Showing 1–50 of 64 results for author: Zhao, B

ZX-DB: A Graph Database for Quantum Circuit Simplification and Rewriting via the ZX-Calculus

Authors: Valter Uotila, Cong Yu, Bo Zhao

Abstract: Quantum computing is an emerging computational paradigm with the potential to outperform classical computers in solving a variety of problems. To achieve this, quantum programs are typically represented as quantum circuits, which must be optimized and adapted for target hardware through quantum circuit compilation. We introduce ZX-DB, a data-driven system that performs quantum circuit simplificati… ▽ More Quantum computing is an emerging computational paradigm with the potential to outperform classical computers in solving a variety of problems. To achieve this, quantum programs are typically represented as quantum circuits, which must be optimized and adapted for target hardware through quantum circuit compilation. We introduce ZX-DB, a data-driven system that performs quantum circuit simplification and rewriting inside a graph database using ZX-calculus, a complete graphical formalism for quantum mechanics. ZX-DB encodes ZX-calculus rewrite rules as standard openCypher queries and executes them on an example graph database engine, Memgraph, enabling efficient, database-native transformations of large-scale quantum circuits. ZX-DB integrates correctness validation via tensor and graph equivalence checks and is evaluated against the state-of-the-art PyZX framework. Experimental results show that ZX-DB achieves up to an order-of-magnitude speedup for independent rewrites, while exposing pattern-matching bottlenecks in current graph database engines. By uniting quantum compilation and graph data management, ZX-DB opens a new systems direction toward scalable, database-supported quantum computing pipelines. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Comments: 9 pages, 16 figures

Twirlator: A Pipeline for Analyzing Subgroup Symmetry Effects in Quantum Machine Learning Ansatzes

Authors: Valter Uotila, Väinö Mehtola, Ilmo Salmenperä, Bo Zhao

Abstract: Leveraging data symmetries has been a key driver of performance gains in geometric deep learning and geometric and equivariant quantum machine learning. While symmetrization appears to be a promising method, its practical overhead, such as additional gates, reduced expressibility, and other factors, is not well understood in quantum machine learning. In this work, we develop an automated pipeline… ▽ More Leveraging data symmetries has been a key driver of performance gains in geometric deep learning and geometric and equivariant quantum machine learning. While symmetrization appears to be a promising method, its practical overhead, such as additional gates, reduced expressibility, and other factors, is not well understood in quantum machine learning. In this work, we develop an automated pipeline to measure various characteristics of quantum machine learning ansatzes with respect to symmetries that can appear in the learning task. We define the degree of symmetry in the learning problem as the size of the subgroup it admits. Subgroups define partial symmetries, which have not been extensively studied in previous research, which has focused on symmetries defined by whole groups. Symmetrizing the 19 common ansatzes with respect to these varying-sized subgroup representations, we compute three classes of metrics that describe how the common ansatz structures behave under varying amounts of symmetries. The first metric is based on the norm of the difference between the original and symmetrized generators, while the second metric counts depth, size, and other characteristics from the symmetrized circuits. The third class of metrics includes expressibility and entangling capability. The results demonstrate varying gate overhead across the studied ansatzes and confirm that increased symmetry reduces expressibility of the circuits. In most cases, increased symmetry increases entanglement capability. These results help select sufficiently expressible and computationally efficient ansatze patterns for geometric quantum machine learning applications. △ Less

Submitted 6 November, 2025; originally announced November 2025.

Comments: 8 pages; 8 figures

Tunable Asymmetric Delay Attack in Quantum Clock Synchronization

Authors: Hui Han, Haotian Teng, Hailong Xu, Jinquan Huang, Yuanmei Xie, Yichen Zhang, Bo Liu, Wanrong Yu, Baokang Zhao, Shuhui Chen

Abstract: Quantum clock synchronization underpins modern secure communications and critical infrastructure, yet its fundamental dependence on channel reciprocity introduces an exploitable vulnerability to asymmetric delay attacks. Current attack strategies rely on static delays, limiting their ability to target application-specific stability requirements. Here, we propose a tunable asymmetric delay attack (… ▽ More Quantum clock synchronization underpins modern secure communications and critical infrastructure, yet its fundamental dependence on channel reciprocity introduces an exploitable vulnerability to asymmetric delay attacks. Current attack strategies rely on static delays, limiting their ability to target application-specific stability requirements. Here, we propose a tunable asymmetric delay attack (T-ADA) that dynamically controls delay parameters to induce manipulate synchronization accuracy. Through experimental implementation, we demonstrate how tailored attack trajectories can selectively compromise system stability across different scenarios. This work uncovers key vulnerabilities in synchronization protocols under customizable attacks and provide a foundation for developing secure and resilient quantum clock synchronization systems. △ Less

Submitted 23 October, 2025; originally announced October 2025.

A Quantum-Inspired Algorithm for Solving Sudoku Puzzles and the MaxCut Problem

Authors: Max B. Zhao, Fei Li

Abstract: We propose and evaluate a quantum-inspired algorithm for solving Quadratic Unconstrained Binary Optimization (QUBO) problems, which are mathematically equivalent to finding ground states of Ising spin-glass Hamiltonians. The algorithm employs Matrix Product States (MPS) to compactly represent large superpositions of spin configurations and utilizes a discrete driving schedule to guide the MPS towa… ▽ More We propose and evaluate a quantum-inspired algorithm for solving Quadratic Unconstrained Binary Optimization (QUBO) problems, which are mathematically equivalent to finding ground states of Ising spin-glass Hamiltonians. The algorithm employs Matrix Product States (MPS) to compactly represent large superpositions of spin configurations and utilizes a discrete driving schedule to guide the MPS toward the ground state. At each step, a driver Hamiltonian -- incorporating a transverse magnetic field -- is combined with the problem Hamiltonian to enable spin flips and facilitate quantum tunneling. The MPS is updated using the standard Density Matrix Renormalization Group (DMRG) method, which iteratively minimizes the system's energy via multiple sweeps across the spin chain. Despite its heuristic nature, the algorithm reliably identifies global minima, not merely near-optimal solutions, across diverse QUBO instances. We first demonstrate its effectiveness on intermediate-level Sudoku puzzles from publicly available sources, involving over $200$ Ising spins with long-range couplings dictated by constraint satisfaction. We then apply the algorithm to MaxCut problems from the Biq Mac library, successfully solving instances with up to $251$ nodes and $3,265$ edges. We discuss the advantages of this quantum-inspired approach, including its scalability, generalizability, and suitability for industrial-scale QUBO applications. △ Less

Submitted 10 October, 2025; originally announced October 2025.

Comments: 29 pages, 10 figures, accepted by Quantum Information & Computation on August 6, 2025

Fast CZ Gate via Energy-Level Engineering in Superconducting Qubits with a Tunable Coupler

Authors: Benzheng Yuan, Chaojie Zhang, Chuanbing Han, Shuya Wang, Peng Xu, Huihui Sun, Qing Mu, Lixin Wang, Bo Zhao, Weilong Wang, Zheng Shan

Abstract: In superconducting quantum circuits, decoherence errors in qubits constitute a critical factor limiting quantum gate performance. To mitigate decoherence-induced gate infidelity, rapid implementation of quantum gates is essential. Here we propose a scheme for rapid controlled-Z (CZ) gate implementation through energy-level engineering, which leverages Rabi oscillations between the |11> state and t… ▽ More In superconducting quantum circuits, decoherence errors in qubits constitute a critical factor limiting quantum gate performance. To mitigate decoherence-induced gate infidelity, rapid implementation of quantum gates is essential. Here we propose a scheme for rapid controlled-Z (CZ) gate implementation through energy-level engineering, which leverages Rabi oscillations between the |11> state and the superposition state in a tunable-coupler architecture. Numerical simulations achieved a 17 ns nonadiabatic CZ gate with fidelity over 99.99%. We further investigated the performance of the CZ gate in the presence of anharmonicity offsets. The results demonstrate that a high-fidelity CZ gate with an error rate below 10^-4 remains achievable even with finite anharmonicity variations. Furthermore, the detrimental impact of spectator qubits in different quantum states on the fidelity of CZ gate is effectively suppressed by incorporating a tunable coupler. This scheme exhibits potential for extending the circuit execution depth constrained by coherence time limitations. △ Less

Submitted 14 October, 2025; v1 submitted 10 October, 2025; originally announced October 2025.

Comments: 7 pages, 5 figures

The quantum communication power of indefinite causal order

Authors: Xuanqiang Zhao, Benchi Zhao, Giulio Chiribella

Abstract: Quantum theory is in principle compatible with scenarios where physical processes take place in an indefinite causal order, a possibility that was shown to yield advantages in several information processing tasks. However, advantages in communication, the most basic form of information processing, have so far remained controversial and hard to prove. Here we develop a framework that can be used to… ▽ More Quantum theory is in principle compatible with scenarios where physical processes take place in an indefinite causal order, a possibility that was shown to yield advantages in several information processing tasks. However, advantages in communication, the most basic form of information processing, have so far remained controversial and hard to prove. Here we develop a framework that can be used to rigorously assess the role of causal order in a scenario where communication links are built by assembling multiple quantum devices. In this setting, we establish a clear-cut advantage of indefinite order in the one-shot transmission of classical messages. On the other hand, we also show that the advantage is not generic to all communication tasks. Notably, we find that indefinite order does not offer any advantage over shared entanglement in the asymptotic scenario where a large number of uses of the same communication device is employed. Overall, our results unveil non-trivial relations between communication, causal order, entanglement, and no-signaling resources in quantum mechanics. △ Less

Submitted 9 October, 2025; originally announced October 2025.

Comments: 25 pages, 1 figure

QUASAR: Quantum Assembly Code Generation Using Tool-Augmented LLMs via Agentic RL

Authors: Cong Yu, Valter Uotila, Shilong Deng, Qingyuan Wu, Tuo Shi, Songlin Jiang, Lei You, Bo Zhao

Abstract: Designing and optimizing task-specific quantum circuits are crucial to leverage the advantage of quantum computing. Recent large language model (LLM)-based quantum circuit generation has emerged as a promising automatic solution. However, the fundamental challenges remain unaddressed: (i) parameterized quantum gates require precise numerical values for optimal performance, which also depend on mul… ▽ More Designing and optimizing task-specific quantum circuits are crucial to leverage the advantage of quantum computing. Recent large language model (LLM)-based quantum circuit generation has emerged as a promising automatic solution. However, the fundamental challenges remain unaddressed: (i) parameterized quantum gates require precise numerical values for optimal performance, which also depend on multiple aspects, including the number of quantum gates, their parameters, and the layout/depth of the circuits. (ii) LLMs often generate low-quality or incorrect quantum circuits due to the lack of quantum domain-specific knowledge. We propose QUASAR, an agentic reinforcement learning (RL) framework for quantum circuits generation and optimization based on tool-augmented LLMs. To align the LLM with quantum-specific knowledge and improve the generated quantum circuits, QUASAR designs (i) a quantum circuit verification approach with external quantum simulators and (ii) a sophisticated hierarchical reward mechanism in RL training. Extensive evaluation shows improvements in both syntax and semantic performance of the generated quantum circuits. When augmenting a 4B LLM, QUASAR has achieved the validity of 99.31% in Pass@1 and 100% in Pass@10, outperforming industrial LLMs of GPT-4o, GPT-5 and DeepSeek-V3 and several supervised-fine-tuning (SFT)-only and RL-only baselines. △ Less

Submitted 1 October, 2025; originally announced October 2025.

Demonstration of quantum error detection in a silicon quantum processor

Authors: Chunhui Zhang, Chunhui Li, Zhen Tian, Yan Jiang, Feng Xu, Shihang Zhang, Hao Wang, Yu-Ning Zhang, Xuesong Bai, Baolong Zhao, Yi-Fei Zhang, Huan Shu, Jiaze Liu, Kunrong Wu, Chao Huang, Keji Shi, Mingchao Duan, Tao Xin, Peihao Huang, Tianluo Pan, Song Liu, Guanyong Wang, Guangchong Hu, Yu He, Dapeng Yu

Abstract: Quantum error detection is essential in realizing large-scale universal quantum computation, especially for quantum error correction (QEC). However, key elements for FTQC have yet to be realized in silicon qubits. Here, we demonstrate quantum error detection on a donor-based silicon quantum processor comprising four-nuclear spin qubits and one electron spin as an auxiliary qubit. The entanglement… ▽ More Quantum error detection is essential in realizing large-scale universal quantum computation, especially for quantum error correction (QEC). However, key elements for FTQC have yet to be realized in silicon qubits. Here, we demonstrate quantum error detection on a donor-based silicon quantum processor comprising four-nuclear spin qubits and one electron spin as an auxiliary qubit. The entanglement capability of this system is validated through the establishment of two-qubit Bell state entanglement between the nuclear spins and the generation of a four-qubit Greenberger-Horne-Zeilinger (GHZ) state, achieving a GHZ state fidelity of 88.5(2.3)%. Furthermore, by executing a four-qubit error detection circuit with the stabilizers, we successfully detect arbitrary single-qubit errors. The encoded Bell state entanglement information is recovered by performing the Pauli-frame update (PFU) via postprocessing. Based on the detected errors, we identify strongly biased noise in our system. Our results mark a significant advance toward FTQC in silicon spin qubits. △ Less

Submitted 29 September, 2025; originally announced September 2025.

Comments: 41 pages

Power and limitations of distributed quantum state purification

Authors: Benchi Zhao, Yu-Ao Chen, Xuanqiang Zhao, Chengkai Zhu, Giulio Chiribella, Xin Wang

Abstract: Quantum state purification, which mitigates noise by exploiting multiple noisy copies of unknown states, has applications in quantum communication and computation with imperfect devices. Despite its importance, the fundamental capabilities and limitations of purification in distributed quantum systems remain largely unexplored. Here, we systematically study distributed quantum state purification u… ▽ More Quantum state purification, which mitigates noise by exploiting multiple noisy copies of unknown states, has applications in quantum communication and computation with imperfect devices. Despite its importance, the fundamental capabilities and limitations of purification in distributed quantum systems remain largely unexplored. Here, we systematically study distributed quantum state purification under local operations and classical communication (LOCC). We prove that no nontrivial two-to-one LOCC purification protocol exists for three fundamental sets: all pure states, all maximally entangled states, and the four Bell states. This no-go theorem demonstrates that no LOCC protocol can reduce noise for every state within these sets, even probabilistically. In stark contrast, we show that single-state purification is achievable, and we provide an explicit analytical LOCC protocol for individual target states. For arbitrary state sets, we develop an optimization-based algorithm that systematically designs LOCC purification protocols, demonstrating its efficacy through concrete examples. These results delineate the fundamental boundaries of LOCC-based purification and provide practical strategies for noise reduction in distributed quantum information processing. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Comments: 22 pages, 7 figures

Dynamic LOCC Circuits for Automated Entanglement Manipulation

Authors: Xia Liu, Jiayi Zhao, Benchi Zhao, Xin Wang

Abstract: Due to the limited qubit number of quantum devices, distributed quantum computing is considered a promising pathway to overcome this constraint. In this paradigm, multiple quantum processors are interconnected to form a cohesive computational network, and the most natural set of free operations is local operations and classical communication (LOCC). However, designing a practical LOCC protocol for… ▽ More Due to the limited qubit number of quantum devices, distributed quantum computing is considered a promising pathway to overcome this constraint. In this paradigm, multiple quantum processors are interconnected to form a cohesive computational network, and the most natural set of free operations is local operations and classical communication (LOCC). However, designing a practical LOCC protocol for a particular task has been a tough problem. In this work, we propose a general and flexible framework called dynamic LOCCNet (DLOCCNet) to simulate and design LOCC protocols. We demonstrate its effectiveness in two key applications: entanglement distillation and distributed state discrimination. The protocols designed by DLOCCNet, in contrast to conventional ones, can solve larger-sized problems with reduced training time, making the framework a practical and scalable tool for current quantum devices. This work advances our understanding of the capabilities and limitations of LOCC while providing a powerful methodology for protocol design. △ Less

Submitted 9 September, 2025; originally announced September 2025.

Comments: 16 pages, 12 figures

Higher-Order Portfolio Optimization with Quantum Approximate Optimization Algorithm

Authors: Valter Uotila, Julia Ripatti, Bo Zhao

Abstract: Portfolio optimization is one of the most studied optimization problems at the intersection of quantum computing and finance. In this work, we develop the first quantum formulation for a portfolio optimization problem with higher-order moments, skewness and kurtosis. Including higher-order moments leads to more detailed modeling of portfolio return distributions. Portfolio optimization with higher… ▽ More Portfolio optimization is one of the most studied optimization problems at the intersection of quantum computing and finance. In this work, we develop the first quantum formulation for a portfolio optimization problem with higher-order moments, skewness and kurtosis. Including higher-order moments leads to more detailed modeling of portfolio return distributions. Portfolio optimization with higher-order moments has been studied in classical portfolio optimization approaches but with limited exploration within quantum formulations. In the context of quantum optimization, higher-order moments generate higher-order terms in the cost Hamiltonian. Thus, instead of obtaining a quadratic unconstrained binary optimization problem, we obtain a higher-order unconstrained binary optimization (HUBO) problem, which has a natural formulation as a parametrized circuit. Additionally, we employ realistic integer variable encoding and a capital-based budget constraint. We consider the classical continuous variable solution with integer programming-based discretization to be the computationally efficient classical baseline for the problem. Our extensive experimental evaluation of 100 portfolio optimization problems shows that the solutions to the HUBO formulation often correspond to better portfolio allocations than the classical baseline. This is a promising result for those who want to perform computationally challenging portfolio optimization on quantum hardware, as portfolio optimization with higher moments is classically complex. Moreover, the experimental evaluation studies QAOA's performance with higher-order terms in this practically relevant problem. △ Less

Submitted 1 September, 2025; originally announced September 2025.

Comments: 12 pages, 6 figures, presented at IEEE International Conference on Quantum Computing and Engineering (QCE) 2025

Agent-Q: Fine-Tuning Large Language Models for Quantum Circuit Generation and Optimization

Authors: Linus Jern, Valter Uotila, Cong Yu, Bo Zhao

Abstract: Large language models (LLMs) have achieved remarkable outcomes in complex problems, including math, coding, and analyzing large amounts of scientific reports. Yet, few works have explored the potential of LLMs in quantum computing. The most challenging problem is to leverage LLMs to automatically generate quantum circuits at a large scale. Fundamentally, the existing pre-trained LLMs lack the know… ▽ More Large language models (LLMs) have achieved remarkable outcomes in complex problems, including math, coding, and analyzing large amounts of scientific reports. Yet, few works have explored the potential of LLMs in quantum computing. The most challenging problem is to leverage LLMs to automatically generate quantum circuits at a large scale. Fundamentally, the existing pre-trained LLMs lack the knowledge of quantum circuits. In this paper, we address this challenge by fine-tuning LLMs and injecting the domain-specific knowledge of quantum computing. We describe Agent-Q, an LLM fine-tuning system to generate and optimize quantum circuits. In particular, Agent-Q implements the mechanisms to generate training data sets and constructs an end-to-end pipeline to fine-tune pre-trained LLMs to generate parameterized quantum circuits for various optimization problems. Agent-Q provides 14,000 quantum circuits covering a large spectrum of the quantum optimization landscape: 12 optimization problem instances and their optimized QAOA, VQE, and adaptive VQE circuits. Based thereon, Agent-Q fine-tunes LLMs and constructs syntactically correct parametrized quantum circuits in OpenQASM 3.0. We have evaluated the quality of the LLM-generated circuits and parameters by comparing them to the optimized expectation values and distributions. Experimental results show superior performance of Agent-Q, compared to several state-of-the-art LLMs and better parameters than random. Agent-Q can be integrated into an agentic workflow, and the generated parametrized circuits with initial parameters can be used as a starting point for further optimization, e.g., as templates in quantum machine learning and as benchmarks for compilers and hardware. △ Less

Submitted 1 September, 2025; v1 submitted 15 April, 2025; originally announced April 2025.

Comments: 12 pages, 8 figures, 3 tables, presented at IEEE International Conference on Quantum Computing and Engineering (QCE) 2025

EDA-Q: Electronic Design Automation for Superconducting Quantum Chip

Authors: Bo Zhao, Zhihang Li, Xiaohan Yu, Benzheng Yuan, Chaojie Zhang, Yimin Gao, Weilong Wang, Qing Mu, Shuya Wang, Huihui Sun, Tian Yang, Mengfan Zhang, Chuanbing Han, Peng Xu, Wenqing Wang, Zheng Shan

Abstract: Electronic Design Automation (EDA) plays a crucial role in classical chip design and significantly influences the development of quantum chip design. However, traditional EDA tools cannot be directly applied to quantum chip design due to vast differences compared to the classical realm. Several EDA products tailored for quantum chip design currently exist, yet they only cover partial stages of the… ▽ More Electronic Design Automation (EDA) plays a crucial role in classical chip design and significantly influences the development of quantum chip design. However, traditional EDA tools cannot be directly applied to quantum chip design due to vast differences compared to the classical realm. Several EDA products tailored for quantum chip design currently exist, yet they only cover partial stages of the quantum chip design process instead of offering a fully comprehensive solution. Additionally, they often encounter issues such as limited automation, steep learning curves, challenges in integrating with actual fabrication processes, and difficulties in expanding functionality. To address these issues, we developed a full-stack EDA tool specifically for quantum chip design, called EDA-Q. The design workflow incorporates functionalities present in existing quantum EDA tools while supplementing critical design stages such as device mapping and fabrication process mapping, which users expect. EDA-Q utilizes a unique architecture to achieve exceptional scalability and flexibility. The integrated design mode guarantees algorithm compatibility with different chip components, while employing a specialized interactive processing mode to offer users a straightforward and adaptable command interface. Application examples demonstrate that EDA-Q significantly reduces chip design cycles, enhances automation levels, and decreases the time required for manual intervention. Multiple rounds of testing on the designed chip have validated the effectiveness of EDA-Q in practical applications. △ Less

Submitted 17 April, 2025; v1 submitted 21 February, 2025; originally announced February 2025.

Comments: 12pages, 11 figures, 4 tables

Variational quantum Hamiltonian engineering

Authors: Benchi Zhao, Keisuke Fujii

Abstract: The Hamiltonian of a quantum system is represented in terms of operators corresponding to the kinetic and potential energies of the system. The expectation value of a Hamiltonian and Hamiltonian simulation are two of the most fundamental tasks in quantum computation. The overheads for realizing the two tasks are determined by the Pauli norm of Hamiltonian, which sums over all the absolute values o… ▽ More The Hamiltonian of a quantum system is represented in terms of operators corresponding to the kinetic and potential energies of the system. The expectation value of a Hamiltonian and Hamiltonian simulation are two of the most fundamental tasks in quantum computation. The overheads for realizing the two tasks are determined by the Pauli norm of Hamiltonian, which sums over all the absolute values of Pauli coefficients. In this work, we propose a variational quantum algorithm (VQA) called variational quantum Hamiltonian engineering (VQHE) to minimize the Pauli norm of Hamiltonian, such that the overhead for executing expectation value estimation and Hamiltonian simulation can be reduced. First, we develop a theory to encode the Pauli norm optimization problem into the vector L1-norm minimization problem. Then we devise an appropriate cost function and utilize the parameterized quantum circuits (PQC) to minimize the cost function. We also conduct numerical experiments to reduce the Pauli norm of the Ising Hamiltonian and molecules' Hamiltonian to show the efficiency of the proposed VQHE. △ Less

Submitted 13 June, 2024; originally announced June 2024.

Comments: 13 pages, 6 figures

Probabilistic channel simulation using coherence

Authors: Benchi Zhao, Kosuke Ito, Keisuke Fujii

Abstract: Channel simulation using coherence, which refers to realizing a target channel with coherent states and free operations, is a fundamental problem in the quantum resource theory of coherence. The limitations of the accuracy of deterministic channel simulation motivate us to consider the more general probabilistic framework. In this paper, we develop the framework for probabilistic channel simulatio… ▽ More Channel simulation using coherence, which refers to realizing a target channel with coherent states and free operations, is a fundamental problem in the quantum resource theory of coherence. The limitations of the accuracy of deterministic channel simulation motivate us to consider the more general probabilistic framework. In this paper, we develop the framework for probabilistic channel simulation using coherence with free operations. When the chosen set of free operations is the maximally incoherent operations, we provide an efficiently computable semidefinite program (SDP) to calculate the maximal success probability and derive the analytic expression of success probability for some special cases. When the chosen set of free operations is the dephasing-covariant incoherent operations (DIO), we show that if the target channel is not a resource nonactivating channel, then one cannot simulate it exactly both deterministically and probabilistically. The SDP for maximal success probability of simulating a channel by DIO is also given correspondingly. △ Less

Submitted 17 December, 2024; v1 submitted 10 April, 2024; originally announced April 2024.

Comments: 8+2pages, 3 figures. Accepted by PR Research

Retrieving non-linear features from noisy quantum states

Authors: Benchi Zhao, Mingrui Jing, Lei Zhang, Xuanqiang Zhao, Yu-Ao CHen, Kun Wang, Xin Wang

Abstract: Accurately estimating high-order moments of quantum states is an elementary precondition for many crucial tasks in quantum computing, such as entanglement spectroscopy, entropy estimation, spectrum estimation, and predicting non-linear features from quantum states. But in reality, inevitable quantum noise prevents us from accessing the desired value. In this paper, we address this issue by systema… ▽ More Accurately estimating high-order moments of quantum states is an elementary precondition for many crucial tasks in quantum computing, such as entanglement spectroscopy, entropy estimation, spectrum estimation, and predicting non-linear features from quantum states. But in reality, inevitable quantum noise prevents us from accessing the desired value. In this paper, we address this issue by systematically analyzing the feasibility and efficiency of extracting high-order moments from noisy states. We first show that there exists a quantum protocol capable of accomplishing this task if and only if the underlying noise channel is invertible. We then establish a method for deriving protocols that attain optimal sample complexity using quantum operations and classical post-processing only. Our protocols, in contrast to conventional ones, incur lower overheads and avoid sampling different quantum operations due to a novel technique called observable shift, making the protocols strong candidates for practical usage on current quantum devices. The proposed method also indicates the power of entangled protocols in retrieving high-order information, whereas in the existing methods, entanglement does not help. We further construct the protocol for large quantum systems to retrieve the depolarizing channels, making the proposed method scalable. Our work contributes to a deeper understanding of how quantum noise could affect high-order information extraction and provides guidance on how to tackle it. △ Less

Submitted 14 May, 2024; v1 submitted 20 September, 2023; originally announced September 2023.

Comments: 27 pages, 8 figures. Accepted by PRX Quantum

Power of quantum measurement in simulating unphysical operations

Authors: Xuanqiang Zhao, Lei Zhang, Benchi Zhao, Xin Wang

Abstract: The manipulation of quantum states through linear maps beyond quantum operations has many important applications in various areas of quantum information processing. Current methods simulate unphysical maps by sampling physical operations according to classically determined probability distributions. In this work, we show that using quantum measurement instead leads to lower simulation costs for ge… ▽ More The manipulation of quantum states through linear maps beyond quantum operations has many important applications in various areas of quantum information processing. Current methods simulate unphysical maps by sampling physical operations according to classically determined probability distributions. In this work, we show that using quantum measurement instead leads to lower simulation costs for general Hermitian-preserving maps. Remarkably, we establish the equality between the simulation cost and the well-known diamond norm, thus closing a previously known gap and assigning diamond norm a universal operational meaning for all Hermitian-preserving maps. We demonstrate our method in two applications closely related to error mitigation and quantum machine learning, where it exhibits a favorable scaling. These findings highlight the power of quantum measurement in simulating unphysical operations, in which quantum interference is believed to play a vital role. Our work paves the way for more efficient sampling techniques and has the potential to be extended to more quantum information processing scenarios. △ Less

Submitted 22 May, 2025; v1 submitted 18 September, 2023; originally announced September 2023.

Comments: 11 pages, 4 figures

Journal ref: Phys. Rev. Research 7, 013334 (2025)

Information recoverability of noisy quantum states

Authors: Xuanqiang Zhao, Benchi Zhao, Zihan Xia, Xin Wang

Abstract: Extracting classical information from quantum systems is an essential step of many quantum algorithms. However, this information could be corrupted as the systems are prone to quantum noises, and its distortion under quantum dynamics has not been adequately investigated. In this work, we introduce a systematic framework to study how well we can retrieve information from noisy quantum states. Given… ▽ More Extracting classical information from quantum systems is an essential step of many quantum algorithms. However, this information could be corrupted as the systems are prone to quantum noises, and its distortion under quantum dynamics has not been adequately investigated. In this work, we introduce a systematic framework to study how well we can retrieve information from noisy quantum states. Given a noisy quantum channel, we fully characterize the range of recoverable classical information. This condition allows a natural measure quantifying the information recoverability of a channel. Moreover, we resolve the minimum information retrieving cost, which, along with the corresponding optimal protocol, is efficiently computable by semidefinite programming. As applications, we establish the limits on the information retrieving cost for practical quantum noises and employ the corresponding protocols to mitigate errors in ground state energy estimation. Our work gives the first full characterization of information recoverability of noisy quantum states from the recoverable range to the recovering cost, revealing the ultimate limit of probabilistic error cancellation. △ Less

Submitted 8 April, 2023; v1 submitted 9 March, 2022; originally announced March 2022.

Comments: 23 pages including appendix, v3 accepted by Quantum

Journal ref: Quantum 7, 978 (2023)

Quantum algorithms for estimating quantum entropies

Authors: Youle Wang, Benchi Zhao, Xin Wang

Abstract: The von Neumann and quantum Rényi entropies characterize fundamental properties of quantum systems and lead to theoretical and practical applications in many fields. Quantum algorithms for estimating quantum entropies, using a quantum query model that prepares the purification of the input state, have been established in the literature. {However, constructing such a model is almost as hard as stat… ▽ More The von Neumann and quantum Rényi entropies characterize fundamental properties of quantum systems and lead to theoretical and practical applications in many fields. Quantum algorithms for estimating quantum entropies, using a quantum query model that prepares the purification of the input state, have been established in the literature. {However, constructing such a model is almost as hard as state tomography.} In this paper, we propose quantum algorithms to estimate the von Neumann and quantum $α$-Rényi entropies of an $n$-qubit quantum state $ρ$ using independent copies of the input state. We also show how to efficiently construct the quantum circuits for {quantum entropy estimation} using primitive single/two-qubit gates. We prove that the number of required copies scales polynomially in $1/ε$ and $1/Λ$, where $ε$ denotes the additive precision and $Λ$ denotes the lower bound on all non-zero eigenvalues. Notably, our method outperforms previous methods in the aspect of practicality since it does not require any quantum query oracles, which are usually necessary for previous methods. Furthermore, we conduct experiments to show the efficacy of our algorithms to single-qubit states and study the noise robustness. We also discuss the applications to some quantum states of practical interest as well as some meaningful tasks such as quantum Gibbs state preparation and entanglement estimation. △ Less

Submitted 4 March, 2022; originally announced March 2022.

Comments: 36 pages, 8 figures

Journal ref: Physical Review Applied 19, 044041, 2023

Optimal quantum dataset for learning a unitary transformation

Authors: Zhan Yu, Xuanqiang Zhao, Benchi Zhao, Xin Wang

Abstract: Unitary transformations formulate the time evolution of quantum states. How to learn a unitary transformation efficiently is a fundamental problem in quantum machine learning. The most natural and leading strategy is to train a quantum machine learning model based on a quantum dataset. Although the presence of more training data results in better models, using too much data reduces the efficiency… ▽ More Unitary transformations formulate the time evolution of quantum states. How to learn a unitary transformation efficiently is a fundamental problem in quantum machine learning. The most natural and leading strategy is to train a quantum machine learning model based on a quantum dataset. Although the presence of more training data results in better models, using too much data reduces the efficiency of training. In this work, we solve the problem on the minimum size of sufficient quantum datasets for learning a unitary transformation exactly, which reveals the power and limitation of quantum data. First, we prove that the minimum size of a dataset with pure states is $2^n$ for learning an $n$-qubit unitary transformation. To fully explore the capability of quantum data, we introduce a practical quantum dataset consisting of $n+1$ elementary tensor product states that are sufficient for exact training. The main idea is to simplify the structure utilizing decoupling, which leads to an exponential improvement in the size of the datasets with pure states. Furthermore, we show that the size of the quantum dataset with mixed states can be reduced to a constant, which yields an optimal quantum dataset for learning a unitary. We showcase the applications of our results in oracle compiling and Hamiltonian simulation. Notably, to accurately simulate a 3-qubit one-dimensional nearest-neighbor Heisenberg model, our circuit only uses $96$ elementary quantum gates, which is significantly less than $4080$ gates in the circuit constructed by the Trotter-Suzuki product formula. △ Less

Submitted 7 March, 2023; v1 submitted 1 March, 2022; originally announced March 2022.

Comments: 11 pages including appendix, v2 added remarks and references, v3 is closed to the published version

Journal ref: Physical Review Applied 19, 034017, 2023

Regulate the direct-indirect electronic band gap transition by electron-phonon interaction in BaSnO3

Authors: Binru Zhao, Qing Huang, Jiangtao Wu, Jinlong Jiao, Mingfang Shu, Gaoting Lin, Qiyang Sun, Ranran Zhang, Masato Hagihala, Shuki Torri, Guohua Wang, Qingyong Ren, Chen Li, Zhe Qu, Haidong Zhou, Jie Ma

Abstract: The neutron powder diffraction, specific heat, thermal conductivity, and Raman scattering measurements were presented to study the interplays of lattice, phonons and electrons of the Sr-doping Ba1-xSrxSnO3 (x was less than or equal to 0.1). Although Ba1-xSrxSnO3 kept the cubic lattice, the Raman spectra suggested a dynamic distortion at low temperature. The density functional theory was applied to… ▽ More The neutron powder diffraction, specific heat, thermal conductivity, and Raman scattering measurements were presented to study the interplays of lattice, phonons and electrons of the Sr-doping Ba1-xSrxSnO3 (x was less than or equal to 0.1). Although Ba1-xSrxSnO3 kept the cubic lattice, the Raman spectra suggested a dynamic distortion at low temperature. The density functional theory was applied to analyze the electronic structures and phonon dispersions of Ba1-xSrxSnO3(x = 0, 0.0125), and the behaviors of electron bands around Fermi levels were discussed. According to the experimental and theoretical results, the Sr-doping played a significant role in tuning the indirect band gap of BaSnO3 and influenced the electron-phonon interaction. △ Less

Submitted 11 April, 2022; v1 submitted 13 November, 2021; originally announced November 2021.

Comments: 23 pages, 7 figures, 3 tables, no supplemental materials

Near-term Efficient Quantum Algorithms for Entanglement Analysis

Authors: Ranyiliu Chen, Benchi Zhao, Xin Wang

Abstract: Entanglement plays a crucial role in quantum physics and is the key resource in quantum information processing. However, entanglement detection and quantification are believed to be hard due to the operational impracticality of existing methods. This work proposes three near-term efficient algorithms exploiting the hybrid quantum-classical technique to address this difficulty. The first algorithm… ▽ More Entanglement plays a crucial role in quantum physics and is the key resource in quantum information processing. However, entanglement detection and quantification are believed to be hard due to the operational impracticality of existing methods. This work proposes three near-term efficient algorithms exploiting the hybrid quantum-classical technique to address this difficulty. The first algorithm finds the Schmidt decomposition--a powerful tool to analyze the properties and structure of entanglement--for bipartite pure states. While the logarithm negativity can be calculated from the Schmidt decomposition, we propose the second algorithm to estimate the logarithm negativity for bipartite pure states, where the width of the parameterized quantum circuits is further reduced. Finally, we generalize our framework for mixed states, leading to our third algorithm which detects entanglement on specific families of states, and determines disdillability in general. All three algorithms share a similar framework where the optimizations are accomplished by maximizing a cost function utilizing local parameterized quantum circuits, with better hardware efficiency and practicality compared to existing methods. The experimental implementation on Quantum Leaf using the IoP CAS superconducting quantum processor exhibits the validity and practicality of our methods for analyzing and quantifying entanglement on near-term quantum devices. △ Less

Submitted 27 August, 2023; v1 submitted 22 September, 2021; originally announced September 2021.

Comments: 16 pages, 9 figures, v3 is close to the published version

Journal ref: Phys. Rev. Applied 20, 024071, 2023

Van der Waals Magnet based Spin-Valve Devices at Room Temperature

Authors: Bing Zhao, Roselle Ngaloy, Anamul Md. Hoque, Bogdan Karpiak, Dmitrii Khokhriakov, Saroj P. Dash

Abstract: The discovery of van der Waals (vdW) magnets opened up a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW magnets are so far limited to cryogenic temperatures, inhibiting its broader practical applications. Here, for the first time, we demonstrate room temperature spin-valve devices using vdW itinerant ferromagnet… ▽ More The discovery of van der Waals (vdW) magnets opened up a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW magnets are so far limited to cryogenic temperatures, inhibiting its broader practical applications. Here, for the first time, we demonstrate room temperature spin-valve devices using vdW itinerant ferromagnet Fe5GeTe2 in heterostructures with graphene. The tunnel spin polarization of the Fe5GeTe2/graphene vdW interface is detected to be significantly large ~ 45 % and negative at room temperature. Lateral spin-valve device design enables electrical control of spin signal and realization of basic building blocks for device application such as efficient spin injection, transport, precession, and detection functionalities. Furthermore, measurements with different magnetic orientations provide unique insights into the magnetic anisotropy of Fe5GeTe2 and its relation with spin polarization and dynamics in the heterostructure. These findings open opportunities for the applications of vdW magnet-based all-2D spintronic devices and integrated spin circuits at ambient temperatures. △ Less

Submitted 1 July, 2021; originally announced July 2021.

Practical distributed quantum information processing with LOCCNet

Authors: Xuanqiang Zhao, Benchi Zhao, Zihe Wang, Zhixin Song, Xin Wang

Abstract: Distributed quantum information processing is essential for building quantum networks and enabling more extensive quantum computations. In this regime, several spatially separated parties share a multipartite quantum system, and the most natural set of operations is Local Operations and Classical Communication (LOCC). As a pivotal part in quantum information theory and practice, LOCC has led to ma… ▽ More Distributed quantum information processing is essential for building quantum networks and enabling more extensive quantum computations. In this regime, several spatially separated parties share a multipartite quantum system, and the most natural set of operations is Local Operations and Classical Communication (LOCC). As a pivotal part in quantum information theory and practice, LOCC has led to many vital protocols such as quantum teleportation. However, designing practical LOCC protocols is challenging due to LOCC's intractable structure and limitations set by near-term quantum devices. Here we introduce LOCCNet, a machine learning framework facilitating protocol design and optimization for distributed quantum information processing tasks. As applications, we explore various quantum information tasks such as entanglement distillation, quantum state discrimination, and quantum channel simulation. We discover protocols with evident improvements, in particular, for entanglement distillation with quantum states of interest in quantum information. Our approach opens up new opportunities for exploring entanglement and its applications with machine learning, which will potentially sharpen our understanding of the power and limitations of LOCC. An implementation of LOCCNet is available in Paddle Quantum, a quantum machine learning Python package based on PaddlePaddle deep learning platform. △ Less

Submitted 22 September, 2021; v1 submitted 28 January, 2021; originally announced January 2021.

Comments: 19 pages

Journal ref: npj Quantum Information volume 7, Article number: 159 (2021)

Quantum Adiabatic Doping for Atomic Fermi-Hubbard Quantum Simulations

Authors: Jue Nan, Jian Lin, Yuchen Luo, Bo Zhao, Xiaopeng Li

Abstract: There have been considerable research efforts devoted to quantum simulations of Fermi-Hubbard model with ultracold atoms loaded in optical lattices. In such experiments, the antiferromagnetically ordered quantum state has been achieved at half filling in recent years. The atomic lattice away from half filling is expected to host d-wave superconductivity, but its low temperature phases have not bee… ▽ More There have been considerable research efforts devoted to quantum simulations of Fermi-Hubbard model with ultracold atoms loaded in optical lattices. In such experiments, the antiferromagnetically ordered quantum state has been achieved at half filling in recent years. The atomic lattice away from half filling is expected to host d-wave superconductivity, but its low temperature phases have not been reached. In a recent work, we proposed an approach of incommensurate quantum adiabatic doping, using quantum adiabatic evolution of an incommensurate lattice for preparation of the highly correlated many-body ground state of the doped Fermi-Hubbard model starting from a unit-filling band insulator. Its feasibility has been demonstrated with numerical simulations of the adiabatic preparation for certain incommensurate particle-doping fractions, where the major problem to circumvent is the atomic localization in the incommensurate lattice. Here we carry out a systematic study of the quantum adiabatic doping for a wide range of doping fractions from particle-doping to hole-doping, including both commensurate and incommensurate cases. We find that there is still a localization-like slowing-down problem at commensurate fillings, and that it becomes less harmful in the hole-doped regime. With interactions, the adiabatic preparation is found to be more efficient for that interaction effect destabilizes localization. For both free and interacting cases, we find the adiabatic doping has better performance in the hole-doped regime than the particle-doped regime. We also study adiabatic doping starting from the half-filling Mott insulator, which is found to be more efficient for certain filling fractions. △ Less

Submitted 5 January, 2021; originally announced January 2021.

Comments: 8 pages,7 figures

Journal ref: Phys. Rev. A 103, 043320 (2021)

Robust Spin Interconnect with Isotropic Spin Dynamics in Chemical Vapour Deposited Graphene Layers and Boundaries

Authors: Dmitrii Khokhriakov, Bogdan Karpiak, Anamul Md. Hoque, Bing Zhao, Subir Parui, Saroj P. Dash

Abstract: The utilization of large-area graphene grown by chemical vapour deposition (CVD) is crucial for the development of scalable spin interconnects in all-spin-based memory and logic circuits. However, the fundamental influence of the presence of multilayer graphene patches and their boundaries on spin dynamics has not been addressed yet, which is necessary for basic understanding and application of ro… ▽ More The utilization of large-area graphene grown by chemical vapour deposition (CVD) is crucial for the development of scalable spin interconnects in all-spin-based memory and logic circuits. However, the fundamental influence of the presence of multilayer graphene patches and their boundaries on spin dynamics has not been addressed yet, which is necessary for basic understanding and application of robust spin interconnects. Here, we report universal spin transport and dynamic properties in specially devised single layer, bi-layer, and tri-layer graphene channels and their layer boundaries and folds that are usually present in CVD graphene samples. We observe uniform spin lifetime with isotropic spin relaxation for spins with different orientations in graphene layers and their boundaries at room temperature. In all the inhomogeneous graphene channels, the spin lifetime anisotropy ratios for spins polarized out-of-plane and in-plane are measured to be close to unity. Our analysis shows the importance of both Elliott-Yafet and Dyakonov-Perel mechanisms, with an increasing role of the latter mechanism in multilayer channels. These results of universal and isotropic spin transport on large-area inhomogeneous CVD graphene with multilayer patches and their boundaries and folds at room temperature prove its outstanding spin interconnect functionality, beneficial for the development of scalable spintronic circuits. △ Less

Submitted 4 December, 2020; originally announced December 2020.

Journal ref: ACS Nano, 14, 11, 15864 (2020)

Positive-energy spectra of atomic hydrogen in a magnetic field with an adiabatic-basis-expansion method

Authors: L. B. Zhao, K. D. Wang, K. Bartschat

Abstract: The problem of photoionization of atomic hydrogen in a white-dwarf-strength magnetic field is revisited to understand the existing discrepancies in the positive-energy spectra obtained by a variety of theoretical approaches reported in the literature. Oscillator strengths for photoionization are calculated with the adiabatic-basis-expansion method developed by Mota-Furtado and O'Mahony [Phys. Rev.… ▽ More The problem of photoionization of atomic hydrogen in a white-dwarf-strength magnetic field is revisited to understand the existing discrepancies in the positive-energy spectra obtained by a variety of theoretical approaches reported in the literature. Oscillator strengths for photoionization are calculated with the adiabatic-basis-expansion method developed by Mota-Furtado and O'Mahony [Phys. Rev. A {\bf 76}, 053405 (2007)]. A comparative study is performed between the adiabatic-basis-expansion method and our previously developed coupled-channel theory [Phys. Rev. A {\bf 94}, 033422 (2016)]. A detailed analysis of the positive-energy spectra obtained here and those from other theoretical approaches shows that the adiabatic-basis-expansion method can produce more accurate positive-energy spectra than other reported approaches for low field strengths. △ Less

Submitted 23 November, 2020; originally announced November 2020.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. A 103, 022807 (2021)

Phase transitions and spin excitations of spin-1 bosons in optical lattice

Authors: Min-Jie Zhu, Bo Zhao

Abstract: We investigate ground state properties of spin-1 bosonic system trapped in optical lattice with extended standard basis operator (SBO) method. For both ferromagnetic ($U_2<0$) and antiferromagnetic ($U_2>0$) systems, we analytically figure out the symmetry properties in Mott-insulator and superfluid phases, which would provide a deeper insight into the MI-SF phase transition process. Then by apply… ▽ More We investigate ground state properties of spin-1 bosonic system trapped in optical lattice with extended standard basis operator (SBO) method. For both ferromagnetic ($U_2<0$) and antiferromagnetic ($U_2>0$) systems, we analytically figure out the symmetry properties in Mott-insulator and superfluid phases, which would provide a deeper insight into the MI-SF phase transition process. Then by applying self-consistent approach to the method, we include the effect of quantum and thermal fluctuations and derive the MI-SF transition phase diagram, which is in quantitative agreement with recent Monte-Carlo simulation at zero temperature, and at finite temperature, we find the underestimation of finite-temperature-effect in the mean-field approximation method. If we further consider the spin excitations in the insulating states of spin-1 system in external field, distinct spin phases are expected. Therefore, in the Mott lobes with $n=1$ and $n=2$ atoms per site, we give analytical and numerical boundaries of the singlet, nematic, partially magnetic and ferromagnetic phases in the magnetic phase diagrams. △ Less

Submitted 1 June, 2017; v1 submitted 24 May, 2017; originally announced May 2017.

Comments: 14 pages, 6 figures

Arbitrary Rotation of a Single Spinwave Qubit in an Atomic-Ensemble Quantum Memory

Authors: Jun Rui, Yan Jiang, Bo Zhao, Xiao-Hui Bao, Jian-Wei Pan

Abstract: We report the first experimental realization of single-qubit manipulation for single spinwaves stored in an atomic ensemble quantum memory. In order to have high-fidelity gate operations, we make use of stimulated Raman transition and controlled Lamor precession jointly. We characterize the gate performances with quantum state tomography and quantum process tomography, both of which imply that hig… ▽ More We report the first experimental realization of single-qubit manipulation for single spinwaves stored in an atomic ensemble quantum memory. In order to have high-fidelity gate operations, we make use of stimulated Raman transition and controlled Lamor precession jointly. We characterize the gate performances with quantum state tomography and quantum process tomography, both of which imply that high-fidelity operations have been achieved. Our work complements the experimental toolbox of atomic-ensemble quantum memories by adding the capability of single-qubit manipulation, thus may have important applications in future scalable quantum networks. △ Less

Submitted 28 January, 2015; originally announced January 2015.

Comments: 5 pages, 2 figures, 2 tables

Experimental demonstration of quantum lithography beyond diffraction limit via Rabi oscillations

Authors: Jun Rui, Yan Jiang, Guo-Peng Lu, Bo Zhao, Xiao-Hui Bao, Jian-Wei Pan

Abstract: Diffraction of light sets the fundamental limit for optical lithography. Many quantum lithography schemes have so far been proposed to overcome this limit either by making use of highly entangled photons, multi-photon processes or multiple Lambda transitions, which are all experimentally high-demanding. Recently, Liao et al. proposed a novel quantum lithography scheme which merely employs Rabi osc… ▽ More Diffraction of light sets the fundamental limit for optical lithography. Many quantum lithography schemes have so far been proposed to overcome this limit either by making use of highly entangled photons, multi-photon processes or multiple Lambda transitions, which are all experimentally high-demanding. Recently, Liao et al. proposed a novel quantum lithography scheme which merely employs Rabi oscillation to surpass the diffraction limit. Here we report a faithful experimental realization of this scheme. Resolution up to ninth of the Rayleigh diffraction limit has been observed. Possibility of creating an arbitrary pattern is also tested experimentally by demonstrating the peak narrowing process using several Rabi floppings together with state-selective optical depletion. Our work may have direct applications in atom pattern engineering for quantum information or quantum simulation applications, and will also possibly boost the adoption of quantum lithography into real-world applications in the near future. △ Less

Submitted 27 January, 2015; originally announced January 2015.

Comments: 5 pages, 3 figures

Journal ref: Phys. Rev. A 93, 033837 (2016)

Operating Spin Echo in the Quantum Regime for an Atomic-Ensemble Quantum Memory

Authors: Jun Rui, Yan Jiang, Sheng-Jun Yang, Bo Zhao, Xiao-Hui Bao, Jian-Wei Pan

Abstract: Spin echo is a powerful technique to extend atomic or nuclear coherence time by overcoming the dephasing due to inhomogeneous broadening. However, applying this technique to an ensemble-based quantum memory at single-quanta level remains challenging. In our experimental study we find that noise due to imperfection of the rephasing pulses is highly directional. By properly arranging the beam direct… ▽ More Spin echo is a powerful technique to extend atomic or nuclear coherence time by overcoming the dephasing due to inhomogeneous broadening. However, applying this technique to an ensemble-based quantum memory at single-quanta level remains challenging. In our experimental study we find that noise due to imperfection of the rephasing pulses is highly directional. By properly arranging the beam directions and optimizing the pulse fidelities, we have successfully managed to operate the spin echo technique in the quantum regime and observed nonclassical photon-photon correlations. In comparison to the case without applying the rephasing pulses, quantum memory lifetime is extended by 5 folds. Our work for the first time demonstrates the feasibility of harnessing the spin echo technique to extend lifetime of ensemble-based quantum memories at single-quanta level. △ Less

Submitted 26 January, 2015; originally announced January 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. Lett. 115, 133002 (2015)

Three `species' of Schrödinger cat states in an infinite-range spin model

Authors: Bo Zhao, Merritt C. Kerridge, David A. Huse

Abstract: We explore a transverse-field Ising model that exhibits both spontaneous symmetry-breaking and eigenstate thermalization. Within its ferromagnetic phase, the exact eigenstates of the Hamiltonian of any large but finite-sized system are all Schrödinger cat states: superpositions of states with `up' and `down' spontaneous magnetization. This model exhibits two dynamical phase transitions {\it within… ▽ More We explore a transverse-field Ising model that exhibits both spontaneous symmetry-breaking and eigenstate thermalization. Within its ferromagnetic phase, the exact eigenstates of the Hamiltonian of any large but finite-sized system are all Schrödinger cat states: superpositions of states with `up' and `down' spontaneous magnetization. This model exhibits two dynamical phase transitions {\it within} its ferromagnetic phase: In the lowest-temperature phase the magnetization can macroscopically oscillate between up and down. The relaxation of the magnetization is always overdamped in the remainder of the ferromagnetic phase, which is divided in to phases where the system thermally activates itself {\it over} the barrier between the up and down states, and where it quantum tunnels. △ Less

Submitted 23 June, 2014; v1 submitted 18 October, 2013; originally announced October 2013.

Comments: 7 pages, added numerical results

Journal ref: Phys. Rev. E 90, 022104 (2014)

Efficient and long-lived quantum memory with cold atoms inside a ring cavity

Authors: Xiao-Hui Bao, Andreas Reingruber, Peter Dietrich, Jun Rui, Alexander Dück, Thorsten Strassel, Li Li, Nai-Le Liu, Bo Zhao, Jian-Wei Pan

Abstract: Quantum memories are regarded as one of the fundamental building blocks of linear-optical quantum computation and long-distance quantum communication. A long standing goal to realize scalable quantum information processing is to build a long-lived and efficient quantum memory. There have been significant efforts distributed towards this goal. However, either efficient but short-lived or long-lived… ▽ More Quantum memories are regarded as one of the fundamental building blocks of linear-optical quantum computation and long-distance quantum communication. A long standing goal to realize scalable quantum information processing is to build a long-lived and efficient quantum memory. There have been significant efforts distributed towards this goal. However, either efficient but short-lived or long-lived but inefficient quantum memories have been demonstrated so far. Here we report a high-performance quantum memory in which long lifetime and high retrieval efficiency meet for the first time. By placing a ring cavity around an atomic ensemble, employing a pair of clock states, creating a long-wavelength spin wave, and arranging the setup in the gravitational direction, we realize a quantum memory with an intrinsic spin wave to photon conversion efficiency of 73(2)% together with a storage lifetime of 3.2(1) ms. This realization provides an essential tool towards scalable linear-optical quantum information processing. △ Less

Submitted 12 July, 2012; originally announced July 2012.

Comments: 6 pages, 4 figures

Journal ref: Nature Physics 8, 517-521 (2012)

Driven-dissipative dynamics of a strongly interacting Rydberg gas

Authors: A. W. Glaetzle, R. Nath, B. Zhao, G. Pupillo, P. Zoller

Abstract: We study the non-equilibrium many-body dynamics of a cold gas of ground state alkali atoms weakly admixed by Rydberg states with laser light. On a timescale shorter than the lifetime of the dressed states, effective dipole-dipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the… ▽ More We study the non-equilibrium many-body dynamics of a cold gas of ground state alkali atoms weakly admixed by Rydberg states with laser light. On a timescale shorter than the lifetime of the dressed states, effective dipole-dipole or van der Waals interactions between atoms can lead to the formation of strongly correlated phases, such as atomic crystals. Using a semiclassical approach, we study the long-time dynamics where decoherence and dissipative processes due to spontaneous emission and blackbody radiation dominate, leading to heating and melting of atomic crystals as well as particle losses. These effects can be substantially mitigated by performing active laser cooling in the presence of atomic dressing. △ Less

Submitted 11 July, 2012; originally announced July 2012.

Comments: 20 pages, 15 figures

Journal ref: Phys. Rev. A 86, 043403 (2012)

Holographic Storage of Biphoton Entanglement

Authors: Han-Ning Dai, Han Zhang, Sheng-Jun Yang, Tian-Ming Zhao, Jun Rui, You-Jin Deng, Li Li, Nai-Le Liu, Shuai Chen, Xiao-Hui Bao, Xian-Min Jin, Bo Zhao, Jian-Wei Pan

Abstract: Coherent and reversible storage of multi-photon entanglement with a multimode quantum memory is essential for scalable all-optical quantum information processing. Although single photon has been successfully stored in different quantum systems, storage of multi-photon entanglement remains challenging because of the critical requirement for coherent control of photonic entanglement source, multimod… ▽ More Coherent and reversible storage of multi-photon entanglement with a multimode quantum memory is essential for scalable all-optical quantum information processing. Although single photon has been successfully stored in different quantum systems, storage of multi-photon entanglement remains challenging because of the critical requirement for coherent control of photonic entanglement source, multimode quantum memory, and quantum interface between them. Here we demonstrate a coherent and reversible storage of biphoton Bell-type entanglement with a holographic multimode atomic-ensemble-based quantum memory. The retrieved biphoton entanglement violates Bell's inequality for 1 microsecond storage time and a memory-process fidelity of 98% is demonstrated by quantum state tomography. △ Less

Submitted 6 April, 2012; originally announced April 2012.

Comments: 5 pages, 4 figures, accepted by Phys. Rev. Lett

Collective Dipole Oscillation of a Spin-Orbit Coupled Bose-Einstein Condensate

Authors: Jin-Yi Zhang, Si-Cong Ji, Zhu Chen, Long Zhang, Zhi-Dong Du, Bo Yan, Ge-Sheng Pan, Bo Zhao, Youjin Deng, Hui Zhai, Shuai Chen, Jian-Wei Pan

Abstract: We present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. Dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region, as a function of spin-orbit coupling parameters as well as oscillation amplitude. Anharmonic properties beyond effective-mass approximation are revealed, such as amplitude… ▽ More We present an experimental study of the collective dipole oscillation of a spin-orbit coupled Bose-Einstein condensate in a harmonic trap. Dynamics of the center-of-mass dipole oscillation is studied in a broad parameter region, as a function of spin-orbit coupling parameters as well as oscillation amplitude. Anharmonic properties beyond effective-mass approximation are revealed, such as amplitude-dependent frequency and finite oscillation frequency at place with divergent effective mass. These anharmonic behaviors agree quantitatively with variational wave-function calculations. Moreover, we experimentally demonstrate a unique feature of spin-orbit coupled system predicted by a sum-rule approach, stating that spin polarization susceptibility--a static physical quantity--can be measured via dynamics of dipole oscillation. The divergence of polarization susceptibility is observed at the quantum phase transition that separates magnetic nonzero-momentum condensate from nonmagnetic zero-momentum phase. The good agreement between the experimental and theoretical results provides a bench mark for recently developed theoretical approaches. △ Less

Submitted 3 July, 2012; v1 submitted 29 January, 2012; originally announced January 2012.

Comments: 5 pages, 4 figures, 0 table

Journal ref: Phys. Rev. Lett. 109, 115301 (2012)

Atomic Rydberg Reservoirs for Polar Molecules

Authors: Bo Zhao, Alexander Glätzle, Guido Pupillo, Peter Zoller

Abstract: We discuss laser dressed dipolar and Van der Waals interactions between atoms and polar molecules, so that a cold atomic gas with laser admixed Rydberg levels acts as a designed reservoir for both elastic and inelastic collisional processes. The elastic scattering channel is characterized by large elastic scattering cross sections and repulsive shields to protect from close encounter collisions. I… ▽ More We discuss laser dressed dipolar and Van der Waals interactions between atoms and polar molecules, so that a cold atomic gas with laser admixed Rydberg levels acts as a designed reservoir for both elastic and inelastic collisional processes. The elastic scattering channel is characterized by large elastic scattering cross sections and repulsive shields to protect from close encounter collisions. In addition, we discuss a dissipative (inelastic) collision where a spontaneously emitted photon carries away (kinetic) energy of the collision partners, thus providing a significant energy loss in a single collision. This leads to the scenario of rapid thermalization and cooling of a molecule in the mK down to the μK regime by cold atoms. △ Less

Submitted 18 December, 2011; originally announced December 2011.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 108, 193007 (2012)

Deterministic spin-wave interferometer based on Rydberg blockade

Authors: Ran Wei, Bo Zhao, Youjin Deng, Yu-Ao Chen, Jian-Wei Pan

Abstract: The spin-wave (SW) NOON state is an $N$-particle Fock state with two atomic spin-wave modes maximally entangled. Attributed to the property that the phase is sensitive to collective atomic motion, the SW NOON state can be utilized as a novel atomic interferometer and has promising application in quantum enhanced measurement. In this paper we propose an efficient protocol to deterministically produ… ▽ More The spin-wave (SW) NOON state is an $N$-particle Fock state with two atomic spin-wave modes maximally entangled. Attributed to the property that the phase is sensitive to collective atomic motion, the SW NOON state can be utilized as a novel atomic interferometer and has promising application in quantum enhanced measurement. In this paper we propose an efficient protocol to deterministically produce the atomic SW NOON state by employing Rydberg blockade. Possible errors in practical manipulations are analyzed. A feasible experimental scheme is suggested. Our scheme is far more efficient than the recent experimentally demonstrated one, which only creates a heralded second-order SW NOON state. △ Less

Submitted 18 May, 2011; v1 submitted 6 March, 2011; originally announced March 2011.

Comments: 5 pages, 2 figures

Journal ref: Phys.Rev.A 83, 063623 (2011)

Teleportation-based realization of an optical quantum two-qubit entangling gate

Authors: Wei-Bo Gao, Alexander M. Goebel, Chao-Yang Lu, Han-Ning Dai, Claudia Wagenknecht, Qiang Zhang, Bo Zhao, Cheng-Zhi Peng, Zeng-Bing Chen, Yu-Ao Chen, Jian-Wei Pan

Abstract: In recent years, there has been heightened interest in quantum teleportation, which allows for the transfer of unknown quantum states over arbitrary distances. Quantum teleportation not only serves as an essential ingredient in long-distance quantum communication, but also provides enabling technologies for practical quantum computation. Of particular interest is the scheme proposed by Gottesman a… ▽ More In recent years, there has been heightened interest in quantum teleportation, which allows for the transfer of unknown quantum states over arbitrary distances. Quantum teleportation not only serves as an essential ingredient in long-distance quantum communication, but also provides enabling technologies for practical quantum computation. Of particular interest is the scheme proposed by Gottesman and Chuang [Nature \textbf{402}, 390 (1999)], showing that quantum gates can be implemented by teleporting qubits with the help of some special entangled states. Therefore, the construction of a quantum computer can be simply based on some multi-particle entangled states, Bell state measurements and single-qubit operations. The feasibility of this scheme relaxes experimental constraints on realizing universal quantum computation. Using two different methods we demonstrate the smallest non-trivial module in such a scheme---a teleportation-based quantum entangling gate for two different photonic qubits. One uses a high-fidelity six-photon interferometer to realize controlled-NOT gates and the other uses four-photon hyper-entanglement to realize controlled-Phase gates. The results clearly demonstrate the working principles and the entangling capability of the gates. Our experiment represents an important step towards the realization of practical quantum computers and could lead to many further applications in linear optics quantum information processing. △ Less

Submitted 2 November, 2010; originally announced November 2010.

Comments: 10 pages, 6 figures

Journal ref: PNAS 107, 20869-20874 (2010)

Residual effect on the robustness of multiqubit entanglement

Authors: Bao-Kui Zhao, Fu-Guo Deng

Abstract: We investigate the relation between the entanglement and the robustness of a multipartite system to a depolarization noise. We find that the robustness of a two-qubit system in an arbitrary pure state depends completely on its entanglement. However, this is not always true in a three-qubit system. There is a residual effect on the robustness of a three-qubit system in an arbitrary superposition of… ▽ More We investigate the relation between the entanglement and the robustness of a multipartite system to a depolarization noise. We find that the robustness of a two-qubit system in an arbitrary pure state depends completely on its entanglement. However, this is not always true in a three-qubit system. There is a residual effect on the robustness of a three-qubit system in an arbitrary superposition of Greenberger-Horne-Zeilinger state and W state. Its entanglement determines the trend of its robustness. However, there is a splitting on its robustness under the same entanglement. Its robustness not only has the same periodicity as its three-tangle but also alters with its three-tangle synchronously. There is also a splitting on the robustness of an $n$-qubit ($n>3$) system although it is more complicated. △ Less

Submitted 14 July, 2010; v1 submitted 1 July, 2010; originally announced July 2010.

Comments: 5 pages, 4 figures; A figure is added, compared with the version published in Phys. Rev. A 82, 014301 (2010)

Journal ref: Phys. Rev. A 82, 014301 (2010)

Quantum interface between frequency-uncorrelated down-converted entanglement and atomic-ensemble quantum memory

Authors: Xian-Min Jin, Jian Yang, Han Zhang, Han-Ning Dai, Sheng-Jun Yang, Tian-Ming Zhao, Jun Rui, Yu He, Xiao Jiang, Fan Yang, Ge-Sheng Pan, Zhen-Sheng Yuan, Youjin Deng, Zeng-Bing Chen, Xiao-Hui Bao, Bo Zhao, Shuai Chen, Jian-Wei Pan

Abstract: Photonic entanglement source and quantum memory are two basic building blocks of linear-optical quantum computation and long-distance quantum communication. In the past decades, intensive researches have been carried out, and remarkable progress, particularly based on the spontaneous parametric down-converted (SPDC) entanglement source and atomic ensembles, has been achieved. Currently, an importa… ▽ More Photonic entanglement source and quantum memory are two basic building blocks of linear-optical quantum computation and long-distance quantum communication. In the past decades, intensive researches have been carried out, and remarkable progress, particularly based on the spontaneous parametric down-converted (SPDC) entanglement source and atomic ensembles, has been achieved. Currently, an important task towards scalable quantum information processing (QIP) is to efficiently write and read entanglement generated from a SPDC source into and out of an atomic quantum memory. Here we report the first experimental realization of a quantum interface by building a 5 MHz frequency-uncorrelated SPDC source and reversibly mapping the generated entangled photons into and out of a remote optically thick cold atomic memory using electromagnetically induced transparency. The frequency correlation between the entangled photons is almost fully eliminated with a suitable pump pulse. The storage of a triggered single photon with arbitrary polarization is shown to reach an average fidelity of 92% for 200 ns storage time. Moreover, polarization-entangled photon pairs are prepared, and one of photons is stored in the atomic memory while the other keeps flying. The CHSH Bell's inequality is measured and violation is clearly observed for storage time up to 1 microsecond. This demonstrates the entanglement is stored and survives during the storage. Our work establishes a crucial element to implement scalable all-optical QIP, and thus presents a substantial progress in quantum information science. △ Less

Submitted 26 April, 2010; originally announced April 2010.

Comments: 28 pages, 4 figures, 1 table

Efficient quantum repeater based on deterministic Rydberg gates

Authors: Bo Zhao, Markus Mueller, Klemens Hammerer, Peter Zoller

Abstract: We propose an efficient quantum repeater architecture with mesoscopic atomic ensembles, where the Rydberg blockade is employed for deterministic local entanglement generation, entanglement swapping and entanglement purification. Compared with conventional atomic-ensemble-based quantum repeater, the entanglement distribution rate is improved by up to two orders of magnitude with the help of the det… ▽ More We propose an efficient quantum repeater architecture with mesoscopic atomic ensembles, where the Rydberg blockade is employed for deterministic local entanglement generation, entanglement swapping and entanglement purification. Compared with conventional atomic-ensemble-based quantum repeater, the entanglement distribution rate is improved by up to two orders of magnitude with the help of the deterministic Rydberg gate. This new quantum repeater scheme is robust and fast, and thus opens up a new way for practical long-distance quantum communication. △ Less

Submitted 9 March, 2010; originally announced March 2010.

Comments: 5 pages, 2 figures

Journal ref: Phys. Rev. A 81, 052329 (2010)

Light pulse in $Λ$-type cold atomic gases

Authors: Ran Wei, Bo Zhao, Youjin Deng, Shuai Chen, Zeng-Bing Chen, Jian-Wei Pan

Abstract: We investigate the behavior of the light pulse in $Lambda$-type cold atomic gases with two counterpropagating control lights with equal strength by directly simulating the dynamic equations and exploring the dispersion relation. Our analysis shows that, depending on the length $L_0$ of the stored wave packet and the decay rate $γ$ of ground-spin coherence, the recreated light can behave differen… ▽ More We investigate the behavior of the light pulse in $Lambda$-type cold atomic gases with two counterpropagating control lights with equal strength by directly simulating the dynamic equations and exploring the dispersion relation. Our analysis shows that, depending on the length $L_0$ of the stored wave packet and the decay rate $γ$ of ground-spin coherence, the recreated light can behave differently. For long $L_0$ and/or large $γ$, a stationary light pulse is produced, while two propagating light pulses appear for short $L_0$ and/or small $γ$. In the $γ\to 0$ limit, the light always splits into two propagating pulses for sufficiently long time. This scenario agrees with a recent experiment [Y.-W.Lin, et al., Phys. Rev. Lett. 102, 213601(2009)] where two propagating light pulses are generated in laser-cooled cold atomic ensembles. △ Less

Submitted 28 February, 2010; v1 submitted 14 January, 2010; originally announced January 2010.

Comments: 7 pages, 7 figures

Journal ref: Phys.Rev.A 81,043403 (2010)

Efficient faithful qubit transmission with frequency degree of freedom

Authors: Xi-Han Li, Bao-Kui Zhao, Yu-Bo Sheng, Fu-Guo Deng, Hong-Yu Zhou

Abstract: We propose an efficient faithful polarization-state transmission scheme by utilizing frequency degree of freedom besides polarization and an additional qubit prepared in a fixed polarization. An arbitrary single-photon polarization state is protected against the collective noise probabilistically. With the help of frequency beam splitter and frequency shifter, the success probability of our fait… ▽ More We propose an efficient faithful polarization-state transmission scheme by utilizing frequency degree of freedom besides polarization and an additional qubit prepared in a fixed polarization. An arbitrary single-photon polarization state is protected against the collective noise probabilistically. With the help of frequency beam splitter and frequency shifter, the success probability of our faithful qubit transmission scheme with frequency degree of freedom can be 1/2 in principle. △ Less

Submitted 16 August, 2009; v1 submitted 30 June, 2009; originally announced July 2009.

Comments: 4 pages, 1 figure

Journal ref: Optics Communications 282 (2009) 4025-4027

Genuine tripartite entanglement in quantum brachistochrone evolution of a three-qubit system

Authors: Bao-Kui Zhao, Fu-Guo Deng, Feng-Shou Zhang, Hong-Yu Zhou

Abstract: We explore the connection between quantum brachistochrone (time-optimal) evolution of a three-qubit system and its residual entanglement called three-tangle. The result shows that the entanglement between two qubits is not required for some brachistochrone evolutions of a three-qubit system. However, the evolution between two distinct states cannot be implemented without its three-tangle, except… ▽ More We explore the connection between quantum brachistochrone (time-optimal) evolution of a three-qubit system and its residual entanglement called three-tangle. The result shows that the entanglement between two qubits is not required for some brachistochrone evolutions of a three-qubit system. However, the evolution between two distinct states cannot be implemented without its three-tangle, except for the trivial cases in which less than three qubits attend evolution. Although both the probability density function of the time-averaged three-tangle and that of the time-averaged squared concurrence between two subsystems become more and more uniform with the decrease in angles of separation between an initial state and a final state, the features of their most probable values exhibit a different trend. △ Less

Submitted 11 November, 2009; v1 submitted 30 June, 2009; originally announced July 2009.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. A 80, 052106 (2009)

Entanglement assisted spin-wave atom interferometer

Authors: Yu-Ao Chen, Xiao-Hui Bao, Zhen-Sheng Yuan, Shuai Chen, Bo Zhao, Jian-Wei Pan

Abstract: We report the observation of phase-super resolution in a motion-sensitive spin-wave (SW) atom interferometer utilizing a NOON-type entanged state. The SW interferometer is implemented by generating a superposition of two SWs and observing the interference between them, where the interference fringe is sensitive to the atomic collective motion. By heralded generation of a second order NOON state… ▽ More We report the observation of phase-super resolution in a motion-sensitive spin-wave (SW) atom interferometer utilizing a NOON-type entanged state. The SW interferometer is implemented by generating a superposition of two SWs and observing the interference between them, where the interference fringe is sensitive to the atomic collective motion. By heralded generation of a second order NOON state in the SW interferometer, we clearly observe the interference pattern with phase super-resolution. The demonstrated SW interferometer can in principle be scaled up to highly entangled quantum state, and thus is of fundamental importance to implement quantum-enhanced-measurement. △ Less

Submitted 15 October, 2009; v1 submitted 23 April, 2009; originally announced April 2009.

Comments: 6 pages, 5 figures (including appendix)

Journal ref: Phys. Rev. Lett 104, 043601 (2010)

Fault tolerant quantum key distribution based on quantum dense coding with collective noise

Authors: Xi-Han Li, Bao-Kui Zhao, Yu-Bo Sheng, Fu-Guo Deng, Hong-Yu Zhou

Abstract: We present two robust quantum key distribution protocols against two kinds of collective noise, following some ideas in quantum dense coding. Three-qubit entangled states are used as quantum information carriers, two of which forming the logical qubit which is invariant with a special type of collective noise. The information is encoded on logical qubits with four unitary operations, which can b… ▽ More We present two robust quantum key distribution protocols against two kinds of collective noise, following some ideas in quantum dense coding. Three-qubit entangled states are used as quantum information carriers, two of which forming the logical qubit which is invariant with a special type of collective noise. The information is encoded on logical qubits with four unitary operations, which can be read out faithfully with Bell-state analysis on two physical qubits and a single-photon measurement on the other physical qubit, not three-photon joint measurements. Two bits of information are exchanged faithfully and securely by transmitting two physical qubits through a noisy channel. When the losses in the noisy channel is low, these protocols can be used to transmit a secret message directly in principle. △ Less

Submitted 12 January, 2010; v1 submitted 31 March, 2009; originally announced April 2009.

Comments: 5 page, no figures

Journal ref: International Journal of Quantum Informaton 7(8), 1479-1489 (2009)

Multipartite entanglement purification with quantum nondemolition detectors

Authors: Yu-Bo Sheng, Fu-Guo Deng, Bao-Kui Zhao, Tie-Jun Wang, Hong-Yu Zhou

Abstract: We present a scheme for multipartite entanglement purification of quantum systems in a Greenberger-Horne-Zeilinger state with quantum nondemolition detectors (QNDs). This scheme does not require the controlled-not gates which cannot be implemented perfectly with linear optical elements at present, but QNDs based on cross-Kerr nonlinearities. It works with two steps, i.e., the bit-flipping error… ▽ More We present a scheme for multipartite entanglement purification of quantum systems in a Greenberger-Horne-Zeilinger state with quantum nondemolition detectors (QNDs). This scheme does not require the controlled-not gates which cannot be implemented perfectly with linear optical elements at present, but QNDs based on cross-Kerr nonlinearities. It works with two steps, i.e., the bit-flipping error correction and the phase-flipping error correction. These two steps can be iterated perfectly with parity checks and simple single-photon measurements. This scheme does not require the parties to possess sophisticated single photon detectors. These features maybe make this scheme more efficient and feasible than others in practical applications. △ Less

Submitted 4 September, 2009; v1 submitted 1 October, 2008; originally announced October 2008.

Comments: 8 pages, 5 figures

Journal ref: Eur. Phys. j. D 55, 235-242 (2009)

Quantum Memory with Optically Trapped Atoms

Authors: Chih-Sung Chuu, Thorsten Strassel, Bo Zhao, Markus Koch, Yu-Ao Chen, Shuai Chen, Zhen-Sheng Yuan, Joerg Schmiedmayer, Jian-Wei Pan

Abstract: We report the experimental demonstration of a quantum memory for collective atomic states in a far-detuned optical dipole trap. Generation of the collective atomic state is heralded by the detection of a Raman scattered photon and accompanied by storage in the ensemble of atoms. The optical dipole trap provides confinement for the atoms during the quantum storage while retaining the atomic coher… ▽ More We report the experimental demonstration of a quantum memory for collective atomic states in a far-detuned optical dipole trap. Generation of the collective atomic state is heralded by the detection of a Raman scattered photon and accompanied by storage in the ensemble of atoms. The optical dipole trap provides confinement for the atoms during the quantum storage while retaining the atomic coherence. We probe the quantum storage by cross-correlation of the photon pair arising from the Raman scattering and the retrieval of the atomic state stored in the memory. Non-classical correlations are observed for storage times up to 60 microseconds. △ Less

Submitted 20 August, 2008; originally announced August 2008.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. Lett 101, 120501 (2008)

A millisecond quantum memory for scalable quantum networks

Authors: Bo Zhao, Yu-Ao Chen, Xiao-Hui Bao, Thorsten Strassel, Chih-Sung Chuu, Xian-Min Jin, Jörg Schmiedmayer, Zhen-Sheng Yuan, Shuai Chen, Jian-Wei Pan

Abstract: Scalable quantum information processing critically depends on the capability of storage of a quantum state. In particular, a long-lived storable and retrievable quantum memory for single excitations is of crucial importance to the atomic-ensemble-based long-distance quantum communication. Although atomic memories for classical lights and continuous variables have been demonstrated with milliseco… ▽ More Scalable quantum information processing critically depends on the capability of storage of a quantum state. In particular, a long-lived storable and retrievable quantum memory for single excitations is of crucial importance to the atomic-ensemble-based long-distance quantum communication. Although atomic memories for classical lights and continuous variables have been demonstrated with milliseconds storage time, there is no equal advance in the development of quantum memory for single excitations, where only around 10 $μ$s storage time was achieved. Here we report our experimental investigations on extending the storage time of quantum memory for single excitations. We isolate and identify distinct mechanisms for the decoherence of spin wave (SW) in atomic ensemble quantum memories. By exploiting the magnetic field insensitive state, ``clock state", and generating a long-wavelength SW to suppress the dephasing, we succeed in extending the storage time of the quantum memory to 1 ms. Our result represents a substantial progress towards long-distance quantum communication and enables a realistic avenue for large-scale quantum information processing. △ Less

Submitted 31 July, 2008; originally announced July 2008.

Comments: 11pages, 4 figures, submitted for publication

Journal ref: Nature Physics 5, 95 - 99 (2009)