Physicists Spin Molecules Inside a Superfluid Using an Optical Centrifuge

Physicists at the University of British Columbia (UBC), in collaboration with the University of Freiburg, have achieved the first-ever controlled rotation of molecules inside a superfluid, opening a powerful new window into the mysterious behavior of quantum liquids.

The research, published in Physical Review Letters, demonstrates a modified optical centrifuge technique that can precisely spin molecules suspended in liquid helium nanodroplets — controlling both direction and speed of rotation.

Why It Matters

Superfluids, such as liquid helium cooled to near absolute zero, flow without any internal friction. Yet they still act as solvents, allowing molecules to dissolve within them. The central question has been: what changes from the perspective of a dissolved molecule when the surrounding fluid transitions from normal to superfluid?

'Controlling the rotation of a molecule dissolved in any fluid is a challenge,' said Dr. Valery Milner of UBC Physics and Astronomy. 'Dissolved molecules interact with the atomic constituents of the fluid, effectively getting bigger and harder to spin up. Imagine making a snowball — it's easy when small, but gets harder as more snow attaches.'

The Technique

Traditional optical centrifuges have spun molecules in gases using rotating laser pulses. The UBC team overcame the limitations by embedding molecules in helium nanodroplets doped with nitric oxide dimers, then introducing a precise delay between laser pulses. The interference produced a slower, steadier rotation rate, dramatically improving the molecules' 'spinnability.'

What's Next

The researchers now plan to vary rotation frequency to identify a critical point where superfluidity begins to break down at the atomic scale — a phenomenon not well understood. The work was supported by the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and the BC Knowledge Development Fund.

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