While pursuing the goal of turning a cloud of ultracold atoms into a completely new kind of circuit element, physicists at the National Institute of Standards and Technology (NIST) have demonstrated that such a cloud—known as a Bose-Einstein condensate (BEC)—can display a sort of “memory.”
The findings, featured as the cover article of the Feb. 12, 2014, issue of Nature, pave the way for a host of novel devices based on “atomtronics,” an emerging field that offers an alternative to conventional electronics.
Just as electronic devices manipulate the flow of electrons, atomtronic devices manipulate the flow of atoms. Since atoms have properties that are very different from electrons, atomtronic devices have the potential to go beyond the capabilities of electronics. The newfound effect of the BEC could be an important tool for constructing atomtronic devices similar to computer memory, according to the research team’s leader, Gretchen Campbell.
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.
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.
A BEC, a gas of atoms cooled to nearly absolute zero, is an exotic form of matter that exhibits superfluidity—flow without resistance. This and other properties make BECs potentially useful in atomtronics. The field is still in its infancy though, so the team is exploring BEC-based analogs to well-understood devices. In this study, they looked at ways to make a BEC rotate, knowledge that might one day produce more sensitive rotation sensors.
The team created a BEC out of about 400,000 sodium atoms suspended by laser beams, which corralled the BEC into a doughnut-shaped cloud about as wide as a strand of hair is thick. Another laser acted as a “slotted spoon” that stirred the cloud, making the doughnut spin like a wheel. While stirring their BEC, the researchers saw some behavior they expected—and some they didn’t.
“A stirred BEC flows only at certain velocities—starting with the spoon at rest, as one stirs more rapidly, the BEC initially stays at rest, then suddenly, at a ‘critical’ stirring rate, starts to flow,” says Campbell, a NIST physicist. “Curiously, the stirring rate at which the BEC jumps into motion is not the same as the stirring rate to get the BEC to jump back to rest; in some cases, one even has to stir backwards.”
A similar effect exists in a magnetic hard disk drive: the magnetic field needed to change a memory bit differs depending on whether you are changing a zero to a one or vice versa. This effect, called “hysteresis,” gives the hard drive stability, allowing it to store computer data. In principle, information also could be stored in the flow state of an atomtronic circuit, and an advantage of a BEC system is that the stability of the hysteresis can be tuned by changing the properties of the laser “spoon.”
What surprised the team was that the most common, albeit imperfect, theory of BECs did not predict correctly how changing the stirring laser—altering the size of the slots in the spoon, as it were—changes the stirring rate at which the BEC switches from one velocity to another. This unexpected finding implies there is something the most common theory of BECs has left out.
“Nevertheless, the demonstration of hysteresis in an atomtronic device opens up lots of possibilities,” Campbell says. “It might now be possible to make a host of atomtronic devices such as switches, more sensitive gyroscopes or maybe even a different type of a quantum computer.”
Atomtronics is an emerging interdisciplinary field that seeks to develop new functional methods by creating devices and circuits where ultracold atoms, often superfluids, have a role analogous to that of electrons in electronics. Hysteresis is widely used in electronic circuits-it is routinely observed in superconducting circuits and is essential in radio-frequency superconducting quantum interference devices. Furthermore, it is as fundamental to superfluidity (and superconductivity) as quantized persistent currents, critical velocity and Josephson effects. Nevertheless, despite multiple theoretical predictions, hysteresis has not been previously observed in any superfluid, atomic-gas Bose-Einstein condensate. Here we directly detect hysteresis between quantized circulation states in an atomtronic circuit formed from a ring of superfluid Bose-Einstein condensate obstructed by a rotating weak link (a region of low atomic density). This contrasts with previous experiments on superfluid liquid helium where hysteresis was observed directly in systems in which the quantization of flow could not be observed, and indirectly in systems that showed quantized flow. Our techniques allow us to tune the size of the hysteresis loop and to consider the fundamental excitations that accompany hysteresis. The results suggest that the relevant excitations involved in hysteresis are vortices, and indicate that dissipation has an important role in the dynamics. Controlled hysteresis in atomtronic circuits may prove to be a crucial feature for the development of practical devices, just as it has in electronic circuits such as memories, digital noise filters (for example Schmitt triggers) and magnetometers (for example superconducting quantum interference devices).
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