Zhou Zheng-Wei

A hybrid quantum encoding algorithm of vector quantization for image compression

Many classical encoding algorithms of vector quantization (VQ) of image compression that can obtain global optimal solution have computational complexity O(N). A pure quantum VQ encoding algorithm with probability of success near 100% has been proposed, that performs operations 45N1/2 times approximately. In this paper, a hybrid quantum VQ encoding algorithm between the classical method and the quantum algorithm is presented. The number of its operations is less than N1/2 for most images, and it is more efficient than the pure quantum algorithm.

Preparation of Two-Qutrit Entangled State in Cavity QED

We propose a scheme to generate a 3×3-dimensional maximally entangled state of two particles. Two three-level atoms interact with a strongly detuned cavity so that the cavity is only virtually excited and efficient decoherence time of the cavity is greatly prolonged. Compared to other protocols, this protocol is simpler and has a higher fidelity.

Cavity-Assisted Single-Mode and Two-Mode Spin-Squeezed States via Phase-Locked Atom-Photon Coupling

We propose a scheme to realize the two-axis countertwisting spin-squeezing Hamiltonian inside an optical cavity with the aid of phase-locked atom-photon coupling. By careful analysis and extensive simulation, we demonstrate that our scheme is robust against dissipation caused by cavity loss and atomic spontaneous emission, and it can achieve significantly higher squeezing than one-axis twisting. We further show how our idea can be extended to generate two-mode spin-squeezed states in two coupled cavities. Because of its easy implementation and high tunability, our scheme is experimentally realizable with current technologies.

Local distinguishability of orthogonal quantum states and generators of SU(N)

We investigate the possibility of distinguishing a set of mutually orthogonal multipartite quantum states by local operations and classical communication (LOCC). We connect this problem with generators of SU(N) and present a condition that is necessary for a set of orthogonal states to be locally distinguishable. We show that even in multipartite cases there exists a systematic way to check whether the presented condition is satisfied for a given set of orthogonal states. Based on the proposed checking method, we find that LOCC cannot distinguish three mutually orthogonal states in which two of them are Greenberger-Horne-Zeilinger-like states.

A survey on quantum information technology

After nearly three decades of rapid development, the quantum information technology in the theoretical and technical studies has gained remarkable achievements. This review gives a brief introduction to the development of various hot research branches of quantum information technology, including quantum cryptography, quantum communication, quantum computing, quantum simulation, quantum metrology, and fundamental theory of quantum information. In addition, this review also discusses various physical systems which have been well applied in the quantum information technology, such as atomic, molecular and optical physics, different branches in solid state physics (superconducting Josephson Junction system, semiconductor quantum dots, Nitrogen-vacancy color centers in diamond), trapped ions, and nuclear magnetic resonance system. With the investigation and accumulation of quantum information technology, the ability to control microscopic world has been significantly improved. Quantum cryptography has been close to the practical application and the long-distance quantum communication has overcome the practical obstacles in principle. Quantum simulation is close to the limit of the classical computer. Also, quantum metrology has gained rapid development. This review not only shows the situation of the international development of quantum information technology, but also highlights the achievements of China in recent years. These achievements demonstrate that China is an indispensable force in the worldwide quantum information community.

Quantum phase transition of Bose-Einstein condensates on a nonlinear ring lattice

We study the phase transitions in a one-dimensional Bose-Einstein condensate on a ring whose atomic scattering length is modulated periodically along the ring. By using a modified Bogoliubov method to treat such a nonlinear lattice in the mean-field approximation, we find that the phase transitions are of different orders when the modulation period is 2 and greater than 2. We further perform a full quantum mechanical treatment based on the time-evolving block decimation algorithm which confirms the mean-field results and reveals interesting quantum behavior of the system. Our studies yield important knowledge of competing mechanisms behind the phase transitions and the quantum nature of this system.

Separability of Bipartite Superoperator Based on Witness

Based on the isomorphic relation between operator space L(H) and Hilbert space H⊗2, Cirac et al. mapped the global superoperator to a mixed state E which has the same separability of the initial superoperator. Zhang et al. [Phys. Rev. A 76 (2007) 012334] provided a calculable lower bound for both the linear and nonlinear witness. We use this bound to detect the entanglement of E to judge the separability of the initial superoperator. With the help of local orthogonal observables, we directly describe the separable condition of superoperator by its each operator. Lastly, using the lower bound of the nonlinear witness, we provide a calculable entanglement factor of bipartite superoperator.

Implementation of a Controlled-Phase Gate and Deutsch-Jozsa Algorithm with Superconducting Charge Qubits in a Cavity

Based on superconducting quantum interference devices (SQUIDs) coupled to a cavity, we propose a scheme for implementing a quantum controlled-phase gate (QPG) and Deutsch–Jozsa (DJ) algorithm by a controllable interaction. In the present scheme, the SQUID works in the charge regime, and the cavity field is ultilized as quantum data-bus, which is sequentially coupled to only one qubit at a time. The interaction between the selected qubit and the data bus, such as resonant and dispersive interaction, can be realized by turning the gate capacitance of each SQUID. Especially, the bus is not excited and thus the cavity decay is suppressed during the implementation of DJ algorithm. For the QPG operation, the mode of the bus is unchanged in the end of the operation, although its mode is really excited during the operations. Finally, for typical experiment data, we analyze simply the experimental feasibility of the proposed scheme. Based on the simple operation, our scheme may be realized in this solid-state system, and our idea may be realized in other systems.

Quantum Entanglement Channel based on Excited States in a Spin Chain

We study the possibility of using a spin chain to construct a quantum entanglement channel that can be used for quantum state transmission in a solid state system. We analyze the spin chains states under various z-directional magnetic field and spin interactions to determine the entanglement between Alice and Bobs spins. We derive the conditions under which this entanglement can be distilled, and find that a spin chain of arbitrary length can be used as a quantum channel for quantum state transmission when the number of spin flips in the chain is large.

Eliminating Next-Nearest-Neighbor Interactions in the Preparation of Cluster State

By analyzing recent schemes for preparing the chain cluster state, we find that the existence of long-range couplings between qubits will induce a quick decay of the fidelity of the cluster state. We propose some schemes for eliminating the effect of the next-nearest-neighbor interactions in the XY and Ising models by dynamical decoupling method. In our approach, the undesired next-nearest-neighbor interactions are effectively suppressed and the fidelity of the generated cluster state is greatly improved by the pulse operations.