Revealing the velocity uncertainties of a levitated particle in the quantum ground state

TL;DR

This paper demonstrates velocity measurements of a levitated nanoparticle in its quantum ground state, highlighting the impact of librational motions and the importance of controlling these motions for quantum state characterization.

Contribution

It introduces a method to measure and analyze the velocity distribution of a levitated nanoparticle in the quantum ground state, accounting for librational motion effects.

Findings

Velocity distributions are broadened by librational motions.

Feedback cooling reduces velocity distribution width.

Librational motion significantly affects translational velocity measurements.

Abstract

We demonstrate time-of-flight measurements for an ultracold levitated nanoparticle and reveal its velocity for the translational motion brought to the quantum ground state. We discover that the velocity distributions obtained with repeated release-and-recapture measurements are significantly broadened via librational motions of the nanoparticle. Under feedback cooling on all the librational motions, we recover the velocity distributions in reasonable agreement with an expectation from the occupation number, with approximately twice the width of the quantum limit. The strong impact of librational motions on the translational motions is understood as a result of the deviation between the libration center and the center of mass, induced by the asymmetry of the nanoparticle. Our results elucidate the importance of the control over librational motions and establish the basis for exploring quantum mechanical properties of levitated nanoparticles in terms of their velocity.