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arXiv:cond-mat/0001341 · Physical Review Letters

Laser Probing of Atomic Cooper Pairs

Päivi Törmä, P. Zoller

Published Jul 17, 2000• 111 citations
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Abstract

We consider a gas of attractively interacting cold fermionic atoms which are manipulated by laser light. The laser induces a transition from an internal state with large negative scattering length to one with almost no interactions. The process can be viewed as a tunneling of atomic population between the superconducting and the normal states of the gas. It can be used to detect the BCS ground state and to measure the superconducting order parameter.

DOI
10.1103/physrevlett.85.487

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We consider a gas of attractively interacting cold fermionic atoms which are manipulated by laser light. The laser induces a transition from an internal state with large negative scattering length to one with almost no interactions. The process can be viewed as a tunneling of atomic population between the superconducting and the normal states of the gas. It can be used to detect the BCS ground state and to measure the superconducting order parameter.
We consider a gas of trapped Cooper-paired fermionic atoms that are manipulated by laser light. The laser induces a transition from an internal state with large negative scattering length (superfluid) to one with weaker interactions (normal gas). We show that the process can be used to detect the presence of the superconducting order parameter. Also, we propose a direct way of measuring the size of the gap in the trap. The efficiency and feasibility of this probing method is investigated in detail in different physical situations.
The recent observation of the onset of degeneracy in a Fermi gas, in which the atoms are cooled to such low temperatures that their associated de Broglie waves start to overlap, has opened up new opportunities in atomic and quantum physics. One prediction is that the interaction between Fermionic atoms can be tuned to form Cooper pairs, just as two electrons do in superconductors when they overcome their Coulombic repulsion and pair up. However, the expectedly small superconducting gap and the nature of the Fermionic system will require new techniques. Törma and Zoller propose using laser pulses to detect the superconducting ground state of the atomic Cooper pair and determine the extent of the pairing. Lasers would be used to tune and manipulate the coupling between the internal energy levels of the atoms, which would alter their interaction strength, and should result in a controllable and measurable phase transition between the superconducting and normal states. — ISO
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.
We consider a gas of cold fermionic atoms having two spin components with interactions characterized by their s-wave scattering length a. At positive scattering length the atoms form weakly bound bosonic molecules which can be evaporatively cooled to undergo Bose-Einstein condensation, whereas at negative scattering length BCS pairing can take place. It is shown that, by adiabatically tuning the scattering length a from positive to negative values, one may transform the molecular Bose-Einstein condensate into a highly degenerate atomic Fermi gas, with the ratio of temperature to Fermi temperature T/T(F) approximately 10(-2). The corresponding critical final value of k(F)/a/, which leads to the BCS transition, is found to be about one-half, where k(F) is the Fermi momentum.
A theoretical treatment of light scattering from a degenerate Fermi gas of trapped ultracold 6Li{}^{6}\mathrm{Li}6Li atoms is presented. We find that the scattered light contains information that directly reflects the quantum pair correlation due to the formation of atomic Cooper pairs resulting from a Bardeen-Cooper-Schrieffer phase transition to a superfluid state. Evidence for pairing should be observable in both the space and time domains.