Frequency-collision analysis for the scalable quantum computation based on high-intensity-driven all-microwave gates

Fixed-frequency transmons have been widely studied for scalable quantum computers due to their simple structure and resilience to charge and magnetic field noise. However, as the number of qubits increases, "frequency collisions" become a significant challenge for integration [1]. The standard cross-resonance (CR) gate, operating within the straddling regime [2], has a narrow tolerance for qubit frequency variations, causing it to suffer severely from this problem.



To mitigate the problem while adhering to fixed-frequency transmons, alternative two-qubit gates, such as using CR gates outside this regime or introducing coupler transmons (CAS gates) [3], have been proposed. However, these approaches rely on high-intensity microwave pulses, making them difficult to analyze with perturbative methods based on effective two-level Hamiltonians.



We propose a new method based on numerical simulations to evaluate frequency collisions in high-intensity-driven all-microwave gates. Our numerical method handles flexible pulse shapes and precisely predicts collisions within the multi-level Jaynes-Cummings model and rotating wave approximation. We evaluate frequency collisions for CR and CAS gates across various parameters, revealing higher-order state transitions that manifest during gate operations.



References

[1] J. B. Hertzberg et al., npj Quantum Information 7, 129 (2021).

[2] S. Sheldon et al., Phys. Rev. A 93, 060302 (2016).

[3] S. Shirai et al., Phys. Rev. Lett. 130, 260601 (2023).