Record density of laser-cooled molecules

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Physics 16, s58

New laser cooling and trapping techniques squeeze large numbers of molecules into confined spaces while keeping them cryogenically cold.

Accurate testing of fundamental physics and correlated quantum systems requires direct laser cooling techniques that can cool dense collections of molecules to quantum degeneracy. Justin Blau and his colleagues at the University of Colorado at Boulder demonstrate progress toward that goal by using a unique magneto-optical trap to compress a molecular cloud while simultaneously cooling it to sub-Doppler temperatures. I’m here. [1]Their approach achieves phase-space densities (a measure of how “quantum” a gas is) that are two orders of magnitude higher than previous efforts.

Cooling the molecular cloud to quantum degeneracy requires a multistep process. First, the cloud is confined and laser-cooled to tens of μK inside a magneto-optical trap (MOT). There, three pairs of counter-propagating laser beams converge on the zero point of the quadrupole magnetic field. The cloud is then transferred to a storage trap (CT), where evaporative cooling is thought to reduce the temperature to tens of nK. The problem with this approach is that the lasers typically used in molecular MOT, which are ‘red-detuned’ with respect to their molecular resonances, cannot fall below the Doppler cooling limit, thus producing relatively warm and diffuse clouds. As a result, the number density of molecules transferred to CT is usually low.

Blau and his colleagues use a procedure called gray molasses cooling to cool yttrium oxide molecules. This technique uses blue-detuned laser light to drive the molecule into a ‘dark’ ground state, where it stops absorbing incident photons. By using light with a specific polarization configuration, he achieves sub-Doppler cooling and generates a position-dependent force that compresses clouds, along with the MOT’s quadrupole field. The researchers say that this volumetric compression will help significantly improve the transfer efficiency to CT from the few percent levels currently possible.

– Rachel Berkowitz

Rachel Berkowitz physics journal Based in Vancouver, Canada.


  1. JJ Blau and others.“Blue detuning magneto-optical traps of molecules”, Physics. Rev. Lett. 130193401 (2023).

subject area

Optical Atomic and Molecular Physics

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