Cold Atoms Make for Hot Physics
By: Professor Steven Rolston

Atomic physics, the study of the properties of atoms and how they interact, has undergone its ups and downs. Careful measurement of atomic spectra was the impetus for the development of quantum mechanics in the early part of the 20th century. By the 1950's the field was rather mature, and it seemed that exciting discoveries would mostly be occurring elsewhere in physics. All that changed with the invention of the laser in the 1960s, which rejuvenated atomic physics. The invention of laser cooling in the 1980s and the observation of Bose-Einstein condensation in the 1995 has kept the momentum going, as well as leading to two Nobel Prizes (1997 and 2001). The Physics Department has recently added two AMO physicists, Luis Orozco and myself, joining William Phillips (1997 Nobel prize winner).
In my laboratories (in the basement of the Computer and Space Science building) we are setting up two experiments, both using ultracold atoms, and both probing physics at the boundaries between disciplines, atomic and plasma physics in one, and atomic and condensed matter in the other.

By carefully scattering millions of photons off an atom, laser cooling can reduce temperatures of atoms from room temperature (300 K) down to a few millionths of a degree above absolute zero. At these temperatures, the thermal velocity of an atom is only a few cm/s. Plasmas, on the other hand, are often measured in terms of thermal energies in the eV range (1 eV corresponds to a temperature of ~12000 K). A few years ago, we realized that we could use our really cold atoms to produce plasmas with temperatures lower than had ever been produced before. A plasma is a gas of charged particles (the most common state of matter in the universe), quite often electrons and ions, which of have been separated from each other by high temperatures (if the thermal energy is greater than the binding energy of the atom, it will be ionized). To produce our ultracold plasmas, we photoionize our ultracold xenon atoms with a laser pulse with a well-defined photon energy. The resulting temperature of the plasma is simply given by the laser photon energy minus the binding energy of the electron to the ion. Because we start with laser-cooled xenon atoms with a temperature of 10 µK, there is no additional thermal energy from the atoms. Of course the electrons would really rather be bound to the ions (the atom is the lowest energy state of the system), so it was not clear at the outset that we could even form a plasma - perhaps it would just immediately recombine into neutral atoms. We discovered a bit of both - we can indeed form ultracold plasmas with temperatures down to a few degrees K, but we also observed a significant fraction (up to 30%) of the charged particles recombine into atoms (into highly excited Rydberg atoms which are energetically the most likely to be formed). Our plasmas are currently unconfined, and as they expand after formation they undergo cooling due to adiabatic expansion as well as heating as a by-product of recombination. We are currently working on an understanding of this dynamic system, and have future plans to introduce magnetic confinement as well as probe other collective aspects of these plasmas. Such ultracold plasmas extend the parameter range of plasma physics, and have stimulated development of state-of-the-art molecular dynamics calculations, which try to follow the individual trajectories of thousands of particles, a computationally challenging task, since the orbit time of an electron around an ion may be picoseconds while the characteristic expansion time may be milliseconds.

Ultracold plasmas experiments are table top physics - pictured here are post-docs Chad Fertig and Jake Roberts, standing in front of the vacuum chamber where the ultracold plasmas are formed.

Our second area of research combines ultracold coherent atoms from a Bose-Einstein condensate (BEC) with periodic potentials created by interfering laser beams, known as optical lattices. This combination allows us to study almost perfect analogs of condensed matter systems, as our optical lattices are defect and impurity free, unlike real world solid-state materials. Using atoms from a BEC allows us to study coherent quantum phenomena, such as how atoms (they should really be thought of as waves in this limit) spread throughout our optical lattices. Interest in quantum computation has stimulated much of the solid-state community to start paying attention to coherence, trying to isolate individual quantum systems from the environment so that they maintain their wavelike properties. Atomic physics has always cared about coherence, and it is much easier to isolate a neutral atom from its environment than an atom imbedded in a crystal. BECs offer a beautiful source of about a million atoms, all in the same quantum state, i.e. completely coherent. We plan on loading these atoms (created with the now standard techniques to make a BEC) into various periodic optical potentials (set by the geometry, frequencies and polarizations of the interfering laser beams). There are a rich variety of topics to explore, and a partial list includes i) quantum phase transitions (phase transitions - such as water freezing into ice - are usually thermally activated processes, but quantum phase transitions occur even at T=0, where the nature of the lowest energy state of the system can suddenly change when a parameter of the system is varied), ii) quantum chaos - the study of quantum systems whose underlying classical description is chaotic, iii) the quantum-classical transition - why does our world appear classical when we know it is most correctly described by quantum mechanics?, and iv) the role of disorder in periodic systems - we will be able to optically add disorder to our lattices with much more control than can ever be managed in a solid state system. This research sits squarely on the boundary between atomic and solid state physics, and we hope as atomic physicists to be able to cleanly address numerous topics of great interest to the condensed matter community.

A representation of the optical lattice potential created by the interference of four laser beams, and atoms trapped at the minima of the potential
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Dr. Steve Rolston is a full professor working in the field of AMO (atomic, molecular & optical) physics here in the Department of Physics at the University of Maryland. If you have any questions, he can be reached at rolston@physics.umd.edu.