The squeezed Kerr oscillator: spectral kissing and phase-flip robustness

TL;DR

This paper demonstrates the realization of a squeezed Kerr oscillator with a driven Josephson circuit, revealing a transition to a doubly-degenerate ground state with Schrödinger-cat states, and achieving high-fidelity quantum readout and enhanced phase-flip lifetime.

Contribution

First experimental realization of a squeezed Kerr oscillator showing spectral kissing and robust phase-flip lifetime, with high-fidelity quantum nondemolition readout.

Findings

Resolved up to the tenth excited state spectroscopically

Observed staircase pattern in coherence lifetime increase

Achieved >99% quantum nondemolition readout fidelity

Abstract

By applying a microwave drive to a specially designed Josephson circuit, we have realized an elementary quantum optics model, the squeezed Kerr oscillator. This model displays, as the squeezing amplitude is increased, a cross-over from a single ground state regime to a doubly-degenerate ground state regime. In the latter case, the ground state manifold is spanned by Schr\"odinger-cat states, i.e. quantum superpositions of coherent states with opposite phases. For the first time, having resolved up to the tenth excited state in a spectroscopic experiment, we confirm that the proposed emergent static effective Hamiltonian correctly describes the system, despite its driven character. We also find that the lifetime of the coherent state components of the cat states increases in steps as a function of the squeezing amplitude. We interpret the staircase pattern as resulting from pairwise level kissing in the excited state spectrum. Considering the Kerr-cat qubit encoded in this ground state manifold, we achieve for the first time quantum nondemolition readout fidelities greater than 99%, and enhancement of the phase-flip lifetime by more than two orders of magnitude, while retaining universal quantum control. Our experiment illustrates the crucial role of parametric drive Hamiltonian engineering for hardware-efficient quantum computation.