Researchers confined a cloud of sodium atom into a ring using light, by combining a ring-shaped trap and a horizontal, sheet-shaped trap. Credit: From Ref.1

A memory effect that is crucial in electronics has been seen for the first time in a cloud of ultracold atoms. The phenomenon represents a milestone in the emerging field of ‘atomtronics’, which seeks to create a whole new class of devices that use the flow of atoms, rather than electrons, in a circuit.

In atomtronics, clouds of atoms are super-cooled to form a collective quantum state known as a Bose-Einstein condensate (BEC). So far, physicists have used these atoms in analogues of basic electrical components such as transistors1 and capacitors2. Such condensates can also become a superfluid — meaning atoms can flow past obstacles without friction — and be set in motion, circulating inside a ring-shaped trap.

Atomtronics has so far been largely theoretical, but it holds potential for developing entirely new quantum devices, says Gretchen Campbell, a physicist at the University of Maryland in College Park. Publishing today in Nature3, her team is the first to directly see an effect known as hysteresis in an atomtronic circuit. Hysteresis is the dependence of a system not just on its current state, but also on its history. A thermostat, for example, might turn a heating system off as the temperature rises to 21 °C, but will not turn it on again until it falls below 18 °C. This prevents small disturbances from causing big changes.

Magnetic hard drives, which store data in 0s and 1s, also exploit hysteresis: turning on a magnetic field can turn a 0 into a 1, but you need to do more than just remove the field to reverse the effect. “We don’t yet know exactly what these atomtronic devices would be,” Campbell says. “But in any real circuit you need hysteresis — something that acts like a memory or a filter."

Paddling against the current

Campbell and her team captured sodium atoms in their ring trap and cooled them to a few billionths of a degree above absolute zero, until they took on the superfluid state. In previous experiments4, the researchers showed how they could use a laser to turn the current off by 'pinching' the stream to stop the flow.

In their latest work, Campbell and her colleagues used the laser as a paddle to stir the atoms in the ring. The condensate starts at rest, and with gentle stirring does not start to flow, because the system’s quantum properties mean that its rate of rotation can vary only in discrete increments. Past a critical stirring rate, however, all the atoms suddenly start flowing and rotate faster than the paddle.

But running the experiment backwards — starting with a current in the superfluid that is then slowed down with a paddle — the researchers found that the point at which the flow abruptly stopped was lower than that at which it had started moving. “This is something we knew should happen, but we weren’t sure if we could technically control our systems precisely enough to be able to see this behaviour,” says Campbell.

"This work is really interesting, because it puts all the components of atomtronics together, studying collective behaviour of atoms and memory effects," says Shan-Wen Tsai, a physicist at the University of California, Riverside.

Atomtronics will not directly replace electronics, because atomic circuits are slower and bigger than their zippy electron counterparts. "You wouldn’t put a billion atomtronics components on a chip," Tsai adds.

But the atomtronic circuit could be useful in applications such as rotation sensors, playing the part that gyroscopes have in spacecraft and aeroplane navigation. The devices could also some day perform rudimentary quantum computations, says Ludwig Mathey, a theoretical physicist at the University of Hamburg in Germany, who works on simulating ultracold atom systems. Future 'quantum computers' promise to perform certain tasks exponentially faster than any traditional computer ever could. “[BECs] have the advantage of being more robust than other kinds of quantum computer,” he says.

And because superfluidity in atoms is analogous to the way electrons flow without resistance in a superconductor, studying the transitions in atomtronics could drive theoretical work in superconductivity, says Campbell. Still, she acknowledges that practical devices are far in the future. “We’re still in the infancy of learning how to control our systems and what we can do. But that is our hope,” she adds.

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