Improved readout of qubit-coupled Gottesman-Kitaev-Preskill states

The Gottesman-Kitaev-Preskill encoding of a qubit in a harmonic oscillator is a promising building block towards fault-tolerant quantum computation. Recently, this encoding was experimentally demonstrated for the first time in trapped-ion and superconducting circuit systems. However, these systems lack some of the Gaussian operations which are critical to efficiently manipulate the encoded qubits. In particular, homodyne detection, which is the go-to method for efficient readout of the encoded qubit in the vast majority of theoretical work, is not readily available, heavily limiting the readout fidelity. Here, we present an alternative read-out strategy designed for qubit-coupled systems. Our method can improve the readout fidelity with several orders of magnitude for such systems and, surprisingly, even surpass the fidelity of homodyne detection in the low squeezing regime.

Automated summary

Encoding a qubit in a harmonic oscillator using the Gottesman-Kitaev-Preskill method is a promising approach for fault-tolerant quantum computation. However, current experimental systems lack some Gaussian operations crucial for efficiently manipulating encoded qubits, leading to limitations in readout fidelity. This study introduces an alternative readout strategy for qubit-coupled systems, significantly improving readout fidelity and even surpassing homodyne detection fidelity in certain conditions.