# Molecules cooled below the Doppler limit

## TL;DR

This paper reports cooling molecules to 50 microkelvin below the Doppler limit using 3D optical molasses, enabling new quantum and fundamental physics experiments with ultracold molecules.

## Contribution

It demonstrates for the first time sub-Doppler cooling of molecules to ultracold temperatures below the Doppler limit.

## Key findings

- Molecules cooled to 50 μK, below the Doppler limit.
- Potential for trapping molecules in optical tweezers for quantum simulation.
- Applications in fundamental physics tests and ultracold chemistry.

## Abstract

The ability to cool atoms below the Doppler limit -- the minimum temperature reachable by Doppler cooling -- has been essential to most experiments with quantum degenerate gases, optical lattices and atomic fountains, among many other applications. A broad set of new applications await ultracold molecules, and the extension of laser cooling to molecules has begun. A molecular magneto-optical trap has been demonstrated, where molecules approached the Doppler limit. However, the sub-Doppler temperatures required for most applications have not yet been reached. Here we cool molecules to 50 uK, well below the Doppler limit, using a three-dimensional optical molasses. These ultracold molecules could be loaded into optical tweezers to trap arbitrary arrays for quantum simulation, launched into a molecular fountain for testing fundamental physics, and used to study ultracold collisions and ultracold chemistry.