Erasure Detection for Bosonic Dual-Rail Qubits with a Strong Beamsplitter Interaction (Part 1/2)

A transmon coupled to one of two high-Q cavities can provide universal control of both cavity modes if they are themselves coupled via a tunable beamsplitter. Recent progress has allowed the strength of the beamsplitter interaction between the cavities to exceed the typical strength of the dispersive interaction to the transmon, while preserving high coherences. In this regime, it is possible for the transmon to learn only joint properties of the two cavities, despite their separation in both frequency and space. In particular, the transmon frequency spectrum exhibits number-split peaks that depend on the combined photon number of the two cavities. 

We use this to enact a mid-circuit erasure check for a dual-rail cavity qubit, detecting the joint vacuum state of the two cavities without perturbing the single-photon manifold. This measurement allows us to convert the most-likely qubit error (single photon loss to the vacuum) into erasures, which dramatically ease the task of quantum error correction and lead to higher thresholds in the surface code.

In part 1 of this two-part talk, we explore how a strong beamsplitter drive transforms the transmon spectrum and how this can be used to construct high-fidelity erasure checks.