UBC Physicists Achieve First-Ever Control of Molecular Rotation in Superfluid Helium

UBC's Optical Centrifuge Revolutionizes Study of Superfluid Helium Dynamics

In a landmark achievement for quantum physics, researchers at the University of British Columbia (UBC) have demonstrated the first-ever controlled rotation of molecules within a frictionless superfluid helium environment. This breakthrough, detailed in a January 22, 2026, publication in Physical Review Letters, introduces a modified optical centrifuge capable of precisely manipulating molecular spin inside helium nanodroplets. The innovation allows scientists to set both the direction and frequency of molecular rotation, offering unprecedented insights into how quantum superfluids interact with embedded particles at the atomic scale.

Led by Associate Professor Valery Milner from UBC's Department of Physics and Astronomy, the team—including PhD candidate Ian MacPhail-Bartley and Research Associate Alexander A. Milner—collaborated with Frank Stienkemeier from the University of Freiburg. Their work addresses long-standing challenges in probing superfluidity, a state where matter flows without viscosity at temperatures near absolute zero.

Understanding Superfluid Helium: A Quantum Marvel

Superfluid helium, particularly helium-4 below 2.17 K (the lambda point), exhibits extraordinary properties: zero viscosity, climbing container walls via the Rollin film effect, and quantized vortices. Unlike classical fluids, superfluids consist of a normal fluid component and an inviscid superfluid component, described by Landau's two-fluid model. Helium nanodroplets—tiny clusters of about 3,000 helium atoms formed by supersonic expansion—serve as ideal, ultra-cold (~0.4 K) matrices for isolating molecules while preserving superfluid behavior.

These droplets act as 'nano-cryostats,' enabling frictionless environments for spectroscopy and dynamics studies. However, molecules dissolved in them interact with surrounding helium atoms, complicating rotational control—much like a snowball growing harder to roll as it accumulates snow, as Milner analogizes. Traditional techniques like infrared spectroscopy reveal free rotation for some species but broadening near roton energies (~180 GHz), hinting at superfluid breakdown.

The Optical Centrifuge: From Gas Phase to Superfluid Frontier

Invented in 1999 by Corkum and colleagues, the optical centrifuge uses counter-rotating femtosecond laser pulses to create a rotating polarization trap, accelerating gas-phase molecules to extreme rotational states ('superrotors'). In gases, molecules align with the electric field and spin up unidirectionally. Adapting this for superfluids required innovation: UBC's constant-frequency centrifuge (cfCFG) employs a Michelson interferometer on chirped pulses (~330 ps, 800 nm) for zero-acceleration rotation below 100 GHz.

A key tweak—a short delay between pulses—induces interference, yielding stable, low rotation rates that overcome helium drag. Nitric oxide dimers ((NO)₂), chosen for droplet stability and heavy NO⁺ fragments in velocity-map imaging (VMI), align planarly with the field.

Step-by-Step: The UBC Experiment Unpacked

1. Droplet Formation: Supersonic expansion of 30 bar He through a 5 μm nozzle at 18 K creates ~3000-atom nanodroplets at 0.4 K.
2. Doping: Pick-up cell introduces (NO)₂ vapor, embedding dimers.
3. Centrifugation: cfCFG (2×10¹² W/cm²) imparts rotation; frequencies tuned to 8.5, 13, 17 GHz.
4. Probing: 120 fs probe pulse (5×10¹⁴ W/cm²) Coulomb-explodes dimers; time-of-flight VMI measures <cos²θ_2D> alignment.
5. Analysis: Oscillations at 2f_CFG confirm in-field rotation; resonant peak at 8.4 GHz reveals renormalized B = 0.092 cm^-1 (vs. gas 0.19 cm^-1).

Field-free decay lasts ~3.2 ns, far longer than expected, indicating coherent superfluid response.

Key Results: Rotation Control Achieved

The experiment confirmed forced rotation up to 36 GHz (2f_CFG), with amplitude dropping at higher speeds due to intensity limits. Resonant excitation drives J=0 to J=2, matching helium-renormalized constants. Post-centrifuge, planar alignment persists nanoseconds, enabling direct decoherence vs. relaxation distinction. This 'control knob' tunes excitation state-by-state, unprecedented in superfluids.

Spotlight on UBC Researchers Driving Quantum Innovation

Valery Milner heads UBC's Ultrafast Coherent Control Group, focusing on laser-molecule interactions. His Google Scholar profile boasts citations on quantum resonances and chiral control. PhD grad Ian MacPhail-Bartley (2024 thesis: 'Control of molecular rotation in superfluid helium') led experiments; Alexander A. Milner contributed laser expertise. Freiburg's Stienkemeier provided nanodroplet know-how. Funded by NSERC, CFI, BC Knowledge Fund.

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Implications for Quantum Physics and Superfluidity

This probes superfluid-defect interactions, vortex nucleation, and roton-phonon coupling at atomic scales. By scanning frequencies, researchers target critical velocities where superfluidity breaks—key to quantum turbulence and Landau critical velocity (~58 m/s in bulk He). Unlike IR linewidths, it isolates dynamics.

Potential Applications and Broader Impacts

Beyond fundamentals, controlled rotors could advance quantum sensing, simulation of exotic matter, or frictionless nanomachines. In Canada, aligns with $50M+ NSERC quantum grants; UBC's Quantum Matter Institute (QMI) received $66.5M CFREF. Ties to national quantum strategy, including recent $360M federal initiative.

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Future Directions: Probing Superfluid Breakdown

Next: Vary f_CFG to find transition frequency where rotation decays rapidly, signaling quantized vortex formation or normal fluid excitation. Could reveal microscopic superfluid hydrodynamics, impacting Bose-Einstein condensate analogies.

Canada's Thriving Quantum Research Ecosystem

UBC exemplifies Canada's quantum leadership: Nine UBC-led NSERC projects ($50M total), QMI partnerships. Complements Perimeter Institute, IQC Waterloo. For students, Canadian university jobs abound in quantum physics.

Photo by Logan Voss on Unsplash

Careers in Quantum Physics: Opportunities at UBC and Beyond

This work highlights demand for expertise in ultrafast lasers, quantum matter. Postdocs, faculty roles via university jobs; adjunct positions at adjunct professor jobs. Career advice at higher ed career advice. Rate professors like Milner on Rate My Professor. Search research jobs today.