Optical parity gate and a wide range of entangled states generation

Generating entangled states efficiently is a hot topic in the area of quantum information science.With the approach presented in this paper,a general parity gate could be realized and a wide range of entangled states,including GHZ state,W state,Dicke state,arbitrary graph state and locally maximally entanglable states,can be generated flexibly.The generation of GHZ state,W state,and Dicke state is probabilistic but heralded and the total success probability is unit.In addition,the arbitrary graph state and locally maximally entanglable states generation is deterministic,flexible,and highly efficient.Especially,with the"simultaneous"generation pattern,the complexity of the graph state generation and locally maximally entanglable states generation could be reduced greatly,providing a more efficient and feasible way to generate the entangled states.     

Preparation of Entanglement of Atoms and of Photons via Cavity Input-Output Process

We propose a method to prepare multipartite entangled states such as cluster states and graph states based on the cavity input-output process and single photon measurement. Two quantum gates, a controlled phase gate and a fusion gate between two atoms trapped in respective cavities, are proposed to prepare atomic cluster states and graph states with one and two dimensions. We also introduce a scheme that can generate an arbitrary multipartite photon duster state which uses two coherent states as a qubit basis.     

Efficient implementation of a nonlocal gate with nonmaximal entanglement

We present two schemes for efficient implementation of a nonlocal gate with nonmaximal entanglement. The main strategy of the schemes is local conversion of pure states, which consists of a generalized measurement described by a positive operator-valued measure (POVM), one-way classical communication, and corresponding unitary operations. First, we discuss the way to generate determinately the nonlocal gate via any pure shared entangled state combined with entanglement-assistance. Then we propose the other way to generate probabilistically the nonlocal gate via any pure entangled state with the aid of ancillary particles.     

Efficient Scheme for One-Way Quantum Computing with Radiofrequency SQUID Qubits byAdiabatic Passage

We proposed an efficient scheme for constructing a quantum controlled phase-shift gate and generating the cluster states with rf superconducting quantum interference devices (SQUIDs) coupled to a microwave cavity through adiabatic evolution of dark eigenstates. During the operation, the spontaneous emission is suppressed since the rf SQUIDs are always in the three lowest flux states. Considering the influence from the cavity decay with achievable
experimental parameters, we numerically analyze the success probability and the fidelity for generating the two-SQUID maximally entangled state and the controlled phase-shift gate by adiabatic passage.     

Efficient scheme for realizing a multiplex-controlled phase gate with photonic qubits in circuit quantum electrodynamics

We propose an efficient scheme to implement a multiplex-controlled phase gate with multiple photonic qubits simultaneously controlling one target photonic qubit based on circuit quantum electrodynamics (QED). For convenience, we denote this multiqubit gate as MCP gate. The gate is realized by using a two-level coupler to couple multiple cavities. The coupler here is a superconducting qubit. This scheme is simple because the gate implementation requires only one step of operation. In addition, this scheme is quite general because the two logic states of each photonic qubit can be encoded with a vacuum state and an arbitrary non-vacuum state |φ> (e.g., a Fock state, a superposition of Fock states, a cat state, or a coherent state, etc.) which is orthogonal or quasi-orthogonal to the vacuum state. The scheme has some additional advantages: because only two levels of the coupler are used, i.e., no auxiliary levels are utilized, decoherence from higher energy levels of the coupler is avoided; the gate operation time does not depend on the number of qubits; and the gate is implemented deterministically because no measurement is applied. As an example, we numerically analyze the circuit-QED based experimental feasibility of implementing a three-qubit MCP gate with photonic qubits each encoded via a vacuum state and a cat state. The scheme can be applied to accomplish the same task in a wide range of physical system, which consists of multiple microwave or optical cavities coupled to a two-level coupler such as a natural or artificial atom.     

An Efficient Scheme for Multiparty Multi-particle State Sharing

We present an efficient scheme for sharing an arbitrary m-qubitstate with n agents. In our scheme, the sender Alice first shares mBell states with the agent Bob, who is designated to recover the originalm-qubit state. Furthermore, Alice introduces n-1 auxiliary particlesin the initial state |0>, applies Hadamard (H) gate and Controlled-Not (CNOT) gate operations on the particles, which make them entangled with one of m particle pairs in Bell states, and then sends them to the controllers (i.e., other n-1 agents), where each controller only holds one particle in hand. After Alice performing m Bell-basis measurements and each controller a single-particle measurement, the recover Bob can obtain the original unknown quantum state by applying the corresponding local unitary operations on his particles.Its intrinsic efficiency for qubits approaches 100%, and the total efficiency really approaches the maximal value.     

Deterministic linear optics quantum computation with single photon qubits

We suggest an efficient scheme for quantum computation with linear optical elements, where the qubits are encoded in single photon states. The scheme reduces the resources required per logical gate by several orders of magnitude, compared to an earlier proposal of Knill, Laflamme, and Milburn, while the resource overhead per gate is independent of the length of the computation. A central feature of the scheme, enabling these improvements, is the prior construction of a "linked" photon state designed according to the particular quantum circuit one wishes to process. Once this state has been successfully prepared, the computation is pursued deterministically by a sequence of teleportation steps.     

Efficient Symmetric Five-Party Quantum State Sharing of an Arbitrary <Emphasis Type="Italic">m</Emphasis>-Qubit State

We present an efficient symmetric scheme for five-party quantum state sharing of an arbitrary m-qubit state with 2m three-particle entangled states. The implementations of this scheme only need to exploit the CNOT gate operations and the single-particle measurements, instead of the three-particle GHZ-state measurements, which makes it more convenient in a practical application than some previous schemes. In addition, its total efficiency can approach the maximal value in theory.     

Bell inequalities for graph states

We investigate the nonlocal properties of graph states. To this aim, we derive a family of Bell inequalities which require three measurement settings for each party and are maximally violated by graph states. In turn, for each graph state there is an inequality maximally violated only by that state. We show that for certain types of graph states the violation of these inequalities increases exponentially with the number of qubits. We also discuss connections to other entanglement properties such as the positivity of the partial transpose or the geometric measure of entanglement.     

Deterministic nondestructive state analysis for polarization-spatial-time-bin hyperentanglement with cross-Kerr nonlinearity

We present a deterministic nondestructive hyperentangled Bell state analysis protocol for photons entangled in three degrees of freedom(DOFs),including polarization,spatial-mode,and time-bin DOFs.The polarization Bell state analyzer and spatial-mode Bell state analyzer are constructed by polarization parity-check quantum nondemolition detector(P-QND)and spatial-mode parity-check quantum nondemolition detector(S-QND)using cross-Kerr nonlinearity,respectively.The time-bin Bell state analyzer is constructed by the swap gate for polarization state and time-bin state of a photon(P-T swap gate)and P-QND.The Bell states analyzer for one DOF will not destruct the Bell states of other two DOFs,so the polarization-spatial-time-bin hyperentangled Bell states can be determinately distinguished without destruction.This deterministic nondestructive state analysis method has useful applications in quantum information protocols.     

Consequent entanglement concentration of a less-entangled electronic cluster state with controlled-not gates

We present a highly efficient entanglement concentration protocol （ECP） for a four-electron system in a less-entangled cluster state. In this ECP, we only require one pair of less-entangled electron cluster states and one ancillary electron to complete the task. With the help of the controlled-not （CNOT） gate, the concentrated maximally entangled state can be retained for further application with some success probability. On the other hand, the discarded items can be reused to obtain a high success probability. All the features make this ECP useful in the current quantum information field.     

Demonstration of a controlled-phase gate for continuous-variable one-way quantum computation

We experimentally demonstrate a controlled-phase gate for continuous variables using a cluster-state resource of four optical modes. The two independent input states of the gate are coupled with the cluster in a teleportation-based fashion. As a result, one of the entanglement links present in the initial cluster state appears in the two unmeasured output modes as the corresponding entangling gate acting on the input states. The genuine quantum character of this gate becomes manifest and is verified through the presence of entanglement at the output for a product two-mode coherent input state. By combining our gate with the recently reported module for single-mode Gaussian operations [R. Ukai et al., Phys. Rev. Lett. 106, 240504 (2011)], it is possible to implement any multimode Gaussian operation as a fully measurement-based one-way quantum computation.     

Scheme for Generating Cluster States with Charge Qubits in a Cavity

Based on superconducting charge qubits （SCCQs） coupled to a single-mode microwave cavity, we propose a scheme for generating charge cluster states. For all SCCQs, the controlled gate voltages are all in their degeneracy points, the quantum information is encoded in two logic states of charge basis. The generation of the multi-qubit cluster state can be achieved step by step on a pair of nearest-neighbor qubits. Considering effective long-rang coupling, we provide an efficient way to one-step generating of a highly entangled cluster state, in which the qubit-qubit coupling is mediated by the cavity mode. Our quantum operations are insensitive to the initial state of the cavity mode by removing the influence of the cavity mode via the periodical evolution of the system. Thus, our operation may be against the decoherence from the cavity.     

Unifying variational methods for simulating quantum many-body systems

We introduce a unified formulation of variational methods for simulating ground state properties of quantum many-body systems. The key feature is a novel variational method over quantum circuits via infinitesimal unitary transformations, inspired by flow equation methods. Variational classes are represented as efficiently contractible unitary networks, including the matrix-product states of density matrix renormalization, multiscale entanglement renormalization (MERA) states, weighted graph states, and quantum cellular automata. In particular, this provides a tool for varying over classes of states, such as MERA, for which so far no efficient way of variation has been known. The scheme is flexible when it comes to hybridizing methods or formulating new ones. We demonstrate the functioning by numerical implementations of MERA, matrix-product states, and a new variational set on benchmarks.     

Active one-way quantum computation with two-photon four-qubit cluster states

By using 2-photon 4-qubit cluster states we demonstrate deterministic one-way quantum computation in a single qubit rotation algorithm. In this operation feed-forward measurements are automatically implemented by properly choosing the measurement basis of the qubits, while Pauli error corrections are realized by using two fast driven Pockels cells. We realized also a C-NOT gate for equatorial qubits and a C-PHASE gate for a generic target qubit. Our results demonstrate that 2-photon cluster states can be used for rapid and efficient deterministic one-way quantum computing.     

Scalable generation of graph-state entanglement through realistic linear optics

We propose a scheme for efficient construction of graph states using realistic linear optics, imperfect photon source, and single-photon detectors. For any many-body entanglement represented by tree-graph states, we prove that the overall preparation and detection efficiency scales nearly polynomially with the size of the graph, no matter how small the efficiencies for the photon source and the detectors.     

Unconventional Geometric Phase-Shift Gates Based on Superconducting Quantum Interference Devices Coupled to a Single-Mode Cavity

We present a scheme to realize geometric phase-shift gate for two superconducting quantum interference device （SQUID） qubits coupled to a single-mode microwave field. The geometric phase-shift gate operation is performed in two lower flux states, and the excited state [2〉 would not participate in the procedure. The SQUIDs undergo no transitions during the gate operation. Thus, the docoherence due to energy spontaneous emission based on the levels of SQUIDs are suppressed. The gate is insensitive to the cavity decay throughout the operation since the cavity mode is displaced along a circle in the phase space, acquiring a phase conditional upon the two lower flux states of the SQUID qubits, and the cavity mode is still in the original vacuum state. Based on the SQUID qubits interacting with the cavity mode, our proposed approach may open promising prospects for quantum iogic in SQUID-system.     

Dynamics of genuine multipartite entanglement under local non-Markovian dephasing

We study dynamics of genuine entanglement for quantum states of three and four qubits under non-Markovian dephasing. Using a computable entanglement monotone for multipartite systems, we find that the GHZ state is quite resilient state whereas the W state is the most fragile. We compare dynamics of chosen quantum states with dynamics of random pure states and weighted graph states.     

Implementation of Quantum Algorithms via Fast Three-Rydberg-Atom CCZ Gates

Multiqubit CCZ gates form one of the building blocks of quantum algorithms and have been involved in achieving many theoretical and experimental triumphs. Designing a simple and efficient multiqubit gate for quantum algorithms is still by no means trivial as the number of qubits increases. Here, by virtue of the Rydberg blockade effect, we propose a scheme to rapidly implement a three-Rydberg-atom CCZ gate via a single Rydberg pulse, and successfully apply the gate to realize the three-qubit refined Deutsch–Jozsa algorithm and three-qubit Grover search. The logical states of the three-qubit gate are encoded to the same ground states to avoid an adverse effect of the atomic spontaneous emission. Furthermore, there is no requirement for individual addressing of atoms in our protocol.     

利用非稳定子态容错实现密集旋转操作

Non-Clifford操作不能在量子纠错码上自然横向实现, 但可通过辅助量子态和在量子纠错码上能横向实现的Clifford操作来容错实现, 从而取得容错量子计算的通用性. 非平庸的单量子比特操作是Non-Clifford操作, 可以分解为绕z轴和绕x轴非平庸旋转操作的组合. 本文首先介绍了利用非稳定子态容错实现绕z轴和绕x轴旋转的操作, 进而设计线路利用魔幻态容错制备非稳定子态集, 最后讨论了运用制备的非稳定子态集模拟任意非平庸单量子比特操作的问题. 与之前工作相比, 制备非稳定子态的线路得到简化, 成功概率提高, 且在高精度模拟任意单量子比特操作时所消耗的非稳定子态数目减少了50%. 关键词： 容错量子计算 非稳定子态 魔幻态 Clifford操作