Ultracold Atoms and Atom Optics

The research group of Randall Hulet uses lasers to cool atoms down to temperatures of just a few-billionths of a degree above absolute zero (see lower left front corner of cover). At these temperatures, atoms cease behaving as individual particles but rather as a many-bodied quantum state. They have shown that the Bose-Einstein condensation of self-attracting atoms, in their case, ⁷Li atoms, collectively collapse in a way that is qualitatively similar to a supernova. For a condensate confined to only one dimension, however, the same attractive interactions lead to the formation of matter-wave solitons, a wave-packets that travels without spreading or changing shape. Hulet's team has observed the formation of soliton trains containing up to 10 individual solitons [K.E. Strecker, et al., Nature 417, 150 (2002)]. This soliton atom laser may be useful as an input source for an atom interferometer.

A laser-based atom trap

In another experiment, the researchers have cooled a gas of ⁶Li atoms, which obey Fermi-Dirac statistics, to the quantum degenerate regime [A.G. Truscott, et al., Science, 291, 2570 (2001)]. They saw that the size of the atom cloud was stabilized by Fermi pressure, similar to the way a white dwarf star is stablized against gravitational collapse. Recently the group has caused this gas of Fermi atoms to form diatomic molecules, which in turn are composite bosons [K.E. Strecker, et al., Phys. Rev. Lett. 91, 080406 (2003)]. The next goal will be form a Bose-Einstein condensate with these molecules and to investigate the BCS transition to a Fermi superfluid. Such a phase transition is analogous to the superfluid transition of ³He and to superconductivity, and has never been produced in the gas phase. Achievement of this goal will open up new directions in strongly correlated fermion research.

Thomas Killian's research group studies ultracold neutral plasmas and high resolution spectroscopy of ultracold atoms. Ultracold neutral plasmas have temperatures as low as 1 K, orders of magnitude colder than traditional neutral plasmas, such as the solar corona (1,000,000 K) or a candle flame (1000 K). They are created by photoionizing laser-cooled strontium atoms just above the ionization threshold. Experiments in this new regime stretch our understanding of basic plasma science and may shed light on the physics of exotic environments such as thermonuclear devices or the cores of gas giant planets. High resolution spectroscopy is the probing of narrow absorption resonances with extremely monochromatic lasers. Such experiments apply the latest advances in laser physics and require working with an atom, such as strontium, that has narrow resonances. Many experiments naturally follow from high resolution spectroscopy of cold and dense gases. The quantum level spacing in an atom trap can exceed the resonance linewidth, and one can manipulate and study matter wave modes like light in optical cavities and waveguides. Understanding atom-atom interactions and decoherence in such systems is important in the emerging field of transporting and controlling coherent samples, and is also of fundamental interest. A narrow transition can also probe the effects of quantum statistics on optical spectra and cold collisions.

optical absorption of an ultracold neutral plasma

The figure above is a false color image of the optical absorption of an ultracold neutral plasma. A laser beam, resonant with the ions in the plasma, passes through the plasma and falls on a CCD camera. The optical depth is the fraction of light that is absorbed by the ions. Such images yield information on the density distribution and dynamics of the system.

Prof. Han Pu's research interest is in the field of theoretical atom optics, which covers different aspects of the physics of Bose-Einstein condensation, quantum degenerate Fermi gases, quantum optics and laser cooling and trapping. One of the most profound revolutions brought about by quantum mechanics is that it does away with the distinction between particles and waves: atoms, in particular, can exhibit all the properties that we associate with wave phenomena when cooled to ultracold temperatures. The development of these ideas leads to the emergence of the field of atom optics, a highly inter-disciplinary field with close ties to atomic physics, quantum optics, quantum information and condensed matter physics.

One of the particular systems studied by Pu's group is the socalled spinor condensate, an atomic condensate with spin degrees of freedom. Due to the nonlinear spin-exchange interaction, this system exhibits a variety of interesting behavior. Because of the close relation between atomic spin and its magnetic moment, a spinor condensate also represents a novel magnetic material. The effect of the long-range magnetic dipole-dipole interaction on the system is currently under study in thegroup. Another system in which they are interested in is an ultracold atomic cloud confined in periodic lattice potentials, an atomic analogy of a solid state crystal.

The novelty of the atomic systems lies in the fact that these systems are very clean with great experimental controllability, such that their properties can be exquisitely tailored. They are interested in a number of phenomena displayed by cold atoms in lattice potentials, such as the ground state phase diagrams, dynamics, quantum phase transitions, etc.