Physicists have used a new optical centrifuge to control the rotation of molecules suspended in liquid helium nano-droplets, bringing them a step closer to demystifying the behaviour of exotic, frictionless superfluids.

It’s the first demonstration of controlled spinning inside a superfluid—researchers can now directly set the direction and frequency of the molecule’s rotation, which is vital in studying how molecules interact with the quantum environment at various rotational frequencies. The method was outlined this week by researchers at the University of British Columbia (UBC) and colleagues at the University of Freiburg in the journal Physical Review Letters.

“Controlling the rotation of a molecule dissolved in any fluid is a challenge,” said Dr. Valery Milner, associate professor with UBC Physics and Astronomy and lead author on the paper.

“Dissolved molecules interact with the atomic or molecular constituents of the fluid, effectively getting bigger and harder to spin up. Imagine making a snowball: It’s very easy to move it when it’s small, but gets harder and harder as more snow gets attached to it.”

Superfluids like liquid helium are exotic states of matter, at near-absolute zero, that flow with no viscosity. But despite the lack of friction, they actually do act as solvents.

“The question of interest in the science of quantum matter, and the one this new approach will help us explore, is what changes from the perspective of the solvated—dissolved—molecule when you make the transition from a normal fluid to this type of quantum superfluid,” adds Dr. Milner.

A new spin on optical centrifuges

Conventional optical centrifuges have already been used to spin and study molecules in gases by shining a rotating laser pulse onto it. Molecules in the gas align with the beam’s electric field and rotate with the pulse. But the technique hasn’t worked yet with molecules suspended in a superfluid.

Dr. Milner and his team embedded the molecules in helium nano-droplets doped with dimers of nitric oxide, and introduced a short time delay between laser pulses. That caused interference that creates a much lower, constant rotation rate that increased the molecule’s “spinnability.”

With the new approach, the team will move on to scan the rotation frequency (using the new ‘control knob’ offered by the novel centrifuge) across a critical frequency, beyond which molecular rotation is expected to decay much faster due to the breakdown of superfluidity.

“It is not well understood how and when—for example at what frequency—this transition will happen at such a tiny atomic scale,” says Dr. Milner. “That’s the key area we’re investigating at the moment.”

The research was supported by the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and the BC Knowledge Development Fund.

Control of Molecular Rotation in Helium Nanodroplets With an Optical Centrifuge
https://doi.org/10.1103/5jnj-97vs

Illustrations and photos

Physicists at the University of British Columbia sent a laser beam of an optical centrifuge into helium nano-droplets doped with dimers of nitric oxide in order to control and study their rotation inside superfluid helium. (Valery Milner, University of British Columbia).