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Physics Today

The Fermionic Cousin of Bose–Einstein Condensate Makes its Debut

Barbara Goss Levi

Published Oct 1, 1999• 1 citation
Publisher

Abstract

Experimenters have been romping in the BEC playground ever since a gaseous Bose–Einstein condensate was first produced four years ago. It was almost inevitable, then, that researchers would also want to explore its fermionic cousin—the quantum degenerate state one expects to get by cooling a gas of atoms with half-integer spins. Whereas the integer-spin atoms (bosons) in a Bose–Einstein condensate all pile into the lowest energy state, the half-integer spin atoms (fermions) are prevented from doing so by the Pauli exclusion principle, which prohibits two identical fermions from occupying the same quantum state. Instead, the atoms fill all the lowest energy levels up to the Fermi energy, just as electrons do in a solid. The crossover into this quantum degenerate state is gradual, compared to the abrupt phase transition into a Bose condensate. But that crossover has now been convincingly detected by Deborah Jin, a physicist at the National Institute of Standards and Technology, and Brian DeMarco, a physics graduate student at the University of Colorado at Boulder, both working at JILA.

DOI
10.1063/1.882853

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Experimenters have been romping in the BEC playground ever since a gaseous Bose–Einstein condensate was first produced four years ago. It was almost inevitable, then, that researchers would also want to explore its fermionic cousin—the quantum degenerate state one expects to get by cooling a gas of atoms with half-integer spins. Whereas the integer-spin atoms (bosons) in a Bose–Einstein condensate all pile into the lowest energy state, the half-integer spin atoms (fermions) are prevented from doing so by the Pauli exclusion principle, which prohibits two identical fermions from occupying the same quantum state. Instead, the atoms fill all the lowest energy levels up to the Fermi energy, just as electrons do in a solid. The crossover into this quantum degenerate state is gradual, compared to the abrupt phase transition into a Bose condensate. But that crossover has now been convincingly detected by Deborah Jin, a physicist at the National Institute of Standards and Technology, and Brian DeMarco, a physics graduate student at the University of Colorado at Boulder, both working at JILA.
A Bose-Einstein condensate (BEC), the first of which was shown experimentally 22 years ago, isn’t your garden variety state of matter. It formed at a fraction above absolute zero and only in atoms that act like bosons, one of two types of fundamental particles. Bosons don’t follow the Pauli exclusion principle, which prohibits two particles from existing in the same quantum state. When bosonic atoms are cooled to form a condensate, they can lose their individuality. They behave like one big collective superatom, analogous to how photons become indistinguishable in a laser beam. But it’s even weirder than that. This velocity-distribution data for a gas of rubidium atoms confirmed the discovery of the Bose–Einstein condensate in 1995. In these three snapshots in time, atoms—cooled to near absolute zero—condensed from less dense areas on the left (red, yellow, and green) to very dense areas at the center and the right (blue and white). Image courtesy of NIST/JILA/CU-Boulder. “In a very good analogy, one can view a BEC as a bell, which begins to ring spontaneously when it is cooled below a certain temperature,” writes physicist Nick Proukakis at the Joint Quantum Centre Durham–Newcastle in the United Kingdom, in Universal Themes of Bose–Einstein Condensation , a forthcoming collection of research essays on progress in BECs (1). That behavior provides physicists with an extraordinary opportunity: to study bizarre quantum effects on a large scale, instead of having to probe individual particles. For the last two decades, physicists have treated BECs something like Play-Doh. They poke it, smash it, tickle it with lasers, and trap it in magnetic fields. They mix condensates together to see what happens, and use it to slow down light (2). They have observed strange behaviors that would have been impossible to predict even two decades ago: solids that flow …
This chapter will focus on the application of kinetic and hydrodynamic approaches to Bose–Einstein condensates. A Bose–Einstein condensate is a state of matter, formed of identical bosons (usually atoms behaving like bosons), so that at a sufficiently low temperature, many of the particles making up the system occupy the lowest energy state (ground state). This is only possible for bosons (particles with an integer spin) unlike fermions (particles with a half-integer spin) due to the Pauli exclusion principle.
About seven decades ago Satyendra Nath Bose and Albert Einstein predicted that a gas of noninteracting integer-spin particles would condense into a macroscopic quantum state when cooled below a critical temperature. Of course Bose–Einstein condensation (BEC) has long since been seen in superfluid He4 and superconductors, but the condensing systems in these examples are far from being noninteracting gases; relatively strong interactions between the condensing particles greatly complicate the theoretical analysis and the experimental behavior. For more than 15 years groups have been cooling and compressing clouds of atoms on a quest to produce and observe a Bose–Einstein condensate in a nearideal gas. They pushed their devices to the limit, seeking to traverse 15 orders of magnitude of phase-space density, and as each technique proved insufficient they developed ingenious variations to create ever colder and denser states.
Experimental methods of laser and evaporative cooling, used in the production of atomic Bose-Einstein condensates have recently been extended to realize quantum degeneracy in trapped Fermi gases. Fermi gases are a new rich system to explore the implications of Pauli exclusion on scattering properties of the system, and ultimately fermionic superfluidity. We have produced a new macroscopic quantum system, in which a degenerate Li-6 Fermi gas coexists with a large and stable Na-23 BEC. This was accomplished using inter-species sympathetic cooling of fermionic 6Li in a thermal bath of bosonic Na-23. We have achieved high numbers of both fermions (less than 10(exp 5) and bosons (less than 10(exp 6), and Li-6 quantum degeneracy corresponding to one half of the Fermi temperature. This is the first time that a Fermi sea was produced with a condensate as a "refrigerator".
The earliest experiments on gaseous Bose–Einstein condensates established that they are coherent collections of atoms, all described by a single wavefunction—just like the known superfluids, liquid helium-4 and superconductors. It was natural to wonder if they would also exhibit some of the behavior that is so characteristic of those larger-scale Bose condensates, such as quantized circulation (vortices) or frictionless flow. Many theorists have proffered ideas on how one would produce vortices in a Bose–Einsten condensate and what their properties might be. One of those ideas has now been experimentally realized. The approach was suggested by James Williams and Murray Holland of JILA and implemented by their experimental colleagues there. (JILA is a jointly operated lab of the University of Colorado at Boulder and the National Institute of Standards and Technology. Williams is now at the University of Toronto.)
The Field of ultracold quantum gases is no longer in its infancy. Since the first gaseous Bose–Einstein condensate was formed in 1995, physicists have made an exhaustive study of the many amazing properties of this new state of matter. Similar progress has been made with degenerate Fermigases – the quantum-mechanical cousins of Bose condensates. In addition to being fascinating in their own right, experiments on quantum gases are relevant to many different areas of physics – from atomic clocks and quantum computing to superfluidity and quantum phase transitions.