Fast, high-fidelity single-qubit gates in strongly coupled multi-qubit systems

Increasing the coupling strength between qubits can speed up multi-qubit entangling gates, which is essential for implementing more complex algorithms on a quantum processor. However, there is a tradeoff: It becomes challenging to implement single-qubit gates in the strong-coupling regime because the relevant transition frequencies become strongly dependent on the states of other qubits. In a fixed-frequency processor, this leads to significant gate errors and fidelity loss if this effect is not taken into account. To avoid this issue, most works to date have focused on the weakly coupled regime where individual qubits retain their identity, albeit at the price of slow entangling operations. Even in the weak-coupling case though, errors due to inter-qubit coupling can still accumulate over the course of several gates applied in sequence. Here, we present a novel method to mitigate errors in single-qubit gates due to coupling to a second qubit that involves building the coupling-dependent frequency shifts directly into the gate design. This is achieved through a combination of pulse shaping and pulse sequencing. Our techniques provide a way to speed up two-qubit entangling gates without sacrificing the speed and fidelity of single-qubit gates.