Probing the optical conductivity of trapped charge-neutral quantum gases

TL;DR

This paper links the optical conductivity of trapped quantum gases to their center-of-mass response, enabling experimental probing of strongly-correlated phases like the Mott insulator.

Contribution

It introduces a linear response method connecting trap displacement to optical conductivity, providing a new way to study excitations in quantum gases.

Findings

Optical Mott gap observed in the conductivity of bosonic atoms.

Exact relation between center-of-mass motion and optical conductivity.

Method applicable to strongly-correlated quantum gases.

Abstract

We study a harmonically confined atomic gas which is subjected to an additional external potential such as an optical lattice. Using a linear response formulation, we determine the response of the gas to a small, time-dependent displacement of the harmonic trap and derive a simple exact relation showing that the centre-of-mass position of the atomic cloud is directly related to the global optical conductivity of the system. We demonstrate the usefulness of this approach by calculating the optical conductivity of bosonic atoms in an optical lattice. In the Mott insulating phase, there is clear evidence of an optical Mott gap, providing a proof-of-principle demonstration that the global optical conductivity gives high-quality information about the exci- tations of strongly-correlated quantum gases.