Extremely cold drop of helium can be levitated forever

A drop of very cold liquid helium can be made to float for an indefinitely long time using strong magnets and quantum effects.

A droplet of helium, made to float in a vacuum
Harris Lab, Yale University

A drop of liquid helium cooled to an extremely low temperature can be made to float in a vacuum for an indefinitely long time. In this state, it could serve as a powerful mini-laboratory for fundamental physics.

At -269°C, liquid helium is already strangely cold for a liquid, but when it is approximately 2 degrees colder it becomes even stranger. At this temperature, quantum effects make its viscosity vanish and turn it into a superfluid. Charles Brown at Yale University and his colleagues worked out how to levitate a millimetre-sized drop of this superfluid in a vacuum forever.

They first placed a helium droplet in a powerful refrigerator whose inside was devoid of all particles of air. Within this very cold vacuum, the helium became a superfluid, so the researchers could levitate it by using strong magnets. Superfluid helium repels magnetic fields, so the right arrangement of magnets can cause the droplet to float.

However, for a drop that has a mass of about a milligram and is a millimetre wide, getting the magnetic field just right means having to use magnets two or three times stronger than those in MRI machines, says Brown. His team used magnets made from coils of perfectly conducting, or superconducting, wires. “These are really at the limit of superconducting magnet technology,” says Jack Harris, also at Yale University, who led the project.

In similar experiments conducted in the past, researchers were able to keep such a droplet afloat for a long time, but never in a fully empty chamber and never as cold as in the new study. In this case, thanks to the use of strong and precisely controlled magnetic fields and a high-quality vacuum, the droplet was colder than the rest of the refrigerator and able to defy gravity for an unprecedented 20 hours, until the researchers turned off the magnets. Over time, the droplet slowly shrank in size owing to evaporation caused by the tiny amount of heat entering the chamber. Mathematically modelling the experiment, the researchers showed that as the drop would continue to shrink, it would become colder, which would cause its evaporation to slow down further. Eventually, it would settle on a size at which it could levitate forever, Brown says.

J. Peter Toennies at the Max Planck Institute for Dynamics and Self-Organization in Germany says that while helium is the oldest superfluid, the relatively huge size of the droplets in the new experiment opens up new possibilities for studying it. A particularly interesting question is whether these large, levitated, superfluid droplets can be used as stable containers or mini-fridges for other molecules that have to be studied at low temperatures, he says.

Harris and his colleagues have already found that superfluid helium droplets are very efficient at trapping light within them. Though these droplets are larger than many quantum objects, all superfluids are fundamentally quantum, so the researchers now want to study how their quantumness influences their response to disturbances like being hit with light.

Exposing one such droplet to an electric field could also help clarify whether the charges of electrons, neutrons and protons in matter actually add up to zero, says Harris. Superfluid helium is incredibly pure, so if an electric field managed to move a droplet, it would be reasonable to conclude that there is some wholly new and unexpected charge imbalance within it, he says.

Journal reference

Physical Review LettersDOI: 10.1103/PhysRevLett.130.216001

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