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Science

Tuning the atomic pairing

Jelena Stajic

Published Jan 25, 2018• 0 citations
Publisher

Abstract

Quantum Gases Cold atomic gases are extremely flexible systems; the ability to tune interactions between fermionic atoms can, for example, cause the gas to undergo a crossover from weakly interacting fermions to weakly interacting bosons via a strongly interacting unitary regime. Murthy et al. studied this crossover in a gas of fermions confined to two dimensions. The formation of atomic pairs occurred at much higher temperatures in the unitary regime than previously thought. Science , this issue p. [452][1] [1]: /lookup/doi/10.1126/science.aan5950

DOI
10.1126/science.359.6374.407-c

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Quantum Gases
Cold atomic gases are extremely flexible systems; the ability to tune interactions between fermionic atoms can, for example, cause the gas to undergo a crossover from weakly interacting fermions to weakly interacting bosons via a strongly interacting unitary regime. Murthy et al. studied this crossover in a gas of fermions confined to two dimensions. The formation of atomic pairs occurred at much higher temperatures in the unitary regime than previously thought.
Science , this issue p. 452
Cold Molecules A dilute atomic gas cooled down to very cold temperatures can enter the so-called quantum degenerate regime, where quantum properties of the gas come to the fore. This regime has been achieved for both bosonic and fermionic atoms, but molecules, with their many internal states, present a special challenge. De Marco et al. cooled a bulk gas of fermionic potassium-rubidium molecules to quantum degeneracy (see the Perspective by Zelevinsky). The authors first cooled atomic potassium and rubidium gases separately, then bound them together into potassium-rubidium molecules, and finally brought the molecules down to their ground state. The density profile of the molecular gas revealed the system's quantum nature, which in turn kept the gas stable by suppressing chemical reactions. Science , this issue p. [853][1]; see also p. [820][2] [1]: /lookup/doi/10.1126/science.aau7230 [2]: /lookup/doi/10.1126/science.aav9149
Quantum Gases At low temperatures, two-dimensional (2D) systems with contact interactions are expected to exhibit quantum anomalies—a breakdown of scaling laws that characterize such systems in the classical regime. Signatures of these anomalies have been observed in the real-space properties of 2D Fermi gases, but the effect is much less pronounced than expected on theoretical grounds. Murthy et al. studied the momentum-space profiles of 2D superfluids of fermionic atoms. They initially perturbed the gas and then monitored the momentum distribution of its atoms. In the regime of strong interactions between the atoms, the momentum profiles deviated markedly from the classical scaling. Science , this issue p. [268][1] [1]: /lookup/doi/10.1126/science.aau4402
The ability to control the interaction strength between atoms within strongly interacting Fermi gases by sweeping a magnetic field across a Feshbach resonance provides a powerful experimental system in which to study many-body physics. One example is the crossover from a Bose-Einstein condensate (BEC) regime, in which the atoms are strongly coupled into pairs, to the weak-coupling regime that mimics Bardeen-Cooper-Schrieffer (BCS) coupling of electrons in superconducting metals. Although behavior on either side of the resonance is fairly well understood, of immediate interest is to find out what happens in the BEC-BCS crossover regime. However, determining the relative contributions of atom pairing mechanisms is an experimental and theoretical challenge.
Quantum Gases Quantum statistics dictates the behavior of identical particles in the quantum world: Bosons like to congregate, whereas fermions avoid one another. However, strong interactions can cause a string of bosons to behave like fermions. This so-called fermionization phenomenon has been studied in equilibrium. Wilson et al. instead focused on dynamical fermionization in a nonequilibrium system consisting of tubes of strongly interacting bosonic rubidium atoms. After letting the tubes expand in the axial direction, the researchers monitored the momentum distribution of the atoms and found that it evolved from bosonic-like to fermionic-like. Science , this issue p. [1461][1] [1]: /lookup/doi/10.1126/science.aaz0242
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.
The dimensionality of an ultracold quantum gas can vastly affect its properties, leading to a great deal of interest in lower dimensional systems. In two dimensions, the transition to superfluidity is a Berezinskii-Kosterlitz-Thouless (BKT) [1,2] phase transition. This is identified by the pairing of vortices with opposite circulation and has been observed in Bose gases [3]. Other phenomena that occur in two-dimensional systems include confinement-induced resonances [4] and the emergence of a two-body bound state for arbitrarily weak attractive interactions [5].