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Science

A quantum breakdown

Jelena Stajic

Published Jul 18, 2019• 0 citations
Publisher

Abstract

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

DOI
10.1126/science.365.6450.245-d

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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
Quantum anomalies are violations of classical scaling symmetries caused by divergences that appear in the quantization of certain classical theories. Although they play a prominent role in the quantum field theoretical description of many-body systems, their influence on experimental observables is difficult to discern. In this study, we discovered a distinctive manifestation of a quantum anomaly in the momentum-space dynamics of a two-dimensional (2D) Fermi superfluid of ultracold atoms. The measured pair momentum distributions of the superfluid during a breathing mode cycle exhibit a scaling violation in the strongly interacting regime. We found that the power-law exponents that characterize long-range phase correlations in the system are modified by the quantum anomaly, emphasizing the influence of this effect on the critical properties of 2D superfluids.
Superfluidity and superconductivity are quantum mechanical features of gases of atoms cooled down close to -273.15 Celsius, where they display neither viscosity nor electrical resistence. Forcing a gas to live in a two-dimensional space almost all the superfluidity-related properties are enhanced and easier to test experimentally. In this thesis we conduct a study of a typical 2D phase, called FFLO, and we built a general setting to address the investigation of Fermi gases when brought to low-dimensionality by a new type of confinement. Results presented include signatures and protocols experimentalists might use to probe the systems we described.
Interacting Fermi systems in two dimensions display intriguing phenomena such as high temperature superfluidity and pseudogap physics. These features emerge from the collective behavior of the constituent particles but are extremely hard to predict from microscopic theory. I will report on experiments that realize microscopic two-dimensional systems with ultracold fermionic lithium atoms. With the ability to deterministically prepare few-body ground states and to observe individual particles in momentum space, they enable a microscopic view of strongly interacting two-dimensional Fermi systems. Surprisingly, we find that characteristic features of many-body Fermi gases can already be found in systems of no more than a dozen particles: In spectroscopy, we observe collective excitations that are the few-body precursor of the Higgs amplitude mode of a superfluid. Moreover, in spin-resolved momentum space probes, we can directly image individual ‘Cooper pairs’ and show the presence of fermionic pairing even in a microscopic setting. These findings may broaden our understanding of finite-size Fermi systems, such as atomic nuclei and superconducting grains.
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].
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