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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Optimization of a cavity-QED system for fast two-qubit gates

Rui Asaoka, Takeru Utsugi, Yuuki Tokunaga, Rina Kanamoto, Takao Aoki

Published Jun 21, 2021• 1 citation
Publisher

Abstract

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] .

DOI
10.1109/cleo/europe-eqec52157.2021.9542055

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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] .
Cavity quantum electrodynamic schemes for quantum gates are amongst the earliest quantum computing proposals. Despite continued progress and the recent demonstration of photon blockade, there are still issues with optimal coupling and gate operation involving high-quality cavities. Here we show that dynamic cavity control allows for scalable cavity-QED based quantum gates using the full cavity bandwidth. This technique allows an order of magnitude increase in operating speed, and two orders reduction in cavity QQQ, over passive systems. Our method exploits Stark shift--based QQQ switching, and is ideally suited to solid-state integrated optical approaches to quantum computing.
Cavity-mediated two-qubit gates, for example between solid-state spins, are attractive for quantum network applications. We propose three schemes to implement a controlled phase-flip gate mediated by a cavity. The main advantage of all these schemes is the possibility to perform them using a cavity with high cooperativity, but not in the strong-coupling regime. We calculate the fidelity of each scheme in detail, taking into account the most important realistic imperfections, and compare them to highlight the optimal conditions for each scheme. Using these results, we discuss which quantum system characteristics might favor one scheme over another.
We analyze the requirements for fault-tolerant quantum computation with atom-atom gates based on cavity quantum electrodynamics (cQED) mediated by a photon with a finite pulse length. For short photon pulses, the distorted shape of the reflected pulses from the cQED system is a serious error source. In the previous study by Goto and Ichimura [Phys. Rev. A 82, 032311 (2010)], only the photon loss is minimized without considering the shape distortion to optimize the system parameters. Here we show an improved optimization method to minimize the infidelity due to the shape distortion and the photon losses in a well-balanced manner for the fault-tolerant scheme using probabilistic gates [Phys. Rev. A 80, 040303(R) (2009)]. Under this optimization, we discuss the fault-tolerant quantum computing requirement for short pulses. Finally, we show that reducing the cavity length is an effective way to reduce the errors of this type of gate in the case of short photon pulses.
High-dimensional quantum systems have been used to reveal interesting fundamental physics and to improve information capacity and noise resilience in quantum information processing. However, it remains a significant challenge to realize universal two-photon quantum gates in high dimensions with high success probability. Here, by considering an ion-cavity QED system, we theoretically propose, to the best of our knowledge, the first high-dimensional, deterministic, and universal two-photon quantum gate. By using an optical cavity embedded with a single trapped Ca + 40 ion, we achieve a high average fidelity larger than 98% for a quantum controlled phase-flip gate in four-dimensional space, spanned by photonic spin angular momenta and orbital angular momenta. Our proposed system can be an essential building block for high-dimensional quantum information processing, and also provides a platform for studying high-dimensional cavity QED. Published by the American Physical Society 2024