Laser Controls Molecular Rotation in Superfluid Helium

Researchers from the University of Freiburg and British Columbia demonstrated precise control over the rotation of individual molecules inside superfluid helium nanodroplets using an optical centrifuge, marking the first time this laser‑based technique, previously used in gases, has been applied to a superfluid environment.

By introducing a timed delay between laser pulses, the team generated controlled, low‑speed molecular rotation, enabling detailed tracking of molecular dynamics and offering new insight into the microscopic behaviour of superfluid helium. Scientific experiments like these, directly involving helium, are set to increase helium demand over the coming years.

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An experimental study with the participation of Professor Dr Frank Stienkemeier from the Institute of Physics at the University of Freiburg shows that the rotation of individual molecules in superfluid helium nanodroplets can be controlled precisely. Led by Prof. Valery Milner from the University of British Columbia, Canada, the team used an innovative optical centrifuge to stimulate molecules with a laser pulse over a continuous frequency range and track their dynamics over time.

This technology has already been used to study molecules in gases, however its use for the study of superfluid, in which frictionless movement is possible, is new. The method that was used applies a brief time-delay between the laser pulses, causing interferences to arise, which produce a controlled, low rotation speed and thus allow the rotatability of the molecules to be characterised. “Our findings bring us a step closer to decoding the properties of microscopic superfluid and the use of nanodroplets as objects for precision measurements at a molecular level,” says Stienkemeier. The study appeared in the academic journal Physical Review Letters on 22 January.

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