A newly designed optical centrifuge allows scientists to control molecular rotation inside superfluid helium nano-droplets.
Physicists have developed a new version of an optical centrifuge that can control how molecules rotate while they are suspended inside liquid helium nano-droplets. The advance brings scientists closer to understanding the unusual properties of superfluids, a rare state of matter that flows without friction.
The experiment marks the first time researchers have been able to deliberately control molecular rotation inside a superfluid. Using the technique, scientists can precisely set both the direction and speed of a molecule’s spin. This level of control is important for investigating how molecules behave within a quantum environment and how their motion changes at different rotational frequencies.
Researchers from the University of British Columbia (UBC) and collaborators at the University of Freiburg described the new method this week in the journal Physical Review Letters.
The Challenge of Rotating Molecules in Fluids
“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 such as liquid helium represent an unusual phase of matter that occurs at temperatures close to absolute zero. In this state, the liquid flows without viscosity. Even though it lacks friction, the fluid still surrounds and interacts with molecules dissolved inside it.
“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
Traditional optical centrifuges have been used to spin molecules in gases. In those experiments, a rotating laser pulse interacts with the molecules. The molecules align with the electric field of the laser beam and begin rotating along with it.
However, this approach had not previously succeeded with molecules placed inside a superfluid.
To overcome this limitation, Milner and his colleagues placed molecules inside helium nano-droplets that were doped with dimers of nitric oxide. The researchers then introduced a short delay between laser pulses. This timing created interference between the pulses, producing a slower and steady rotation that made the molecules easier to spin.
The result was a significant increase in the molecules’ “spinnability,” allowing scientists to control their motion even within the surrounding superfluid.
Probing the Limits of Superfluid Behavior
With the new method in hand, the team plans to vary the rotation frequency using what Milner describes as a new “control knob” provided by the modified centrifuge. By gradually increasing the rotation rate, researchers hope to identify a critical frequency where the molecules suddenly lose rotational stability.
Beyond that threshold, molecular rotation is expected to decay much faster as the superfluid state begins to break down.
“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.”
Reference: “Control of Molecular Rotation in Helium Nanodroplets with an Optical Centrifuge” by Ian MacPhail-Bartley, Alexander A. Milner, Frank Stienkemeier and Valery Milner, 22 January 2026, Physical Review Letters.
DOI: 10.1103/5jnj-97vs
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.
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