From Outside to Inside the Atom

LU Dao-ming

Considering three two-level atoms initially in the W state, then two of them are placed into two initially empty cavities respectively and made resonantly interacted. The two-atom entanglement evolution inside cavities is investigated. The effects of rotational manipulation and state-selective measurement of the atoms outside the cavities on the two-atom entanglement evolution inside cavities are discussed. The results obtained using the numerical method show that the two-atom entanglement inside cavities is controlled by manipulating the atom outside the cavity.

John P. Cox

Zhou Bing-j

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.

Kalervo V. Laurikainen

Arthur Holly Compton

Bingju Zhou, Zhao-Hui Peng, Chun-Xia Jia +2 more

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.