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Physical Review A

Quantum SWAP gate in an optical cavity with an atomic cloud

Gong-Wei Lin, Xu‐Bo Zou, Ming-Yong Ye, Xiu‐Min Lin, Guang‐Can Guo

Published Jun 13, 2008• 29 citations
Publisher

Abstract

A scheme is presented for realizing a two-qubit quantum SWAP gate for intracavity fields. In the scheme, a $\mathit{V}$-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 $\sqrt{N}$ ($\mathit{N}$ 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.

DOI
10.1103/physreva.77.064301

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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.
A scheme is presented for realizing frequency-up conversion and a two-qubit quantum swap gate for intracavity fields. In the scheme, a V-type atomic ensemble prepared in their ground states collectively 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​ (NNN is the number of atoms). The numerical simulation shows that the quantum swap gate still has a high fidelity under the influence of the atomic spontaneous emission and the decay of the cavity modes.
We propose a scheme for implementing a two-qubit quantum phase gate for intracavity fields. In the scheme, two qubits are encoded in zero- and one-photon Fock states of two intracavity modes, and a four-level N\mathsf{N}N-type atomic ensemble trapped in a cavity mediates the conditional phase gate within a given interaction time. We also discuss the influence of the atomic spontaneous emission and the decay of the cavity modes on the photon loss and gate fidelity, showing the scheme is within the current experiment technology.
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.
A scheme is proposed to realize a two-qubit quantum phase gate for the intracavity modes. In the scheme, two qubits are encoded in zero- and one-photon Fock states of two intracavity modes, and the three-level $\ensuremath{\Lambda}$-type atom trapped in a high-QQQ cavity mediates the conditional phase gate within a given interaction time. The influence of cavity decay and atomic spontaneous emission on the gate fidelity is also discussed.
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.