Tuning of dipolar interactions and evaporative cooling in a three-dimensional molecular quantum gas

Abstract

Ultracold polar molecules possess long-range, anisotropic, and tunable dipolar interactions, providing unique opportunities to probe novel quantum phenomena 1-4 . However, experimental progress has been hindered by excessive two-body loss, which also limits further cooling via evaporation. Recent work shows the loss can be mitigated by confining molecules in a two-dimensional geometry 5,6 . However, a general approach for tuning molecular interactions in a full three-dimensional (3D) stable system has been lacking. Here, we demonstrate the use of an electric field-induced shielding resonance [6][7][8] to suppress the reactive loss by a factor of 30 while preserving elastic, long-range dipolar interactions in a bulk gas of ultracold 40 K 87 Rb molecules in 3D. The favorable ratio of elastic to inelastic collisions enables direct thermalization, the rate of which depends on the angle between the collisional axis and the dipole orientation controlled by an external electric field. This is a direct manifestation of the anisotropic dipolar interaction. We further achieve dipolar-interaction-mediated evaporative cooling in 3D. This work demonstrates control of a long-lived bulk quantum gas system with tunable long-range interactions, paving the way for the study of collective quantum many-body physics.

Self-bound dipolar droplets and supersolids in molecular Bose-Einstein condensates

“…Apart from this numerical procedure, and in analogy with the absence of two-body losses, we will assume that three-body losses are zero. This assumption is supported by recent experiments with KRb molecules [37,49] and will be further discussed in Sec. VI.…”

“…At a particular dc electric field, the molecules are protected from short-range losses via the creation of a long-range repulsive barrier in the entrance channel [99][100][101][102]. This was successfully observed in recent experiments with ultracold KRb molecules, increasing the lifetime of the gas to several seconds [37] and enabling evaporative cooling to take place [49].…”

“…It also includes progress in direct laser cooling of molecules, where the achievable phase space densities have increased by almost ten orders of magnitude over the last few years [39][40][41][42][43][44]. Further cooling for various molecular species is expected to be possible through collisional cooling [45][46][47][48][49]. With this progress, a Bose-Einstein condensate (BEC) of strongly dipolar ground state molecules is now within experimental reach.…”

Accurate determination of the scattering length of erbium atoms

“…The quantity α(a s , θ) exhibits an anisotropic character via its angle dependence, as already observed for dipolar fermionic atoms [19] and molecules [36].…”

“…The cross-dimensional-thermalization technique is a very powerful method to experimentally determine the scattering length. First successfully applied to alkali atoms [30][31][32][33], this technique has proved to be very general and, more recently, has been used for more complex atomic species, such as chromium [34], specific isotopes of erbium [19] and dysprosium [21], and molecular systems [35,36].…”