A mid-circuit erasure check on a dual-rail cavity qubit using the joint-photon number-splitting regime of circuit QED

TL;DR

This paper demonstrates a hardware-efficient method for quantum error detection in dual-rail cavity qubits using a joint-photon number-splitting regime enabled by a parametric beamsplitter, improving multi-mode control in circuit QED.

Contribution

The authors introduce a novel joint-photon number-splitting technique that extends single-oscillator control to two oscillators, enabling minimal-connectivity erasure checks for dual-rail qubits in circuit QED.

Findings

Achieved leakage error detection with a missed erasure fraction of (9.0 ± 0.5)×10⁻⁴

Implemented an erasure rate of 2.92 ± 0.01%

Measured a Pauli error rate of 0.31 ± 0.01%

Abstract

Quantum control of a linear oscillator using a static dispersive coupling to a nonlinear ancilla underpins a wide variety of experiments in circuit QED. Extending this control to more than one oscillator while minimizing the required connectivity to the ancilla would enable hardware-efficient multi-mode entanglement and measurements. We show that the spectrum of an ancilla statically coupled to a single mode can be made to depend on the joint photon number in two modes by applying a strong parametric beamsplitter coupling between them. This `joint-photon number-splitting' regime extends single-oscillator techniques to two-oscillator control, which we use to realize a hardware-efficient erasure check for a dual-rail qubit encoded in two superconducting cavities. By leveraging the beamsplitter coupling already required for single-qubit gates, this scheme permits minimal connectivity between circuit elements. Furthermore, the flexibility to choose the pulse shape allows us to limit the susceptibility to different error channels. We use this scheme to detect leakage errors with a missed erasure fraction of $(9.0 \pm 0.5)\times10^{-4}$, while incurring an erasure rate of $2.92 \pm 0.01\%$ and a Pauli error rate of $0.31 \pm 0.01\%$, both of which are dominated by cavity errors.