Speed limits for two-qubit gates with weakly anharmonic qubits

作者：Sahel Ashhab, Fumiki Yoshihara, Tomoko Fuse, Naoki Yamamoto, Adrian Lupaşcu, Kouichi Semba · 发表于：Physical Review A · 年份：2022 · DOI：10.1103/physreva.105.042614 · 被引用次数：18 · 研究领域：Quantum Information and Cryptography、Quantum and electron transport phenomena、Quantum Computing Algorithms and Architecture

We consider the implementation of two-qubit gates when the physical systems used to realize the qubits possess additional quantum states in the accessible energy range. We use optimal control theory to determine the maximum achievable gate speed for two-qubit gates in the qubit subspace of the many-level Hilbert space, and we analyze the effect of the additional quantum states on the gate speed. We identify two competing mechanisms. On one hand, higher energy levels are generally more strongly coupled to each other. Under suitable conditions, this stronger coupling can be utilized to make two-qubit gates significantly faster than the reference value based on simple qubits. On the other hand, a weak anharmonicity constrains the speed at which the system can be adequately controlled: according to the intuitive picture, faster operations require stronger control fields, which are more likely to excite higher levels in a weakly anharmonic system, which in turn leads to faster decoherence and uncontrolled leakage outside the qubit space. To account for this constraint, we modify the pulse optimization algorithm to avoid pulses that lead to appreciable population of the higher levels. In this case, we find that the presence of the higher levels can lead to a significant reduction in the maximum achievable gate speed. We also compare the optimal-control gate speeds with those obtained using the cross-resonance or selective-darkening gate protocol. We find that this protocol, with so...