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Quantum Simulation of Intrinsic Dissipation in Mesoscopic Transport with Ultracold Atoms

Simon Christian Wili

Published Jan 1, 2026• 0 citations
Publisher

Abstract

Transport measurements provide a powerful probe of quantum manybody systems, offering insights into the non-equilibrium dynamics of particles, spin, and entropy. Because transport inherently probes many-body systems away from equilibrium, its theoretical description remains a significant challenge, particularly in the presence of strong correlations or beyond the linear response regime. A close interplay between theoretical efforts and systematic experiments is therefore essential to deepen our understanding. In this thesis, we extend and employ a cold-atom transport setup to investigate the flow of a unitary Fermi gas through a mesoscopic channel connecting two superfluid reservoirs. The present work focuses on residual dissipative processes occurring in superfluid flow away from equilibrium. Specifically, we investigate how the geometry and dimensionality of the channel influence the mechanisms of dissipation under an applied chemical-potential bias. This system serves as a highly tunable quantum simulator of a voltage-biased superconducting wire. We resolve distinct dissipative regimes as the channel is tuned from the quasi-one-dimensional to the quasi-two-dimensional limit. In narrow channels, dissipation is driven by phase slips described by the Langer–Ambegaokar–McCumber–Halperin theory. In wide channels, dissipation is instead dominated by vortex dynamics, consistent with a finite-size vortex model. In the dimensional crossover between these limits, we observe a non-monotonic dependence of dissipation on channel width, characterized by a local minimum. These measurements provide a systematic experimental validation of theoretically predicted scaling laws in both the one- and two-dimensional limits. Beyond establishing a quantitative benchmark for theoretical descriptions of the delicate dimensional crossover, our findings suggest strategies for minimizing dissipation in mesoscopic superconducting devices.

DOI
10.3929/ethz-c-000807567

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Transport measurements provide a powerful probe of quantum manybody systems, offering insights into the non-equilibrium dynamics of particles, spin, and entropy. Because transport inherently probes many-body systems away from equilibrium, its theoretical description remains a significant challenge, particularly in the presence of strong correlations or beyond the linear response regime. A close interplay between theoretical efforts and systematic experiments is therefore essential to deepen our understanding. In this thesis, we extend and employ a cold-atom transport setup to investigate the flow of a unitary Fermi gas through a mesoscopic channel connecting two superfluid reservoirs. The present work focuses on residual dissipative processes occurring in superfluid flow away from equilibrium. Specifically, we investigate how the geometry and dimensionality of the channel influence the mechanisms of dissipation under an applied chemical-potential bias. This system serves as a highly tunable quantum simulator of a voltage-biased superconducting wire. We resolve distinct dissipative regimes as the channel is tuned from the quasi-one-dimensional to the quasi-two-dimensional limit. In narrow channels, dissipation is driven by phase slips described by the Langer–Ambegaokar–McCumber–Halperin theory. In wide channels, dissipation is instead dominated by vortex dynamics, consistent with a finite-size vortex model. In the dimensional crossover between these limits, we observe a non-monotonic dependence of dissipation on channel width, characterized by a local minimum. These measurements provide a systematic experimental validation of theoretically predicted scaling laws in both the one- and two-dimensional limits. Beyond establishing a quantitative benchmark for theoretical descriptions of the delicate dimensional crossover, our findings suggest strategies for minimizing dissipation in mesoscopic superconducting devices.
Resistance in standard conductors decreases with increasing cross-section. Yet, in low-dimensional superconductors and superfluids residual resistance arises from topological fluctuations of the order parameter manifesting as phase slips in one-dimensional (1D) and vortices in two-dimensional (2D) systems. How resistance and dissipation evolve as geometry interpolates between these regimes remains an open question. This evolution is masked in solid-state experiments by disorder, impurities, and geometric imperfections, and poses theoretical challenges due to competing dissipative processes and pronounced finite-size effects. Here, we use a defect-free unitary Fermi gas in a digitally programmable transport geometry to isolate geometric effects on superfluid dissipation and discover a paradox: in the crossover from 1D to 2D, the resistance reaches a minimum. There, widening a channel increases its resistance. Narrower, quasi-1D channels show dissipation described by Langer-Ambegaokar-McCumber-Halperin theory of phase slips. In this regime, varying the channel width yields the predicted exponential scaling of the activation factor over more than ten orders of magnitude. Wider, quasi-2D channels show dissipation consistent with a finite-size vortex model. The minimal dissipation in the dimensional crossover reflects a transition in the dominant dissipative mechanism, with both phase slips and vortices simultaneously suppressed. Our measurements suggest a route to minimizing dissipation in superconducting devices and provide a benchmark for theoretical efforts aimed at describing the dimensional crossover.
We study the emergence of dissipation in an atomic Josephson junction between weakly coupled superfluid Fermi gases. We find that vortex-induced phase slippage is the dominant microscopic source of dissipation across the Bose-Einstein condensate-Bardeen-Cooper-Schrieffer crossover. We explore different dynamical regimes by tuning the bias chemical potential between the two superfluid reservoirs. For small excitations, we observe dissipation and phase coherence to coexist, with a resistive current followed by well-defined Josephson oscillations. We link the junction transport properties to the phase-slippage mechanism, finding that vortex nucleation is primarily responsible for the observed trends of conductance and critical current. For large excitations, we observe the irreversible loss of coherence between the two superfluids, and transport cannot be described only within an uncorrelated phase-slip picture. Our findings open new directions for investigating the interplay between dissipative and superfluid transport in strongly correlated Fermi systems, and general concepts in out-of-equilibrium quantum systems.
Quantum systems out of equilibrium offer the possibility of understanding intriguing and challenging problems in modern physics. Studying transport properties is not only valuable to unveil fundamental properties of quantum matter but it is also an excellent tool for developing new quantum devices which inherently employ quantum-mechanical effects. In this contribution, we present our experimental studies on quantum transport using ultracold Fermi gases of 6Li atoms. We realize the analogous of a Josephson junction by bisecting fermionic superfluids by a thin optical barrier. We observe coherent dynamics in both the population and in the relative phase between the two reservoirs. For critical parameters, the superfluid dynamics exhibits both coherent and resistive flow due to phase-slippage events manifesting as vortices propagating into the bulk. We uncover also a regime of strong dissipation where the junction operation is irreversibly affected by vortex proliferation. Our studies open new directions for investigating dissipation and superfluid transport in strongly correlated fermionic systems.
The transport properties of strongly interacting fermionic systems can reveal exotic states of matter, but experiments and theory have predominantly focused on bulk systems in the hydrodynamic limit describable with linear response coefficients such as electrical and thermal conductivity. In a ballistic channel connecting two superfluid reservoirs, recent experiments revealed a far-from-equilibrium regime beyond linear hydrodynamics where particle and entropy currents respond nonlinearly to biases of chemical potential and temperature, and their ratio is robust to the channel geometry. However, the origin of this robustness and its relation to the strong interparticle interactions remain unknown. Here, we study the coupled transport of particles and entropy tuning the interaction across the Bardeen-Cooper-Schrieffer to Bose-Einstein condensate (BCS-BEC) crossover, the reservoir degeneracy across the superfluid phase transition, as well as the local potentials and confinement of the channel. Surprisingly, the entropy advectively transported per particle depends only on the interactions and reservoir degeneracy and not on the details of the channel, suggesting that this property has its origin in the universal equilibrium properties of the reservoirs. In contrast, the magnitudes of the advective and diffusive entropy currents vary significantly with the channel details. The advective current increases monotonically towards the BEC side, which can be largely explained by the estimated superfluid gap in the channel. The Wiedemann-Franz law that links the advective and diffusive currents in Fermi liquids is most egregiously violated at unitarity, suggesting a change in the nature of the excitations responsible for entropy diffusion near unitarity. These observations pose fundamental questions regarding transport phenomena in strongly interacting Fermi systems far from equilibrium.
We study the onset of dissipation in an atomic Josephson junction between Fermi superfluids in the molecular Bose-Einstein condensation limit of strong attraction. Our simulations identify the critical population imbalance and the maximum Josephson current delimiting dissipationless and dissipative transport, in quantitative agreement with recent experiments. We unambiguously link dissipation to vortex ring nucleation and dynamics, demonstrating that quantum phase slips are responsible for the observed resistive current. Our work directly connects microscopic features with macroscopic dissipative transport, providing a comprehensive description of vortex ring dynamics in three-dimensional inhomogeneous constricted superfluids at zero and finite temperatures.
We have experimentally measured transport of superfluid, bosonic atoms in a mesoscopic system: a small channel connecting two large reservoirs. Starting far from equilibrium (superfluid in a single reservoir), we observe first resistive flow transitioning at a critical current into superflow, characterized by oscillations. We reproduce this full evolution with a simple electronic circuit model. We compare our fitted conductance to two different microscopic phenomenological models. We also show that the oscillations are consistent with LC oscillations as estimated by the kinetic inductance and effective capacitance in our system. Our experiment provides an attractive platform to begin to probe the mesoscopic transport properties of a dilute, superfluid, Bose gas.