Osamu Hirota

Some remarks on a conditional unitary operator

Abstract This Letter presents a general definition of a conditional unitary operator and some possible applications of it in quantum optics. Specific examples of this conditional unitary operator are studied.

Realization of a Conditional Unitary Process by Systems of Collett and Knight

Abstract This paper presents a physical interpretation of a conditional unitary process discussed in recent articles by one of the authors. It is verified that a generation process of a coherent squeezed state with arbitrary amplitude always corresponds to the conditional unitary process. Some examples of a quasi-free system are given.

Elimination of nonlinear PM and FM noises in optical amplifier repeater systems by squeezer

Recently, noise phenomenon, so called Gordon-Haus effect, caused by nonlinearity of fiber and spontaneous emission from optical amplifier has received much attention. In general, it is very difficult to eliminate such an effect, because the Kerr effect of fibers and the spontaneous emission noise of a conventional optical amplifier are inherent in those physics. In this paper, we show that the Gordon-Haus effect can be perfectly eliminated by applying the squeezer which is studied in the research field of squeezed state.

A quantum theoretical analysis of nonlinear phase noise

The authors clarify noise generation mechanisms due to optical fiber nonlinearity in laser amplifiers and squeezers in coherent communication systems, and discuss the mechanism of reduction of phase noise due to optical fiber nonlinearity by using a squeezer in optical coherent repeater systems. They show that considering the classical and the quantum behaviors of the Kerr effect separately makes it easier to understand the mechanism instinctively. They expect further technical development of squeezer in order to realize long-haul coherent communications systems.<<ETX>>

A realization of quantum state switch

The authors discuss the concepts of received quantum state control and quantum state switches, and propose a new quantum state switch system. It is shown that the new system is a quantum conditional unitary system and can overcome the standard quantum limit.<<ETX>>

Quantum mechanical conditional unitary process for linear beam splitter

The authors consider the example of the linear depression process. It was evident that the result of the conditional unitary process exactly agrees with that of the conventional unitary process. This new process can provide progress towards a new theory in quantum communications.<<ETX>>

Effect of complex nonlinear susceptibility in generation of squeezed state light

The authors analyse the effects of complex nonlinear susceptibilities on several processes of squeezed state generation and show that the effect on degenerate parametric amplification was less than the effects on SHG and degenerate four-wave mixing. In designing a quantum communication system with squeezed states, the effects of the complex nonlinear susceptibilities should be considered.<<ETX>>

The distance between two signals on the quantum and the classical communication theory

In order to build a new standard of evaluation for the distinction of signals, the authors propose the orthogonality on quasi-phase space, which corresponds to the inner product on the Hilbert space. Hence the quasi-phase space is the generalized signal space where some concepts of quantum theory work. This orthogonality has great advantages of making possible to define the distance between two signals without a precise representation of the quantum state and providing a very convenient method of analysis of quantum communication systems. Moreover, since the mathematical structure of the orthogonality is similar to that of the Bhattacharyya distance in the classical theory, the authors discuss whether it is possible or not to apply the orthogonality to the conventional communication system.<<ETX>>