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In cold atomic gases, superfluid transport arises from a coherent many-body state where particles share a single quantum phase across macroscopic distances. This coherence enables frictionless flow, allowing a superfluid to move without energy dissipation under ideal conditions. However, real systems show a rich dependence on temperature, interparticle interactions, and external trapping geometry. As the gas cools toward quantum degeneracy, phase correlations extend over larger regions, enhancing the rigidity of the superfluid response. The transport properties then reflect how smoothly the order parameter’s phase winds and twists in space and time. Thus, measuring transport coefficients becomes a sensitive probe of the underlying coherence length and the spectrum of collective modes that govern the fluid.

Experimental platforms based on ultracold atoms provide clean realizations of phase-coherent transport. Laser cooling creates Bose-Einstein condensates or degenerate Fermi gases, where a macroscopic wavefunction describes particle motion. By shaping traps and applying weak or modulated potentials, researchers induce flows and monitor how vortices, solitons, or phonons interact with the medium. The observed resistance to damping hinges on how uniformly the phase aligns across the cloud and how quickly phase fluctuations dissipate. The interplay between coherence and dissipation thus becomes central to understanding real-world transport, guiding theoretical models that bridge microscopic interactions and emergent hydrodynamic behavior in these quantum fluids.

The first connection between phase coherence and transport appears in the response to slow, smooth motion of the container containing a cold gas. When the phase is coherent over the entire system, the velocity field is effectively irrotational and uniform, yielding a robust, nearly lossless flow. Any breakdown of coherence introduces phase slips and defects that act as barriers to smooth transport. The resulting dissipation reveals itself as a finite viscosity-like behavior, even in a nominally superfluid state. The transition between coherent and incoherent transport depends on temperature, trap geometry, and interaction strength, creating a tunable landscape for studying frictionless flow.

A second facet concerns how phase coherence controls the propagation of collective excitations. Phonons, rotons, and other modes carry momentum and energy through the fluid, and their characteristics are tied to the order parameter’s phase correlations. In a phase-coherent gas, these excitations propagate with well-defined dispersion, enabling predictable transport. As coherence wanes, scattering among excitations grows, and damping rises. This shift alters conductivity-like quantities and the effective hydrodynamic response, highlighting how microscopic coherence translates into macroscopic transport properties. Observations across different isotopes and interaction regimes reinforce the universality of these ideas in cold-gas systems.

Theoretical treatments emphasize the role of the superfluid fraction as a direct indicator of coherent transport. In a trapped cloud, the fraction of particles participating in the condensate determines how much of the mass flows without friction. As the temperature increases or as disorder and inhomogeneity emerge, a portion of the fluid becomes normal, reducing the superfluid density and altering transport. Models incorporate the coupling between the phase stiffness and boundary conditions, predicting how the system’s response to imposed flows evolves with coherence. These predictions align with measurements of collective oscillations and critical velocities in diverse cold-gas experiments.

Importantly, phase coherence is not only a static property but also a dynamical resource. When external perturbations drive the system, the phase adjusts to minimize energy, guiding the restoration of a coherent flow after disturbances. The timescale for reestablishing coherence competes with dissipation, and this competition dictates transient transport phenomena. In weakly interacting gases, long coherence times support persistent currents, while stronger interactions introduce nonlinearities that reshape transport through phase-amplitude coupling. This dynamic view links coherence maintenance to robust, controllable transport in experimental setups.

Observations of persistent currents in toroidal traps offer striking evidence for phase-coherent transport. In these geometries, the quantization of circulation ties directly to the global phase state. When coherence endures, the system supports flux quantization and stable, low-dissipation rotation. Loss of coherence manifests as phase slips and decay of the current, marking a clear boundary between ideal transport and dissipative behavior. The dependence on temperature, trap size, and interaction strength provides a rich parameter space for testing approximate theories of superfluid hydrodynamics in finite systems.

Another avenue examines how phase coherence affects transport through weak links or junctions. The Josephson-like coupling between neighboring regions of a gas allows supercurrents to flow with a phase difference across the junction. The maximal current achievable without dissipation is set by the phase coherence length and the coupling strength. As coherence shrinks, the critical current drops and phase fluctuations become more important, producing a crossover to resistive transport. This framework connects microscopic coherence phenomena to measurable transport signatures across a range of experimental architectures.

In optical-lattice experiments, phase coherence across lattice sites governs collective tunneling and mass transport. When a lattice is shallow and coherence spans many sites, atoms can move coherently, producing band-like transport with reduced friction. Deep lattices fragment coherence, and transport becomes dominated by hopping with higher dissipation. The transition between these regimes mirrors the balance between phase rigidity and thermal fluctuations. By tuning lattice depth and interaction strength, researchers map how transport changes in step with coherence properties, offering a clean testbed for phase-driven hydrodynamics.

Disorder, too, plays a crucial role in coherence-mediated transport. Quasiperiodic potentials and random perturbations fragment phase correlations, creating localized regions with hindered flow. Yet even in the presence of disorder, remnants of coherence can sustain percolation-like transport pathways. Understanding how phase coherence survives or collapses amid randomness helps explain real-world materials and informs the design of coherent quantum devices. The cold-gas platform thus serves as a bridge between fundamental coherence physics and practical transport phenomena.

Looking forward, precision measurements of phase correlations provide a direct route to predicting transport coefficients in quantum fluids. Interference fringes, time-of-flight images, and Bragg spectroscopy reveal how the phase and its gradients evolve under drive. The resulting data feed into hydrodynamic models that incorporate quantum corrections, yielding improved descriptions of viscosity, diffusion, and thermal transport. By continuously refining the link between coherence length and mobility, researchers can tailor transport properties for specific applications, from quantum simulators to sensors.

In sum, phase coherence acts as the master regulator of superfluid transport in cold gases. It determines how smoothly mass moves, how robust the flow remains under perturbations, and how excitations convey momentum and energy. The coherence landscape is tunable through temperature, interactions, geometry, and disorder, enabling systematic exploration of transport physics across quantum regimes. This deepens our understanding of macroscopic quantum phenomena and guides the development of technologies that capitalize on dissipationless flow and coherent control.