarking framework features an efficient adversarial attack algorithm designed for QNNs, enabling quantitative characterization of adversarial robustness and robustness bounds. From our analysis, we verify that adversarial training reduces sensitivity to targeted perturbations by regularizing input gradients, significantly enhancing QNN's robustness. Additionally, our analysis reveals that QNNs exhibit superior adversarial robustness compared to classical neural networks, an advantage attributed to inherent quantum noise. Furthermore, the empirical upper bound extracted from our attack experiments shows a minimal deviation ($3 \times 10^{-3}$) from the theoretical lower bound, providing strong experimental confirmation of the attack's effectiveness and the tightness of fidelity-based robustness bounds. This work establishes a critical experimental framework for assessing and improving quantum adversarial robustness, paving the way for secure and reliable QML applications. △ Less

Submitted 19 January, 2026; v1 submitted 22 May, 2025; originally announced May 2025.

Comments: There are 8 pages with 5 figures in the main text

Report number: 1674-7348

Journal ref: SCIENCE CHINA Physics, Mechanics & Astronomy Volume 69, Issue 6: 260315 (2026)

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

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

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

Submitted 14 April, 2025; originally announced April 2025.

Free-Space Twin-Field Quantum Key Distribution

Authors: Yu-Huai Li, Ting Zeng, Min-Yan Wang, Cong Jiang, Jin Lin, Hao-Bin Fu, Xin-Yang Zheng, Jiu-Peng Chen, Zeng-Sen Lin, Cheng-Lin Li, Jian-Yu Guan, Yang Li, Qi Shen, Hao Li, Lixing You, Zhen Wang, Fei Zhou, Juan Yin, Sheng-Kai Liao, Ji-Gang Ren, Xiang-Bin Wang, Yuan Cao, Qiang Zhang, Cheng-Zhi Peng, Jian-Wei Pan

Abstract: Twin-field quantum key distribution (TF-QKD) elevates the secure key rate from a linear to a square-root dependence on channel loss while preserving measurement-device-independent security. This protocol is uniquely positioned to enable global-scale quantum networks, even under extreme channel loss. While fiber-based TF-QKD implementations have advanced rapidly since its proposal, free-space reali… ▽ More Twin-field quantum key distribution (TF-QKD) elevates the secure key rate from a linear to a square-root dependence on channel loss while preserving measurement-device-independent security. This protocol is uniquely positioned to enable global-scale quantum networks, even under extreme channel loss. While fiber-based TF-QKD implementations have advanced rapidly since its proposal, free-space realizations have remained elusive due to atmospheric turbulence-induced phase distortions. Here, we report the first experimental demonstration of free-space TF-QKD over 14.2 km urban atmospheric channels, surpassing the effective atmospheric thickness -- a critical threshold for satellite compatibility. We achieve a secret key rate exceeding the repeaterless capacity bound, a milestone for practical quantum communication. Our approach eliminates the need for an auxiliary channel to stabilize a closed interferometer, instead leveraging open-channel time and phase control of optical pulses. This work represents a pivotal advance toward satellite-based global quantum networks, combining high-speed key distribution with inherent resistance to real-world channel fluctuations. △ Less

Submitted 22 March, 2025; originally announced March 2025.

Comments: 17 pages, 3 figures

Journal ref: Nature Photonics 20, 783 (2026)

Approximation Methods for Simulation and Equivalence Checking of Noisy Quantum Circuits

Authors: Mingyu Huang, Ji Guan, Wang Fang, Mingsheng Ying

Abstract: In the current NISQ (Noisy Intermediate-Scale Quantum) era, simulating and verifying noisy quantum circuits is crucial but faces challenges such as quantum state explosion and complex noise representations, constraining simulation and equivalence checking to circuits with a limited number of qubits. This paper introduces an approximation algorithm for simulating and assessing the equivalence of no… ▽ More In the current NISQ (Noisy Intermediate-Scale Quantum) era, simulating and verifying noisy quantum circuits is crucial but faces challenges such as quantum state explosion and complex noise representations, constraining simulation and equivalence checking to circuits with a limited number of qubits. This paper introduces an approximation algorithm for simulating and assessing the equivalence of noisy quantum circuits, specifically designed to improve scalability under low-noise conditions. The approach utilizes a novel tensor network diagram combined with singular value decomposition to approximate the tensors of quantum noises. The implementation is based on Google's TensorNetwork Python package for contraction. Experimental results on realistic quantum circuits with realistic hardware noise models indicate that our algorithm can simulate and check the equivalence of QAOA (Quantum Approximate Optimization Algorithm) circuits with around 200 qubits and 20 noise operators, outperforming state-of-the-art approaches in scalability and speed. △ Less

Submitted 10 December, 2025; v1 submitted 13 March, 2025; originally announced March 2025.

Coherence Properties of Rare-Earth Spins in Micrometer-Thin Films

Authors: Zihua Chai, Zhaocong Wang, Xinghang Chen, Quanshen Shen, Zeyu Gao, Junyu Guan, Hanyu Zhang, Ya Wang, Yang Tan, Feng Chen, Kangwei Xia

Abstract: Rare-earth ions in bulk crystals are excellent solid-state quantum systems in quantum information science, owing to the exceptional optical and spin coherence properties. However, the weak fluorescence of single rare-earth ions present a significant challenge for scalability, necessitating the integration into micro-cavities. Thin films serve as a promising material platform for the integration, y… ▽ More Rare-earth ions in bulk crystals are excellent solid-state quantum systems in quantum information science, owing to the exceptional optical and spin coherence properties. However, the weak fluorescence of single rare-earth ions present a significant challenge for scalability, necessitating the integration into micro-cavities. Thin films serve as a promising material platform for the integration, yet the fabrication without compromising the properties of the materials and rare-earth ions remains challenging. In this work, we fabricate micrometer-thin yttrium aluminum garnet (YAG) films from bulk crystals using ion implantation techniques. The resulting films preserve the single-crystalline structure of the original bulk crystal. Notably, the embedded rare-earth ions are photo-stable and exhibit bulk-like spin coherence properties. Our results demonstrate the compatibility of bulk-like spin properties with the thin-film fabrication technique, facilitating the efficient integration of rare-earth ions into on-chip photonic devices and advancing the applications of rare-earth ions systems in quantum technologies. △ Less

Submitted 12 March, 2025; originally announced March 2025.

Differential Privacy of Quantum and Quantum-Inspired Classical Recommendation Algorithms

Authors: Chenjian Li, Mingsheng Ying, Ji Guan

Abstract: We study the differential privacy (DP) of the quantum recommendation algorithm of Kerenidis--Prakash and its quantum-inspired classical counterpart. Under standard low-rank and incoherence assumptions on the preference matrix, we show that the randomness already present in the algorithms' measurement/$\ell_2$-sampling steps can act as a privacy-curating mechanism, yielding $(\varepsilon,δ)$-DP wit… ▽ More We study the differential privacy (DP) of the quantum recommendation algorithm of Kerenidis--Prakash and its quantum-inspired classical counterpart. Under standard low-rank and incoherence assumptions on the preference matrix, we show that the randomness already present in the algorithms' measurement/$\ell_2$-sampling steps can act as a privacy-curating mechanism, yielding $(\varepsilon,δ)$-DP without injecting additional DP noise through the interface. Concretely, for a system with $m$ users and $n$ items and rank parameter $k$, we prove $\varepsilon=\mathcal O(\sqrt{k/n})$ and $δ= \mathcal O\big(k^2/\min^2\{m,n\}\big)$; in the typical regime $k=\mathrm{polylog}(m,n)$ this simplifies to $\varepsilon=\tilde{\mathcal O}(1/\sqrt n)$ and $δ=\tilde{\mathcal O}\big(1/\min^2\{m,n\}\big)$. Our analysis introduces a perturbation technique for truncated SVD under a single-entry update, which tracks the induced change in the low-rank reconstruction while avoiding unstable singular-vector comparisons. Finally, we validate the scaling on real-world rating datasets and compare against classical DP recommender baselines. △ Less

Submitted 26 February, 2026; v1 submitted 7 February, 2025; originally announced February 2025.

Comments: 18 pages, 3 figures in total(including appendix)

RH: An Architecture for Redesigning Quantum Circuits on Quantum Hardware Devices

Authors: Runhong He, Ji Guan, Xin Hong, Guolong Cui, Shengbin Wang, Shenggang Ying

Abstract: In this paper we present an architecture that enables the redesign of large-scale quantum circuits on quantum hardware based on the entangling quantum generative adversarial network (EQ-GAN). Specifically, by prepending a random quantum circuit module to the standard EQ-GAN framework, we extend its capability from quantum state learning to unitary transformation learning. The completeness of this… ▽ More In this paper we present an architecture that enables the redesign of large-scale quantum circuits on quantum hardware based on the entangling quantum generative adversarial network (EQ-GAN). Specifically, by prepending a random quantum circuit module to the standard EQ-GAN framework, we extend its capability from quantum state learning to unitary transformation learning. The completeness of this architecture is theoretically proved. Moreover, an efficient local random circuit is proposed, which significantly enhances the practicality of our architecture. For concreteness, we apply this architecture to three crucial applications in circuit optimization, including the equivalence checking of (non-) parameterized circuits, as well as the variational reconstruction of quantum circuits. The feasibility of our approach is demonstrated by excellent results in both classical and noisy intermediate-scale quantum (NISQ) hardware implementations. We believe our work will facilitate the implementation and validation of the advantages of quantum algorithms. △ Less

Submitted 16 May, 2025; v1 submitted 30 December, 2024; originally announced December 2024.

Comments: 9 pages,11 figures

113 km absolute ranging with nanometer precision

Authors: Yan-Wei Chen, Meng-Zhe Lian, Jin-Jian Han, Ting Zeng, Min Li, Guo-Dong Wei, Yong Wang, Yi Sheng, Ali Esamdin, Lei Hou, Qi Shen, Jian-Yu Guan, Jian-Jun Jia, Ji-Gang Ren, Cheng-Zhi Peng, Qiang Zhang, Hai-Feng Jiang, Jian-Wei Pan

Abstract: Accurate long-distance ranging is crucial for diverse applications, including satellite formation flying, very-long-baseline interferometry, gravitational-wave observatory, geographical research, etc. The integration of the time-of-flight mesurement with phase interference in dual-comb method enables high-precision ranging with a rapid update rate and an extended ambiguity range. Pioneering experi… ▽ More Accurate long-distance ranging is crucial for diverse applications, including satellite formation flying, very-long-baseline interferometry, gravitational-wave observatory, geographical research, etc. The integration of the time-of-flight mesurement with phase interference in dual-comb method enables high-precision ranging with a rapid update rate and an extended ambiguity range. Pioneering experiments have demonstrated unprecedented precision in ranging, achieving 5 nm @ 60 ms for 1.1 m and 200 nm @ 0.5 s for 25 m. However, long-distance ranging remains technically challenging due to high transmission loss and noise. In this letter, we propose a two-way dual-comb ranging (TWDCR) approach that enables successful ranging over a distance of 113 kilometers. We employ air dispersion analysis and synthetic repetition rate technique to extend the ambiguity range of the inherently noisy channel beyond 100 km. The achieved ranging precision is 11.5 $μ$m @ 1.3 ms, 681 nm @ 1 s, and 82 nm @ 21 s, as confirmed through a comparative analysis of two independent systems. The advanced long-distance ranging technology is expected to have immediate implications for space research initiatives, such as the space telescope array and the satellite gravimetry. △ Less

Submitted 7 December, 2024; originally announced December 2024.

Comments: 21 pages, 5 figures, 1 table

VeriQR: A Robustness Verification Tool for Quantum Machine Learning Models

Authors: Yanling Lin, Ji Guan, Wang Fang, Mingsheng Ying, Zhaofeng Su

Abstract: Adversarial noise attacks present a significant threat to quantum machine learning (QML) models, similar to their classical counterparts. This is especially true in the current Noisy Intermediate-Scale Quantum era, where noise is unavoidable. Therefore, it is essential to ensure the robustness of QML models before their deployment. To address this challenge, we introduce \textit{VeriQR}, the first… ▽ More Adversarial noise attacks present a significant threat to quantum machine learning (QML) models, similar to their classical counterparts. This is especially true in the current Noisy Intermediate-Scale Quantum era, where noise is unavoidable. Therefore, it is essential to ensure the robustness of QML models before their deployment. To address this challenge, we introduce \textit{VeriQR}, the first tool designed specifically for formally verifying and improving the robustness of QML models, to the best of our knowledge. This tool mimics real-world quantum hardware's noisy impacts by incorporating random noise to formally validate a QML model's robustness. \textit{VeriQR} supports exact (sound and complete) algorithms for both local and global robustness verification. For enhanced efficiency, it implements an under-approximate (complete) algorithm and a tensor network-based algorithm to verify local and global robustness, respectively. As a formal verification tool, \textit{VeriQR} can detect adversarial examples and utilize them for further analysis and to enhance the local robustness through adversarial training, as demonstrated by experiments on real-world quantum machine learning models. Moreover, it permits users to incorporate customized noise. Based on this feature, we assess \textit{VeriQR} using various real-world examples, and experimental outcomes confirm that the addition of specific quantum noise can enhance the global robustness of QML models. These processes are made accessible through a user-friendly graphical interface provided by \textit{VeriQR}, catering to general users without requiring a deep understanding of the counter-intuitive probabilistic nature of quantum computing. △ Less

Submitted 18 July, 2024; originally announced July 2024.

Optimal Mechanisms for Quantum Local Differential Privacy

Authors: Ji Guan

Abstract: Centralized differential privacy has been successfully applied to quantum computing and information processing to protect privacy and avoid leaks in the connections between neighboring quantum states. Consequently, quantum local differential privacy (QLDP) has been newly proposed to preserve quantum data privacy akin to the classical scenario where all states are viewed as neighboring states. Howe… ▽ More Centralized differential privacy has been successfully applied to quantum computing and information processing to protect privacy and avoid leaks in the connections between neighboring quantum states. Consequently, quantum local differential privacy (QLDP) has been newly proposed to preserve quantum data privacy akin to the classical scenario where all states are viewed as neighboring states. However, the exploration of the QLDP framework is still in its early stages, primarily conceptual, which poses challenges for its practical implementation in safeguarding quantum state privacy. This paper delves into optimal QLDP mechanisms to balance privacy and utility to enhance the practical use of the QLDP framework. QLDP utilizes a parameter $ε$ to manage privacy leaks and ensure the privacy of individual quantum states. The optimization of the QLDP value $ε$, denoted as $ε^*$, for any quantum mechanism is addressed as an optimization problem. The introduction of quantum noise is shown to provide privacy protections similar to classical scenarios, with quantum depolarizing noise identified as the optimal unital privatization mechanism within the QLDP framework. Unital mechanisms represent a diverse set of quantum mechanisms that encompass frequently employed quantum noise types. Quantum depolarizing noise optimizes both fidelity and trace distance utilities, which are crucial metrics in the field of quantum computation and information, and can be viewed as a quantum counterpart to classical randomized response methods. The study further explores the trade-off between utility and privacy across different quantum noise mechanisms, including unital and non-unital quantum noise mechanisms, through both analytical and numerically experimental approaches. This highlights the optimization of quantum depolarizing noise in the QLDP framework. △ Less

Submitted 16 September, 2025; v1 submitted 18 July, 2024; originally announced July 2024.

Measurement-based Verification of Quantum Markov Chains

Authors: Ji Guan, Yuan Feng, Andrea Turrini, Mingsheng Ying

Abstract: Model-checking techniques have been extended to analyze quantum programs and communication protocols represented as quantum Markov chains, an extension of classical Markov chains. To specify qualitative temporal properties, a subspace-based quantum temporal logic is used, which is built on Birkhoff-von Neumann atomic propositions. These propositions determine whether a quantum state is within a su… ▽ More Model-checking techniques have been extended to analyze quantum programs and communication protocols represented as quantum Markov chains, an extension of classical Markov chains. To specify qualitative temporal properties, a subspace-based quantum temporal logic is used, which is built on Birkhoff-von Neumann atomic propositions. These propositions determine whether a quantum state is within a subspace of the entire state space. In this paper, we propose the measurement-based linear-time temporal logic MLTL to check quantitative properties. MLTL builds upon classical linear-time temporal logic (LTL) but introduces quantum atomic propositions that reason about the probability distribution after measuring a quantum state. To facilitate verification, we extend the symbolic dynamics-based techniques for stochastic matrices described by Agrawal et al. (JACM 2015) to handle more general quantum linear operators (super-operators) through eigenvalue analysis. This extension enables the development of an efficient algorithm for approximately model checking a quantum Markov chain against an MLTL formula. To demonstrate the utility of our model-checking algorithm, we use it to simultaneously verify linear-time properties of both quantum and classical random walks. Through this verification, we confirm the previously established advantages discovered by Ambainis et al. (STOC 2001) of quantum walks over classical random walks and discover new phenomena unique to quantum walks. △ Less

Submitted 9 May, 2024; originally announced May 2024.

Dual-comb spectroscopy over 100km open-air path

Authors: Jin-Jian Han, Wei Zhong, Ruo-Can Zhao, Ting Zeng, Min Li, Jian Lu, Xin-Xin Peng, Xi-Ping Shi, Qin Yin, Yong Wang, Ali Esamdin, Qi Shen, Jian-Yu Guan, Lei Hou, Ji-Gang Ren, Jian-Jun Jia, Yu Wang, Hai-Feng Jiang, XiangHui Xue, Qiang Zhang, Xian-Kang Dou, Jian-Wei Pan

Abstract: Satellite-based greenhouse gases (GHG) sensing technologies play a critical role in the study of global carbon emissions and climate change. However, none of the existing satellite-based GHG sensing technologies can achieve the measurement of broad bandwidth, high temporal-spatial resolution, and high sensitivity at the same time. Recently, dual-comb spectroscopy (DCS) has been proposed as a super… ▽ More Satellite-based greenhouse gases (GHG) sensing technologies play a critical role in the study of global carbon emissions and climate change. However, none of the existing satellite-based GHG sensing technologies can achieve the measurement of broad bandwidth, high temporal-spatial resolution, and high sensitivity at the same time. Recently, dual-comb spectroscopy (DCS) has been proposed as a superior candidate technology for GHG sensing because it can measure broadband spectra with high temporal-spatial resolution and high sensitivity. The main barrier to DCS's display on satellites is its short measurement distance in open air achieved thus far. Prior research has not been able to implement DCS over 20 km of open-air path. Here, by developing a bistatic setup using time-frequency dissemination and high-power optical frequency combs, we have implemented DCS over a 113 km turbulent horizontal open-air path. Our experiment successfully measured GHG with 7 nm spectral bandwidth and a 10 kHz frequency and achieved a CO2 sensing precision of <2 ppm in 5 minutes and <0.6 ppm in 36 minutes. Our results represent a significant step towards advancing the implementation of DCS as a satellite-based technology and improving technologies for GHG monitoring △ Less

Submitted 31 October, 2023; v1 submitted 30 October, 2023; originally announced October 2023.

Comments: 24 pages, 6 figures

Detecting Violations of Differential Privacy for Quantum Algorithms

Authors: Ji Guan, Wang Fang, Mingyu Huang, Mingsheng Ying

Abstract: Quantum algorithms for solving a wide range of practical problems have been proposed in the last ten years, such as data search and analysis, product recommendation, and credit scoring. The concern about privacy and other ethical issues in quantum computing naturally rises up. In this paper, we define a formal framework for detecting violations of differential privacy for quantum algorithms. A det… ▽ More Quantum algorithms for solving a wide range of practical problems have been proposed in the last ten years, such as data search and analysis, product recommendation, and credit scoring. The concern about privacy and other ethical issues in quantum computing naturally rises up. In this paper, we define a formal framework for detecting violations of differential privacy for quantum algorithms. A detection algorithm is developed to verify whether a (noisy) quantum algorithm is differentially private and automatically generate bugging information when the violation of differential privacy is reported. The information consists of a pair of quantum states that violate the privacy, to illustrate the cause of the violation. Our algorithm is equipped with Tensor Networks, a highly efficient data structure, and executed both on TensorFlow Quantum and TorchQuantum which are the quantum extensions of famous machine learning platforms -- TensorFlow and PyTorch, respectively. The effectiveness and efficiency of our algorithm are confirmed by the experimental results of almost all types of quantum algorithms already implemented on realistic quantum computers, including quantum supremacy algorithms (beyond the capability of classical algorithms), quantum machine learning models, quantum approximate optimization algorithms, and variational quantum eigensolvers with up to 21 quantum bits. △ Less

Submitted 9 September, 2023; originally announced September 2023.

Journal ref: In Proceedings of the 2023 ACM SIGSAC Conference on Computer and Communications Security (CCS 2023)

cuQuantum SDK: A High-Performance Library for Accelerating Quantum Science

Authors: Harun Bayraktar, Ali Charara, David Clark, Saul Cohen, Timothy Costa, Yao-Lung L. Fang, Yang Gao, Jack Guan, John Gunnels, Azzam Haidar, Andreas Hehn, Markus Hohnerbach, Matthew Jones, Tom Lubowe, Dmitry Lyakh, Shinya Morino, Paul Springer, Sam Stanwyck, Igor Terentyev, Satya Varadhan, Jonathan Wong, Takuma Yamaguchi

Abstract: We present the NVIDIA cuQuantum SDK, a state-of-the-art library of composable primitives for GPU-accelerated quantum circuit simulations. As the size of quantum devices continues to increase, making their classical simulation progressively more difficult, the availability of fast and scalable quantum circuit simulators becomes vital for quantum algorithm developers, as well as quantum hardware eng… ▽ More We present the NVIDIA cuQuantum SDK, a state-of-the-art library of composable primitives for GPU-accelerated quantum circuit simulations. As the size of quantum devices continues to increase, making their classical simulation progressively more difficult, the availability of fast and scalable quantum circuit simulators becomes vital for quantum algorithm developers, as well as quantum hardware engineers focused on the validation and optimization of quantum devices. The cuQuantum SDK was created to accelerate and scale up quantum circuit simulators developed by the quantum information science community by enabling them to utilize efficient scalable software building blocks optimized for NVIDIA GPU platforms. The functional building blocks provided cover the needs of both state vector- and tensor network- based simulators, including approximate tensor network simulation methods based on matrix product state, projected entangled pair state, and other factorized tensor representations. By leveraging the enormous computing power of the latest NVIDIA GPU architectures, quantum circuit simulators that have adopted the cuQuantum SDK demonstrate significant acceleration, compared to CPU-only execution, for both the state vector and tensor network simulation methods. Furthermore, by utilizing the parallel primitives available in the cuQuantum SDK, one can easily transition to distributed GPU-accelerated platforms, including those furnished by cloud service providers and high-performance computing systems deployed by supercomputing centers, extending the scale of possible quantum circuit simulations. The rich capabilities provided by the SDK are conveniently made available via both Python and C application programming interfaces, where the former is directly targeting a broad Python quantum community and the latter allows tight integration with simulators written in any programming language. △ Less

Submitted 3 August, 2023; originally announced August 2023.

Comments: paper accepted at QCE 2023, journal reference will be updated whenever available

MSC Class: 68Q12; 68Q09; 81P68;

Approximation Algorithm for Noisy Quantum Circuit Simulation

Authors: Mingyu Huang, Ji Guan, Wang Fang, Mingsheng Ying

Abstract: Simulating noisy quantum circuits is vital in designing and verifying quantum algorithms in the current NISQ (Noisy Intermediate-Scale Quantum) era, where quantum noise is unavoidable. However, it is much more inefficient than the classical counterpart because of the quantum state explosion problem (the dimension of state space is exponential in the number of qubits) and the complex (non-unitary)… ▽ More Simulating noisy quantum circuits is vital in designing and verifying quantum algorithms in the current NISQ (Noisy Intermediate-Scale Quantum) era, where quantum noise is unavoidable. However, it is much more inefficient than the classical counterpart because of the quantum state explosion problem (the dimension of state space is exponential in the number of qubits) and the complex (non-unitary) representation of noises. Consequently, only noisy circuits with up to about 50 qubits can be simulated approximately well. This paper introduces a novel approximation algorithm for simulating noisy quantum circuits when the noisy effectiveness is insignificant to improve the scalability of the circuits that can be simulated. The algorithm is based on a new tensor network diagram for the noisy simulation and uses the singular value decomposition to approximate the tensors of quantum noises in the diagram. The contraction of the tensor network diagram is implemented on Google's TensorNetwork. The effectiveness and utility of the algorithm are demonstrated by experimenting on a series of practical quantum circuits with realistic superconducting noise models. As a result, our algorithm can approximately simulate quantum circuits with up to 225 qubits and 20 noises (within about 1.8 hours). In particular, our method offers a speedup over the commonly-used approximation (sampling) algorithm -- quantum trajectories method. Furthermore, our approach can significantly reduce the number of samples in the quantum trajectories method when the noise rate is small enough. △ Less

Submitted 23 November, 2023; v1 submitted 30 November, 2022; originally announced November 2022.

Verifying Fairness in Quantum Machine Learning

Authors: Ji Guan, Wang Fang, Mingsheng Ying

Abstract: Due to the beyond-classical capability of quantum computing, quantum machine learning is applied independently or embedded in classical models for decision making, especially in the field of finance. Fairness and other ethical issues are often one of the main concerns in decision making. In this work, we define a formal framework for the fairness verification and analysis of quantum machine learni… ▽ More Due to the beyond-classical capability of quantum computing, quantum machine learning is applied independently or embedded in classical models for decision making, especially in the field of finance. Fairness and other ethical issues are often one of the main concerns in decision making. In this work, we define a formal framework for the fairness verification and analysis of quantum machine learning decision models, where we adopt one of the most popular notions of fairness in the literature based on the intuition -- any two similar individuals must be treated similarly and are thus unbiased. We show that quantum noise can improve fairness and develop an algorithm to check whether a (noisy) quantum machine learning model is fair. In particular, this algorithm can find bias kernels of quantum data (encoding individuals) during checking. These bias kernels generate infinitely many bias pairs for investigating the unfairness of the model. Our algorithm is designed based on a highly efficient data structure -- Tensor Networks -- and implemented on Google's TensorFlow Quantum. The utility and effectiveness of our algorithm are confirmed by the experimental results, including income prediction and credit scoring on real-world data, for a class of random (noisy) quantum decision models with 27 qubits ($2^{27}$-dimensional state space) tripling ($2^{18}$ times more than) that of the state-of-the-art algorithms for verifying quantum machine learning models. △ Less

Submitted 22 July, 2022; originally announced July 2022.

VeriQBench: A Benchmark for Multiple Types of Quantum Circuits

Authors: Kean Chen, Wang Fang, Ji Guan, Xin Hong, Mingyu Huang, Junyi Liu, Qisheng Wang, Mingsheng Ying

Abstract: In this paper, we introduce VeriQBench -- an open source benchmark for quantum circuits. It offers high-level quantum circuit abstractions of various circuit types, including 1) combinational, 2) dynamic, 3) sequential, and 4) variational quantum circuits, which cover almost all existing types of quantum circuits in the literature. Meanwhile, VeriQBench is a versatile benchmark which can be used i… ▽ More In this paper, we introduce VeriQBench -- an open source benchmark for quantum circuits. It offers high-level quantum circuit abstractions of various circuit types, including 1) combinational, 2) dynamic, 3) sequential, and 4) variational quantum circuits, which cover almost all existing types of quantum circuits in the literature. Meanwhile, VeriQBench is a versatile benchmark which can be used in verifying quantum software for different applications, as is evidenced by the existing works including quantum circuit verification (e.g., equivalence checking [Hon+21a; WLY21] and model checking [Yin21]), simulation (e.g., fault simulation), testing (e.g., test pattern generation [CY22]) and debugging (e.g., runtime assertions [Li+20b]). All the circuits are described in OpenQASM and are validated on Qiskit and QCOR simulators. With the hope that it can be used by other researchers, VeriQBench is released at: https://github.com/Veri-Q/Benchmark. △ Less

Submitted 22 June, 2022; originally announced June 2022.

New Quantum Algorithms for Computing Quantum Entropies and Distances

Authors: Qisheng Wang, Ji Guan, Junyi Liu, Zhicheng Zhang, Mingsheng Ying

Abstract: We propose a series of quantum algorithms for computing a wide range of quantum entropies and distances, including the von Neumann entropy, quantum Rényi entropy, trace distance, and fidelity. The proposed algorithms significantly outperform the prior best (and even quantum) ones in the low-rank case, some of which achieve exponential speedups. In particular, for $N$-dimensional quantum states of… ▽ More We propose a series of quantum algorithms for computing a wide range of quantum entropies and distances, including the von Neumann entropy, quantum Rényi entropy, trace distance, and fidelity. The proposed algorithms significantly outperform the prior best (and even quantum) ones in the low-rank case, some of which achieve exponential speedups. In particular, for $N$-dimensional quantum states of rank $r$, our proposed quantum algorithms for computing the von Neumann entropy, trace distance and fidelity within additive error $\varepsilon$ have time complexity of $\tilde O(r/\varepsilon^2)$, $\tilde O(r^5/\varepsilon^6)$ and $\tilde O(r^{6.5}/\varepsilon^{7.5})$, respectively. By contrast, prior quantum algorithms for the von Neumann entropy and trace distance usually have time complexity $Ω(N)$, and the prior best one for fidelity has time complexity $\tilde O(r^{12.5}/\varepsilon^{13.5})$. The key idea of our quantum algorithms is to extend block-encoding from unitary operators in previous work to quantum states (i.e., density operators). It is realized by developing several convenient techniques to manipulate quantum states and extract information from them. The advantage of our techniques over the existing methods is that no restrictions on density operators are required; in sharp contrast, the previous methods usually require a lower bound on the minimal non-zero eigenvalue of density operators. △ Less

Submitted 30 May, 2024; v1 submitted 25 March, 2022; originally announced March 2022.

Comments: Final version. 58 pages, 5 tables, 1 figure. Minor corrections to Theorem 3.1, Theorem 3.4, and Corollary 3.5

Journal ref: IEEE Transactions on Information Theory, 70(8): 5653-5680, 2024

Model Checking Quantum Continuous-Time Markov Chains

Authors: Ming Xu, Jingyi Mei, Ji Guan, Nengkun Yu

Abstract: Verifying quantum systems has attracted a lot of interests in the last decades. In this paper, we initialised the model checking of quantum continuous-time Markov chain (QCTMC). As a real-time system, we specify the temporal properties on QCTMC by signal temporal logic (STL). To effectively check the atomic propositions in STL, we develop a state-of-art real root isolation algorithm under Schanuel… ▽ More Verifying quantum systems has attracted a lot of interests in the last decades. In this paper, we initialised the model checking of quantum continuous-time Markov chain (QCTMC). As a real-time system, we specify the temporal properties on QCTMC by signal temporal logic (STL). To effectively check the atomic propositions in STL, we develop a state-of-art real root isolation algorithm under Schanuel's conjecture; further, we check the general STL formula by interval operations with a bottom-up fashion, whose query complexity turns out to be linear in the size of the input formula by calling the real root isolation algorithm. A running example of an open quantum walk is provided to demonstrate our method. △ Less

Submitted 1 May, 2021; originally announced May 2021.

Quantum Algorithm for Fidelity Estimation

Authors: Qisheng Wang, Zhicheng Zhang, Kean Chen, Ji Guan, Wang Fang, Junyi Liu, Mingsheng Ying

Abstract: For two unknown mixed quantum states $ρ$ and $σ$ in an $N$-dimensional Hilbert space, computing their fidelity $F(ρ,σ)$ is a basic problem with many important applications in quantum computing and quantum information, for example verification and characterization of the outputs of a quantum computer, and design and analysis of quantum algorithms. In this paper, we propose a quantum algorithm that… ▽ More For two unknown mixed quantum states $ρ$ and $σ$ in an $N$-dimensional Hilbert space, computing their fidelity $F(ρ,σ)$ is a basic problem with many important applications in quantum computing and quantum information, for example verification and characterization of the outputs of a quantum computer, and design and analysis of quantum algorithms. In this paper, we propose a quantum algorithm that solves this problem in $\operatorname{poly}(\log (N), r, 1/\varepsilon)$ time, where $r$ is the lower rank of $ρ$ and $σ$, and $\varepsilon$ is the desired precision, provided that the purifications of $ρ$ and $σ$ are prepared by quantum oracles. This algorithm exhibits an exponential speedup over the best known algorithm (based on quantum state tomography) which has time complexity polynomial in $N$. △ Less

Submitted 28 September, 2022; v1 submitted 16 March, 2021; originally announced March 2021.

Comments: Final version with an improvement over the previous version. 19 pages, 2 tables, 1 algorithm

Journal ref: IEEE Transactions on Information Theory, 69(1): 273-282, 2023

Quantum Random Number Generation with Uncharacterized Laser and Sunlight

Authors: Yu-Huai Li, Xuan Han, Yuan Cao, Xiao Yuan, Zheng-Ping Li, Jian-Yu Guan, Juan Yin, Qiang Zhang, Xiongfeng Ma, Cheng-Zhi Peng, Jian-Wei Pan

Abstract: The entropy or randomness source is an essential ingredient in random number generation. Quantum random number generators generally require well modeled and calibrated light sources, such as a laser, to generate randomness. With uncharacterized light sources, such as sunlight or an uncharacterized laser, genuine randomness is practically hard to be quantified or extracted owing to its unknown or c… ▽ More The entropy or randomness source is an essential ingredient in random number generation. Quantum random number generators generally require well modeled and calibrated light sources, such as a laser, to generate randomness. With uncharacterized light sources, such as sunlight or an uncharacterized laser, genuine randomness is practically hard to be quantified or extracted owing to its unknown or complicated structure. By exploiting a recently proposed source-independent randomness generation protocol, we theoretically modify it by considering practical issues and experimentally realize the modified scheme with an uncharacterized laser and a sunlight source. The extracted randomness is guaranteed to be secure independent of its source and the randomness generation speed reaches 1 Mbps, three orders of magnitude higher than the original realization. Our result signifies the power of quantum technology in randomness generation and paves the way to high-speed semi-self-testing quantum random number generators with practical light sources. △ Less

Submitted 9 January, 2021; originally announced January 2021.

Comments: 24 pages, 5 figures

Journal ref: npj Quantum Information 5, 97 (2019)

Field demonstration of distributed quantum sensing without post-selection

Authors: Si-Ran Zhao, Yu-Zhe Zhang, Wen-Zhao Liu, Jian-Yu Guan, Weijun Zhang, Cheng-Long Li, Bing Bai, Ming-Han Li, Yang Liu, Lixing You, Jun Zhang, Jingyun Fan, Feihu Xu, Qiang Zhang, Jian-Wei Pan

Abstract: Distributed quantum sensing can provide quantum-enhanced sensitivity beyond the shot-noise limit (SNL) for sensing spatially distributed parameters. To date, distributed quantum sensing experiments have been mostly accomplished in laboratory environments without a real space separation for the sensors. In addition, the post-selection is normally assumed to demonstrate the sensitivity advantage ove… ▽ More Distributed quantum sensing can provide quantum-enhanced sensitivity beyond the shot-noise limit (SNL) for sensing spatially distributed parameters. To date, distributed quantum sensing experiments have been mostly accomplished in laboratory environments without a real space separation for the sensors. In addition, the post-selection is normally assumed to demonstrate the sensitivity advantage over the SNL. Here, we demonstrate distributed quantum sensing in field and show the unconditional violation (without post-selection) of SNL up to 0.916 dB for the field distance of 240 m. The achievement is based on a loophole free Bell test setup with entangled photon pairs at the averaged heralding efficiency of 73.88%. Moreover, to test quantum sensing in real life, we demonstrate the experiment for long distances (with 10-km fiber) together with the sensing of a completely random and unknown parameter. The results represent an important step towards a practical quantum sensing network for widespread applications. △ Less

Submitted 5 November, 2020; originally announced November 2020.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. X 11, 031009 (2021)

Chromatic interferometry with small frequency differences

Authors: Luo-Yuan Qu, Lu-Chuan Liu, Jordan Cotler, Fei Ma, Jian-Yu Guan, Ming-Yang Zheng, Quan Yao, Xiu-Ping Xie, Yu-Ao Chen, Qiang Zhang, Frank Wilczek, Jian-Wei Pan

Abstract: By developing a `two-crystal' method for color erasure, we can broaden the scope of chromatic interferometry to include optical photons whose frequency difference falls outside of the 400 nm to 4500 nm wavelength range, which is the passband of a PPLN crystal. We demonstrate this possibility experimentally, by observing interference patterns between sources at 1064.4 nm and 1063.6 nm, correspondin… ▽ More By developing a `two-crystal' method for color erasure, we can broaden the scope of chromatic interferometry to include optical photons whose frequency difference falls outside of the 400 nm to 4500 nm wavelength range, which is the passband of a PPLN crystal. We demonstrate this possibility experimentally, by observing interference patterns between sources at 1064.4 nm and 1063.6 nm, corresponding to a frequency difference of about 200 GHz. △ Less

Submitted 17 September, 2020; originally announced September 2020.

Comments: 5 pages, 3 figures

Journal ref: Opt. Express 28, 32294 (2000)

An HHL-Based Algorithm for Computing Hitting Probabilities of Quantum Random Walks

Authors: Ji Guan, Qisheng Wang, Mingsheng Ying

Abstract: We present a novel application of the HHL (Harrow-Hassidim-Lloyd) algorithm -- a quantum algorithm solving systems of linear equations -- in solving an open problem about quantum random walks, namely computing hitting (or absorption) probabilities of a general (not only Hadamard) one-dimensional quantum random walks with two absorbing boundaries. This is achieved by a simple observation that the p… ▽ More We present a novel application of the HHL (Harrow-Hassidim-Lloyd) algorithm -- a quantum algorithm solving systems of linear equations -- in solving an open problem about quantum random walks, namely computing hitting (or absorption) probabilities of a general (not only Hadamard) one-dimensional quantum random walks with two absorbing boundaries. This is achieved by a simple observation that the problem of computing hitting probabilities of quantum random walks can be reduced to inverting a matrix. Then a quantum algorithm with the HHL algorithm as a subroutine is developed for solving the problem, which is faster than the known classical algorithms by numerical experiments. △ Less

Submitted 27 March, 2021; v1 submitted 8 September, 2020; originally announced September 2020.

Journal ref: Quantum Information and Computation, 21(5-6): 395-408, 2021

Robustness Verification of Quantum Classifiers

Authors: Ji Guan, Wang Fang, Mingsheng Ying

Abstract: Several important models of machine learning algorithms have been successfully generalized to the quantum world, with potential speedup to training classical classifiers and applications to data analytics in quantum physics that can be implemented on the near future quantum computers. However, quantum noise is a major obstacle to the practical implementation of quantum machine learning. In this wo… ▽ More Several important models of machine learning algorithms have been successfully generalized to the quantum world, with potential speedup to training classical classifiers and applications to data analytics in quantum physics that can be implemented on the near future quantum computers. However, quantum noise is a major obstacle to the practical implementation of quantum machine learning. In this work, we define a formal framework for the robustness verification and analysis of quantum machine learning algorithms against noises. A robust bound is derived and an algorithm is developed to check whether or not a quantum machine learning algorithm is robust with respect to quantum training data. In particular, this algorithm can find adversarial examples during checking. Our approach is implemented on Google's TensorFlow Quantum and can verify the robustness of quantum machine learning algorithms with respect to a small disturbance of noises, derived from the surrounding environment. The effectiveness of our robust bound and algorithm is confirmed by the experimental results, including quantum bits classification as the "Hello World" example, quantum phase recognition and cluster excitation detection from real world intractable physical problems, and the classification of MNIST from the classical world. △ Less

Submitted 31 May, 2021; v1 submitted 17 August, 2020; originally announced August 2020.

Sending-or-Not-Sending with Independent Lasers: Secure Twin-Field Quantum Key Distribution Over 509 km

Authors: Jiu-Peng Chen, Chi Zhang, Yang Liu, Cong Jiang, Weijun Zhang, Xiao-Long Hu, Jian-Yu Guan, Zong-Wen Yu, Hai Xu, Jin Lin, Ming-Jun Li, Hao Chen, Hao Li, Lixing You, Zhen Wang, Xiang-Bin Wang, Qiang Zhang, Jian-Wei Pan

Abstract: Twin field quantum key distribution promises high key rates at long distance to beat the rate distance limit. Here, applying the sending or not sending TF QKD protocol, we experimentally demonstrate a secure key distribution breaking the absolute key rate limit of repeaterless QKD over 509 km, 408 km ultra-low loss optical fibre and 350 km standard optical fibre. Two independent lasers are used as… ▽ More Twin field quantum key distribution promises high key rates at long distance to beat the rate distance limit. Here, applying the sending or not sending TF QKD protocol, we experimentally demonstrate a secure key distribution breaking the absolute key rate limit of repeaterless QKD over 509 km, 408 km ultra-low loss optical fibre and 350 km standard optical fibre. Two independent lasers are used as the source with remote frequency locking technique over 500 km fiber distance; Practical optical fibers are used as the optical path with appropriate noise filtering; And finite key effects are considered in the key rate analysis. The secure key rates obtained at different distances are more than 5 times higher than the conditional limit of repeaterless QKD, a bound value assuming the same detection loss in the comparison. The achieved secure key rate is also higher than that a traditional QKD protocol running with a perfect repeaterless QKD device and even if an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve high secure key rate at long distribution distance, and hence practically useful for field implementation of intercity QKD. △ Less

Submitted 17 October, 2019; originally announced October 2019.

Comments: 17 pages, 10 figures and 8 tables

Journal ref: Phys. Rev. Lett. 124, 070501 (2020)

Quantum Coherence Witness with Untrusted Measurement Devices

Authors: You-Qi Nie, Hongyi Zhou, Jian-Yu Guan, Qiang Zhang, Xiongfeng Ma, Jun Zhang, Jian-Wei Pan

Abstract: Coherence is a fundamental resource in quantum information processing, which can be certified by a coherence witness. Due to the imperfection of measurement devices, a conventional coherence witness may lead to fallacious results. We show that the conventional witness could mistake an incoherent state as a state with coherence due to the inaccurate settings of measurement bases. In order to make t… ▽ More Coherence is a fundamental resource in quantum information processing, which can be certified by a coherence witness. Due to the imperfection of measurement devices, a conventional coherence witness may lead to fallacious results. We show that the conventional witness could mistake an incoherent state as a state with coherence due to the inaccurate settings of measurement bases. In order to make the witness result reliable, we propose a measurement-device-independent coherence witness scheme without any assumptions on the measurement settings. We introduce the decoy-state method to significantly increase the capability of recognizing states with coherence. Furthermore, we experimentally demonstrate the scheme in a time-bin encoding optical system. △ Less

Submitted 14 August, 2019; originally announced August 2019.

Comments: 13 pages, 7 figures, including Supplemental Material, accepted for publication in Physical Review Letters

Journal ref: Phys. Rev. Lett. 123, 090502 (2019)

Color Erasure Detectors Enable Chromatic Interferometry

Authors: Luo-Yuan Qu, Jordan Cotler, Fei Ma, Jian-Yu Guan, Ming-Yang Zheng, Xiuping Xie, Yu-Ao Chen, Qiang Zhang, Frank Wilczek, Jian-Wei Pan

Abstract: By engineering and manipulating quantum entanglement between incoming photons and experimental apparatus, we construct single-photon detectors which cannot distinguish between photons of very different wavelengths. These color erasure detectors enable a new kind of intensity interferometry, with potential applications in microscopy and astronomy. We demonstrate chromatic interferometry experimenta… ▽ More By engineering and manipulating quantum entanglement between incoming photons and experimental apparatus, we construct single-photon detectors which cannot distinguish between photons of very different wavelengths. These color erasure detectors enable a new kind of intensity interferometry, with potential applications in microscopy and astronomy. We demonstrate chromatic interferometry experimentally, observing robust interference using both coherent and incoherent photon sources. △ Less

Submitted 19 March, 2020; v1 submitted 6 May, 2019; originally announced May 2019.

Comments: 21 pages, 7 figures

Journal ref: Phys. Rev. Lett. 123, 243601 (2019)

Experimental Twin-Field Quantum Key Distribution Through Sending-or-Not-Sending

Authors: Yang Liu, Zong-Wen Yu, Weijun Zhang, Jian-Yu Guan, Jiu-Peng Chen, Chi Zhang, Xiao-Long Hu, Hao Li, Cong Jiang, Jin Lin, Teng-Yun Chen, Lixing You, Zhen Wang, Xiang-Bin Wang, Qiang Zhang, Jian-Wei Pan

Abstract: Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demanding is rather tough because it requests sin… ▽ More Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demanding is rather tough because it requests single photon level interference of two remote independent lasers. Here, we adopt the technology developed in the frequency and time transfer to lock two independent lasers' wavelengths and utilize additional phase reference light to estimate and compensate the fiber fluctuation. Further with a single photon detector with high detection rate, we demonstrate twin field quantum key distribution through the sending-or-not-sending protocol with realistic phase drift over 300 km optical fiber spools. We calculate the secure key rates with finite size effect. The secure key rate at 300 km ($1.96\times10^{-6}$) is higher than that of the repeaterless secret key capacity ($8.64\times10^{-7}$). △ Less

Submitted 12 September, 2019; v1 submitted 17 February, 2019; originally announced February 2019.

Comments: 34 pages, 10 figures and 9 tables

Journal ref: Phys. Rev. Lett. 123, 100505 (2019)

Model Checking Applied to Quantum Physics

Authors: Ji Guan, Yuan Feng, Andrea Turrini, Mingsheng Ying

Abstract: Model checking has been successfully applied to verification of computer hardware and software, communication systems and even biological systems. In this paper, we further push the boundary of its applications and show that it can be adapted for applications in quantum physics. More explicitly, we show how quantum statistical and many-body systems can be modeled as quantum Markov chains, and some… ▽ More Model checking has been successfully applied to verification of computer hardware and software, communication systems and even biological systems. In this paper, we further push the boundary of its applications and show that it can be adapted for applications in quantum physics. More explicitly, we show how quantum statistical and many-body systems can be modeled as quantum Markov chains, and some of their properties that interest physicists can be specified in linear-time temporal logics. Then we present an efficient algorithm to check these properties. A few case studies are given to demonstrate the use of our algorithm to actual quantum physical problems. △ Less

Submitted 8 February, 2019; originally announced February 2019.

A Photonic Topological Mode Bound to a Vortex

Authors: Adrian J Menssen, Jun Guan, David Felce, Martin J Booth, Ian A Walmsley

Abstract: Topological photonics sheds light on some of the surprising phenomena seen in condensed matter physics that arise with the appearance of topological invariants. Optical waveguides provide a well-controlled platform to investigate effects that relate to different topological phases of matter, providing insight into phenomena such as topological insulators and superconductors by direct simulation of… ▽ More Topological photonics sheds light on some of the surprising phenomena seen in condensed matter physics that arise with the appearance of topological invariants. Optical waveguides provide a well-controlled platform to investigate effects that relate to different topological phases of matter, providing insight into phenomena such as topological insulators and superconductors by direct simulation of the states that are protected by the topology of the system. Here, we observe a mode associated with a topological defect in the bulk of a 2D photonic material by introducing a vortex distortion to an hexagonal lattice and analogous to graphene. These observations are made possible by advances in our experimental methods. We were able to manufacture uniform large two-dimensional photonic crystal structures, containing thousands of identical waveguides arranged in two dimensions, and we developed a new method to excite multiples of these waveguides with a well-defined light field. This allows us to probe the detailed spatial features of topological defect modes for the first time. The observed modes lie mid-gap at zero energy and are closely related to Majorana bound states in superconducting vortices. This is the first experimental demonstration of a mode that is a solution to the Dirac equation in the presence of a vortex, as proposed by Jackiw and Rossi. △ Less

Submitted 5 September, 2019; v1 submitted 14 January, 2019; originally announced January 2019.

Journal ref: Phys. Rev. Lett. 125, 117401 (2020)

Device independent quantum random number generation

Authors: Yang Liu, Qi Zhao, Ming-Han Li, Jian-Yu Guan, Yanbao Zhang, Bing Bai, Weijun Zhang, Wen-Zhao Liu, Cheng Wu, Xiao Yuan, Hao Li, W. J. Munro, Zhen Wang, Lixing You, Jun Zhang, Xiongfeng Ma, Jingyun Fan, Qiang Zhang, Jian-Wei Pan

Abstract: Randomness is critical for many information processing applications, including numerical modeling and cryptography. Device-independent quantum random number generation (DIQRNG) based on the loophole free violation of Bell inequality produces unpredictable genuine randomness without any device assumption and is therefore an ultimate goal in the field of quantum information science. However, due to… ▽ More Randomness is critical for many information processing applications, including numerical modeling and cryptography. Device-independent quantum random number generation (DIQRNG) based on the loophole free violation of Bell inequality produces unpredictable genuine randomness without any device assumption and is therefore an ultimate goal in the field of quantum information science. However, due to formidable technical challenges, there were very few reported experimental studies of DIQRNG, which were vulnerable to the adversaries. Here we present a fully functional DIQRNG against the most general quantum adversaries. We construct a robust experimental platform that realizes Bell inequality violation with entangled photons with detection and locality loopholes closed simultaneously. This platform enables a continuous recording of a large volume of data sufficient for security analysis against the general quantum side information and without assuming independent and identical distribution. Lastly, by developing a large Toeplitz matrix (137.90 Gb $\times$ 62.469 Mb) hashing technique, we demonstrate that this DIQRNG generates $6.2469\times 10^7$ quantum-certified random bits in 96 hours (or 181 bits/s) with uniformity within $10^{-5}$. We anticipate this DIQRNG may have profound impact on the research of quantum randomness and information-secured applications. △ Less

Submitted 27 July, 2018; v1 submitted 24 July, 2018; originally announced July 2018.

Comments: 35 pages, 10 figures. The section "randomness extraction" in supplementary is revised in order to avoid text overlap

Journal ref: Nature 562, 548 (2018)

Experimental test of measurement dependent local Bell inequality with human free will

Authors: Yang Liu, Xiao Yuan, Cheng Wu, Weijun Zhang, Jian-Yu Guan, Jiaqiang Zhong, Hao Li, Ming-Han Li, Carlos Abellan, Morgan W. Mitchell, Sheng-Cai Shi, Jingyun Fan, Lixing You, Zhen Wang, Xiongfeng Ma, Qiang Zhang, Jian-Wei Pan

Abstract: A Bell test can rule out local realistic models, and has potential applications in communications and information tasks. For example, a Bell inequality violation can certify the presence of intrinsic randomness in measurement outcomes, which then can be used to generate unconditional randomness. A Bell test requires, however, measurements that are chosen independently of other physical variables i… ▽ More A Bell test can rule out local realistic models, and has potential applications in communications and information tasks. For example, a Bell inequality violation can certify the presence of intrinsic randomness in measurement outcomes, which then can be used to generate unconditional randomness. A Bell test requires, however, measurements that are chosen independently of other physical variables in the test, as would be the case if the measurement settings were themselves unconditionally random. This situation seems to create a "bootstrapping problem" that was recently addressed in The BIG Bell Test, a collection of Bell tests and related tests using human setting choices. Here we report in detail our experimental methods and results within the BIG Bell Test. We perform a experimental test of a special type of Bell inequality - the measurement dependent local inequality. With this inequality, even a small amount of measurement independence makes it possible to disprove local realistic models. The experiment uses human-generated random numbers to select the measurement settings in real time, and implements the measurement setting with space-like separation from the distant measurement. The experimental result shows a Bell inequality violation that cannot be explained by local hidden variable models with independence parameter (as defined in [Putz et al. Phys. Rev. Lett. 113, 190402 (2014).] ) l > 0.10 +/- 0.05. This result quantifies the degree to which a hidden variable model would need to constrain human choices, if it is to reproduce the experimental results. △ Less

Submitted 2 July, 2018; originally announced July 2018.

Comments: 14 pages, 2 figures

Journal ref: Phys. Rev. A 99, 022115 (2019)

Challenging local realism with human choices

Authors: The BIG Bell Test Collaboration, C. Abellán, A. Acín, A. Alarcón, O. Alibart, C. K. Andersen, F. Andreoli, A. Beckert, F. A. Beduini, A. Bendersky, M. Bentivegna, P. Bierhorst, D. Burchardt, A. Cabello, J. Cariñe, S. Carrasco, G. Carvacho, D. Cavalcanti, R. Chaves, J. Cortés-Vega, A. Cuevas, A. Delgado, H. de Riedmatten, C. Eichler, P. Farrera , et al. (83 additional authors not shown)

Abstract: A Bell test is a randomized trial that compares experimental observations against the philosophical worldview of local realism. A Bell test requires spatially distributed entanglement, fast and high-efficiency detection and unpredictable measurement settings. Although technology can satisfy the first two of these requirements, the use of physical devices to choose settings in a Bell test involves… ▽ More A Bell test is a randomized trial that compares experimental observations against the philosophical worldview of local realism. A Bell test requires spatially distributed entanglement, fast and high-efficiency detection and unpredictable measurement settings. Although technology can satisfy the first two of these requirements, the use of physical devices to choose settings in a Bell test involves making assumptions about the physics that one aims to test. Bell himself noted this weakness in using physical setting choices and argued that human `free will' could be used rigorously to ensure unpredictability in Bell tests. Here we report a set of local-realism tests using human choices, which avoids assumptions about predictability in physics. We recruited about 100,000 human participants to play an online video game that incentivizes fast, sustained input of unpredictable selections and illustrates Bell-test methodology. The participants generated 97,347,490 binary choices, which were directed via a scalable web platform to 12 laboratories on five continents, where 13 experiments tested local realism using photons, single atoms, atomic ensembles, and superconducting devices. Over a 12-hour period on 30 November 2016, participants worldwide provided a sustained data flow of over 1,000 bits per second to the experiments, which used different human-generated data to choose each measurement setting. The observed correlations strongly contradict local realism and other realistic positions in bipartite and tripartite scenarios. Project outcomes include closing the `freedom-of-choice loophole' (the possibility that the setting choices are influenced by `hidden variables' to correlate with the particle properties), the utilization of video-game methods for rapid collection of human generated randomness, and the use of networking techniques for global participation in experimental science. △ Less

Submitted 9 November, 2018; v1 submitted 11 May, 2018; originally announced May 2018.

Comments: This version includes minor changes resulting from reviewer and editorial input. Abstract shortened to fit within arXiv limits

Journal ref: Nature, volume 557, pages 212-216 (2018)

The Structure of Decoherence-free Subsystems

Authors: Ji Guan, Yuan Feng, Mingsheng Ying

Abstract: Decoherence-free subsystems have been successfully developed as a tool to preserve fragile quantum information against noises. In this letter, we develop a structure theory for decoherence-free subsystems. Based on it, we present an effective algorithm to construct a set of maximal decoherence-free subsystems in the sense that any other such subsystem is a subspace of one of them. As an applicatio… ▽ More Decoherence-free subsystems have been successfully developed as a tool to preserve fragile quantum information against noises. In this letter, we develop a structure theory for decoherence-free subsystems. Based on it, we present an effective algorithm to construct a set of maximal decoherence-free subsystems in the sense that any other such subsystem is a subspace of one of them. As an application of these techniques in quantum many body systems, we propose a simple and numerically robust method to determine if two irreducible tensors are repeated, a key step in deciding if they are equivalent in generating matrix product states. △ Less

Submitted 13 February, 2018; originally announced February 2018.

$Q|SI\rangle$: A Quantum Programming Environment

Authors: Shusen Liu, Xin Wang, Li Zhou, Ji Guan, Yinan Li, Yang He, Runyao Duan, Mingsheng Ying

Abstract: This paper describes a quantum programming environment, named $Q|SI\rangle$. It is a platform embedded in the .Net language that supports quantum programming using a quantum extension of the $\mathbf{while}$-language. The framework of the platform includes a compiler of the quantum $\mathbf{while}$-language and a suite of tools for simulating quantum computation, optimizing quantum circuits, and a… ▽ More This paper describes a quantum programming environment, named $Q|SI\rangle$. It is a platform embedded in the .Net language that supports quantum programming using a quantum extension of the $\mathbf{while}$-language. The framework of the platform includes a compiler of the quantum $\mathbf{while}$-language and a suite of tools for simulating quantum computation, optimizing quantum circuits, and analyzing and verifying quantum programs. Throughout the paper, using $Q|SI\rangle$ to simulate quantum behaviors on classical platforms with a combination of components is demonstrated. The scalable framework allows the user to program customized functions on the platform. The compiler works as the core of $Q|SI\rangle$ bridging the gap from quantum hardware to quantum software. The built-in decomposition algorithms enable the universal quantum computation on the present quantum hardware. △ Less

Submitted 25 October, 2017; originally announced October 2017.

Comments: 30 pages, software available at http://www.qcompiler.com

Efficient quantum repeater in perspectives of both entanglement concentration rate and LOCC complexity

Authors: Zhaofeng Su, Ji Guan, Lvzhou Li

Abstract: Quantum entanglement is an indispensable resource for many significant quantum information processing tasks. However, because of the noise in quantum channels, it is difficult to distribute quantum entanglement over a long distance in practice. A solution for this challenge is the quantum repeater which can extend the distance of entanglement distribution. In this scheme, the time consumption of c… ▽ More Quantum entanglement is an indispensable resource for many significant quantum information processing tasks. However, because of the noise in quantum channels, it is difficult to distribute quantum entanglement over a long distance in practice. A solution for this challenge is the quantum repeater which can extend the distance of entanglement distribution. In this scheme, the time consumption of classical communication and local operations takes an important place in perspective of time efficiency. Motivated by this observation, we exploit the basic quantum repeater scheme in perspectives of not only the optimal rate of entanglement concentration but also the complexity of local operations and classical communication. Firstly, we consider the case where two two-qubit pure states are prepared. We construct a protocol with the optimal entanglement concentration rate and less consumption of local operations and classical communication. We also find a criteria for the projective measurements to achieve the optimal probability. Secondly, we exploit the case where two general pure states are prepared and general measurements are considered. We get an upper bound on the probability for a successful measurement operation to produce a maximally entangled state without any further local operations. △ Less

Submitted 8 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. A 97, 012325 (2018)

Experimental preparation and verification of quantum money

Authors: Jian-Yu Guan, Juan Miguel Arrazola, Ryan Amiri, Weijun Zhang, Hao Li, Lixing You, Zhen Wang, Qiang Zhang, Jian-Wei Pan

Abstract: A quantum money scheme enables a trusted bank to provide untrusted users with verifiable quantum banknotes that cannot be forged. In this work, we report an experimental demonstration of the preparation and verification of unforgeable quantum banknotes. We employ a security analysis that takes experimental imperfections fully into account. We measure a total of $3.6\times 10^6$ states in one verif… ▽ More A quantum money scheme enables a trusted bank to provide untrusted users with verifiable quantum banknotes that cannot be forged. In this work, we report an experimental demonstration of the preparation and verification of unforgeable quantum banknotes. We employ a security analysis that takes experimental imperfections fully into account. We measure a total of $3.6\times 10^6$ states in one verification round, limiting the forging probability to $10^{-7}$ based on the security analysis. Our results demonstrate the feasibility of preparing and verifying quantum banknotes using currently available experimental techniques. △ Less

Submitted 18 September, 2017; originally announced September 2017.

Comments: 12 pages, 4 figures

Journal ref: Phys. Rev. A 97, 032338 (2018)

Super-activating Quantum Memory with Entanglement

Authors: Ji Guan, Yuan Feng, Mingsheng Ying

Abstract: Noiseless subsystems were proved to be an efficient and faithful approach to preserve fragile information against decoherence in quantum information processing and quantum computation. They were employed to design a general (hybrid) quantum memory cell model that can store both quantum and classical information. In this paper, we find an interesting new phenomenon that the purely classical memory… ▽ More Noiseless subsystems were proved to be an efficient and faithful approach to preserve fragile information against decoherence in quantum information processing and quantum computation. They were employed to design a general (hybrid) quantum memory cell model that can store both quantum and classical information. In this paper, we find an interesting new phenomenon that the purely classical memory cell can be super-activated to preserve quantum states, whereas the null memory cell can only be super-activated to encode classical information. Furthermore, necessary and sufficient conditions for this phenomenon are discovered so that the super-activation can be easily checked by examining certain eigenvalues of the quantum memory cell without computing the noiseless subsystems explicitly. In particular, it is found that entangled and separable stationary states are responsible for the super-activation of storing quantum and classical information, respectively. △ Less

Submitted 20 November, 2018; v1 submitted 2 August, 2017; originally announced August 2017.

Experimental measurement-device-independent quantum random number generation

Authors: You-Qi Nie, Jian-Yu Guan, Hongyi Zhou, Qiang Zhang, Xiongfeng Ma, Jun Zhang, Jian-Wei Pan

Abstract: The randomness from a quantum random number generator (QRNG) relies on the accurate characterization of its devices. However, device imperfections and inaccurate characterizations can result in wrong entropy estimation and bias in practice, which highly affects the genuine randomness generation and may even induce the disappearance of quantum randomness in an extreme case. Here we experimentally d… ▽ More The randomness from a quantum random number generator (QRNG) relies on the accurate characterization of its devices. However, device imperfections and inaccurate characterizations can result in wrong entropy estimation and bias in practice, which highly affects the genuine randomness generation and may even induce the disappearance of quantum randomness in an extreme case. Here we experimentally demonstrate a measurement-device-independent (MDI) QRNG based on time-bin encoding to achieve certified quantum randomness even when the measurement devices are uncharacterized and untrusted. The MDI-QRNG is randomly switched between the regular randomness generation mode and a test mode, in which four quantum states are randomly prepared to perform measurement tomography in real-time. With a clock rate of 25 MHz, the MDI-QRNG generates a final random bit rate of 5.7 Kbps. Such implementation with an all-fiber setup provides an approach to construct a fully-integrated MDI-QRNG with trusted but error-prone devices in practice. △ Less

Submitted 6 December, 2016; originally announced December 2016.

Comments: 15 pages, 5 figures, accepted for publication as a Rapid Communication in Physical Review A

Journal ref: Phys. Rev. A 94, 060301(R) (2016)

Ground test of satellite constellation based quantum communication

Authors: Sheng-Kai Liao, Hai-Lin Yong, Chang Liu, Guo-Liang Shentu, Dong-Dong Li, Jin Lin, Hui Dai, Shuang-Qiang Zhao, Bo Li, Jian-Yu Guan, Wei Chen, Yun-Hong Gong, Yang Li, Ze-Hong Lin, Ge-Sheng Pan, Jason S. Pelc, M. M. Fejer, Wen-Zhuo Zhang, Wei-Yue Liu, Juan Yin, Ji-Gang Ren, Xiang-Bin Wang, Qiang Zhang, Cheng-Zhi Peng, Jian-Wei Pan

Abstract: Satellite based quantum communication has been proven as a feasible way to achieve global scale quantum communication network. Very recently, a low-Earth-orbit (LEO) satellite has been launched for this purpose. However, with a single satellite, it takes an inefficient 3-day period to provide the worldwide connectivity. On the other hand, similar to how the Iridium system functions in classic comm… ▽ More Satellite based quantum communication has been proven as a feasible way to achieve global scale quantum communication network. Very recently, a low-Earth-orbit (LEO) satellite has been launched for this purpose. However, with a single satellite, it takes an inefficient 3-day period to provide the worldwide connectivity. On the other hand, similar to how the Iridium system functions in classic communication, satellite constellation (SC) composed of many quantum satellites, could provide global real-time quantum communication. In such a SC, most of the satellites will work in sunlight. Unfortunately, none of previous ground testing experiments could be implemented at daytime. During daytime, the bright sunlight background prohibits quantum communication in transmission over long distances. In this letter, by choosing a working wavelength of 1550 nm and developing free-space single-mode fibre coupling technology and ultralow noise up-conversion single photon detectors, we overcome the noise due to sunlight and demonstrate a 53-km free space quantum key distribution (QKD) in the daytime through a 48-dB loss channel. Our system not only shows the feasibility of satellite based quantum communication in daylight, but also has the ability to naturally adapt to ground fibre optics, representing an essential step towards a SC-based global quantum network. △ Less

Submitted 29 November, 2016; originally announced November 2016.

Comments: 14 pages, 2 figures and 1 table

Journal ref: Nat. Photonics 11, 509 (2017)