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Optics Frontier Conference (OFS)

Realization of quantum SWAP gate and preparation for entangled coherent state based on optical microcavity

Shi-Qing Tang

Published Jun 18, 2021• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2599771

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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 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.
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.
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.
W-type optical entangled cat states play a crucial role in quantum information processing, quantum communication, and quantum technology. In this Letter, we propose an efficient protocol for preparing a W-type optical entangled cat state in circuit quantum electrodynamics (QED). The W-state preparation requires only a few basic operations. The system complexity is substantially reduced by using only one coupler qutrit. Our numerical simulation demonstrates that the high-fidelity preparation of the W-type optical entangled cat state is feasible with current circuit QED technology. This proposal is universal and can be extended to generate the W-type optical entangled cat state, by employing three microwave or optical cavities coupled to a natural or artificial three-level atom.
We propose a theoretical model for realizing a photonic two-qubit phase gate in cavity QED using a one-step process. The fidelity and probability of success of the conditional quantum phase gate is very high in the presence of cavity decay. Our scheme only employs one two-level atom, and thus is much simpler than other schemes involving multi-level atoms. This proposal can also be applied to generate two-mode squeezed states; therefore, we give three examples, i.e., the two-mode squeezed vacuum state, two-mode squeezed odd coherent state, and two-mode squeezed even coherent state, to estimate the variance of Duan’s criterion when taking into account cavity decay. It is shown that the variance is smaller than 2 for the three squeezed states in most cases. Furthermore, we utilize logarithmic negativity to measure the entanglement, and find that these squeezed states have very high degrees of entanglement.