Control of a long-lived multimode bosonic memory with a weakly coupled transmon ancilla*

High-Q 3D multimode cavities coupled to a nonlinear ancilla circuit are a promising platform for quantum computing. This architecture has the advantages of long cavity coherence times and the ability to realize multiplexed control of these modes with minimal control lines. Critical challenges for the viability of this architecture include crosstalk errors that emerge from the dispersive interaction and ancilla errors which propagate to the cavity modes and limit cavity coherence via the inverse Purcell effect.

We explore strategies to mitigate these errors by weakening the coupling between the transmon and the cavity modes and employing multimode versions of control protocols that use conditional displacements for fast gate operations [1]. These gates reduce crosstalk errors by using large cavity displacements as an interaction switch which increases the contrast between the rates of gate operations involving the target modes and those of spurious coherent errors from photons in non-target modes. We will also discuss progress toward realizing a new multimode processor architecture which mitigates crosstalk errors at the hardware level using tunable couplers.

[1] Eickbusch, A. et al. Nat. Phys. 18, 1464–1469 (2022).