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Tuning the interaction between spins coupled to a superconductor on the atomic level

Felix Küster, Filipe S. M. Guimarães, Sascha Brinker, S. Parkin, Samir Lounis, Paolo Sessi, Ana M. Montero

Published Jan 1, 2021• 0 citations
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Momentum-Space Josephson Effects

Junpeng Hou, Xi-Wang Luo, Kuei Sun +5 more

The Josephson effect is a prominent phenomenon of quantum supercurrents that has been widely studied in superconductors and superfluids. Typical Josephson junctions consist of two real-space superconductors (superfluids) coupled through a weak tunneling barrier. Here we propose a momentum-space Josephson junction in a spin-orbit coupled Bose-Einstein condensate, where states with two different momenta are coupled through Raman-assisted tunneling. We show that Josephson currents can be induced not only by applying the equivalent of "voltages," but also by tuning tunneling phases. Such tunneling-phase-driven Josephson junctions in momentum space are characterized through both full mean field analysis and a concise two-level model, demonstrating the important role of interactions between atoms. Our scheme provides a platform for experimentally realizing momentum-space Josephson junctions and exploring their applications in quantum-mechanical circuits.

arXiv:1710.06369•Physical Review Letters•Mar 23, 2018•42 citations

Poking at Atomic Cooper Pairs

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

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Spin-Imbalance in One and Three-Dimensional Fermi Gases

Randall G. Hulet

The FFLO modulated superconductor state was independently proposed by Fulde and Ferrell, and Larkin and Ovchinnikov, as a way of accommodating the excess spin in a spin-polarized superconductor. The pairs in the FFLO state have non-zero center-of-mass momentum, which can produce a periodic structure with a spatially-modulated order parameter. Although there is some evidence for FFLO pairing in certain heavy fermion compounds that are able to accommodate both magnetic and superconducting order, conclusive experimental proof remains elusive. Motivated by the search for exotic paired states, we have performed experiments with spin-imbalanced ultracold atomic Fermi gases in both 3D and 1D. We use two hyperfine levels of 6Li to emulate the spin-up and down states. The s-wave pairing interactions are controlled via a magnetically-tuned collisional (Feshbach) resonance. In 3D, we find that the gas phase separates into an evenly paired BCS-like core, surrounded by the excess spin-up atoms [1]. For the 1D experiment, a two-dimensional optical lattice was used to create an array of 1D tubes that are each filled with ∼200 atoms. The weak axial confinement again produces a phase separation, but in contrast to 3D we find a partially-polarized central core surrounded by either fully-paired or fully-polarized wings, depending on the degree of overall spin-polarization [2]. Theory predicts that the partially-polarized phase is an FFLO superfluid. We are currently trying to obtain direct evidence for FFLO pairing, which may be revealed in the pair momentum distribution found by releasing the atoms axially and allowing them to expand in time-of-flight. We are also exploring the 1D-3D dimensional crossover that occurs when the coupling between tubes is reduced.

Bulletin of the American Physical Society•Feb 28, 2012•0 citations
The Josephson effect is a prominent phenomenon of quantum supercurrents that has been widely studied in superconductors and superfluids. Typical Josephson junctions consist of two real-space superconductors (superfluids) coupled through a weak tunneling barrier. Here we propose a momentum-space Josephson junction in a spin-orbit coupled Bose-Einstein condensate, where states with two different momenta are coupled through Raman-assisted tunneling. We show that Josephson currents can be induced not only by applying the equivalent of "voltages," but also by tuning tunneling phases. Such tunneling-phase-driven Josephson junctions in momentum space are characterized through both full mean field analysis and a concise two-level model, demonstrating the important role of interactions between atoms. Our scheme provides a platform for experimentally realizing momentum-space Josephson junctions and exploring their applications in quantum-mechanical circuits.
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
The FFLO modulated superconductor state was independently proposed by Fulde and Ferrell, and Larkin and Ovchinnikov, as a way of accommodating the excess spin in a spin-polarized superconductor. The pairs in the FFLO state have non-zero center-of-mass momentum, which can produce a periodic structure with a spatially-modulated order parameter. Although there is some evidence for FFLO pairing in certain heavy fermion compounds that are able to accommodate both magnetic and superconducting order, conclusive experimental proof remains elusive. Motivated by the search for exotic paired states, we have performed experiments with spin-imbalanced ultracold atomic Fermi gases in both 3D and 1D. We use two hyperfine levels of 6Li to emulate the spin-up and down states. The s-wave pairing interactions are controlled via a magnetically-tuned collisional (Feshbach) resonance. In 3D, we find that the gas phase separates into an evenly paired BCS-like core, surrounded by the excess spin-up atoms [1]. For the 1D experiment, a two-dimensional optical lattice was used to create an array of 1D tubes that are each filled with ∼200 atoms. The weak axial confinement again produces a phase separation, but in contrast to 3D we find a partially-polarized central core surrounded by either fully-paired or fully-polarized wings, depending on the degree of overall spin-polarization [2]. Theory predicts that the partially-polarized phase is an FFLO superfluid. We are currently trying to obtain direct evidence for FFLO pairing, which may be revealed in the pair momentum distribution found by releasing the atoms axially and allowing them to expand in time-of-flight. We are also exploring the 1D-3D dimensional crossover that occurs when the coupling between tubes is reduced.