Friday, February 14, 2014

Observation opens possibilities for building practical atomtronic device

COLLEGE PARK, MD -- 'Atomtronics' is an emerging technology that uses ensembles of atoms to build analogs to electronic circuit elements. While modern electronics rely on the charge properties of the electron, atomic systems can be engineered using lasers and magnetic fields to behave like electrons. This feature makes atomic systems an exciting platform for studying and generating alternatives to charge-based electronics.

Cover of Nature highlighting this research. Credit: Nature Press OfficeUsing a superfluid atomtronic circuit, physicists at the University of Maryland and the National Institute of Standards and Technology (NIST) have demonstrated hysteresis in an ultracold atomic gas for the first time. This research, led by Gretchen Campbell of the UMD-NIST Joint Quantum Institute (JQI), was published in the Feb. 13, 2014 issue of the journal Nature, whose cover features an artistic impression of the atomtronic system.

"Hysteresis is ubiquitous in electronics," said Stephen Eckel, a postdoctoral researcher at JQI. "This effect is used in writing information to hard drives as well as other memory devices. It's also used in certain types of sensors and in noise filters such as the Schmitt trigger."

Hysteresis is something we encounter in our daily lives. When we set a desired temperature on the air conditioning thermostat in our homes, and the room air exceeds this temperature, a fan switches on to cool the room. Then, the fan lowers the temperature to one lower than the one we set before turning off. The mismatch between the temperatures that turn the fan on and off is an example of hysteresis and prevents fast switching of the fan, which would be highly inefficient. Here, the hysteresis is programmed into the electronic circuit.

For the current study, physicists observed hysteresis that is an inherent natural property of a quantum fluid. Researchers cooled 400,000 sodium atoms to condensation, forming a type of quantum matter called a Bose-Einstein condensate (BEC), which has a temperature around 0.000000100 Kelvin (0 Kelvin is absolute zero). The atoms reside in a doughnut-shaped trap that is only marginally bigger than a human red blood cell. A focused laser beam intersects the ring trap and is used to stir the quantum fluid around the ring.

While BECs are made from a dilute gas of atoms less dense than air, they have unusual collective properties, making them more like a fluid—or in this case a "superfluid." First discovered in liquid helium in 1937, this form of matter, under some conditions, can flow persistently, undeterred by friction. A consequence of this behavior is that the fluid flow or rotational velocity around the team's ring trap is quantized, meaning it can only spin at certain speeds. This is unlike a non-quantum (classical) system, where its rotation can vary continuously and the viscosity of the fluid plays a substantial role.

Schematic of laser setup (red and blue beams) to create flat, toroidal-shaped Bose-Einstein condensate (yellow). Credit: Gretchen Campbell

Because of the characteristic lack of viscosity in a superfluid, stirring this system induces drastically different behavior. Here, physicists stir the quantum fluid, yet the fluid does not speed up continuously. At a critical stir rate, the fluid jumps from having no rotation to rotating at a fixed velocity. The stable velocities are a multiple of a quantity that is determined by the trap size and the atomic mass.

Building on their previous work demonstrating persistent currents and this quantized velocity behavior in superfluid atomic gases, the researchers explored what happens when they stop the rotation, or reverse the system back to its initial velocity state. Without hysteresis, they could achieve this by reducing the stir rate below the critical value, causing the rotation to cease. In fact, they observed that they had to go far below the critical stir rate, and in some cases reverse the direction of stirring, to see the fluid return to the lower quantum velocity state.

Controlling this hysteresis opens up new possibilities for building a practical atomtronic device. For instance, there are specialized superconducting electronic circuits that are precisely controlled by magnetic fields, and in turn, small magnetic fields affect the behavior of the circuit itself. These devices, called superconducting quantum interference devices (SQuIDs), are used as magnetic field sensors.

"Our current circuit is analogous to a specific kind of SQuID called a radio frequency SQuID," said Campbell, who is a JQI Fellow with appointments at both NIST and UMD. "In our atomtronic version of the SQuID, the focused laser beam induces rotation when the speed of the laser beam 'spoon' hits a critical value. We can control where that transition occurs by varying the properties of the spoon. Thus, the atomtronic circuit could be used as an inertial sensor."

This two-velocity state quantum system has the ingredients for making a qubit. However, this idea has some significant obstacles to overcome before it could be a viable choice. Atomtronics is a young technology and physicists are still trying to understand these systems and their potential. One current focus for Campbell's team includes exploring the properties and capabilities of the novel device by adding complexities such as a second ring.

This research was supported by the National Science Foundation Physics Frontier Center at JQI.

-UMDCP/CMNS-

Emily Edwards, eedwards@umd.edu, 301-405-2291

This article was written by Emily Edwards/JQI.

The research paper, “Hysteresis in a quantized superfluid 'atomtronic' circuit,” Stephen Eckel, Jeffrey G. Lee, Fred Jendrzejewski, Noel Murray, Charles W. Clark, Christopher J. Lobb, William D. Phillips, Mark Edwards & Gretchen K. Campbell, was published Feb. 13, 2014 in Nature.