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Acta Physica Sinica

THE PREPARATION OF FOCK STATES BY QUANTUM MEASUREMENTS IN A KERR MEDIA AND THE INFLUENCE OF SELF-PHASE MODULATION

ZHOU WEI, Xue Qiu-Han, Guo Guang-Can, 中国科学技术大学物理系,合肥230026

Published Jan 1, 1996• 0 citations
Publisher

Abstract

A scheme for the preparation of a Fock state is proposed. We study two lights interacting in a Kerr media with self-phase modulation. The signal light and the probe light whose initial states are both coherent states will be entangled via interaction, then when a quadrature component of the probe light is measured, the photon number distribution of the signal light is modulated. Repeating the process, we find the signal will collapse into a Fock state finally, and the physical principle of this evolution is based on the cross-phase modulation, the self-phase modulation may hinder the preparation of a Fock state.

DOI
10.7498/aps.45.1297

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A scheme for the preparation of a Fock state is proposed. We study two lights interacting in a Kerr media with self-phase modulation. The signal light and the probe light whose initial states are both coherent states will be entangled via interaction, then when a quadrature component of the probe light is measured, the photon number distribution of the signal light is modulated. Repeating the process, we find the signal will collapse into a Fock state finally, and the physical principle of this evolution is based on the cross-phase modulation, the self-phase modulation may hinder the preparation of a Fock state.
A scheme is presented for preparing maximally entangled states among six modes.The scheme only utilizes weak cross-Kerr nonlinearity and momentum quadrature homodyne measurements on the probe mode of intense coherent light fields.It is relatively easy to generate Dicke states and W states of light fields in the scheme.It is not necessary that the cross-Kerr nonlinearity is very large,as long as the coherent light is bright enough.Therefore,this scheme is within the reach of current technology.In addition,only weak coherent light beams,with which single-photon sources have been replaced in the signal modes,are needed in the present scheme since it is difficult to realize single-photon sources in experiments.
The generation of an entangled coherent state is one of the most important ingredients of quantum information processing using coherent states. Recently, numerous schemes to achieve this task have been proposed. In order to generate travelling-wave entangled coherent states, cross-phase-modulation, optimized by optical Kerr effect enhancement in a dense medium in an electromagnetically induced transparency (EIT) regime, seems to be very promising. In this scenario, we propose a fully quantized model of a double-EIT scheme recently proposed [D. Petrosyan and G. Kurizki, Phys. Rev. A 65, 33 833 (2002)]: the quantization step is performed adopting a fully Hamiltonian approach. This allows us to write effective equations of motion for two interacting quantum fields of light that show how the dynamics of one field depends on the photon-number operator of the other. The preparation of a Schr\"odinger cat state, which is a superposition of two distinct coherent states, is briefly exposed. This is based on nonlinear interaction via double EIT of two light fields (initially prepared in coherent states) and on a detection step performed using a 50:50 beam splitter and two photodetectors. In order to show the entanglement of an entangled coherent state, we suggest to measure the joint quadrature variance of the field. We show that the entangled coherent states satisfy the sufficient condition for entanglement based on quadrature variance measurement. We also show how robust our scheme is against a low detection efficiency of homodyne detectors.
An analysis is presented of the time evolution of an optical field during a quantum nondemolition measurement of photon number using the cross-Kerr interaction between the signal and probe fields. It is shown that the signal field state collapses into a Fock state only asymptotically (in the infinite time limit), remaining in a superposition of two Fock states (Fock-state qubit) throughout most of the measurement period. Estimates are obtained both for the time required to measure photon number to the desired accuracy and for the Fock-state qubit lifetime.
The authors of this paper propose an optical scheme for the generation of entangled coherent states which include the multi-mode and high-dimensional entangled coherent states.The scheme is based on the interaction of nonlinear Kerr medium and optical fields.The authors find that muliti-mode and high-dimensional entangled coherent states can be generated through proper selections of the fields′ initial states,the time of the interaction and the generalized bell measurement.The authors also find that the interaction of nonlinear Kerr media can generate entanglement when the states of input modes are prepared in coherent state and superposed Fock states.