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Science

Standing Room Only at the Quantum Scale

Kenneth M. O’Hara, John E. Thomas

Published Mar 30, 2001• 2 citations
Publisher

Abstract

Atoms may be bosons and fermions depending on their isotopic mass, and this determines their behavior at very low temperatures. Bosons form quantum degenerate gases called Bose-Einstein condensates. In their Perspective, O9Hara and Thomas highlight the work by Truscott et al., who have made a degenerate fermionic gas through a method called sympathetic cooling. Such studies will shed light on many processes from superconductivity to the mechanisms driving neutron stars.

DOI
10.1126/science.1060025

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Evaporative cooling of an atom gas of bosons, whereby the hottest atoms are progressively removed, is an effective and widely used method for forming Bose-Einstein condensates. For Fermi gases, however, collisions between identical fermions are forbidden, and new tricks, such as mixing two Fermi gases with different spin states, have been used to form Fermi condensates. Truscott et al. (p. 2570; see the Perspective by O'Hara and Thomas) now create quantum-degenerate Bose and Fermi condensates simultaneously from a boson-fermion mixture. Heat is removed from the fermions through elastic collisions with the bosons, a process called sympathetic cooling. They show that different statistics associated with the two types of gases can actually be visualized.

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The quantum mechanical phenomenon of Bose-Einstein condensation appears throughout physics with manifestations ranging from superconductivity in metals to superfluidity in nuclei and neutron stars. In most cases the condensation emerges due to correlations or pairing of fermionic particles. We explore this quantum behavior in a uniquely controllable and clean system consisting of an ultracold gas of atoms. In particular, by tuning the interactions between the fermionic atoms that make up our ultracold gas we can create condensates consisting of pairs of correlated atoms. These experiments introduce the ability to access the intriguing crossover regime where the quantum behavior of fermions becomes intertwined with that of bosons.

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We give an overview of the experiments at the Laser Centre of the Vrije Universiteit in Amsterdam on ultracold gases of metastable helium-4 and helium-3 as well as mixtures of both isotopes. We describe our experimental setup and discuss our experiments on Bose-Einstein condensation of 4He* (more than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi degeneracy (more than 1 million atoms below the Fermi temperature as well as realization of a degenerate boson-fermion mixture). We also present our results on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear losses) or a mixture of both isotopes (heteronuclear losses).

arXiv:cond-mat/0610414•Oct 16, 2006•0 citations
Atoms may be bosons and fermions depending on their isotopic mass, and this determines their behavior at very low temperatures. Bosons form quantum degenerate gases called Bose-Einstein condensates. In their Perspective,
O9Hara and Thomas
highlight the work by
Truscott
et al., who have made a degenerate fermionic gas through a method called sympathetic cooling. Such studies will shed light on many processes from superconductivity to the mechanisms driving neutron stars.
Evaporative cooling of an atom gas of bosons, whereby the hottest atoms are progressively removed, is an effective and widely used method for forming Bose-Einstein condensates. For Fermi gases, however, collisions between identical fermions are forbidden, and new tricks, such as mixing two Fermi gases with different spin states, have been used to form Fermi condensates. Truscott et al. (p. 2570; see the Perspective by O'Hara and Thomas) now create quantum-degenerate Bose and Fermi condensates simultaneously from a boson-fermion mixture. Heat is removed from the fermions through elastic collisions with the bosons, a process called sympathetic cooling. They show that different statistics associated with the two types of gases can actually be visualized.
The quantum mechanical phenomenon of Bose-Einstein condensation appears throughout physics with manifestations ranging from superconductivity in metals to superfluidity in nuclei and neutron stars. In most cases the condensation emerges due to correlations or pairing of fermionic particles. We explore this quantum behavior in a uniquely controllable and clean system consisting of an ultracold gas of atoms. In particular, by tuning the interactions between the fermionic atoms that make up our ultracold gas we can create condensates consisting of pairs of correlated atoms. These experiments introduce the ability to access the intriguing crossover regime where the quantum behavior of fermions becomes intertwined with that of bosons.
In a gas of identical atoms, each atom is described by a localized de Broglie wave packet. If the gas is trapped and cooled, the atoms move closer together and their de Broglie wavelengths get longer. As the gas is cooled further, the wave packets become larger and eventually start to overlap and interfere with each other. What happens next in these so-called quantum-degenerate gases depends on whether the atoms are bosons or fermions. Quantum gases of both types have proved to be a rich source of new physics in recent years and this trend is continuing with experiments on gases that contain both types of atom, such as the observation of the collapse of a degenerate Fermi gas by researchers at the European Laboratory for Nonlinear Spectroscopy in Florence, Italy (G Modugno et al. 2002 Science 297 2240).
The laser cooling mechanisms described in Chapter 4 operate irrespective of the statistics of the atom, and they can therefore be used to cool Fermi species. The statistics of a neutral atom is determined by the number of neutrons in the nucleus, which must be odd for a fermionic atom. Since alkali atoms have odd atomic number Z , their fermionic isotopes have even mass number A . Such isotopes are relatively less abundant than those with odd A since they have both an unpaired neutron and an unpaired proton, which increases their energy by virtue of the odd–even effect. In early experiments, 40 K and 6 Li atoms were cooled to about one-quarter of the Fermi temperature. More recently, fermionic alkali atoms have been cooled to temperatures well below one-tenth of their Fermi temperature, and in addition a degenerate gas of the fermionic species 173 Yb (with I = 5/2) has been prepared. In the classical limit, at low densities and/or high temperatures, clouds of fermions and bosons behave alike. The factor governing the importance of quantum degeneracy is the phase-space density ϖ introduced in Eq. (2.24), and in the classical limit ϖ ≪ 1. When ϖ becomes comparable with unity, gases become degenerate: bosons condense in the lowest single-particle state, while fermions tend towards a state with a filled Fermi sea. As one would expect on dimensional grounds, the degeneracy temperature for fermions – the Fermi temperature T F – is given by the same expression as the Bose–Einstein transition temperature for bosons, apart from a numerical factor of order unity.
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.
We give an overview of the experiments at the Laser Centre of the Vrije Universiteit in Amsterdam on ultracold gases of metastable helium-4 and helium-3 as well as mixtures of both isotopes. We describe our experimental setup and discuss our experiments on Bose-Einstein condensation of 4He* (more than 10 million atoms in a BEC) and sympathetic cooling of 3He* towards Fermi degeneracy (more than 1 million atoms below the Fermi temperature as well as realization of a degenerate boson-fermion mixture). We also present our results on Penning Ionization in ~1 mK clouds containing a single isotope (homonuclear losses) or a mixture of both isotopes (heteronuclear losses).