Validity of resonant two-qubit gates in the ultrastrong coupling regime of circuit QED

TL;DR

This paper analyzes the performance of two-qubit resonant gates in circuit QED as the system transitions from strong to ultrastrong coupling, highlighting fidelity limits and the need for new schemes at high coupling ratios.

Contribution

It provides a theoretical analysis of two-qubit gate fidelity in the USC regime, including effects of counter-rotating terms and proposing the necessity for novel ultrafast gate schemes.

Findings

Gate fidelity remains above 96% at g/ω_r ≈ 10%

Counter-rotating terms significantly affect gate performance in USC

Standard schemes need modification for ultrafast gates in USC

Abstract

We investigate theoretically the performance of two-qubit resonant gates in the crossover from the strong to the ultrastrong coupling (USC) regime of light-matter interaction in circuit QED. Two controlled-PHASE gate schemes---that works well within the rotating wave-approximation (RWA)---are analyzed while taking into account the effects of counter-rotating terms appearing in the Hamiltonian. Our numerical results show that the fidelity of the gate operation is above 96 % when the ratio between the coupling strength and the resonator frequency, g/\omega_r, is of about 10 %. Novel schemes are required in order to implement ultrafast quantum gates when increasing the ratio g/\omega_r.