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arXiv:2211.04151 · Physical Review A

Optimal cavity design for minimizing errors in cavity-QED-based atom-photon entangling gates with finite temporal duration

Takeru Utsugi, Rui Asaoka, Yuuki Tokunaga, Takao Aoki

Published Jan 23, 2025• 5 citations
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Abstract

The authors present an optimal cavity design for atom-photon entangling gates based on cavity quantum electrodynamics, considering finite photon-pulse duration. By minimizing both the photon loss probability and the error rate due to temporal mode mismatch, they suggest that a cavity has an optimal length for the atom-photon gate, providing a guideline for implementing quantum information processing.

DOI
10.1103/physreva.111.l011701

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The authors present an optimal cavity design for atom-photon entangling gates based on cavity quantum electrodynamics, considering finite photon-pulse duration. By minimizing both the photon loss probability and the error rate due to temporal mode mismatch, they suggest that a cavity has an optimal length for the atom-photon gate, providing a guideline for implementing quantum information processing.
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Cavity quantum electrodynamics (cQED) enables deterministic controlled gates between neutral atom and photon qubits, such as the controlled phase flip (CPF) gate [1] , which is a key component for implementing quantum computation. The gate fidelity drops sharply when the length of an input-photon pulse is shorter than the inverse of the bandwidth of a cavity, because of the distortion of the output pulse. This limits the operational speed of the gates. One way to boost the limit of gate speed may be to reduce cavity length, since this increases the bandwidth of the cavity with keeping high cooperativity [2] .
We propose a scheme for two-qubit gates between a flying photon and an atom in a cavity. The atom-photon gate setup consists of a cavity and a Mach-Zehnder interferometer with doubly degenerate ground- and excited-state energy levels mediating the atom-light interaction. We provide an error analysis of the gate and model important errors, including spatial mode mismatch between the photon and the cavity, spontaneous emission, cavity losses, detunings, and random fluctuations of the cavity parameters and frequencies. Error analysis shows that the gate protocol is more robust against experimental errors compared to previous atom-photon gates and achieves higher fidelity.
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.
We address a fundamental issue in quantum mechanics and quantum information theory---the generation of an entangled pair of qubits that interact solely through a third, semiclassical degree of freedom---in the framework of cavity quantum electrodynamics. We show that finite, although not maximal, entanglement is obtainable in the classical limit, at the price of a diverging effective interaction time. The optimal atomic entanglement derives from a trade-off between the atomic entanglement in a sub-wave packet and the purity of the atomic state. Decoherence by photon loss sets an upper limit on the degree of excitation of the cavity mode, beyond which the achievable entanglement decreases as the inverse mean photon number to the sixth power.