Jiang Nian-Quan

Continuous-variable controlled-Z gate using an atomic ensemble

The continuous-variable controlled-Z gate is a canonical two-mode gate for universal continuous-variable quantum computation. It is considered as one of the most fundamental continuous-variable quantum gates. Here we present a scheme for realizing continuous-variable controlled-Z gate between two optical beams using an atomic ensemble. The gate is performed by simply sending the two beams propagating in two orthogonal directions twice through a spin-squeezed atomic medium. Its fidelity can run up to one if the input atomic state is infinitely squeezed. Considering the noise effects due to atomic decoherence and light losses, we show that the observed fidelities of the scheme are still quite high within presently available techniques.

Criterion for Genuine Multipartite Entanglement Quantum Channels

We introduce a character matrix for the N-qubit subsystem of a 2N-qubit state and show the criterion for genuine entanglement channel existing between two N-qubit subsystems in the state. The criterion allows us to check conveniently whether genuine quantum channels exist or not in the 2N-qubit state without calculating its N-qubit reduced density matrices.

New Three-Mode Squeezing Operators Gained via Tripartite Entangled State Representation

We show that the Agarwal–Simon representation of single-mode squeezed states can be generalized to find new form of three-mode squeezed states. We use the tripartite entangled state representations |p,y,z〉 and |x,u,v〉 to realize this goal.

Producing and Distinguishing χ-Type Four-Qubit States in Flux Qubits

We propose an effective method to produce four-qubit χ-type entangled states by using flux qubits coupled to an LC circuit which acts as a quantum data bus (QDB). In our scheme, the interaction is mediated by the exchange of virtual rather than real photons because of the large detuning between flux qubits and QDB, and then QDB-induced loss can be effectively avoided. The experimental feasibility of the scheme is also presented.

A simple scheme to generate χ-type four-charge entangled states in circuit QED

We propose a simple scheme to generate ?-type four-charge entangled states by using SQUID-based charge qubits capacitively coupled to a transmission line resonator (TLR). The coupling between the superconducting qubit and the TLR can be effectively controlled by properly adjusting the control parameters of the charge qubit. The experimental feasibility of our scheme is also shown.

Tractable Quantification of Entanglement for Multipartite Pure States

We present kth-order entanglement measure and global kth-order entanglement measure for multipartite pure states, and extend Bennetts measure of partial entropy for bipartite pure states to a multipartite case. These measures are computable and can effectively classify and quantify the entanglement of multipartite pure states.

Efficient Atomic One-Qubit Phase Gate Realized by a Cavity QED and Identical Atoms System

We present a scheme to implement a one-qubit phase gate with a two-level atom crossing an optical cavity in which some identical atoms are trapped. One can conveniently acquire an arbitrary phase shift of the gate by properly choosing the number of atoms trapped in the cavity and the velocity of the atom crossing the cavity. The present scheme provides a very simple and efficient way for implementing one-qubit phase gate.

New Approach for Normalizing Photon-Added and Photon-Subtracted Squeezed States

We present a new concise approach for normalizing m-photon-added squeezed state and m-photon-subtracted squeezed state , i.e., we construct the generating function respectively, after calculating them and comparing the result with the standard form of generating function of Legendre polynomials Pm, we find , and , where r is the squeezing parameter.

Long-time limit behavior of the solution to an atom's master equation

We study the long-time limit behavior of the solution to an atoms master equation. For the first time we derive that the probability of the atom being in the α-th (α = j + 1 − jz, j is the angular momentum quantum number, jz is the z-component of angular momentum) state is {(1 − K/G)/[1 − (K/G)2j+1]}(K/G)α−1 as t → +∞, which coincides with the fact that when K/G > 1, the larger the α is, the larger the probability of the atom being in the α-th state (the lower excited state) is. We also consider the case for some possible generalizations of the atomic master equation.

Implementing 1 → M Economical Phase-Covariant Telecloning in Cavity QED

We propose an experimentally feasible scheme to implement the economical 1 → M phase-covariant telecloning based on cavity QED. By the resonant interaction of the atoms with cavity field of a high-Q cavity and the different coupling strength between atoms and cavity field, the scheme can generate quantum entanglement channel in one step. What is more, the operation time and steps do not increase with the increase of atoms.