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arXiv:physics/0104043

Coherent control of superfluidity in a Fermi gas of atoms

Matt Mackie, Jyrki Piilo, Kalle‐Antti Suominen, Juha Javanainen

Published Apr 13, 2001• 0 citations
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Abstract

We theoretically examine photoassociation of a two-component Fermi degenerate gas, focusing on light-induced atom-atom interactions as a means to raise the critical temperature of the BCS transition to a superfluid state. As it stands, photoassociation-induced superfluidity is limited by spontaneous decay to experimentally inconvenient light intensities [Mackie et al., Opt. Express 8, 118 (2000)]. We therefore propose to use coherent control in photoassociation to a pair of molecular levels to cancel spontaneous emission, whereby the BCS transition should occur within reach of current experimental techniques.

DOI
10.48550/arxiv.physics/0104043

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We theoretically examine photoassociation of a two-component Fermi degenerate gas, focusing on light-induced atom-atom interactions as a means to raise the critical temperature of the BCS transition to a superfluid state. As it stands, photoassociation-induced superfluidity is limited by spontaneous decay to experimentally inconvenient light intensities [Mackie et al., Opt. Express 8, 118 (2000)]. We therefore propose to use coherent control in photoassociation to a pair of molecular levels to cancel spontaneous emission, whereby the BCS transition should occur within reach of current experimental techniques.
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We study the coherence properties of a trapped two-component gas of fermionic atoms below the BCS critical temperature. We propose an optical method to investigate the Cooper pair coherence across different regions of the superfluid. Near-resonant laser light is used to induce transitions between the two coupled hyperfine states. The beam is split so that it probes two spatially separate regions of the gas. Absorption of the light in this interferometric scheme depends on the Cooper pair coherence between the two regions.
The main topic of these lectures is the physics of quantum degenerate atomic Fermi gases both under uniform confinement and in harmonic traps. To begin with, a general introduction on ideal Fermi gases is proposed. This "academic" limit is relevant in experiments where ultracold fermionic atoms are all polarized in a single spin state: the low atom momenta together with the fermionic nature of the atoms forbid atomic s-wave scattering and the atoms can be considered as free. Interactions are then introduced for the case of a gas with two spin components. Interactions can be tuned via Feshbach resonances and control the microscopic features of the superfluid phase, ranging from the BCS limit (formation of Cooper pairs) to the BEC limit (Bose-Einstein condensation of dimers). Various theoretical approaches for the description of the BCS-BEC crossover and their predictions in comparison with the experimental results are discussed.
We propose that the evolution of superfluidity from the Bardeen-Cooper-Schrieffer (BCS) regime to the Bose-Einstein condensation (BEC) regime can be realized using ultracold Fermi gas coupled to a single-mode cavity. By the functional integral formalism, we derive an effective atom-only action, which mimics the two-component Fermi gas with tunable two-body interaction. First, we address the features of the cavity-mediated interaction. We find that the matter-light coupling creates an effective sss-wave scattering whose sign and amplitude are controlled by parameters of the cavity. Second, we discuss the fermionic superfluidity on the mean-field level, including the order parameter, chemical potential, quasiparticle excitation spectrum, momentum distribution, and dissociation temperature. It is shown that by varying the atom-cavity detuning, a BCS to BEC crossover occurs. In addition, the influences of the atomic collaborative effect and external pumping field on the pairing correlation are also studied.
We theoretically study the propagation of light in a Fermi-Dirac gas in the presence of a superfluid state. BCS pairing between atoms in different spin levels increases the optical linewidth and line shift of a quantum degenerate Fermi-Dirac gas already at very low densities and introduces a collisional local-field shift that may dramatically dominate the Lorentz-Lorenz shift. These optical properties could possibly signal the presence of the superfluid state and determine the value of the BCS order parameter.