Revolutionising navigational systems

When helium gets cold, it starts to get really cool. At temperatures close to absolute zero, it becomes a liquid that never rests.

A laser field was used to draw energy from waves traveling on the superfluid’s surface. Credit: Christopher Baker.
A laser field was used to draw energy from waves traveling on the superfluid’s surface.
Credit: Christopher Baker.

For the first time, researchers from the University of Queensland have cooled a helium film using a laser, and in the process found a way to measure waves on its surface.

These bizarre liquids could be used to create the most accurate inertial sensors to date, upgrading satellite-free navigational equipment to improve submarine guidance, the control of drones, and the way a smartphone finds its location indoors.

Superfluids such as supercooled helium, are quantum liquids with a peculiar property — their viscosity is effectively zero, so the flow of a superfluid never stops.

With no viscous resistance to flow, tossing a pebble into a pool of supercooled helium would produce ripples that would, in theory, never fade. This unique property could benefit a number of advanced applications, but to behave this way the liquids have to be at temperatures approaching absolute zero.

Lasers are already widely used to cool gases and solid objects, but have never before been applied to cool a superfluid. To become the first team to do so, an international team based at the ARC Centre of Excellence for Engineered Quantum Systems fabricated microphotonic devices, using photolithography systems at ANFF-Q, which can confine a bright laser field capable of drawing energy from a liquid helium film.

This laser field exerts a force pushing back in the opposite direction to these waves, squeezing energy from the liquid; and as a result cooling it. The research also showed that combining superfluids with microphotonics in this way allows extremely precise measurements of superfluid waves.

It is this ability to monitor the waves which provides a pathway towards replacing state-of-the-art inertial used in navigation systems, according to the project’s chief investigator, Professor Warwick Bowen.

“Inertial sensors allow navigation by dead reckoning,” Warwick explained. “They tell you your velocity, and therefore allow you to navigate along a trajectory without external systems such as GPS. They’re important, for example, in submarine navigation where the sea water blocks GPS signals, or when navigating using a mobile phone indoors.”

Today’s highest precision inertial sensors are laser gyroscopes, which are essentially optical interferometers. Acceleration of the interferometer causes a change in the synchronisation of two laser beams resulting in a measurable change in the interference pattern produced by the light. Calculations can then be made to determine the velocity and direction of the motion.

In quantum mechanics, matter can also exhibit wavelike properties due to wave-particle duality. These matterwaves can interfere to provide a similar measurement of acceleration. However, the wavelength of matter-waves is around ten billion times smaller than that of light waves and therefore can, in principle, allow for much more sensitive measurements of acceleration.

“Previous experiments have shown that ultra-precise inertial sensing is possible using superfluid helium. However, these experiments relied upon bulky architectures somewhat akin to a plumbing system for water,” Warwick said.

“The ability to cool, measure, and control superfluid waves on a silicon chip brings a new level of scalability and integrability to such sensors.”