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Cornell University


Cold Gases

At room temperatures the behavior of a gas of atoms is dominated by their random thermal motion. Averaged over time this gives simple descriptions in terms of thermodynamic variables such as Temperature and Pressure. As the temperature is lowered, this thermal motion is reduced. The Heisenberg uncertainty principle prevents the atoms from coming to a stop. Instead, at nanokelvin temperatures, quantum mechanics dictates the properties of these atomic gases. We study this strange and beautiful form of quantum matter. For some of our underlying motivations, please see essays written by individual group members:

Basic Physics of Cold Gases

It seems silly to write too much here. Instead we recommend that you look at some of the wonderful (interactive) essays on the subject which are already on the web:


These are a bit more technical, but are very useful.
  • Ultracold Atom News: The site for all your ultracold atom information
  • Center for Ultracold Atoms is a joint venture between MIT and Harvard. Has links to the home pages of almost all cold-gas researchers at MIT and Harvard.
  • Istituto Nazionale per la Fisica della Materia Bose-Einstein Condensation in Trento Italy has a large concentration of theory on cold atoms.
  • JILA [formerly the Joint Institute for Laboratory Astrophysics -- a collaboration between NIST (National Institute for Standards and Technology) and the University of Colorado)] has one of the highest concentrations of cold gas research in the country.
  • Randy Hulet runs one of the premier cold atoms experimental groups from Rice University in Texas

Vortices and Topological Defects

Order Parameters

Much of physics deals with the competition between order and disorder. Energetic typically drive matter towards an ordered state: the energy of a collection of sodium atoms is minized if it forms a crystal. On the other hand, energy is not the only concept playing a role in determining the behavior of a physical system. If you are at finite temperature, then entropy favors a disordered state. Similarly, at zero temperature "quantum fluctuations" may favor a disordered state. [For example, a classical crystal has every atom localized. A corrolory of the Heisenberg uncertainty principle is that it is hard to truly localize particles. Consequently you have materials such as Helium, which remain liquid down to arbitrarily low temperatures (at least at atmospheric pressure).]

Ordered states of matter typically have more degrees of freedom than unordered states: one would not notice if you uniformly translated all of the atoms in a liquid, but such a move would be important in a crystal. The object which encodes these degrees of freedom is the "order parameter". Order parameter textures occur when the order parameter varies slowly through space. A particularly interesting form of textures are those that are topological, meaning that they require the order parameter to be discontinuous. For example a dislocation in a crystal is topological.

BEC-BCS Crossover

The many-body physics of strongly interacting Fermions lies at the heart of modern condensed matter and nuclear physics. Atomic physics has emerged as a field which can provide key insights into the collective effects which happen in these systems. Experimentalists take dilute clouds of alkali atoms (with densities n~1013cm-3), such as 7Li, and cool them to quantum degeneracy (T~nK). They trap N~106 of these atoms in a harmonic optical trap. Most experiments trap these atoms in two different hyperfine states, (↑ and ↓). These atoms interact with magnetically tunable short-range interactions, parameterized by a scattering length a. In the unitary limit, where a is tuned to ∞, the system is as strongly interacting as is allowed by conservation laws, and the physical properties are then independent of microscopic details. Thus, by studying a unitary gas of atoms, one learns about the equation of state of a generic strongly interacting Fermi gas. For example, one can relate these results to the equation of state of the high density nuclear matter found in the center of neutron stars. A related state of matter is formed at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven. At low temperatures, experiments find that these strongly interacting fermions are superfluid. When a0, the low energy scattering corresponds to attractive interactions, and the ground state is analogous to a Bardeen, Cooper, and Schreifer (BCS) superconductor. When a0$ the low energy scattering corresponds to repulsive interactions, however there exists a two-body (molecular) bound state in vacuum. In this limit the ground state corresponds to a Bose-Einstein Condensate (BEC) of these molecules. Experimentalists find a smooth crossover between these states when a changes sign by passing through ∞. The superfluid state of strongly interacting fermions is protected by a large energy gap, and is therefore quite conventional. One consequence is that despite the strong interactions, mean field theory has been quite successful at predicting the properties of the superfluid state. The normal state, however, may be much more exotic. At low temperatures, experimentalists suppress s-wave superfluidity by polarizing the atomic gas. The spin relaxation time is extremely long in these systems, allowing them to produce arbitrarily large polarizations. They have observed that the normal state is strongly interacting, and theorists have used scaling arguments to extract features of the equation of state from the experimental results.

RF Spectroscopy

Spectroscopy has long been a probe of atomic structure, and is often used to learn about solid state systems. Radio frequency spectroscopy seems to be especially useful to probing the properties of cold atoms. We have a couple presentations on this:
  • This presentation was used during a course Erich Mueller taught: RF.key
  • This one was used for a seminar at OSU: clock -- OSU.ppt
  • March Meeting 2008
  • PQE