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    SPECIAL TOPIC — Quantum computation and quantum simulation

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    A concise review of Rydberg atom based quantum computation and quantum simulation
    Xiaoling Wu(吴晓凌), Xinhui Liang(梁昕晖), Yaoqi Tian(田曜齐), Fan Yang(杨帆), Cheng Chen(陈丞), Yong-Chun Liu(刘永椿), Meng Khoon Tey(郑盟锟), and Li You(尤力)
    Chin. Phys. B, 2021, 30 (2): 020305.   DOI: 10.1088/1674-1056/abd76f
    Abstract749)   HTML25)    PDF (3178KB)(596)      
    Quantum information processing based on Rydberg atoms emerged as a promising direction two decades ago. Recent experimental and theoretical progresses have shined exciting light on this avenue. In this concise review, we will briefly introduce the basics of Rydberg atoms and their recent applications in associated areas of neutral atom quantum computation and simulation. We shall also include related discussions on quantum optics with Rydberg atomic ensembles, which are increasingly used to explore quantum computation and quantum simulation with photons.
    Low-temperature environments for quantum computation and quantum simulation
    Hailong Fu(付海龙), Pengjie Wang(王鹏捷), Zhenhai Hu(胡禛海), Yifan Li(李亦璠), and Xi Lin(林熙)
    Chin. Phys. B, 2021, 30 (2): 020702.   DOI: 10.1088/1674-1056/abd762
    Abstract383)   HTML13)    PDF (957KB)(213)      
    This review summarizes the requirement of low temperature conditions in existing experimental approaches to quantum computation and quantum simulation.
    Quantum computation and simulation with vibrational modes of trapped ions
    Wentao Chen(陈文涛), Jaren Gan, Jing-Ning Zhang(张静宁), Dzmitry Matuskevich, and Kihwan Kim(金奇奂)
    Chin. Phys. B, 2021, 30 (6): 060311.   DOI: 10.1088/1674-1056/ac01e3
    Abstract367)   HTML6)    PDF (5874KB)(164)      
    Vibrational degrees of freedom in trapped-ion systems have recently been gaining attention as a quantum resource, beyond the role as a mediator for entangling quantum operations on internal degrees of freedom, because of the large available Hilbert space. The vibrational modes can be represented as quantum harmonic oscillators and thus offer a Hilbert space with infinite dimensions. Here we review recent theoretical and experimental progress in the coherent manipulation of the vibrational modes, including bosonic encoding schemes in quantum information, reliable and efficient measurement techniques, and quantum operations that allow various quantum simulations and quantum computation algorithms. We describe experiments using the vibrational modes, including the preparation of non-classical states, molecular vibronic sampling, and applications in quantum thermodynamics. We finally discuss the potential prospects and challenges of trapped-ion vibrational-mode quantum information processing.
    Phase-sensitive Landau-Zener-Stückelberg interference in superconducting quantum circuit
    Zhi-Xuan Yang(杨智璇), Yi-Meng Zhang(张一萌), Yu-Xuan Zhou(周宇轩), Li-Bo Zhang(张礼博), Fei Yan(燕飞), Song Liu(刘松), Yuan Xu(徐源), and Jian Li(李剑)
    Chin. Phys. B, 2021, 30 (2): 024212.   DOI: 10.1088/1674-1056/abd753
    Abstract345)   HTML3)    PDF (1157KB)(137)      
    Superconducting circuit quantum electrodynamics (QED) architecture composed of superconducting qubit and resonator is a powerful platform for exploring quantum physics and quantum information processing. By employing techniques developed for superconducting quantum computing, we experimentally investigate phase-sensitive Landau-Zener-Stückelberg (LZS) interference phenomena in a circuit QED. Our experiments cover an extensive range of LZS transition parameters and demonstrate the LZS induced Rabi-like oscillation as well as phase-dependent steady-state population.
    Quantum computation and simulation with superconducting qubits
    Kaiyong He(何楷泳), Xiao Geng(耿霄), Rutian Huang(黄汝田), Jianshe Liu(刘建设), and Wei Chen(陈炜)
    Chin. Phys. B, 2021, 30 (8): 080304.   DOI: 10.1088/1674-1056/ac16cf
    Abstract338)   HTML8)    PDF (3866KB)(238)      
    Superconducting circuits based on Josephson junctions are regarded as one of the most promising technologies for the implementation of scalable quantum computers. This review presents the basic principles of superconducting qubits and shows the progress of quantum computing and quantum simulation based on superconducting qubits in recent years. The experimental realization of gate operations, readout, error correction codes, as well as some quantum algorithms are summarized, followed by an introduction of quantum simulation. And then some important applications in fields including condensed matter physics, quantum annealing, and quantum chemistry are discussed.
    Realization of adiabatic and diabatic CZ gates in superconducting qubits coupled with a tunable coupler
    Huikai Xu(徐晖凯), Weiyang Liu(刘伟洋), Zhiyuan Li(李志远), Jiaxiu Han(韩佳秀), Jingning Zhang(张静宁), Kehuan Linghu(令狐克寰), Yongchao Li(李永超), Mo Chen(陈墨), Zhen Yang(杨真), Junhua Wang(王骏华), Teng Ma(马腾), Guangming Xue(薛光明), Yirong Jin(金贻荣), and Haifeng Yu(于海峰)
    Chin. Phys. B, 2021, 30 (4): 044212.   DOI: 10.1088/1674-1056/abf03a
    Abstract302)   HTML0)    PDF (1576KB)(128)      
    High fidelity two-qubit gates are fundamental for scaling up the superconducting qubit number. We use two qubits coupled via a frequency-tunable coupler which can adjust the coupling strength, and demonstrate the CZ gate using two different schemes, adiabatic and diabatic methods. The Clifford based randomized benchmarking (RB) method is used to assess and optimize the CZ gate fidelity. The fidelities of adiabatic and diabatic CZ gates are 99.53(8)% and 98.72(2)%, respectively. We also analyze the errors induced by the decoherence. Comparing to 30 ns duration time of adiabatic CZ gate, the duration time of diabatic CZ gate is 19 ns, revealing lower incoherence error rate $r'_{incoherent, int} = 0.0197(5)$ compared to $r_{incoherent, int} = 0.0223(3)$.
    Review of quantum simulation based on Rydberg many-body system
    Zheng-Yuan Zhang(张正源), Dong-Sheng Ding(丁冬生), and Bao-Sen Shi(史保森)
    Chin. Phys. B, 2021, 30 (2): 020307.   DOI: 10.1088/1674-1056/abd744
    Abstract296)   HTML8)    PDF (3357KB)(218)      
    Quantum simulation has been developed extensively over the past decades, widely applied to different models to explore dynamics in the quantum regime. Rydberg atoms have strong dipole-dipole interactions and interact with each other over a long distance, which makes it straightforward to build many-body interacting quantum systems to simulate specific models. Additionally, neutral atoms are easily manipulated due to their weak interactions. These advantages make Rydberg many-body system an ideal platform to implement quantum simulations. This paper reviews several quantum simulations for different models based on Rydberg many-body systems, including quantum Ising models in one dimension and two dimensions mainly for quantum magnetism, XY model for excitation transport, SSH model for symmetry-protected topological phases, and critical self-organized behaviors in many-body systems. Besides, some challenges and promising directions of quantum simulations based on Rydberg many-body system are discussed in this paper.
    Selected topics of quantum computing for nuclear physics
    Dan-Bo Zhang(张旦波), Hongxi Xing(邢宏喜), Hui Yan(颜辉), Enke Wang(王恩科), and Shi-Liang Zhu(朱诗亮)
    Chin. Phys. B, 2021, 30 (2): 020306.   DOI: 10.1088/1674-1056/abd761
    Abstract285)   HTML6)    PDF (915KB)(173)      
    Nuclear physics, whose underling theory is described by quantum gauge field coupled with matter, is fundamentally important and yet is formidably challenge for simulation with classical computers. Quantum computing provides a perhaps transformative approach for studying and understanding nuclear physics. With rapid scaling-up of quantum processors as well as advances on quantum algorithms, the digital quantum simulation approach for simulating quantum gauge fields and nuclear physics has gained lots of attention. In this review, we aim to summarize recent efforts on solving nuclear physics with quantum computers. We first discuss a formulation of nuclear physics in the language of quantum computing. In particular, we review how quantum gauge fields (both Abelian and non-Abelian) and their coupling to matter field can be mapped and studied on a quantum computer. We then introduce related quantum algorithms for solving static properties and real-time evolution for quantum systems, and show their applications for a broad range of problems in nuclear physics, including simulation of lattice gauge field, solving nucleon and nuclear structures, quantum advantage for simulating scattering in quantum field theory, non-equilibrium dynamics, and so on. Finally, a short outlook on future work is given.
    Fabrication and characterization of all-Nb lumped-element Josephson parametric amplifiers
    Hang Xue(薛航), Zhirong Lin(林志荣), Wenbing Jiang(江文兵), Zhengqi Niu(牛铮琦), Kuang Liu(刘匡), Wei Peng(彭炜), and Zhen Wang(王镇)
    Chin. Phys. B, 2021, 30 (6): 068503.   DOI: 10.1088/1674-1056/abeee6
    Abstract282)   HTML0)    PDF (606KB)(112)      
    Josephson parametric amplifiers (JPAs) with nearly quantum-limited noise performance have become indispensable devices for the measurements of superconducting quantum information. We have developed an all-Nb lumped-element flux-driven JPA operating in the three-wave mixing mode. Our Nb-based JPA comprises Nb/Al-AlOx/Nb Josephson junctions, a parallel-plate capacitor with SiO2 dielectric sandwiched between two Nb layers, a bottom coplanar waveguides layer, and a top Nb wiring layer. We experimentally demonstrate a 20 dB gain over a 190 MHz bandwidth, a mean 1 dB compression of -123 dBm, and near quantum-limited noise performance. This fabrication process can be further used to design impedance transformed parametric amplifiers for multiple-qubit readout.
    Quantum dynamics on a lossy non-Hermitian lattice
    Li Wang(王利), Qing Liu(刘青), and Yunbo Zhang(张云波)
    Chin. Phys. B, 2021, 30 (2): 020506.   DOI: 10.1088/1674-1056/abd765
    Abstract280)   HTML2)    PDF (3294KB)(178)      
    We investigate quantum dynamics of a quantum walker on a finite bipartite non-Hermitian lattice, in which the particle can leak out with certain rate whenever it visits one of the two sublattices. Quantum walker initially located on one of the non-leaky sites will finally totally disappear after a length of evolution time and the distribution of decay probability on each unit cell is obtained. In one regime, the resultant distribution shows an expected decreasing behavior as the distance from the initial site increases. However, in the other regime, we find that the resultant distribution of local decay probability is very counterintuitive, in which a relatively high population of decay probability appears on the edge unit cell which is the farthest from the starting point of the quantum walker. We then analyze the energy spectrum of the non-Hermitian lattice with pure loss, and find that the intriguing behavior of the resultant decay probability distribution is intimately related to the existence and specific property of the edge states, which are topologically protected and can be well predicted by the non-Bloch winding number. The exotic dynamics may be observed experimentally with arrays of coupled resonator optical waveguides.
    Quantum algorithm for a set of quantum 2SAT problems
    Yanglin Hu(胡杨林), Zhelun Zhang(张哲伦), and Biao Wu(吴飙)
    Chin. Phys. B, 2021, 30 (2): 020308.   DOI: 10.1088/1674-1056/abd741
    Abstract270)   HTML4)    PDF (543KB)(107)      
    We present a quantum adiabatic algorithm for a set of quantum 2-satisfiability (Q2SAT) problem, which is a generalization of 2-satisfiability (2SAT) problem. For a Q2SAT problem, we construct the Hamiltonian which is similar to that of a Heisenberg chain. All the solutions of the given Q2SAT problem span the subspace of the degenerate ground states. The Hamiltonian is adiabatically evolved so that the system stays in the degenerate subspace. Our numerical results suggest that the time complexity of our algorithm is O(n3.9) for yielding non-trivial solutions for problems with the number of clauses m=dn(n-1)/2 (\(d\lesssim 0.1\)). We discuss the advantages of our algorithm over the known quantum and classical algorithms.
    A two-dimensional quantum walk driven by a single two-side coin
    Quan Lin(林泉), Hao Qin(秦豪) Kun-Kun Wang(王坤坤), Lei Xiao(肖磊), and Peng Xue(薛鹏)
    Chin. Phys. B, 2020, 29 (11): 110303.   DOI: 10.1088/1674-1056/abaee8
    Abstract255)   HTML    PDF (823KB)(157)      

    We study a two-dimensional quantum walk with only one walker alternatively walking along the horizontal and vertical directions driven by a single two-side coin. We find the analytical expressions of the first two moments of the walker’s position distribution in the long-time limit, which indicates that the variance of the position distribution grows quadratically with walking steps, showing a ballistic spreading typically for quantum walks. Besides, we analyze the correlation by calculating the quantum mutual information and the measurement-induced disturbance respectively as the outcome of the walk in one dimension is correlated to the other with the coin as a bridge. It is shown that the quantum correlation between walker spaces increases gradually with the walking steps.

    Nonlocal advantage of quantum coherence in a dephasing channel with memory
    Ming-Liang Hu(胡明亮), Yu-Han Zhang(张宇晗), and Heng Fan(范桁)
    Chin. Phys. B, 2021, 30 (3): 030308.   DOI: 10.1088/1674-1056/abcf4a
    Abstract244)   HTML1)    PDF (587KB)(152)      
    We investigate nonlocal advantage of quantum coherence (NAQC) in a correlated dephasing channel modeled by the multimode bosonic reservoir. We obtain analytically the dephasing and memory factors of this channel for the reservoir having a Lorentzian spectral density, and analyze how they affect the NAQC defined by the l1 norm and relative entropy. It is shown that the memory effects of this channel on NAQC are state-dependent, and they suppress noticeably the rapid decay of NAQC for the family of input Bell-like states with one excitation. For the given transmission time of each qubit, we also obtain the regions of the dephasing and memory factors during which there is NAQC in the output states.
    Realization of arbitrary two-qubit quantum gates based on chiral Majorana fermions
    Qing Yan(闫青) and Qing-Feng Sun(孙庆丰)
    Chin. Phys. B, 2021, 30 (4): 040303.   DOI: 10.1088/1674-1056/abe296
    Abstract238)   HTML3)    PDF (815KB)(156)      
    Quantum computers are in hot-spot with the potential to handle more complex problems than classical computers can. Realizing the quantum computation requires the universal quantum gate set {T, H, CNOT} so as to perform any unitary transformation with arbitrary accuracy. Here we first briefly review the Majorana fermions and then propose the realization of arbitrary two-qubit quantum gates based on chiral Majorana fermions. Elementary cells consist of a quantum anomalous Hall insulator surrounded by a topological superconductor with electric gates and quantum-dot structures, which enable the braiding operation and the partial exchange operation. After defining a qubit by four chiral Majorana fermions, the single-qubit T and H quantum gates are realized via one partial exchange operation and three braiding operations, respectively. The entangled CNOT quantum gate is performed by braiding six chiral Majorana fermions. Besides, we design a powerful device with which arbitrary two-qubit quantum gates can be realized and take the quantum Fourier transform as an example to show that several quantum operations can be performed with this space-limited device. Thus, our proposal could inspire further utilization of mobile chiral Majorana edge states for faster quantum computation.
    Quench dynamics in 1D model with 3rd-nearest-neighbor hoppings
    Shuai Yue(岳帅), Xiang-Fa Zhou(周祥发), and Zheng-Wei Zhou(周正威)
    Chin. Phys. B, 2021, 30 (2): 026402.   DOI: 10.1088/1674-1056/abd742
    Abstract230)   HTML4)    PDF (1731KB)(96)      
    The non-equilibrium dynamics of a one-dimensional (1D) topological system with 3rd-nearest-neighbor hopping has been investigated by analytical and numerical methods. An analytical form of topological defect density under the periodic boundary conditions (PBC) is obtained by using the Landau-Zener formula (LZF), which is consistent with the scaling of defect production provided by the Kibble-Zurek mechanism (KZM). Under the open boundary conditions (OBC), quench dynamics becomes more complicated due to edge states. The behaviors of the system quenching across different phases show that defect production no longer satisfies the KZM paradigm since complicated couplings exist under OBC. Some new dynamical features are revealed.
    An easily-prepared impedance matched Josephson parametric amplifier
    Ya-Peng Lu(卢亚鹏), Quan Zuo(左权), Jia-Zheng Pan(潘佳政), Jun-Liang Jiang(江俊良), Xing-Yu Wei(魏兴雨), Zi-Shuo Li(李子硕), Wen-Qu Xu(许问渠), Kai-Xuan Zhang(张凯旋), Ting-Ting Guo(郭婷婷), Shuo Wang(王硕), Chun-Hai Cao(曹春海), Wei-Wei Xu(许伟伟), Guo-Zhu Sun(孙国柱), and Pei-Heng Wu(吴培亨)
    Chin. Phys. B, 2021, 30 (6): 068504.   DOI: 10.1088/1674-1056/ac0420
    Abstract228)   HTML5)    PDF (1410KB)(229)      
    An impedance matched parametric amplifier (IMPA) with Josephson junctions is fabricated and characterized. A hybrid structure containing coplanar and strip structures is implemented to realize an impedance taper line and a plate capacitor in an LC nonlinear resonator based on Josephson junctions. The upper plate of the capacitor is isolated with SiNx without grounding as well as the strips. Such easily-prepared designs greatly reduce the requirements for lithography alignment and precision, which makes the fabrication process more reliable. The experimental results show that in such IMPA a gain higher than 25 dB with a bandwidth of about 100 MHz can be obtained. This broadband amplifier operates close to the quantum limit. By adjusting the working point, a higher bandwidth of about 400 MHz can be obtained with a gain of about 17 dB.
    Quantum computation and error correction based on continuous variable cluster states
    Shuhong Hao(郝树宏), Xiaowei Deng(邓晓玮), Yang Liu(刘阳), Xiaolong Su(苏晓龙), Changde Xie(谢常德), and Kunchi Peng(彭堃墀)
    Chin. Phys. B, 2021, 30 (6): 060312.   DOI: 10.1088/1674-1056/abeb0a
    Abstract223)   HTML3)    PDF (1213KB)(154)      
    Measurement-based quantum computation with continuous variables, which realizes computation by performing measurement and feedforward of measurement results on a large scale Gaussian cluster state, provides a feasible way to implement quantum computation. Quantum error correction is an essential procedure to protect quantum information in quantum computation and quantum communication. In this review, we briefly introduce the progress of measurement-based quantum computation and quantum error correction with continuous variables based on Gaussian cluster states. We also discuss the challenges in the fault-tolerant measurement-based quantum computation with continuous variables.
    Quantum walk under coherence non-generating channels
    Zishi Chen(陈子石) and Xueyuan Hu(胡雪元)
    Chin. Phys. B, 2021, 30 (3): 030305.   DOI: 10.1088/1674-1056/abd74d
    Abstract213)   HTML3)    PDF (645KB)(133)      
    We investigate the probability distribution of the quantum walk under coherence non-generating channels. We define a model called generalized classical walk with memory. Under certain conditions, generalized classical random walk with memory can degrade into classical random walk and classical random walk with memory. Based on its various spreading speed, the model may be a useful tool for building algorithms. Furthermore, the model may be useful for measuring the quantumness of quantum walk. The probability distributions of quantum walks are generalized classical random walks with memory under a class of coherence non-generating channels. Therefore, we can simulate classical random walk and classical random walk with memory by coherence non-generating channels. Also, we find that for another class of coherence non-generating channels, the probability distributions are influenced by the coherence in the initial state of the coin. Nevertheless, the influence degrades as the number of steps increases. Our results could be helpful to explore the relationship between coherence and quantum walk.
    Quantum simulations with nuclear magnetic resonance system
    Chudan Qiu(邱楚丹), Xinfang Nie(聂新芳), and Dawei Lu(鲁大为)
    Chin. Phys. B, 2021, 30 (4): 048201.   DOI: 10.1088/1674-1056/abe299
    Abstract208)   HTML1)    PDF (2301KB)(118)      
    Thanks to the quantum simulation, more and more problems in quantum mechanics which were previously inaccessible are now open to us. Capitalizing on the state-of-the-art techniques on quantum coherent control developed in past few decades, e.g., the high-precision quantum gate manipulating, the time-reversal harnessing, the high-fidelity state preparation and tomography, the nuclear magnetic resonance (NMR) system offers a unique platform for quantum simulation of many-body physics and high-energy physics. Here, we review the recent experimental progress and discuss the prospects for quantum simulation realized on NMR systems.
    Fabrication of microresonators by using photoresist developer as etchant
    Shu-Qing Song(宋树清), Jian-Wen Xu(徐建文), Zhi-Kun Han(韩志坤), Xiao-Pei Yang(杨晓沛), Yu-Ting Sun(孙宇霆), Xiao-Han Wang(王晓晗), Shao-Xiong Li(李邵雄), Dong Lan(兰栋), Jie Zhao(赵杰), Xin-Sheng Tan(谭新生), and Yang Yu(于扬)
    Chin. Phys. B, 2021, 30 (6): 060313.   DOI: 10.1088/1674-1056/abf112
    Abstract207)   HTML1)    PDF (1338KB)(126)      
    In superconducting circuit, microwave resonators and capacitors are crucial components, and their quality has a strong impact on circuit performance. Here we develop a novel wet etching process to define these two components using common photoresist developer as etchant. This method reduces subsequent steps and can be completed immediately after development. By measuring the internal quality factor of resonators, we show that it is possible to achieve similar or better performance when compared with samples made by standard etching processes. This easy-to-implement method may boost the yield hence providing an alternative fabrication process for microwave resonators and capacitors.