Fast Control of Multimode Cavities with Conditional Displacements

One promising implementation of quantum computers is a high-Q 3D multimode cavity coupled to superconducting transmon circuits. A key limitation of this architecture are the ancillary transmons, which limit the fidelity of gate operations and lower cavity coherence via the inverse Purcell effect. In order to mitigate these errors, a recently proposed pulse scheme uses large oscillator displacements to achieve an effective conditional displacement interaction [1]. Such a displacement acts as a “switch” to temporarily turn on the oscillator’s interaction strength with the qubit. In doing so, this technique maintains gate speeds while mitigating ancilla errors by weakening the bare dispersive interaction.

In this talk, we extend these protocols for control of multiple cavity modes weakly coupled to an ancilla qubit. In this multimode context, these gates increase the contrast between the rates of gate operations with the target modes, and those of spurious coherent errors from photons in non-target modes. We present multimode generalizations of echoed-conditional displacements sequences–entangling multiple cavity modes with sequential conditional displacements of the individual modes. We also produce multimode gates from related pulses generated by numerical optimization via GRAPE. Our work presents a pathway for potentially implementing cross-talk resilient multimode processors that are also more immune to ancilla errors.

[1] Eickbusch et al., Nature Physics (2022)