BERKELEY – Physicists at the University of California, Berkeley, have the universe's coldest substance running in circles.
TheUC Berkeley team has created a Bose-Einstein condensate of rubidiumatoms and nudged it into a circular racetrack 2 millimeters across,creating a particle storage ring analogous to the accelerator storagerings of high energy physics. This ring, the first to contain aBose-Einstein gas, is full of cold particles at a temperature of onlyone-millionth of a degree above absolute zero, traveling with energiesa billion trillion times less than the particles in a high-energystorage ring.
The creation of a Bose-Einstein condensate (BEC) ina storage ring is reported in a paper accepted last week by the journalPhysical Review Letters.
Though such slow-moving rubidium atomswould be useless for producing the exotic collision particles that arethe bread and butter of high-energy accelerators, cold collisions ofsuch atoms might reveal new quantum physics, said Dan Stamper-Kurn,assistant professor of physics at UC Berkeley and leader of the study.
"Thisexperiment was a very fortunate accident," Stamper-Kurn said. "Coldcollisions could give very precise information about the quantum statesof these cold particles and the potential between the particles."
Apartfrom basic physics, however, the millimeter storage rings could be usedas sensitive gyroscopes to detect minute changes in rotation. If a ringcould be created with two BECs traveling in opposite directions, thequantum interference pattern the two matter waves create would shiftwith rotation, allowing exquisitely sensitive detection of rotation foruse in research or navigation systems for satellites or aircraft.Similar sensitive quantum rotation detectors were proposed severalyears ago by other UC Berkeley physicists using superfluid heliumflowing in a ring.
Other possible areas of study includequantized circulation, which is seen in superfluids andsuperconductors, and fluid analogs of general relativity.
Stamper-Kurnand Subhadeep Gupta, a Miller post-doctoral fellow at UC Berkeley, gotthe idea for a cold storage ring while building a more elaborate devicethat would create supercold atoms inside a mirror cavity to study theinteractions between light and BECs. The first BEC was generated only10 years ago, and Stamper-Kurn was part of the Massachusetts Instituteof Technology team that first created this new form of matter. Thatteam's leader, Wolfgang Ketterle, shared the 2001 Nobel Prize inPhysics.
As first proposed 80 years ago by Albert Einstein, basedon previous work by Satyendra Nath Bose, if a gas of neutral atoms iscooled to a low enough temperature, all atoms of the gas would fallinto the same quantum state. In other words, all of the million orbillion atoms in the gas would end up in the same place at the sametime, a weird quantum state dubbed a Bose-Einstein condensate.
Thesupercold atoms are created from a hot gas of neutral atoms that islaser cooled, collected in a magneto-optic trap, cooled further byevaporation, and then spun off into a magnetic trap for a few secondsof study before it warms up and dissipates. Like most such BECrefrigerators, the UC Berkeley device trapped about a million rubidiumatoms in a microscopic nebula at the center of the magnetic trap.
Becauseof the UC Berkeley team's particular design, the researchers found iteasy to magnetically push the clump of atoms into a circular trajectoryinside the magnetic trap. Over the course of several circuits of thetrack, the clump tended to spread out into a beam analogous to theparticle beams in accelerators, the first of which was created byErnest O. Lawrence at UC Berkeley in 1931.
"The atoms fill thering in a matter of seconds," said Gupta. Since photographing the BECdestroys it, Gupta and his team repeated the experiment every twominutes until they had captured every stage in the evolution of thecircling beam of atoms.
The atoms circled the racetrack at aspeed of about 50 to 150 millimeters per second, which is equal to anenergy of about one nano-electron volt (eV) per atom, or one billionthof an electron volt. High-energy particle accelerators routinely bumpparticles to energies of a few tera-electron volts, or a trillion eV -a billion trillion times more energetic than the cold rubidium atoms.
Theatoms made as many as 20 laps in the two seconds before dissipating -enough time for Gupta and graduate students Kater Murch, Kevin Mooreand Tom Purdy to study them.
Stamper-Kurn and his colleagues arepursuing further experiments with the storage ring while continuing tobuild the originally planned device, which will be used to study cavityquantum electrodynamics and possible applications in quantum computers.
Thework was sponsored by the National Science Foundation, the DefenseAdvanced Research Projects Agency, the David and Lucile PackardFoundation and the University of California.
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