Quantum simulation of BEC–BCS crossover dynamics using dressed states in an optical cavity
ORAL
Abstract
In Bardeen–Cooper–Schrieffer (BCS) superconductors, fermions with opposite momenta form weakly bound Cooper pairs through an effective attractive interaction, giving rise to superconductivity at equilibrium. At the opposite extreme, in the Bose–Einstein condensation (BEC) regime, fermions bind into tightly bound pairs that behave as composite bosons. The BCS–BEC crossover connects these limits and fundamentally changes the nature of pairing, collective excitations, and relaxation dynamics.
Following a rapid quench of system parameters, theory predicts non-equilibrium dynamics of the superconducting order parameter, including Higgs-mode oscillations whose decay and damping depend sensitively on the interaction regime. In particular, the Higgs-mode decay is predicted to follow a power law, with an exponent that changes across the BCS–BEC crossover, reflecting the qualitative difference between weakly and strongly paired superfluids.
Here we realize a cavity-QED quantum simulator that allows us to observe key aspects of BEC–BCS crossover dynamics using dressed internal states of ultracold Rb atoms. Using the Anderson pseudo-spin mapping, the presence or absence of a Cooper pair is encoded in a two-level dressed-state basis, while photon-mediated interactions through the optical cavity generate effective attractive pairing interactions. By initializing the collective Bloch vector with different orientations in the dressed-state basis, we effectively tune between BCS-like and BEC-like regimes and perform controlled quenches across the crossover.
We observe post-quench Higgs-mode dynamics of the superconducting order parameter, with decay and damping behavior consistent with theoretical predictions for type-II superconductors across the BEC–BCS crossover. Our results demonstrate a flexible platform for studying non-equilibrium superconducting dynamics and establish cavity-mediated dressed-state systems as a new tool for quantum simulation.
Following a rapid quench of system parameters, theory predicts non-equilibrium dynamics of the superconducting order parameter, including Higgs-mode oscillations whose decay and damping depend sensitively on the interaction regime. In particular, the Higgs-mode decay is predicted to follow a power law, with an exponent that changes across the BCS–BEC crossover, reflecting the qualitative difference between weakly and strongly paired superfluids.
Here we realize a cavity-QED quantum simulator that allows us to observe key aspects of BEC–BCS crossover dynamics using dressed internal states of ultracold Rb atoms. Using the Anderson pseudo-spin mapping, the presence or absence of a Cooper pair is encoded in a two-level dressed-state basis, while photon-mediated interactions through the optical cavity generate effective attractive pairing interactions. By initializing the collective Bloch vector with different orientations in the dressed-state basis, we effectively tune between BCS-like and BEC-like regimes and perform controlled quenches across the crossover.
We observe post-quench Higgs-mode dynamics of the superconducting order parameter, with decay and damping behavior consistent with theoretical predictions for type-II superconductors across the BEC–BCS crossover. Our results demonstrate a flexible platform for studying non-equilibrium superconducting dynamics and establish cavity-mediated dressed-state systems as a new tool for quantum simulation.
* This material is based upon work supported by the US Department of Energy, Officeof Science, National Quantum Information Science Research Centers, Quantum SystemsAccelerator, and the Vannevar-Bush Faculty Fellowship.
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Publication: Planned paper - "Quantum simulation of BEC–BCS crossover dynamics using dressed states in an optical cavity"
Presenters
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Eliot Bohr
- JILA