Cold Gases, Hot Topic
By Ernie Tretkoff
Ultracold quantum gases were a hot topic at the APS March Meeting in Montréal, where several groups reported their latest results in the race to explore a new state of atomic matter. In addition to providing insight into the quantum nature of atoms in this not yet well-understood regime, the investigations could lead to better understanding of high temperature superconductivity.
At extremely low temperatures, bosons, (particles with integer spin), can all pile together into a single quantum state, known as a Bose-Einstein Condensate (BEC). The BEC, in which many atoms act like a single entity, was first demonstrated in 1995.
Unlike bosons, fermions (particles with half-integer spin), obey the Pauli exclusion principle, which dictates that they cannot share the same quantum state. But if two fermion atoms pair up, they can act like a boson. Indeed, last fall several groups coaxed fermion atoms into molecules that collapsed into a BEC.
In addition to forming strongly bound molecules, fermions can combine in weakly-bound Cooper pairs, as electrons do in a Bardeen-Cooper-Schreiffer (BCS) superconductor, or as helium-3 atoms do in a superfluid. In these pairs, the fermions are correlated in momentum space, not position space.
Between the molecule and Cooper pair extremes lies a whole spectrum of interaction strengths, known as the BEC-BCS crossover region. Researchers have recently begun to explore this terrain, and several groups have demonstrated that they can tune inter-atom forces by adjusting an external magnetic field.
Several competing researchers described their experiments at the March Meeting, including Deborah Jin of NIST, Wolfgang Ketterle of MIT, Randy Hulet of Rice, and Johannes Hecker Denschlag of the University of Innsbruck.
Between the BEC and BCS extremes lies the Feshbach resonance, in which the energy of two free atoms equals that of a bound molecule. On one side of the resonance, the interactions between atoms are strongly attractive; on the other side the interactions are strongly repulsive. By varying the magnetic field, the researchers can smoothly and reversibly tune the interaction strength around this resonance.
"This is an amazing level of control," said Ketterle. "By changing the magnetic field we can go from a boson system to a fermi system."
The NIST group, which experiments with potassium atoms, approaches the crossover region from the BCS side; the MIT, Rice, and Innsbruck groups start with lithium molecules on the BEC side of the Feschbach resonance.
Although researchers can't yet tune the interaction all the way to the extreme BCS side, where true Cooper pairs would form and the gas would become a superfluid, the current data covers a large part of this crossover region. This intermediate region, with the ability to control the inter-atom interactions, may prove especially interesting, said Jin. "Ultimately, understanding the region we're in will tell us more about the connection between BEC and BCS," said Jin. "What's interesting is that we have this new knob that we don't typically have."
The research might even lead to better understanding of high-temperature superconductivity, because the strength of pairing in this crossover region corresponds to that expected for room-temperature superconductors.
Also, because these extremely low-density, low-temperature gases are relatively simple systems, they may aid in the understanding of more complicated condensed matter systems, said Ketterle.
The race to explore ultra-cold quantum gases has produced an astonishing number of developments recently, noted Randy Hulet. "I think the most exciting thing is the pace of discovery. We're going to be understanding lots of new physics."
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