ScienceWeek

CONDENSED MATTER: ON ULTRACOLD MOLECULES

The term "Feshbach resonance" refers to a transient "sticking" of two colliding atoms, the sticking involving a resonance coupling that occurs when the molecular state has nearly zero energy. The term "optical trapping" refers to the confinement of entities in a restricted geometry by the controlled action of light. In this report, the term "inelastic" refers to a collision process in which the total kinetic energy of the colliding particles is not the same after the collision as before it, and the term "coherent beams of atoms" refers to beams composed of atoms moving in unison.

The following points are made by Paul S. Julienne (Nature 2003 424:24):

1) At the quantum level, any particle can also be considered to be a wave, its momentum corresponding to a wavelength. In ultracold atomic gases, at temperatures much lower than a millionth of a degree above absolute zero, the wavelength of the atoms becomes larger than the mean distance between them, giving rise to some remarkable quantum mechanical properties that are at the forefront of contemporary physics research(1).

2) The properties of such cold quantum gases depend on whether the atoms are bosons or fermions. Bosonic atoms have integer values of the quantum number known as "spin", and can form a Bose–Einstein condensate in which the trapped atoms occupy the single ground state of the system. Fermionic atoms, on the other hand, have half-integer spin values and each identical fermion must occupy a different quantum level. But if two fermions paired up to make a bosonic molecule, an exotic kind of superfluidity --flow without resistance -- could be the result(2-4).

3) Regal et al(5) have described an important step in this direction. Their experiment began with a quantum gas of fermionic potassium atoms, a mixture of equal numbers of atoms with spin quantum numbers -9/2 and -5/2. The different spin states are necessary for the fermions to undergo collisions with each other with low collision energy. To create molecules from these atoms, Regal et al took advantage of a special molecular state known as a "Feshbach resonance", which may be thought of as a weakly bound pair of atoms. The energy of such a state can be tuned, using a magnetic field, to lie close to that of two separated atoms. By ramping the magnetic field so that the energy of the resonance state moved from being above to being below the energy of two separated atoms, colliding pairs of atoms were induced to form a lower-energy molecule that is bound with respect to the separated atoms. Regal et al were able to convert up to half of the atoms to diatomic molecules.

4) Producing a molecular gas in this ultracold regime has proved to be a challenge. There have been several proposals to make molecules, and possibly a molecular Bose–Einstein condensate, by combining bosonic atoms in a Bose–Einstein condensate using a molecular resonance state. Experiments have come tantalizingly close. Wynar et al (2000) were the first to infer the formation of molecules nearly at rest, by observing the loss of condensed rubidium-87 atoms as they were photoassociated into molecules using a two-color pulse of light. Donley et al (2002) have also given evidence for coherent atom–molecule interconversion in experiments with a rubidium-85 condensate that involved manipulating the energy of a Feshbach-resonance state using a sequence of magnetic-field pulses. However, neither experiment could detect any molecules directly: the imaging methods used for atoms do not work for molecules, because they have very different light-scattering properties.

References (abridged):

1. Nature Insight on Ultracold Matter Nature 416, 205-246 (2002)

2. Houbiers, M. & Stoof, H. T. C. Phys. Rev. A 59, 1556-1561 (1999)

3. Bruun, G., Castin, Y., Dum, R. & Burnett, K. Eur. Phys. D 7, 433-439 (1999)

4. Holland, M., Kokkelmans, S. J. J. M. F., Chiofalo, M. L. & Walser, R. Phys. Rev. Lett. 87, 120406 (2001)

5. Regal, C. A., Ticknor, C., Bohn, J. L. & Jin, D. S. Nature 424, 47-50 (2003)

Nature http://www.nature.com/nature

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QUANTUM ENCOUNTERS OF THE COLD KIND

The following points are made by K. Burnett et al (Nature 2002 416:225):

1) Since the introduction of laser-cooling techniques for neutral atoms in the early 1980s, the study of collisional interactions between atoms and molecules has been extended to the regime of ultracold temperatures. With nanokelvin temperatures now attainable, our ability to probe the interactions, both experimentally and theoretically, has also progressed. Understanding of the subtle and often highly quantum-mechanical effects that are manifest at such low energies has advanced to the point where new precision measurements are matched by highly accurate theoretical calculations. Low-energy phenomena such as Bose Einstein condensation and the photoassociation of atoms into bound molecules are now accurately described with no free parameters.

2) The behavior of atoms and their interactions at ultracold temperatures is a fascinating area of study. These interactions and their effects distinguish them from those encountered in collisions at room temperature. The realization that these interactions would be both subtle and interesting began in the 1970s with studies(1) of spin-polarized hydrogen and long-range molecules, and expanded as laser cooling(2-4) reached temperatures in the millikelvin and then microkelvin ranges. With the advent of evaporative cooling(5) and the production of atomic Bose Einstein condensates (BECs), we now require a detailed understanding of atomic interactions at nanokelvin temperatures. These applications have driven a tremendous growth of interest in the field.

3) It was realized early on that the quantal nature of ultracold atomic interactions would have a profound role and provide a challenge to theorists and experimentalists in the field of collision dynamics. At the low energies involved, the precise nature of the interatomic forces -- at ludicrously large distances from the point of view of "normal molecular physics" --would have to be determined, requiring new ways to examine them. It was also evident that nuclear spin dynamics, usually irrelevant to collision dynamics, would complicate the analysis enormously. In fact, the term complicated does not do service to the range of new physics that the hyperfine interactions bring up in this regime.

References (abridged):

1. Weiner, J., Zilio, S., Bagnato, V. S. & Julienne, P. S. Experiments and theory in cold and ultracold collisions. Rev. Mod. Phys. 71, 1-85 (1999)

2. Cohen-Tannoudji, C. Manipulating atoms with photons. Rev. Mod. Phys. 70, 707-719 (1997)

3. Chu, S. The manipulation of neutral particles. Rev. Mod. Phys. 70, 685-706 (1997)

4. Phillips, W. D. Laser cooling and trapping of neutral atoms. Rev. Mod. Phys. 70, 721-741 (1997)

5. Ketterle, W. & Van Druten, N. J. Evaporative cooling of trapped atoms. Adv. At. Mol. Opt. Phys. 37, 181-236 (1996)

Nature http://www.nature.com/nature

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ULTRACOLD MATTER: SUPERFLUIDITY IN FERMI GASES

The following points are made by L. Pitaevskii and S. Stringari (Science 2002 298:2144):

1) Over the past decade, studies of ultracold atomic gas clouds have yielded unprecedented insights into the quantum statistical properties of matter, with most studies have focused on boson gases. The elementary constituents of matter can be divided into fermions and bosons. Fermions are particles whose intrinsic angular momentum (or spin) is an odd multiple of h/2(pi), where (h) is the Planck constant. In contrast, the angular momentum of bosons is an even multiple of h/2(pi). The dramatically different thermodynamic properties of fermions and bosons at low temperature are a direct result of quantum statistical effects.

2) The fundamental constituents of atoms (electrons, neutrons, and protons) are fermions. However, pairs of fermions -- and, in general, systems composed of an even number of fermions -- behave like bosons. Because of their bosonic properties, hydrogen and several alkali elements can be used to study the phenomenon of Bose-Einstein condensation (2). But some isotopic species of these alkali atoms, like 6Li and 40K, with an odd number of fermions, instead exhibit fermionic behavior.

3) The first signatures of quantum statistical effects in atomic Fermi gases were reported in 1999 (3). An important motivation for these studies is the search for the transition to the superfluid phase (4), analogous to the transition exhibited by superconductors and liquid 3He. According to the standard theory of fermion superfluidity, this transition should take place at extremely low temperatures, well below the Fermi temperature T(F) (the typical temperature where quantum effects show up). Attempts to reach such temperatures with trapped atomic gases have encountered major difficulties because the cooling mechanisms become less and less efficient with decreasing temperature.

4. In contrast to other systems (such as atomic nuclei, liquid 3He, and superconductors), the trapping and interaction mechanisms in atomic gases can be manipulated in a controlled manner, allowing the interaction between atoms to be tuned (5). By changing the strength of the magnetic field, the value and even the sign of the scattering length can be changed. The scattering length can be extremely large, much larger than the average distance between atoms. As a result, the number of collisions increases dramatically, enhancing the efficiency of the cooling mechanisms, which are based on evaporation.

References (abridged):

1. K. M. O'Hara, S. L. Hemmer, M. E. Gehm, S. R. Granade, J. E. Thomas, Science 298, 2179 (2002)

2. M. H. Anderson et al., Science 269, 198 (1995)

3. B. DeMarco, D. S. Jin, Science 285, 1703 (1999)

4, G. Shlyapnikov, in Proceedings of the 19th International Conference on Atomic Physics, Cambridge, MA (World Scientific Publishing), in press.

5. S. Inouye et al., Nature 392, 151 (1998)

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