  The Cross-resonance (CR) gate architecture that exploits fixed-frequency
transmon qubits and fixed couplings is a leading candidate for quantum
computing. Nonetheless, without the tunability of qubit parameters such as
qubit frequencies and couplings, gate operations can be limited by the presence
of quantum crosstalk arising from the always-on couplings. When increasing
system sizes, this can become even more serious considering frequency
collisions caused by fabrication uncertainties. Here, we introduce a CR
gate-based transmon architecture with passive mitigation of both quantum
crosstalk and frequency collisions. Assuming typical parameters, we show that
ZZ crosstalk can be suppressed while maintaining XY couplings to support fast,
high-fidelity CR gates. The architecture also allows one to go beyond the
existing literature by extending the operating regions in which fast,
high-fidelity CR gates are possible, thus alleviating the frequency-collision
issue. To examine the practicality, we analyze the CR gate performance in
multiqubit lattices and provide an intuitive model for identifying and
mitigating the dominant source of error. For the state-of-the-art precision in
setting frequencies, we further investigate its impact on the gates. We find
that ZZ crosstalk and frequency collisions can be largely mitigated for
neighboring qubits, while interactions beyond near neighbor qubits can
introduce new frequency collisions. As the strength is typically at the sub-MHz
level, adding weak off-resonant drives to selectively shift qubits can mitigate
the collisions. This work could be useful for suppressing quantum crosstalk and
improving gate fidelities in large-scale quantum processors based on
fixed-frequency qubits and fixed couplings.

Quantum Physics