Myung Shik Kim

Quantum-information processing for a coherent superposition state via a mixed entangled coherent channel

An entangled two-mode coherent state is studied within the framework of

Dynamics of nonlocality for a two-mode squeezed state in a thermal environment

2\times 2

Asymmetric quantum channel for quantum teleportation

dimensional Hilbert space. An entanglement concentration scheme based on joint Bell-state measurements is worked out. When the entangled coherent state is embedded in vacuum environment, its entanglement is degraded but not totally lost. It is found that the larger the initial coherent amplitude, the faster entanglement decreases. We investigate a scheme to teleport a coherent superposition state while considering a mixed quantum channel. We find that the decohered entangled coherent state may be useless for quantum teleportation as it gives the optimal fidelity of teleportation less than the classical limit 2/3.

Quantum noise in optical beam propagation in distributed amplifiers

We investigate the time evolution of nonlocality for a two-mode squeezed state in the thermal environment. The initial two-mode pure squeezed state is nonlocal with a stronger nonlocality for a larger degree of squeezing. It is found that the larger the degree of initial squeezing is, the more rapidly the squeezed state loses its nonlocality. We explain this by the rapid destruction of quantum coherence for the strongly squeezed state.

Photon statistics in two-photon linear amplifiers

There are a few obstacles, which bring about imperfect quantum teleportation of a continuous variable state, such as unavailability of maximally entangled two-mode squeezed states, inefficient detection and imperfect unitary transformation at the receiving station. We show that all those obstacles can be understood by a combination of an {\it asymmetrically-decohered} quantum channel and perfect apparatuses for other operations. For the asymmetrically-decohered quantum channel, we find some counter-intuitive results; one is that teleportation does not necessarily get better as the channel is initially squeezed more and another is when one branch of the quantum channel is unavoidably subject to some imperfect operations, blindly making the other branch as clean as possible may not result in the best teleportation result. We find the optimum strategy to teleport an unknown field for a given environment or for a given initial squeezing of the channel.

Reconstruction of quantum entanglement in cavity QED

Quantum effects on optical beam propagation in a gain medium having distributed loss are analyzed using an amplifier-attenuator array model. For the amplifier-attenuator set, the Wigner function of the output field is related to that of the input field by a convolution law. Using this, we derive the Fokker-Planck equation for the beam propagation in the distributed amplifier with loss, and the added quantum noise is analyzed by solving this equation. When the amplification just compensates the attenuation, the mean amplitude of the signal does not change but the mean energy increases due to the added noise. Even in this case, the photon number variance can be reduced when a proper phase-sensitive reservoir is incorporated.

Wigner functions and quantum interference in optics

Abstract An analytical form of the general solution of the Fokker-Planck equation is given for the two-photon linear amplifier. With use of the general solution photon statistics are investigated for the amplified field by the two-photon linear amplifier. Unlike a phase-insensitive amplifier, the phase-sensitive two-photon linear amplifier can reduce the photon number fluctuations. However we show that it is not possible to reduce the photon number fluctuations below the nonclassical limit, in other words a super-Poissonian input field cannot become sub-Poissonian by the two-photon linear amplification. As we show that if the input field has a positive well-defined P representation the field can always be expressed by a positive well-defined P representation, we find an important conclusion that a classical input never evolves to a nonclassical field even in the phase-sensitive amplifier.

PHASE-SENSITIVE RESERVOIR MODELED BY BEAM SPLITTERS

Summary form only given, as follows. The subject of state reconstruction has recently become a major field of study in quantum optics [see Special issue on Quantum state preparation and measurement, J. Mod Opt., 11/12, (1997), edited by W.P. Schleich and M.G. Raymer]. The quasiprobability functions and density matrices have been measured experimentally for single-mode running fields using the homodyne scheme. However, most of the earlier reconstruction schemes are for single-mode fields. In this presentation the author is interested in reconstruction of a two-mode entangled cavity field.

Quasiprobabilities and the nonclassicality of fields

A new, approximate generalized Wigner function based on discrete coherent state superpositions is introduced. It is shown that in contrast to the exact generalized Wigner function that may not exist in some region of its parameter the proposed approximate function exists everywhere.