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An ideal Josephson junction in an ultracold two-dimensional Fermi gas

Niclas Luick https://orcid.org/0000-0003-3283-8695, Lennart Sobirey https://orcid.org/0000-0002-5567-9327, Markus Bohlen https://orcid.org/0000-0003-4108-0409, Vijay Pal Singh https://orcid.org/0000-0001-8881-978X, Ludwig Mathey, Thomas Lompe https://orcid.org/0000-0002-7880-1639 [email protected], and Henning Moritz https://orcid.org/0000-0002-4854-5806
Science • 3 Jul 2020 • Vol 369, Issue 6499 • pp. 89-91 • DOI: 10.1126/science.aaz2342
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A gas junction

In superconductors, electrons form a macroscopic wave function that has a definite phase. If two superconductors with different wave function phases are placed in contact with each other through an insulating link, a current will flow through this so-called Josephson's junction without any external voltage. Luick et al. and Kwon et al. observed an analogous phenomenon in a setup that involved two reservoirs of superfluid Fermi gases. Both groups measured the so-called current-phase relation: the dependence of the magnitude of the current on the relative phase. By tuning an external magnetic field, they were able to study how the interactions between fermions affected the nature of the superfluid state.
Science, this issue p. 89, p. 84

Abstract

The role of reduced dimensionality in high-temperature superconductors is still under debate. Recently, ultracold atoms have emerged as an ideal model system to study such strongly correlated two-dimensional (2D) systems. Here, we report on the realization of a Josephson junction in an ultracold 2D Fermi gas. We measure the frequency of Josephson oscillations as a function of the phase difference across the junction and find excellent agreement with the sinusoidal current phase relation of an ideal Josephson junction. Furthermore, we determine the critical current of our junction in the crossover from tightly bound molecules to weakly bound Cooper pairs. Our measurements clearly demonstrate phase coherence and provide strong evidence for superfluidity in a strongly interacting 2D Fermi gas.
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Supplementary Material

Summary

Supplementary Text
Figs. S1 to S4
References (44–49)

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References and Notes

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Science
Volume 369 | Issue 6499
3 July 2020

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Copyright © 2020, American Association for the Advancement of Science.
This is an article distributed under the terms of the Science Journals Default License.

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Received: 23 August 2019
Accepted: 7 May 2020
Published in print: 3 July 2020

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Acknowledgments

We thank K. Hueck and B. Lienau for their contributions during earlier stages of the experiment; T. Enss, A. Recati, and M. Zaccanti for stimulating discussions; and G. Roati and F. Scazza for careful reading of the manuscript and valuable suggestions regarding the interpretation of Fig. 3. Funding: This work was supported by the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 335431 and by the DFG in the framework of SFB 925 and the excellence clusters “The Hamburg Centre for Ultrafast Imaging” - EXC 1074 - project ID 194651731 and “Advanced Imaging of Matter” - EXC 2056 - project ID 390715994. M.B. acknowledges support by Labex ICFP of École Normale Supérieure Paris. Author contributions: N.L. and L.S. performed the experiments and data analysis with support from M.B. and T.L. V.P.S. and L.M. developed numerical and analytical models and contributed to the interpretation of our experimental data. N.L. and T.L. wrote the manuscript, and L.S. created the figures with input from all authors. T.L. and H.M. supervised the project. All authors contributed to the discussion and interpretation of our results. Competing interests: The authors declare no competing interests. Data and materials availability: All data presented in this paper and simulation scripts are deposited at Zenodo (42, 43).

Authors

Affiliations

Niclas Luick https://orcid.org/0000-0003-3283-8695
Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
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Lennart Sobirey https://orcid.org/0000-0002-5567-9327
Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
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Markus Bohlen https://orcid.org/0000-0003-4108-0409
Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, 24 rue Lhomond, 75005 Paris, France.
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Vijay Pal Singh https://orcid.org/0000-0001-8881-978X
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
Zentrum für Optische Quantentechnologien, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
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Ludwig Mathey
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
Zentrum für Optische Quantentechnologien, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
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Thomas Lompe* https://orcid.org/0000-0002-7880-1639 [email protected]
Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
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Henning Moritz https://orcid.org/0000-0002-4854-5806
Institut für Laserphysik, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
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*Corresponding author. Email: [email protected]

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