Spin-motion entanglement and state diagnosis with squeezed oscillator wavepackets

TL;DR

This paper demonstrates the creation and characterization of superpositions of squeezed wavepackets in a trapped ion, revealing spin-motion entanglement and revivals, with potential applications in quantum metrology and information processing.

Contribution

It introduces a method to generate and analyze superpositions of squeezed oscillator states using internal-state dependent forces on a trapped ion.

Findings

Successful generation of superpositions of squeezed wavepackets

Observation of spin-motion entanglement and state revivals

Differences in displacement along squeezed and anti-squeezed axes

Abstract

Mesoscopic superpositions of distinguishable coherent states provide an analog to the Schr\"odinger's cat thought experiment. For mechanical oscillators these have primarily been realised using coherent wavepackets, for which the distinguishability arises due to the spatial separation of the superposed states. Here, we demonstrate superpositions composed of squeezed wavepackets, which we generate by applying an internal-state dependent force to a single trapped ion initialized in a squeezed vacuum state with 9 dB reduction in the quadrature variance. This allows us to characterise the initial squeezed wavepacket by monitoring the onset of spin-motion entanglement, and to verify the evolution of the number states of the oscillator as a function of the duration of the force. In both cases, we observe clear differences between displacements aligned with the squeezed and anti-squeezed axes. We observe coherent revivals when inverting the state-dependent force after separating the wavepackets by more than 19 times the ground-state root mean squared extent, which corresponds to 56 times the root mean squared extent of the squeezed wavepacket along the displacement direction. Aside from their fundamental nature, these states may be useful for quantum metrology or quantum information processing with continuous variables.