Optimal Controlled Teleportation

We give the analytic expressions of maximal probabilities of successfully controlled teleportating an unknown qubit via every kind of tripartite states. Besides, another kind of localizable entanglement is also determined. Furthermore, we give the sufficient and necessary condition that a three-qubit state can be collapsed to an EPR pair by a measurement on one qubit, and characterize the three-qubit states that can be used as quantum channel for controlled teleporting a qubit of unknown information with unit probability and with unit fidelity.Comment: 4 page

A Simultaneous Quantum Secure Direct Communication Scheme between the Central Party and Other M Parties

We propose a simultaneous quantum secure direct communication scheme between one party and other three parties via four-particle GHZ states and swapping quantum entanglement. In the scheme, three spatially separated senders, Alice, Bob and Charlie, transmit their secret messages to a remote receiver Diana by performing a series local operations on their respective particles according to the quadripartite stipulation. From Alice, Bob, Charlie and Diana's Bell measurement results, Diana can infer the secret messages. If a perfect quantum channel is used, the secret messages are faithfully transmitted from Alice, Bob and Charlie to Diana via initially shared pairs of four-particle GHZ states without revealing any information to a potential eavesdropper. As there is no transmission of the qubits carrying the secret message in the public channel, it is completely secure for the direct secret communication. This scheme can be considered as a network of communication parties where each party wants to communicate secretly with a central party or server.Comment: 4 pages, no figur

Improving the security of secure direct communication based on secret transmitting order of particles

We analyzed the security of the secure direct communication protocol based on secret transmitting order of particles recently proposed by Zhu, Xia, Fan, and Zhang [Phys. Rev. A 73, 022338 (2006)], and found that this scheme is insecure if an eavesdropper, say Eve, wants to steal the secret message with Trojan horse attack strategies. The vital loophole in this scheme is that the two authorized users check the security of their quantum channel only once. Eve can insert another spy photon, an invisible photon or a delay one in each photon which the sender Alice sends to the receiver Bob, and capture the spy photon when it returns from Bob to Alice. After the authorized users check the security, Eve can obtain the secret message according to the information about the transmitting order published by Bob. Finally, we present a possible improvement of this protocol.Comment: 4 pages, no figur

Probabilistic teleportation of unknown two-particle state via POVM

We propose a scheme for probabilistic teleportation of unknown two-particle state with partly entangled four-particle state via POVM. In this scheme the teleportation of unknown two-particle state can be realized with certain probability by performing two Bell state measurements, a proper POVM and a unitary transformation.Comment: 5 pages, no figur

Electron Temperature Evolution in Expanding Ultracold Neutral Plasmas

We have used the free expansion of ultracold neutral plasmas as a time-resolved probe of electron temperature. A combination of experimental measurements of the ion expansion velocity and numerical simulations characterize the crossover from an elastic-collision regime at low initial Gamma_e, which is dominated by adiabatic cooling of the electrons, to the regime of high Gamma_e in which inelastic processes drastically heat the electrons. We identify the time scales and relative contributions of various processes, and experimentally show the importance of radiative decay and disorder-induced electron heating for the first time in ultracold neutral plasmas

Experimental Realization of an Exact Solution to the Vlasov Equations for an Expanding Plasma

We study the expansion of ultracold neutral plasmas in the regime in which inelastic collisions are negligible. The plasma expands due to the thermal pressure of the electrons, and for an initial spherically symmetric Gaussian density profle, the expansion is self-similar. Measurements of the plasma size and ion kinetic energy using fluorescence imaging and spectroscopy show that the expansion follows an analytic solution of the Vlasov equations for an adiabatically expanding plasma.Comment: 4 pages, 4 figure

Energy relaxation of exciton-polariton condensates in quasi-1D microcavities

We present a time-resolved study of energy relaxation and trapping dynamics of polariton condensates in a semiconductor microcavity ridge. The combination of two non-resonant, pulsed laser sources in a GaAs ridge-shaped microcavity gives rise to profuse quantum phenomena where the repulsive potentials created by the lasers allow the modulation and control of the polariton flow. We analyze in detail the dependence of the dynamics on the power of both lasers and determine the optimum conditions for realizing an all-optical polariton condensate transistor switch. The experimental results are interpreted in the light of simulations based on a generalized Gross-Pitaevskii equation, including incoherent pumping, decay and energy relaxation within the condensate.Comment: 15 pages, 20 figure