Remote preparation of an arbitrary multi-qubit state via two-qubit entangled states

We propose a novel scheme for remote preparation of an arbitrary n-qubit state with the aid of an appropriate local \(2^n\times 2^n\) unitary operation and n maximally entangled two-qubit states. The analytical expression of local unitary operation, which is constructed in the form of iterative process, is presented for the preparation of n-qubit state in detail. We obtain the total successful probabilities of the scheme in the general and special cases, respectively. The feasibility of our scheme in preparing remotely multi-qubit states is explicitly demonstrated by theoretical studies and concrete examples, and our results show that the novel proposal could enlarge the applied range of remote state preparation.     

An efficient scheme for multi-party quantum state sharing of an arbitrary multi-qubit state with one GHZ channel

We present an innovative and extremely efficient scheme to share an arbitrary multi-qubit state between n agents with only 1 GHZ channel under control of m agents in a network. Compared with existing ones in this literature, our scheme requires less communication resources, least qubits and only three physical favorable simple operations (single-qubit measurement, Bell-basis measurement and CNOT gate operations) are included, leading to a higher overall efficiency.     

Deterministic remote preparation of arbitrary multi-qubit equatorial states via two-qubit entangled states

We propose an efficient scheme for remotely preparing an arbitrary n-qubit equatorial state via n two-qubit maximally entangled states. Compared to the former scheme (Wei et al. in Quantum Inf Process 16:260, 2017) that has the 50% successful probability when the amplitude factors of prepared states are \(2^{-n{/}2}\), the probability would be increased to 100% by using of our modified proposal. The feasibility of our scheme for remote preparation arbitrary multi-qubit equatorial states is explicitly demonstrated by theoretical studies and concrete examples.     

An efficient scheme for five-party quantum state sharing of an arbitrary m-qubit state using multiqubit cluster states

We present an efficient scheme for five-party quantum state sharing (QSTS) of an arbitrary m-qubit state with multiqubit cluster states. Unlike the three-partite QSTS schemes using the same quantum channel [Phys. Rev. A 78, 062333 (2008)], our scheme for sharing of quantum information among five parties utilizing a cluster state as an entangled resource. It is found that the six-partite cluster state can be used for QSTS of an entangled state, the five-partite cluster state can be used for QSTS of an arbitrary two-qubit state and also can be used for QSTS of an arbitrary m-qubit state. It involves two-qubit Bell-basis or three-qubit GHZ-basis measurements, not multipartite joint measurements, which makes it more convenient than some previous schemes. In addition, the total efficiency really approaches the maximal value.     

Multi-party quantum state sharing of an arbitrary multi-qubit state via \chi -type entangled states

By using the \(\chi \) -type entangled states, a novel scheme for multi-party quantum state sharing (MQSTS) of an arbitrary multi-qubit state is investigated. It is shown that the MQSTS scheme can be faithfully realized by performing appropriate Bell state measurements, Z basis measurements and local unitary operations, rather than multi-qubit entanglement or multi-particle joint measurements. Thus, our MQSTS scheme is more convenient in a practical application than some previous schemes. Furthermore, its intrinsic efficiency for qubits approaches 100 %, and the total efficiency really approaches the maximal value, which is higher than those of the previous MQSTS schemes. Finally, we analyze the security from the views of participant attack and outside attack in detail.     

Nondestructive Greenberger-Horne-Zeilinger-state analyzer

We propose a method to construct a nondestructive n-qubit Greenberger– Horne–Zeilinger (GHZ)-state analyzer. The method is applied to any systems in which two-qubit parity gates, controlled-phase gates, or controlled-NOT gates can be realized. We also present a simplified two-photon parity gate with which a nondestructive n-photon GHZ-state analyzer could be largely simplified. The nondestructive GHZ-state analyzer is expected to find useful applications for economical quantum-information processing.     

Optimal remote preparation of arbitrary multi-qubit real-parameter states via two-qubit entangled states

In this paper, we present an efficient scheme for remote state preparation of arbitrary n-qubit states with real coefficients. Quantum channel is composed of n maximally two-qubit entangled states, and several appropriate mutually orthogonal bases including the real parameters of prepared states are delicately constructed without the introduction of auxiliary particles. It is noted that the successful probability is 100% by using our proposal under the condition that the parameters of prepared states are all real. Compared to general states, the probability of our protocol is improved at the cost of the information reduction in the transmitted state.     

Classical communication cost and probabilistic remote two-qubit state preparation via POVM and W-type states

I present a new scheme for probabilistic remote preparation of a general two-qubit state by using two W-type states as the shared quantum channel and a proper POVM instead of the usual positive measurement. Also I explore the scheme??s applications to five special ensembles of two-qubit states. The success probability and the classical communication cost in different cases are calculated minutely, respectively, which show that the remote two-qubit preparation can be realized with higher probability after consuming some more classical bits provided that the two-qubit state to be prepared is chosen from the special ensembles.     

多量子位Grover量子搜索算法的NMR仿真实现

核磁共振（NMR）技术目前是能有效实现量子计算的物理体系之一。多量子算符代数理论可以将幺正变换分解为一系列有限的单量子门和对角双量子门的组合。本文以核磁共振和多量子算符代数理论为基础,提出了实现多量子位Grover量子搜索算法的核磁共振脉冲序列设计方法,并在量子计算仿真程序上进行了3量子位的Grover量子搜索算法的实验验证。     

On the role of the four-qubit state in two-qubit gate teleportation

The full analysis of quantum protocols requires the knowledge of the role of quantum states, bases of measurement and quantum gates involved. In what concerns the famous two-qubit quantum gate teleportation protocol, the role of the basis of measurement was considered in a recent work by Mendes and Ramos. In this work, we analyze the role of the four-qubit state used as resource. We show that the quantum two-qubit gate teleportation divides the set of pure four-qubit states in two classes. For one class, deterministic and probabilistic teleportation can be achieved, while for the other class, probabilistic remote two-qubit gate preparation is achieved.     

A centralized quantum switch network based on probabilistic channels

We present a practical scheme for deterministically teleporting quantum information via probabilistic communication channels in a centralized quantum switch network. In the network, a central quantum switch agent is assigned for regulating probabilistic channels so as to construct a direct deterministic channel between the sender and the receiver. This scheme is further extended to a hierarchical network and a tree network involving multiple agents. The advantage of the scheme is that all required multi qubit gates from distributed terminal agents are uniformly performed by a central agent, with which the physical design of terminal nodes is greatly simplified and more reliable deterministic teleportation can be realized in a centralized quantum probabilistic network.     

Spin Squeezing Criterion with Local Unitary Invariance

We propose a spin squeezing criterion for arbitrary multi-qubit states that is invariant under local unitary operations. We find that, for arbitrary pure two-qubit states, spin squeezing is equivalent to entanglement, and multi-qubit states are entangled if this new spin squeezing parameter is less than unity. PACS: 03.67.-a; 03.65.Ud     

Testing Hardy’s ladder proof of nonlocality by joint measurements of qubits

We propose an experimentally feasible scheme to test Hardy’s ladder proof of nonlocality with two qubits (two-level atoms) dispersively coupled to a driven cavity. First, we find that the required nonmaximally entangled two-qubit pure state can be prepared by only one-step two-qubit operation from the ground state $|00\rangle $ | 00 〉 , assisted by two single-qubit gates. Next, we perform two single-qubit operations to encode the local information into the prepared nonmaximally entangled state. Finally, the nonlocal correlations between the two qubits can be directly detected by the joint measurement of the two-qubit register in one of selected computational basis, implemented by probing the steady-state transmitted spectra of the driven cavity. Consequently, the Hardy’s ladder proof of nonlocality can be effectively tested. The feasibility of our proposal with the current experimental technology is also analyzed.     

Single-photon controlled multi-photon polarization unitary gate based on weak cross-Kerr nonlinearities

With the help of weak cross-Kerr nonlinearities, we propose a single-photon controlled multi-photon polarization unitary gate, which can fulfill the task of n single-photon controlled one-photon polarization unitary gates, but only by adopting a nondestructive measurement and an auxiliary coherent state. Moreover, simple linear optical elements and mature existing techniques containing Homodyne measurement and classical feed-forward are applied. So this scheme provides an efficient and feasible approach for optimally fulfilling single-photon controlled multi-photon unitary gate.     

Generation and protection of steady-state quantum correlations due to quantum channels with memory

We have proposed a scheme of the generation and preservation of two-qubit steady-state quantum correlations through quantum channels where successive uses of the channels are correlated. Different types of noisy channels with memory, such as amplitude damping, phase damping, and depolarizing channels, have been taken into account. Some analytical or numerical results are presented. The effect of channels with memory on dynamics of quantum correlations has been discussed in detail. The results show that steady-state entanglement between two initial qubits whose initial states are prepared in a specific family states without entanglement subject to amplitude damping channel with memory can be generated. The entanglement creation is related to the memory coefficient of channel \(\mu \). The stronger the memory coefficient of channel \( \mu \) is, the more the entanglement creation is, and the earlier the separable state becomes the entangled state. Besides, we compare the dynamics of entanglement with that of quantum discord when a two-qubit system is initially prepared in an entangled state. We show that entanglement dynamics suddenly disappears, while quantum discord dynamics displays only in the asymptotic limit. Furthermore, two-qubit quantum correlations can be preserved at a long time in the limit of \(\mu \rightarrow 1\).     

Towards Scalable Linear-Optical Quantum Computers

Scalable quantum computation with linear optics was considered to be impossible due to the lack of efficient two-qubit logic gates, despite the ease of implementation of one-qubit gates. Two-qubit gates necessarily need a non-linear interaction between the two photons, and the efficiency of this non-linear interaction is typically very small in bulk materials. However, it has recently been shown that this barrier can be circumvented with effective non-linearities produced by projective measurements, and with this work linear-optical quantum computing becomes a new avenue towards scalable quantum computation. We review several issues concerning the principles and requirements of this scheme. PACS: 03.67.Lx, 03.67.Pp, 42.50.Dv, 42.65.Lm     

Bidirectional controlled teleportation by using nine-qubit entangled state in noisy environments

A theoretical scheme is proposed to implement bidirectional quantum controlled teleportation (BQCT) by using a nine-qubit entangled state as a quantum channel, where Alice may transmit an arbitrary two-qubit state called qubits \(A_1\) and \(A_2\) to Bob; and at the same time, Bob may also transmit an arbitrary two-qubit state called qubits \(B_1\) and \(B_2\) to Alice via the control of the supervisor Charlie. Based on our channel, we explicitly show how the bidirectional quantum controlled teleportation protocol works. And we show this bidirectional quantum controlled teleportation scheme may be determinate and secure. Taking the amplitude-damping noise and the phase-damping noise as typical noisy channels, we analytically derive the fidelities of the BQCT process and show that the fidelities in these two cases only depend on the amplitude parameter of the initial state and the decoherence noisy rate.     

Efficient joint remote preparation of an arbitrary two-qubit state via cluster and cluster-type states

In this paper, we present a possible improvement of the successful probability of joint remote state preparation via cluster states following some ideals from probabilistic joint remote state preparation (Wang et al. in Opt Commun, 284:5835, 2011). The success probability can be improved from $1/4$ to 1 via the same quantum entangled channel by adding some classical information and performing some unitary operations. Moreover, we also discussed the scheme for joint remote preparation via cluster-type states. Compared with other schemes, our schemes have the advantage of having high successful probability for joint preparation of an arbitrary two-qubit state via cluster states and cluster-type states.     

量子态的概率隐形传态

利用多方控制代理的网络,提出了一个量子隐形传态多量子位的协议。协议中所使用的量子信道有别于已有协议中所使用的量子最大纠缠态。得到的研究结果是,只有在发送方和多方代理合作时,接受方才能得到概率隐形传态的量子信息。因为使用的量子信道是非最大纠缠态,所以该协议更具有实际意义。     

Controlled teleportation

In this article, we review the recent development of controlled teleportation which can be used for sharing quantum information and has important applications in remote quantum computation. We introduce the principles of a couple of controlled teleportation schemes with maximally entangled quantum channels and those with pure entangled quantum channels (non-maximally entangled states). The schemes based on maximally entangled states have the advantage of having maximal efficiency although there are differences in their implementations in experiment. In the controlled teleportation schemes using non-maximally entangled states as the quantum channels, the receiver can reconstruct the originally unknown state by adding an auxiliary particle and performing a unitary evolution. No matter what the unknown state is (a single qubit state or an m-qudit state), the auxiliary particle required is only a two-level quantum system.