Transition from a polaronic condensate to a degenerate Fermi gas of heteronuclear molecules

TL;DR

This paper reports the observation of a quantum phase transition in a Bose-Fermi mixture, transforming a polaronic condensate into a molecular Fermi gas, and producing a quantum-degenerate sample of heteronuclear molecules with a large dipole moment.

Contribution

It provides experimental evidence for a novel quantum phase transition in a Bose-Fermi mixture, expanding understanding beyond traditional BEC-BCS crossover phenomena.

Findings

Depletion of condensate fraction indicates transition

Emergence of a molecular Fermi gas with strong correlations

Production of sodium-potassium molecules with 2.7 Debye dipole moment

Abstract

The interplay of quantum statistics and interactions in atomic Bose--Fermi mixtures leads to a phase diagram markedly different from pure fermionic or bosonic systems. However, investigating this phase diagram remains challenging when bosons condense. Here, we observe evidence for a quantum phase transition from a polaronic to a molecular phase in a density-matched degenerate Bose--Fermi mixture. The condensate fraction, representing the order parameter of the transition, is depleted by interactions and the build-up of strong correlations results in the emergence of a molecular Fermi gas. By driving through the transition, we ultimately produce a quantum-degenerate sample of sodium-potassium molecules exhibiting a large molecule-frame dipole moment of 2.7 Debye. The observed phase transition represents a new phenomenon complementary to the paradigmatic BEC-BCS crossover observed in Fermi systems.