Observing dynamical phases of BCS superconductors in a cavity QED simulator

Dylan J. Young, Anjun Chu, Eric Yilun Song, Diego Barberena, David Wellnitz, Zhijing Niu, Vera M. Schäfer, Robert J. Lewis-Swan, Ana Maria Rey () and James K. Thompson ()
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Dylan J. Young: University of Colorado
Anjun Chu: University of Colorado
Eric Yilun Song: University of Colorado
Diego Barberena: University of Colorado
David Wellnitz: University of Colorado
Zhijing Niu: University of Colorado
Vera M. Schäfer: University of Colorado
Robert J. Lewis-Swan: University of Oklahoma
Ana Maria Rey: University of Colorado
James K. Thompson: University of Colorado

Nature, 2024, vol. 625, issue 7996, 679-684

Abstract: Abstract In conventional Bardeen–Cooper–Schrieffer superconductors1, electrons with opposite momenta bind into Cooper pairs due to an attractive interaction mediated by phonons in the material. Although superconductivity naturally emerges at thermal equilibrium, it can also emerge out of equilibrium when the system parameters are abruptly changed2–8. The resulting out-of-equilibrium phases are predicted to occur in real materials and ultracold fermionic atoms, but not all have yet been directly observed. Here we realize an alternative way to generate the proposed dynamical phases using cavity quantum electrodynamics (QED). Our system encodes the presence or absence of a Cooper pair in a long-lived electronic transition in 88Sr atoms coupled to an optical cavity and represents interactions between electrons as photon-mediated interactions through the cavity9,10. To fully explore the phase diagram, we manipulate the ratio between the single-particle dispersion and the interactions after a quench and perform real-time tracking of the subsequent dynamics of the superconducting order parameter using nondestructive measurements. We observe regimes in which the order parameter decays to zero (phase I)3,4, assumes a non-equilibrium steady-state value (phase II)2,3 or exhibits persistent oscillations (phase III)2,3. This opens up exciting prospects for quantum simulation, including the potential to engineer unconventional superconductors and to probe beyond mean-field effects like the spectral form factor11,12, and for increasing the coherence time for quantum sensing.

Date: 2024
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DOI: 10.1038/s41586-023-06911-x

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