Dual-rail cavity qubit measurements with erasure detection, Part 1: Design

The dual-rail cavity qubit has recently been proposed for the implementation of an erasure qubit in which the logical information is encoded within the {|01>, |10>} subspace of two superconducting microwave cavities [1]. Here, we focus on the recent experimental realization of a key element of the dual-rail cavity qubit concept: state preparation and measurement (SPAM). State preparation simply amounts to loading a single photon into one of the cavities. In contrast to conventional qubit measurement, the projective logical measurements can result not only in a “0” or “1” outcome, but also can be declared as an “erasure”, which occurs if the cavities have undergone photon loss or if the measurement outcome is ambiguous, indicating an error. By using standard circuit quantum electrodynamics techniques, namely measurements of cavity photon number via a transmon, we demonstrate SPAM fidelities exceeding those in any other qubit platform to date. In part 1 of this two-part talk, we describe the dual-rail cavity qubit hardware, design of our SPAM protocol, and show that repeated measurements of cavity photon number can be used to trade off erasure rates for exceedingly high SPAM fidelities.

[1] Teoh et al., PNAS 120 (41), e2221736120, 2023

[2] Chou et al., arXiv:2307.03169, 2023