Ultracold polar molecules near quantum degeneracy

TL;DR

This paper reports the creation of a near quantum-degenerate gas of polar KRb molecules in their ground state, achieved through precise laser control and phase stabilization, enabling studies of strongly dipolar quantum gases.

Contribution

First demonstration of transferring weakly bound heteronuclear molecules to their absolute ground state with phase-coherent lasers and optical frequency comb referencing.

Findings

Prepared a dense, ultracold gas of 40,000 polar molecules below 400 nK.

Achieved a degeneracy parameter of T/T_F=3, close to quantum degeneracy.

Measured a large molecular dipole moment of 0.5 Debye, opening avenues for dipolar interaction studies.

Abstract

We report the creation and characterization of a near quantum-degenerate gas of polar $^{40}$K-$^{87}$Rb molecules in their absolute rovibrational ground state. Starting from weakly bound heteronuclear KRb Feshbach molecules, we implement precise control of the molecular electronic, vibrational, and rotational degrees of freedom with phase-coherent laser fields. In particular, we coherently transfer these weakly bound molecules across a 125 THz frequency gap in a single step into the absolute rovibrational ground state of the electronic ground potential. Phase coherence between lasers involved in the transfer process is ensured by referencing the lasers to two single components of a phase-stabilized optical frequency comb. Using these methods, we prepare a dense gas of $4\cdot10^4$ polar molecules at a temperature below 400 nK. This fermionic molecular ensemble is close to quantum degeneracy and can be characterized by a degeneracy parameter of $T/T_F=3$. We have measured the molecular polarizability in an optical dipole trap where the trap lifetime gives clues to interesting ultracold chemical processes. Given the large measured dipole moment of the KRb molecules of 0.5 Debye, the study of quantum degenerate molecular gases interacting via strong dipolar interactions is now within experimental reach.