Characterizing the coherence in the dual-rail subspace of two superconducting cavities (Part 2)

Quantum error correction is vital to the realization of fault-tolerant quantum computing since every quantum system used to store and process quantum information is prone to physical errors. Depending on the architecture, some types of errors are more likely than others, leading to an error hierarchy characteristic for each platform. In superconducting cavities, single photon loss is the dominant error channel, while intrinsic dephasing times can exceed energy relaxation timescales by orders of magnitude. To this end, we introduce the dual-rail cavity qubit, where logical information is encoded in the single-photon subspace of two superconducting cavities. With this encoding, leakage out of the computational subspace is detectable and thus convertible into erasure errors. However, since the cavities are intrinsically linear, non-linear auxiliary systems are required for control and logical readout, but these non-linear elements bring with them additional decoherence channels capable of spoiling the error hierarchy.

In this talk, we present the mechanisms of decoherence in the dual-rail subspace intrinsic to the cavities, and how errors in the auxiliary systems can propagate into the logical subspace. Thanks to the high level of control over the system, we can check for errors in the individual hardware components and mitigate error propagation. We compare the prediction of our error model to measurements performed on a single dual-rail cavity qubit.