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Chinese Physics B

Feasible schemes for quantum swap gates of optical qubits via cavity QED

Deng-Yu Zhang, Shi-Qing Tang, Xin-Wen Wang, Xie Li-Jun, Gao Feng

Published Apr 1, 2011• 4 citations
Publisher

Abstract

Feasible schemes for implementing quantum swap gates of both coherent-state qubits and photonic qubits are proposed using a Λ-type atomic ensemble trapped in a bimodal optical cavity. In both protocols, the decoherence from atomic spontaneous emission is negligible due to the fact that the excited states of the atoms are adiabatically eliminated under large detuning condition and the swap gates can be created in a single step. In our schemes, the required atoms—cavity interaction time decreases with the increase of the number of atoms, which is very important in view of decoherence. The experimental feasibilities of the schemes are also discussed.

DOI
10.1088/1674-1056/20/4/040308

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Feasible schemes for implementing quantum swap gates of both coherent-state qubits and photonic qubits are proposed using a Λ-type atomic ensemble trapped in a bimodal optical cavity. In both protocols, the decoherence from atomic spontaneous emission is negligible due to the fact that the excited states of the atoms are adiabatically eliminated under large detuning condition and the swap gates can be created in a single step. In our schemes, the required atoms—cavity interaction time decreases with the increase of the number of atoms, which is very important in view of decoherence. The experimental feasibilities of the schemes are also discussed.
A two-qubit quantum swap gate scheme based on coherent state qubit is proposed in cavity quantum electrodynamics system. Our scheme adopt the interaction between the Λ-type atomic ensemble and the two-mode optical cavity, and the swap gate can be realized directly only one step, so as to save a lot of quantum resources and reduce the number of single qubit rotation operations in the actual quantum information processing. It is found that under the condition of large detuning limit, the excited states of atoms are adiabatically eliminated, and the decoherence caused by the spontaneous emission of atoms is effectively suppressed. All atoms in the optical cavity are universally addressed, which reduces the experimental operation difficulty of the scheme. The fidelity of the quantum logic gate is maintained at a very high level when the photon loss of the cavity field is considered. Compared with the single-atom scheme, it is found that the atom-cavity interaction time required by the atomic ensemble scheme is inversely proportional to the number of atoms, so that the speed of the quantum logic gate can be increased by N times. Therefore, in principle, as long as enough atoms interacting with the optical field, an effective coupling of any atom-cavity field can be generated, which is conducive to selecting the appropriate quantum gate operation time in practical operation and effectively avoiding decoherence.
Based on the cavity quantum electrodynamics (CQED), we propose two schemes for realization of basic quantum information processing tasks, i.e., constructing a quantum SWAP gate directly and preparation for entangled coherent states. In the present paper, we show that the quantum swap gates of photonic qubits can be implemented only by one step with a three-level Λ-type atom trapped in a bimodal optical cavity. The purpose for the direct construction of swap gates is to save a great many quantum resources in practical quantum information processing and reduce the number of required unitary operation. For the latter, we obtain the entangled coherent states via the large detuning interaction, in the process of the unitary evolution of dressed states. This scheme provides a suitable cavity QED to prepare quantum entanglement for researchers, and provides a wider horizon for the generation of quantum entanglement based on cavity QED, which, to some extent, possesses reference value and guiding effect in the experiment, and the quantum entanglement can be used for the potential application in the quantum information and quantum communication in the future.
A scheme is presented for realizing a two-qubit quantum SWAP gate for intracavity fields. In the scheme, a V\mathit{V}V-type atomic ensemble prepared in its ground state mediates the interaction between the two-cavity modes. Under certain conditions, the cavity-field degree of freedom is decoupled from the atomic degrees of freedom, and the effective coupling strength between the two-cavity modes scales up with N\sqrt{N}N​ (N\mathit{N}N is the number of effective atoms). The numerical simulation shows that the quantum SWAP gate may have a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity modes.
The cross fusion of quantum mechanics and information science forms quantum information science. Quantum logic gates and quantum entanglement are very important building blocks in quantum information processing. In this paper, we propose one-step schemes for realizing quantum swap gates and generating two-mode entangled coherent states via circuit QED. In our scheme, due to the adiabatic elimination of the excited state of the qutrit under the condition of large detuning, the decoherence of the spontaneous emission of the qutrit can be ignored. The fidelity of the quantum swap gate remains at a very high level. In addition, we also explore the nonclassical properties of two-mode entangled coherent states prepared in our scheme by addressing the second-order correlation function and intermodal squeezing. In particular, two classes of entangled coherent states demonstrate distinct entanglement and nonclassical behavior.