Guo Guangcan

Background noise of satellite-to-ground quantum key distribution

The influence of background light on satellite-to-ground free-space quantum key distribution (QKD) is investigated. By comparing the number of noise photons to that of the signal photons per pulse, the technical requirements for a practical system are evaluated. We show that satellite-to-ground QKD is feasible with currently available technology even under full moonlight.

Generation of a Non-Classical Correlated Photon Pair via Spontaneous Four-Wave Mixing in a Cold Atomic Ensemble

Counter propagated write and read lasers can be used to generate non-classical correlated photon pairs in an atomic ensemble. We experimentally investigate how the detuning of the write laser affects the non-classical correlation function between the Stokes photon and the anti-Stokes photon, which are generated via a spontaneous four-wave mixing process using an off-axis configuration in a cold 85Rb atomic ensemble. The change of the time-resolved second-order correlated function between the Stokes and anti-Stokes photons is presented. The experimental result suggests that a suitable choice of detuning should be considered in such an experiment.

Experimental Decoy State Quantum Key Distribution Over 120 km Fibre

Decoy state quantum key distribution (QKD), being capable of beating PNS attack and being unconditionally secure, has become attractive recently. However, in many QKD systems, disturbances of transmission channel make the quantum bit error rate (QBER) increase, which limits both security distance and key bit rate of real-world decoy state QKD systems. We demonstrate the two-intensity decoy QKD with a one-way Faraday-Michelson phase modulation system, which is free of channel disturbance and keeps an interference fringe visibility (99%) long period, over a 120km single mode optical fibre in telecom (1550 nm) wavelength. This is the longest distance fibre decoy state QKD system based on the two-intensity protocol.Decoy State Quantum Key Distribution (QKD), being capable of beating PNS attack and unconditionally secure, have become an attractive one recently. But, in many QKD systems, disturbances of transmission channel make quantum bit error rate (QBER) increase which limits both security distance and key bit rate of real-life decoy state QKD systems. We demonstrate the two-intensity decoy QKD with one-way Faraday-Michelson phase modulation system, which is free of channel disturbance and keeps interference fringe visibility (99%) long period, near 130KM single mode optical fiber in telecom (1550 nm) wavelength. This is longest distance fiber decoy state QKD system based on two intensity protocol.

Probabilistic teleportation of an unknown atomic state

We propose two schemes for teleporting an unknown state. In our schemes, a three-particle Greenberger-Horne-Zeilinger state is used as a quantum channel. We show that the probabilistic teleportation of an unknown quantum state can be realized.

Realization of a Two-Dimensional Magneto-optical Trap with a High Optical Depth *

We build up a novel setup of a two-dimensional (2D) 85Rb magneto-optical trap (MOT) with a high optical depth (OD) of 38. Such a MOT trap of 85Rb has several advantages as compared to the normal three-dimensional ellipsoidal MOT. Firstly, it will greatly enhance atom-photon interaction due to the large OD. Then, the dephasing caused by the magnetic gradient will be decreased in the long axis of the 2D MOT, which we want to avoid from in the experiments. The metastable ground level dephasing rate was γ21 = 0.008γ31, which is much less than that in a normal MOT. The total number of atoms in this MOT was measured to be 9.1 × 108.

Quantum entanglement and quantum operation

It is a simple introduction to quantum entanglement and quantum operations. The authors focus on some applications of quantum entanglement and relations between two-qubit entangled states and unitary operations. It includes remote state preparation by using any pure entangled states, nonlocal operation implementation using entangled states, entanglement capacity of two-qubit gates and two-qubit gates construction.

Experimental preparation of the Werner state via spontaneous parametric down-conversion

We present an experiment for preparing a Werner state via spontaneous parametric down-conversion and controlled decoherence of photons in this paper. In this experiment two independent {beta}-barium borate crystals are used to produce down-conversion light beams, which are mixed to prepare the Werner state.

Conditions for optimal construction of two-qubit nonlocal gates

Optimal implementation of quantum gates is crucial for designing a quantum computer. The necessary condition for optimal construction of a two-qubit unitary operation is obtained. It can be proved that the B gate is the unique gate that can construct a two-qubit universal circuit with only two applications, i.e., this condition is also sufficient in the case of two applications of the elementary two-qubit gate. It is also shown that many perfect entanglers cannot simulate an arbitrary two-qubit gate with only three applications. We show how a two-qubit gates construction power is related to the construction power of its mirror gate, and introduce a specific kind of two-qubit entangling gate, i.e., a super controlled gate, which can construct an arbitrary two-qubit gate by three applications.

A Proposal of Teleportation for Three-Particle Entangled State

We propose a scheme for teleporting a three-particle entangled Greenberger-Horne-Zeilinger state to three distant users. The scheme operates essentially through the sharing of Einstein-Podolsky-Rosen pairs followed by only local measurements and unitary operations. It can be regarded as a method for generating the entanglement of particles distributed in a communication network.

Partial Teleportation of Entanglement Through Natural Thermal Entanglement in Two-Qubit Heisenberg XXX Chain

Natural thermal entanglement between two qubits with XXX Heisenberg interaction is studied. For the antiferromagnet, increasing coupling strength or decreasing temperature under critical point increases the entanglement. Based on the thermal entanglement as quantum channel, entanglement and information of an input entangled state are transferred via partial teleportation. We find that the entanglement transferred will be lost during the process, and for the entanglement fidelity the partial teleportation is superior to classical communication as concurrence of entangled channel beyond 1/4. We show that both correlation information in input entangled state and individual information of the teleported particle are linearly dissipated. With more entanglement in quantum channel, more entanglement and correlation information can be transferred.