From Supernova to Smoke Ring: Recent Experiments Underscore Weirdness of the Bose-Einstein Condensate

Bose-Einstein condensates can sometimes explode like a pocket-sized supernova, but when they're not doing that they can look like the inside of a cigar bar, awash in insubstantial smoke rings. These two disparate images of one of modern physics's most mysterious creations come out of recent research efforts at the Commerce Department's National Institute of Standards and Technology in cooperation with the University of Colorado at Boulder. Details of the work, which include some quite unexpected observations, were announced for the first time today at the American Physical Society March Meeting in Seattle, Wash.

Bose-Einstein condensation of atomic gases was predicted in theory by Albert Einstein in 1924, and was first observed by NIST and University of Colorado scientists in 1995 at their joint institute, JILA, located on the CU-Boulder campus. When the condensate is produced, virtually all the atoms in the gas fall into the lowest-energy quantum mechanical state. Spread out in space, they become superimposed on one another, each indistinguishable from the other, creating what has been called a "superatom." A collection of millions of atoms, extending over the entire gas, behaves like a single atom described by the "matter wave" picture of quantum mechanics.

A legitimately "new state of matter," Bose-Einstein condensates exhibit a variety of odd, large-scale quantum effects. Techniques in the laser-cooling of atoms pioneered at NIST were an important step in achieving the extraordinarily cold temperatures-only a few billionths of a degree above absolute zero-needed to produce a BEC, and have opened many novel possibilities for manipulating and studying of the behavior of quantum wave functions. BECs can exhibit the interference phenomenon of wave motion, for example, in which the crests and troughs of two waves cancel each other. They can also produce "matter wave lasers," which emit bright beams of atoms with identical speeds.

Building Your Own Supernova

Working at JILA, physicist Carl Wieman's [pronounced wy-man] team has explored tuning the self-interaction of atoms in a BEC. By making a BEC in a particular isotope of rubidium, rubidium-85, and then changing the magnetic field in which the BEC is sitting, the team is able to adjust the wavefunction's self-interaction between repulsion and attraction. If the self-interaction is repulsive, all the parts of the wavefunction push each other away. If it is attractive, they all pull towards each other, like gravity. Achieving a pure BEC in rubidium-85 required the cloud of atoms to be cooled to about 3 billionths of a degree above absolute zero, the lowest temperature ever achieved.

Making the self-interaction mildly repulsive causes the condensate to swell up in a controlled manner, as predicted by theory. However, when the magnetic field is adjusted to make the interaction attractive, dramatic and very unexpected effects are observed.

The condensate first shrinks as expected, but rather than gradually clumping together in a mass, there is instead a sudden explosion of atoms outward. This "explosion," which actually corresponds to a tiny amount of energy by normal standards, continues for a few thousandths of a second. Left behind is a small cold remnant condensate surrounded by the expanding gas of the explosion. About half the original atoms in the condensate seem to have vanished in that they are not seen in either the remnant or the expanding gas cloud.

Since the phenomenon looks very much like a tiny supernova, or exploding star, Wieman's team dubbed it a "Bosenova." The most surprising thing about the Bosenova is that the fundamental physical process behind the explosion is still a mystery.

"Understand that atoms have been very well studied. Essentially all the behavior of isolated atoms in general and BECs in particular we thought were quite well understood, and could be predicted accurately by theoretical calculations," said Wieman. "Even for those features that cannot be accurately predicted, the basic physical processes are still qualitatively well understood.

"But the theoretical calculations of what would happen in this situation predict behaviors that are totally unlike what we've observed, so the basic process responsible for the Bosenova must be something new and different from what has been proposed," Weiman said.

Wieman's team has made a detailed study of the Bosenova, including how it progresses in time, how it depends on the strength of the self-interaction of the condensate, and many other parameters. The fate of the missing atoms is still an open question, but the researchers suspect that they wind up either accelerated so greatly that they escape the trap undetected, or perhaps form molecules that are invisible to the detection system.

Solitons to "Smoke Rings"

In related work, experimental and theoretical teams under Eric Cornell of both NIST and CU-Boulder, and Charles Clark of NIST, have for the first time demonstrated vortex rings created in a Bose-Einstein condensate. Such vortices have been conjectured to occur in superfluids such as liquid helium or a BEC, but have never been observed directly.

A superfluid-typically liquid helium when it is within about 2 degrees of absolute zero-is one of the stranger manifestations of quantum theory on a macroscopic scale. Flowing without viscosity or friction, like current in a superconductor, a superfluid will flow uphill despite gravity, or up and over the walls of a container.

A vortex is a whirling pattern of liquid, a circular flow around a central core (the most common examples are tornados and whirlpools.) The core can be made to close upon itself, making a "vortex ring" - a familiar example is the smoke ring.

Certain experimental results led theorists to suspect that closed vortex rings occur in superfluids and play a crucial role in their behavior, but in a dense superfluid like liquid helium the rings are difficult to observe. A BEC, on the other hand, is an ultra-low density gas, but it exhibits the properties of a superfluid. The creation of permanent vortex motions is one of the striking features of superfluids and superconductors, and in 1999 the NIST/CU team at JILA first produced a simple vortex structure in a Bose-Einstein condensate. Computer simulations by the theoretical team predicted that vortex rings should occur in a BEC and be large enough to be seen.

To blow smoke rings in a BEC, the research team first created a strange phenomenon called a "dark soliton" in a spherical BEC of rubidium-87. A soliton is a special kind of wave that can travel for a long distance, maintaining its shape and velocity without dissipating its energy. The ripples caused by throwing a pebble in a pond are ordinary waves. A tsunami is a soliton. A dark soliton requires some imagination-it's a soliton wave made up of the absence of something.

Using a combination of microwave radiation and a specially-tuned laser beam played rapidly back and forth across the BEC, the researchers split the rubidium atoms into two different quantum states, creating a dark soliton in the center of the atoms of one state, filled with atoms in the second state. In photos the soliton is visible as a dark plane bisecting the glowing sphere of the BEC. "Filling" the soliton with other atoms is a special trick to give stability to what is otherwise a very unstable phenomenon.

At the proper moment a tuned laser beam is used to blow the stabilizing atoms out of the soliton. At almost the same moment, the trapping field that confines the BEC is turned off. The suddenly unstable soliton collapses into more stable vortex structures, and since the BEC is a sphere, it is encouraged to decay into vortex rings. Without its filling, the soliton and its decay products would still be too small to observe, but with the trap turned off the condensate rapidly expands in all directions, enlarging its internal structures like the patterns on an inflating balloon. In computer simulations the soliton can clearly be seen to decay into a series of concentric vortex rings, exactly like a particularly complicated set of smoke rings. The experimental photos strongly resemble the simulation images.

The teams plan further studies of the stability, lifetime and dynamics of vortex rings in BECs.

Details of these projects will be found in the following forthcoming papers:

• "Watching dark solitons decay into vortex rings in a Bose-Einstein condensate", B. P. Anderson, P. C. Haljan, C. A. Regal, D. L. Feder, L. A. Collins, C. W. Clark, and E. A. Cornell (to appear in Physical Review Letters, April 2, 2001)
• "Controlled Collapse of a Bose-Einstein Condensate" J. L. Roberts, N. R. Claussen, S. L. Cornish, E. A. Donley, E. A. Cornell, C. E. Wieman (submitted to Physical Review Letters.)

As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST strengthens the U.S. economy and improves the quality of life by working with industry to develop and apply technology, measurements and standards through four partnerships: the NIST Laboratories, the Baldrige National Quality Program, the Manufacturing Extension Partnership and the Advanced Technology Program.

JILA is a joint research institute of the National Institute of Standards and Technology and the University of Colorado. Located on the main university campus in Boulder, Colo., the Institute is a center for teaching and research in the areas of atomic, chemical, optical, laser, gravitational, and solar physics; semiconductors; precision measurement; astrophysics; and astronomy.

Editor's note: Digital movies of these two NIST-JILA experiments are available on the web:

• Implosion and explosion of a Bose-Einstein condensate "bosenova"
• Decay of a dark soliton into vortex rings in a Bose-Einstein condensate

Additional Contact: University of Colorado at Boulder, caughey [at] spot.colorado.edu (Peter Caughey), 303-492-4007