Digraph states and their neural network representations

With the rapid development of machine learning, artificial neural networks provide a powerful tool to represent or approximate many-body quantum states. It was proved that every graph state can be generated by a neural network. Here, we introduce digraph states and explore their neural network representations (NNRs). Based on some discussions about digraph states and neural network quantum states (NNQSs), we construct explicitly an NNR for any digraph state, implying every digraph state is an NNQS. The obtained results will provide a theoretical foundation for solving the quantum many-body problem with machine learning method whenever the wave-function is known as an unknown digraph state or it can be approximated by digraph states.     

神经网络量子态及其应用

神经网络量子态是由人工神经网络所表示的量子态。得益于机器学习,尤其是深度学习近年来取得的突破性进展,神经网络量子态的研究得到了广泛的关注,成为当前的热点前沿方向。文章将介绍不同的神经网络量子态,其物理性质与典型应用场景,最新进展,以及面临的挑战。     

Representations of hypergraph states with neural networks

The quantum many-body problem(QMBP) has become a hot topic in high-energy physics and condensed-matter physics. With an exponential increase in the dimensions of Hilbert space, it becomes very challenging to solve the QMBP, even with the most powerful computers. With the rapid development of machine learning, artificial neural networks provide a powerful tool that can represent or approximate quantum many-body states. In this paper, we aim to explicitly construct the neural network representations of hypergraph states. We construct the neural network representations for any k-uniform hypergraph state and any hypergraph state,respectively, without stochastic optimization of the network parameters. Our method constructively shows that all hypergraph states can be represented precisely by the appropriate neural networks introduced in [Science 355(2017) 602] and formulated in [Sci. China-Phys.Mech. Astron. 63(2020) 210312].     

机器学习方法在量子多体物理中的应用

机器学习方法近年来在许多不同的领域受到广泛的关注,文章回顾了机器学习方法在量子多体物理中的应用的几个有代表性的例子,着重讨论了机器学习方法对于解决量子多体物理中“指数墙”困难的可能的潜在意义。除此以外,量子多体物理中的若干方法和思想反过来可能对理解机器学习领域面临的核心问题有重要的启发作用。     

基于纠缠态的量子通信网络的量子信道建立速率模型

针对基于纠缠态的量子通信网络, 提出了网络模型. 基于网络模型, 首先分析了基础链路的量子信道建立速率. 然后根据基础链路的量子信道建立速率, 针对不同的量子信道建立方法, 对中继长链路上的量子信道建立速率进行分析, 得到在逐点方法和分段方法下所对应的量子信道建立速率. 最后, 利用逾渗模型, 对大规模纠缠态量子通信网络中任意两点间的量子信道建立速率进行分析, 推导出n个节点量子通信网络中, 量子信道建立速率为Ω (1/n).     

Direct measurement for general quantum states using parametric quantum circuits

In the field of quantum information,the acquisition of information for unknown quantum states is very important.When we only need to obtain specific elements of a state density matrix,the traditional quantum state tomography will become very complicated,because it requires a global quantum state reconstruction.Direct measurement of the quantum state allows us to obtain arbitrary specific matrix elements of the quantum state without state reconstruction,so direct measurement schemes have obtained...     

Purification in entanglement distribution with deep quantum neural network

Entanglement distribution is important in quantum communication. Since there is no information with value in this process, purification is a good choice to solve channel noise. In this paper, we simulate the purification circuit under true environment on Cirq, which is a noisy intermediate-scale quantum (NISQ) platform. Besides, we apply quantum neural network (QNN) to the state after purification. We find that combining purification and quantum neural network has good robustness towards quantum noise. After general purification, quantum neural network can improve fidelity significantly without consuming extra states. It also helps to obtain the advantage of entangled states with higher dimension under amplitude damping noise. Thus, the combination can bring further benefits to purification in entanglement distribution.     

A quantum network for implementation of the optimal quantum cloning

This paper presents a quantum network to implement the optimal 1→2 quantum cloning in 2 dimensions, including the optimal asymmetric universal, the optimal symmetric phase-covariant, and the asymmetric real state cloning. By only choosing different angles of the single-qubit rotations, the quantum network can implement three optimal quantum cloning.     

Noise-free frequency conversion of quantum states

In this paper,the frequency conversion of quantum states based on the intracavity nonlinear interaction is proposed.The fidelity of an input state after frequency conversion is calculated,and it is shown the noise-free frequency conversion of a quantum state can be achieved by injecting a strong signal field.The dependences of conversion efficiency on the pump parameter,extra losses and input state amplitude are also analysed.     

A full quantum network scheme

In this paper, we study the thermodynamic features of a two-dimensional charged black hole. Weinhold curvature and Ruppeiner curvature are explored as information geometry, respectively. Moreover, based on the Legendre invariant proposed by Hernando Quevedo, the geometro-thermodynamics behavior of this black hole is investigated.     

有限温度量子多体系统与热态张量网络

量子多体系统热力学性质的精确模拟在理论和实验方面都具有重要的价值。局域相互作用量子多体系统的热态满足互信息(mutual information)面积律,对于这样的系统,热态张量网络可以提供满足面积律的精确“波函数”拟设,提供了模拟有限温度系统的有力手段。文章介绍了关联格点模型在有限温度下的热态张量网络刻画及相关模拟方法。作者按照世界线热态张量网络和级数展开热态张量网络来分别介绍,并讨论了自由能极小变分原理与重正化群剪裁的优化原则。世界线框架内,人们发展了转移矩阵重正化群,基于纯化策略的有限温度密度矩阵重正化群,以及张量网络的线性重正化群等方法。在此基础上,介绍作者新近提出的级数展开热态张量网络方法,该方法受随机级数展开量子蒙特卡罗方法的启发,突破了世界线方法的局限,提高了有限温度计算重正化群模拟的精度标准,并且在计算阻挫量子自旋链模型时不会有负符号问题。此外,文章讨论了在两维格点系统上推广有限温度张量网络计算的进展和未来展望。     

量子纠缠态的普适远程克隆

提出一种任意两粒子纠缠态1→2普适远程克隆方案. 此方案仅需一个特殊的四粒子纠缠态作为量子信道, 就可使处于空间不同位置的两个接收者分别以5/6的保真度得到任意输入态的近似拷贝, 该保真度远高于已有方案中的保真度. 将方案推广到任意两粒子纠缠态1→N(N>2)普适远程克隆的情况, 可使处于不同地点的N个接收者分别以(2N+1)/(3N)的保真度得到输入态的近似拷贝. 另外, 提出一种以上述单个特殊四粒子纠缠态作为量子信道, 在多目标量子比特受控非门和 关键词： 量子纠缠态 普适远程克隆 保真度     

Preservation of quantum states via a super-Zeno effect on ensemble quantum computers

Following a recent proposal by Dhar et al (2006 Phys. Rev. Lett. 96 100405), we demonstrate experimentally the preservation of quantum states in a two-qubit system based on a super-Zeno effect using liquid-state nuclear magnetic resonance techniques. Using inverting radiofrequency pulses and delicately selecting time intervals between two pulses, we suppress the effect of decoherence of quantum states. We observe that preservation of the quantum state |11\rg with the super-Zeno effect is three times more efficient than the ordinary one with the standard Zeno effect.     

Controlled quantum state sharing of arbitrary two-qubit states with five-qubit cluster states

In this paper, we propose a controlled quantum state sharing scheme to share an arbitrary two-qubit state using a five-qubit cluster state and the Bell state measurement. In this scheme, the five-qubit cluster state is shared by a sender (Alice), a controller (Charlie), and a receiver (Bob), and the sender only needs to perform the Bell-state measurements on her particles during the quantum state sharing process, the controller performs a single-qubit projective measurement on his particles, then the receiver can reconstruct the arbitrary two-qubit state by performing some appropriate unitary transformations on his particles after he has known the measured results of the sender and the controller.     

Geometric global quantum discord of two-qubit states

We consider the geometric global quantum discord(GGQD) of two-qubit systems. By analyzing the symmetry of geometric global quantum discord we give an approach for deriving analytical formulae of the extremum problem which lies at the core of computing the GGQD for arbitrary two-qubit states. Furthermore, formulae of GGQD of arbitrary two-qubit states and some concrete examples are presented.     

Reconstructing unknown quantum states using variational layerwise method

In order to gain comprehensive knowledge of an arbitrary unknown quantum state, one feasible way is to reconstruct it, which can be realized by finding a series of quantum operations that can refactor the unitary evolution producing the unknown state. We design an adaptive framework that can reconstruct unknown quantum states at high fidelities, which utilizes SWAP test, parameterized quantum circuits (PQCs) and layerwise learning strategy. We conduct benchmarking on the framework using numerical simulations and reproduce states of up to six qubits at more than 96% overlaps with original states on average using PQCs trained by our framework, revealing its high applicability to quantum systems of different scales theoretically. Moreover, we perform experiments on a five-qubit IBM Quantum hardware to reconstruct random unknown single qubit states, illustrating the practical performance of our framework. For a certain reconstructing fidelity, our method can effectively construct a PQC of suitable length, avoiding barren plateaus of shadow circuits and overuse of quantum resources by deep circuits, which is of much significance when the scale of the target state is large and there is no a priori information on it. This advantage indicates that it can learn credible information of unknown states with limited quantum resources, giving a boost to quantum algorithms based on parameterized circuits on near-term quantum processors.     

多进制量子图态纠缠的确定

图态是可以与数学上的图对应起来的多组分纠缠态,图的顶点在此扮演多进制量子位而连线则表示两个多进制量子位之间的相互作用．图态在量子纠错码、多体量子计算和单向量子计算中起重要作用．本文系统研究多进制图态纠缠,使用迭代计算等方法计算了局域幺正变换和图同构下不等价的所有9点图以下的三进制图态的纠缠及一部分四进制和五进制图态的纠缠,纠缠测度可以是几何纠缠、相对熵纠缠和鲁棒性纠缠．我们对计算结果进行了分类,并分析了所得到的最近分离态．     

量子态的非相干光时域测量

提出了利用非相干光实现对量子态的时域高分辨率测量的新方法.以测量Autler-Townes效应为例,通过对信号解析表达式的详细讨论,阐述了利用这种方法测量量子态的物理过程,揭示出被测量的演化规律及测量过程对量子态的影响.研究结果表明利用这种方法既可以有效地、高分辨地测量很小的量子态能级间距和很大的原子能级间距,同时又可以测量相干量子态的横向弛豫速率.通过对时域和频域测量方法的比较表明利用非相干光对量子态的时域测量可以全面揭示量子态演化信息. 关键词： 非相干光 量子态测量 Autler-Townes 效应     

Efficient quantum secret sharing scheme with two-particle entangled states

This paper proposes a protocol for multi-party quantum secret sharing utilizing four non-orthogonal two-particle entangled states following some ideas in the schemes proposed by Liu et al. (2006 Chin. Phys. Lett. 23 3148) and Zhang et al. (2009 Chin. Phys. B 18 2149) respectively. The theoretical efficiency for qubits of the new protocol is improved from 50% to approaching 100%. All the entangled states can be used for generating the private key except those used for the eavesdropping check. The validity of a probable attack called opaque cheat attack to this kind of protocols is considered in the paper for the first time.     

Preparation of multi-photon Fock states and quantum entanglement properties in circuit QED

We demonstrate the controllable generation of multi-photon Fock states in circuit quantum electrodynamics （circuit QED）. The external bias flux regulated by a counter can effectively adjust the bias time on each superconducting flux qubit so that each flux qubit can pass in turn through the circuit cavity and thereby avoid the effect of decoherence. We further investigate the quantum correlation dynamics of coupling superconducting qubits in a Fock state. The results reveal that the lower the photon number of the light field in the number state, the stronger the interaction between qubits is, then the more beneficial to maintaining entanglement between qubits it will be.