Describing a Quantum Channel by State Tomography of a Single Probe State

A general law is presented for (composite) quantum systems which directly describes the time evolution of quantum states (with one or both components) through an arbitrary noisy quantum channel. It is shown that the time evolution of all quantum states through a quantum channel can be completely captured by the evolution of a single 'probe state'. Thus in order to grasp the information of the final output states subject to a quantum channel, especially an unknown one, it only requires quantum state tomography of a single probe state, which dramatically simplifies the practical operations in experiment.Comment: 3 pages, To be publised in EP

Driving three atoms into a singlet state in an optical cavity via adiabatic passage of a dark state

In this paper, we propose an efficient scheme to drive three atoms in an optical cavity into a singlet state via adiabatic passage. Appropriate Rabi frequencies of the classical fields are selected to realize the present scheme. The scheme is robust against deviations in the pulse delay and laser intensity through some simple analysis of the adiabatic condition. It is notable that the estimated range of the effective adiabaticity condition coincides with the numerical results. When taking dissipation into account, we show that the process is immune to atomic spontaneous emission as the atomic excited states are never populated in adiabatic evolution. Moreover, under certain conditions, the cavity decay can also be efficiently suppressed

Amplification effects in optomechanics via weak measurement

We revisit the scheme of single-photon weak-coupling optomechanics using post-selection, proposed by Pepper, Ghobadi, Jeffrey, Simon and Bouwmeester [Phys. Rev. Lett. 109, 023601 (2012)], by analyzing the exact solution of the dynamical evolution. Positive and negative amplification effects of the displacement of the mirror's position can be generated when the Kerr phase is considered. This effect occurs when the post-selected state of the photon is orthogonal to the initial state, which can not be explained by the usual weak measurement results. The amplification effect can be further modulated by a phase shifter, and the maximal displacement state can appear within a short evolution time

Measurable Concurrence of Mixed States

We show that bipartite concurrence for rank-2 mixed states of qubits is written by an observable which can be exactly and directly measurable in experiment by local projective measurements, provided that four copies of the composite quantum system are available. In addition, for a tripartite quantum pure state of qubits, the 3-tangle is also shown to be measurable only by projective measurements on the reduced density matrices of a pair of qubits conditioned on four copies of the state.Comment: 3 page