Ultracold Feshbach molecules in an orbital optical lattice

TL;DR

This paper reports the first creation of ultracold Feshbach molecules in the second Bloch band of an optical lattice, enabling exploration of orbital BEC-BCS crossover physics with detailed measurements of binding energies and lifetimes.

Contribution

It introduces the experimental realization of Feshbach molecules in higher Bloch bands, expanding the study of quantum gases beyond the lowest band.

Findings

Long lifetimes (~300 ms) for molecules at unitarity in the lowest band.

Observation of bound dimers in the second band for negative scattering lengths.

Preparation of conditions suitable for studying orbital BEC-BCS crossover.

Abstract

Quantum gas systems provide a unique experimental platform to study a fundamental paradigm of quantum many-body physics: the crossover between Bose-Einstein condensed (BEC) molecular pairs and Bardeen Cooper Schrieffer (BCS) superfluidity. Some studies have considered quantum gas samples confined in optical lattices, however, focusing on the case, when only the lowest Bloch band is populated, such that orbital degrees of freedom are excluded. In this work, for the first time, ultracold Feshbach molecules of fermionic $^{40}K$ atoms are selectively prepared in the second Bloch band of an optical square lattice, covering a wide range of interaction strengths including the regime of unitarity. Binding energies and band relaxation dynamics are measured by means of a method resembling mass spectrometry. The longest lifetimes arise for strongly interacting Feshbach molecules at the onset of unitarity with values around 300 ms for the lowest band and 100 ms for the second band. In the case of strong confinement in a deep lattice potential, we observe bound dimers also for negative values of the s-wave scattering length, extending previous findings for molecules in the lowest band. Our work prepares the stage for orbital BEC-BCS crossover physics.