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arXiv:quant-ph/0411090 · Journal of Modern Optics

Dynamics of a Raman coupled model: Entanglement and quantum computation

Jonas Larson, B. M. Garraway

Published Jun 18, 2004• 15 citations
arXivHTMLPDF

Abstract

The evolution of a Raman coupled three-level A atom with two quantized cavity modes is studied in the large detuning case; i.e. when the upper atomic level can be adiabatically eliminated. Particularly the situation when the two modes are prepared in initial coherent or squeezed states, with a large average number of photons, is investigated. It is found that the atom, after specific interaction times, disentangles from the two modes, leaving them, in certain cases, in entangled Schrödinger cat states. These disentanglement times can be controlled by adjusting the ratio between average numbers of photons in the two modes. It is also shown how this effective model may be used for implementing quantum information processing. Especially it is demonstrated how to generate various entangled states, such as EPR and GHZ states, and quantum logic operations, such as the control-not and the phase gate.

DOI
10.1080/09500340408232483

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The evolution of a Raman coupled three-level A atom with two quantized cavity modes is studied in the large detuning case; i.e. when the upper atomic level can be adiabatically eliminated. Particularly the situation when the two modes are prepared in initial coherent or squeezed states, with a large average number of photons, is investigated. It is found that the atom, after specific interaction times, disentangles from the two modes, leaving them, in certain cases, in entangled Schrödinger cat states. These disentanglement times can be controlled by adjusting the ratio between average numbers of photons in the two modes. It is also shown how this effective model may be used for implementing quantum information processing. Especially it is demonstrated how to generate various entangled states, such as EPR and GHZ states, and quantum logic operations, such as the control-not and the phase gate.
We describe the dynamics of an exactly solvable Raman coupled model interacting with two modes of a quantized cavity electromagnetic field. The model consists of a three-level atom in the \ensuremath{\Lambda} configuration whose excited state is considered to be far off resonance and is adiabatically eliminated. The effective Hamiltonian for this system contains products of the transition operators between the two ground-state levels and a field annihilation operator from one mode and a creation operator from the other. This Hamiltonian provides an example of a zero-photon process. We study the atomic inversion and investigate the production of nonclassical effects such as antibunched light, violations of the Cauchy-Schwartz inequality, and squeezing.
An exact solution of the problem of two two-level atoms with degenerate Raman two-photon transitions interacting with one-mode coherent or thermal radiation field in cavity is presented. Asymptotic solution for system state vector is obtained in the approximation of large initial coherent fields. The atom-field is investigated on the basis of the reduced atomic entropy dynamics. The possibility of the system being initially in a pure disentangled state to revive into this state during the evolution process is shown. Conditions and times of disentanglement are derived. The atom-atom entanglement is investigated with using negativity. The possibility of sudden death and birth of atom-atom entanglement is predicted for a coherent field with large mean photon numbers.
We analyze the problem of a single mode field interacting with a pair of two level atoms. The atoms enter and exit the cavity at different times. Instead of using constant coupling, we use time-dependent couplings which represent the spatial dependence of the mode. Although the system evolution is adiabatic for most of the time, a previously unstudied energy crossing plays a key role in the system dynamics when the atoms have a time delay. We show that conditional atom-cavity entanglement can be generated, while for large photon numbers the entangled system has a behavior which can be mapped onto the single atom Jaynes-Cummings model. Exploring the main features of this system we propose simple and fairly robust methods for entangling atoms independently of the cavity, for quantum state mapping, and for implementing SWAP and controlled-NOT (CNOT) gates with atomic qubits.
In the process of a two-mode coherent state field interacting with an atom via intensity-dependent coupling,the evolution of entanglement between the atom and field or between the two-mode field is investigated by using quantum reduced entropy and relative entropy respectively.The results show that we can regulate the above mentioned two kinds of entanglement by choosing the system parameters of the initial state,or controlling the action time between the atom and field.The entanglement of the two-mode field will increase or decrease,but it's impossible to disentangle completely.We can get the EPR state and the separable state between the field and atom periodically in the high field conditions(the average photon number of the field is very large).
Interaction of a V-type three-level atom with two mode fields is studied in detail. Under large-detuning condition, the ground state can be adiabatically eliminated, i.e., this model may be converted into a non-degenerate Raman coupled model which an effective two-level atom interacts with two mode cavity fields. We derive the modified effective Hamiltonian and their wave functions of both non-degenerate Raman coupled and a degenerate Raman coupled model respectively. Based on state-selective measurements on V-type three-level atoms crossing the cavity, We present a new scheme of generation of Schrdinger cat states which are made up of superposition of the coherent states with controllable weighting factors and phases, through Raman interaction of degenerate V-type three atoms with one cavity field.
The exact dynamics of a model consisting of two two-level atoms interacting with the electromagnetic field mode of an ideal resonator through degenerate Raman transitions are found for coherent and thermal field states. The exact solution is used to calculate atom-atom negativity. It is shown that for separable initial states of atoms, their interaction with the resonator field does not lead to the occurrence of atom-atom entanglement. It was found that for the Bell initial states of atoms in the case of a coherent resonator field, the effect of sudden death of entanglement takes place for large average values of the number of photons in the resonator, while in case of thermal noise, this effect is absent for any intensities of the resonator field.