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Chinese Physics B

Preparation of optimal entropy squeezing state of atomic qubit inside the cavity via two-photon process and manipulation of atomic qubit outside the cavity

Bingju Zhou, Zhao-Hui Peng, Chun-Xia Jia, Chunlei Jiang, Xiaojuan Liu

Published Nov 28, 2014• 2 citations
Publisher

Abstract

Considering two atomic qubits initially in Bell states, we send one qubit into a vacuum cavity with two-photon resonance and leave the other one outside. Using quantum information entropy squeezing theory, the time evolutions of the entropy squeezing factor of the atomic qubit inside the cavity are discussed for two cases, i.e., before and after rotation and measurement of the atomic qubit outside the cavity. It is shown that the atomic qubit inside the cavity has no entropy squeezing phenomenon and is always in a decoherent state before the operating atomic qubit outside the cavity. However, the periodical entropy squeezing phenomenon emerges and the optimal entropy squeezing state can be prepared for the atomic qubit inside the cavity by adjusting the rotation angle, choosing the interaction time between the atomic qubit and the cavity, controlling the probability amplitudes of subsystem states. Its physical essence is cutting the entanglement between the atomic qubit and its environment, causing the atomic qubit inside the cavity to change from the initial decoherent state into maximum coherent superposition state, which is a possible way of recovering the coherence of a single atomic qubit in the noise environment.

DOI
10.1088/1674-1056/23/12/120305

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Considering two atomic qubits initially in Bell states, we send one qubit into a vacuum cavity with two-photon resonance and leave the other one outside. Using quantum information entropy squeezing theory, the time evolutions of the entropy squeezing factor of the atomic qubit inside the cavity are discussed for two cases, i.e., before and after rotation and measurement of the atomic qubit outside the cavity. It is shown that the atomic qubit inside the cavity has no entropy squeezing phenomenon and is always in a decoherent state before the operating atomic qubit outside the cavity. However, the periodical entropy squeezing phenomenon emerges and the optimal entropy squeezing state can be prepared for the atomic qubit inside the cavity by adjusting the rotation angle, choosing the interaction time between the atomic qubit and the cavity, controlling the probability amplitudes of subsystem states. Its physical essence is cutting the entanglement between the atomic qubit and its environment, causing the atomic qubit inside the cavity to change from the initial decoherent state into maximum coherent superposition state, which is a possible way of recovering the coherence of a single atomic qubit in the noise environment.
Considering two atom bits in Bell state initially,one bit was injected into a vacuum cavity to produce resonance while leaving the other outside.The controllable coherence of an atom bit inside the cavity was investigated by manipulating the atom bit outside the cavity.For the cases of before and after the Hadamard,Hadamard-like,Y,and phase logic gate manipulations on the atom bit outside the cavity,the correlation of two kinds of initial Bell state was discussed by numerical calculation.The time evolution curves of off-diagonal elements in the reduced density matrix for the atom bit inside the cavity was studied.The results show that the off-diagonal elements of the density matrix for the atom bit inside the cavity are zero at any moment before operating the atom bit outside the cavity.That is,the atom bit inside the cavity is always in a mixed state or classical states,and its coherence is always suppressed.However,after Hadamard gate and Hadamard-like gate operations and ground state measurements,offdiagonal elements in the reduced density matrix of the atom bit inside the cavity show the 2π period time evolution,and the coherence can be restored except(2n+1)π/2.At nπthe maximal coherence superposition state and general coherence superposition state were prepared,and a R(π/2)rotation correlation existed between the two kinds of Bell states.In addition,performing Y gate and phase gate operations on the outside atom bit,the coherence for the inside atom bit cannot restore.The necessary condition for the restoration of the coherence of the inside atom bit was found out as following:while logic gate operations manipulate the interactions between the atom bit inside the cavity and its environment,and the two atom bits are separated from the field,a strongly coherent entanglement is necessary between the two atom bits.
From the quantum information point of view we investigate the entropy squeezing properties of a two_level atom with arbitrary initial state interacting with the coherent field via the two_photon process. The influence of the initial state of the system on the atomic information entropy squeezing is discussed. The numerical results obtained from the uncertainly relation of quantum information entropy are compared with those obtained from the uncertainly relation of Heisenberg. It is shown that the number of squeezed component of the atomic dipole, squeezed frequency, squeezed amplitude and squeezed direction of the atomic information entropy squeezing can be controlled by choosing the initial atomic distribution angle, the atomic mixing degree and the relative phase of the field mode, respectively. Quantum information entropy is a remarkable precise measure for the atomic squeezing.
From the viewpoint of quantum information, this paper studies preparation and control of atomic optimal entropy squeezing states (AOESS) for a moving two-level atom under control of the two-mode squeezing vacuum fields. Necessary conditions of preparation of the AOESS are analysed, and numerical verification of the AOESS is finished. It shows that the AOESS can be prepared by controlling the time of the atom interaction with the field, cutting the entanglement between the atom and field, and adjusting squeezing factor of the field. An atomic optimal entropy squeezing sudden generation in different components can alternately be realized by controlling the field-mode structure parameter.
The information entropy squeezing properties of the atom in motion interacting with the two-mode squeezing vacuum field via two-photon transition are studied by means of quantum theory.The influences of the velocity of atomic motion,the field structure and the parameter of squeezing field on the information entropy squeezing properties of the atom are investigated as the initial atom is in any state.The results indicate that the value of squeezed component of the atomic dipole,squeezed frequency,amplitude and squeezed direction of the atomic information entropy can be controlled by choosing the atomic initial state,the velocity of atomic motion,the field structure and the squeezing factor of field and the squeezing phase angle of the field,respectively.In the longer time,squeezing effects of the atom in motion can be obtained by choosing the system parameters.The mixing degree of atomic initial state has no influence on the entropy squeezing of the atom in motion.