Speaker
Description
Josephson junctions provide a fundamental platform to study the interplay between macroscopic phase coherence and dissipation, with relevance ranging from quantum technologies and superconductors to neutron stars. Ultracold atomic gases, with tunable interactions and controllable geometries, enable exploration of Josephson dynamics across the BEC–BCS crossover within a single system.
We investigate the dynamical regimes of atomic Josephson junctions in both simply[1-5] and multiply connected [6] geometries. Below a critical velocity, coherent Josephson oscillations occur; above it, dissipation sets in through mechanisms that depend on the interaction regime and temperature. In simply connected elongated 3D systems, dissipation is governed by vortex rings and sound emission on the BEC side [1-2], and by pair breaking in the BCS regime [3], with additional thermal damping at finite temperature [4-5]. Increasing the barrier strength suppresses vortex motion[1-2] and can induce macroscopic quantum self-trapping [2].
In multiply connected (ring) geometries, the system supports quantized persistent currents. Here, dissipation due to vortex emission can be controlled by increasing the number of Josephson junctions. This motivates atomtronic Josephson “necklaces,” ring-shaped superfluids with multiple tunneling links. We study finite-circulation states and show that adding junctions enhances the maximum sustainable circulation (critical current), as the quantization of the circulation in this geometry distributes the total phase across links, reducing the phase drop and superfluid velocity at each junction. This increased stability contrasts with the decreasing superfluid fraction predicted by Leggett’s criterion. Our results are supported by experiments in both elongated (single-junction) [1] and annular geometries with up to 16 junctions [6].
References:
[1] K. Xhani et al., Physical Review Letters 124, 045301 (2020).
[2] K. Xhani et al., New Journal of Physics 22 (2020) 123006.
[3] G. Wlazłowski, K. Xhani et al., Physical Review Letters 130, 023003 (2023).
[4] K. Xhani et al., Phys. Rev. Research 4, 033205 (2022).
[5] K. Xhani et al., Atoms 2025, 13(8), 68 (2025);
[6] L. Pezzè, K. Xhani, C. Daix* et al., Nature Communications 15, 4831 (2024).