Abstract The piston shock problem is a prototypical example of strongly nonlinear fluid flow that enables the experimental exploration of fluid dynamics in extreme regimes. Here we investigate this problem for a nominally dissipationless, superfluid Bose-Einstein condensate and observe rich dynamics including the formation of a plateau region, a non-expanding shock front, and rarefaction waves. Many aspects of the observed dynamics follow predictions of classical dissipative—rather than superfluid dispersive—shock theory. The emergence of dissipative-like dynamics is attributed to the decay of large amplitude excitations at the shock front into turbulent vortex excitations, which allow us to invoke an eddy viscosity hypothesis. Our experimental observations are accompanied by numerical simulations of the mean-field, Gross-Pitaevskii equation that exhibit quantitative agreement with no fitting parameters. This work provides an avenue for the investigation of quantum shock waves and turbulence in channel geometries, which are currently the focus of intense research efforts.
Abstract The piston shock problem is a prototypical example of strongly nonlinear fluid flow that enables the experimental exploration of fluid dynamics in extreme regimes. Here we investigate this problem for a nominally dissipationless, superfluid Bose-Einstein condensate and observe rich dynamics including the formation of a plateau region, a non-expanding shock front, and rarefaction waves. Many aspects of the observed dynamics follow predictions of classical dissipative—rather than superfluid dispersive—shock theory. The emergence of dissipative-like dynamics is attributed to the decay of large amplitude excitations at the shock front into turbulent vortex excitations, which allow us to invoke an eddy viscosity hypothesis. Our experimental observations are accompanied by numerical simulations of the mean-field, Gross-Pitaevskii equation that exhibit quantitative agreement with no fitting parameters. This work provides an avenue for the investigation of quantum shock waves and turbulence in channel geometries, which are currently the focus of intense research efforts.
The piston shock problem is a prototypical example of strongly nonlinear fluid flow that enables the experimental exploration of fluid dynamics in extreme regimes. Here we investigate this problem for a nominally dissipationless, superfluid Bose-Einstein condensate and observe rich dynamics including the formation of a plateau region, a non-expanding shock front, and rarefaction waves. Many aspects of the observed dynamics follow predictions of classical dissipative-rather than superfluid dispersive-shock theory. The emergence of dissipative-like dynamics is attributed to the decay of large amplitude excitations at the shock front into turbulent vortex excitations, which allow us to invoke an eddy viscosity hypothesis. Our experimental observations are accompanied by numerical simulations of the mean-field, Gross-Pitaevskii equation that exhibit quantitative agreement with no fitting parameters. This work provides an avenue for the investigation of quantum shock waves and turbulence in channel geometries, which are currently the focus of intense research efforts.
The piston shock problem is a prototypical example of strongly nonlinear fluid flow that enables the experimental exploration of fluid dynamics in extreme regimes. We investigate this problem for the case of a nominally inviscid, superfluid Bose-Einstein condensate and observe rich dynamics including the formation of a plateau region, a sharp shock front, and rarefaction waves. Interestingly, many aspects of the observed dynamics follow the predictions of classical viscous, rather than superfluid dispersive, shock theory. The emergence of viscous dissipation in the fluid is attributed to turbulent vortex dynamics. Our experimental observations are accompanied by numerical simulations revealing a turbulent energy spectrum E that develops into the critical balance regime E∼k−2, typical of wavebreaking scenarios in other systems. This work provides a new avenue for the investigation of quantum shock waves and turbulence in channel geometries, both of which are currently the focus of intense research efforts.
A Bose-Einstein condensate (BEC) is a quantum fluid that gives rise to interesting shock-wave nonlinear dynamics. Experiments depict a BEC that exhibits behavior similar to that of a shock wave in a compressible gas, e.g., traveling fronts with steep gradients. However, the governing Gross-Pitaevskii (GP) equation that describes the mean field of a BEC admits no dissipation, hence classical dissipative shock solutions do not explain the phenomena. Instead, wave dynamics with small dispersion is considered and it is shown that this provides a mechanism for the generation of a dispersive shock wave (DSW). Computations with the GP equation are compared to experiment with excellent agreement. A comparison between a canonical one-dimensional (1D) dissipative and dispersive shock problem shows significant differences in shock structure and shock-front speed. Numerical results associated with the three-dimensional experiment show that three- and two-dimensional approximations are in excellent agreement and 1D approximations are in good qualitative agreement. Using 1D DSW theory, it is argued that the experimentally observed blast waves may be viewed as dispersive shock waves.
Light flow in nonlinear media can exhibit quantum hydrodynamical features which are profoundly different from those of classical fluids. We show that rather extremes regime of quantum hydrodynamics can be accessed by exploring two paradigmatic problems in fluid dynamics. The first one is the piston problem for light, and its generalization, named after the celebrated mathematician Riemann, where the piston acts on a concomitant abrupt change of photon density. Our experiment reveals regimes featuring optical rarefaction (retracting piston) or shock (pushing piston) wave pairs, and most importantly the transition to a peculiar type of flow, occurring above a precise critical piston velocity, where the light shocks are smoothly interconnected by a large contrast, periodic, fully nonlinear wave. The second one is the temporal photonic analogue of the dam-break phenomenon for shallow water by exploiting a fiber optics setup. We clearly observe the decay of the step-like input (photonic dam) into a pair of oppositely propagating rarefaction wave and dispersive shock wave. Our results show evidence for a critical transition of the dispersive shock into a self-cavitating state.Our results show that fiber-based optical systems are peerless testbeds to investigate the extension of gas dynamics problems to superfluid regimes by taking benefit of the analogy between optics and fluid dynamics supported by the universality of the nonlinear Schrodinger equation.
Dilute gas Bose-Einstein condensates are ultracold quantum gases that display many peculiar hydrodynamic properties, such as superfluidity, i.e. dissipation-less flow, a variety of solitonic textures and quantized vortex structures. Small amplitude excitations within a Bose-Einstein condensate are described by the Bogoliubov dispersion, and have been extensively studied in the past. This dissertation extends previous studies by focusing on strong, nonlinear excitations and shock structures generated in elongated Rb-87 Bose-Einstein condensates, elucidating novel dynamics in these quantum systems.\nThis dissertation is separated into two major parts. In the first part, the building and characterization of a new Bose-Einstein condensate apparatus at Washington State University is described. This apparatus has been built to generate ultracold clouds of Rb-87 and, more recently, K-41atoms. A description of the setups for both isotopes are provided. The apparatus reliably produces Bose-Einstein condensates of 7 x 10^5 Rb-87 atoms every 20 seconds.\nIn the second part of this dissertation, three experiments in a channel geometry are described that have been conducted with the new apparatus. In this part of the dissertation, quantum hydrodynamic properties are probed by using time-dependent optical potentials to generate nonlinear excitations and shock structures in an elongated Bose-Einstein condensate. An emergence of viscous-like shock dynamics, unidirectionality of a non-magnetic spin switch device, and the structure of dispersive shock waves in new types of higher order dispersions are observed. The work described in this dissertation establishes a novel platform for studying strong nonlinear effects in ultracold quantum gases.
Light flow in nonlinear media can exhibit quantum hydrodynamical features which are profoundly different from those of classical fluids. Here, we show that a rather extreme regime of quantum hydrodynamics can be accessed by exploring the piston problem (a paradigm in gas dynamics) for light, and its generalization, named after the celebrated mathematician Riemann, where the piston acts on a concomitant abrupt change of photon density. Our experiment reveals regimes featuring optical rarefaction (retracting piston) or shock (pushing piston) wave pairs, and most importantly the transition to a peculiar type of flow, occurring above a precise critical piston velocity, where the light shocks are smoothly interconnected by a large contrast, periodic, fully nonlinear wave. The transition to such extreme hydrodynamic state is generic for superfluids, but to date remained elusive to any other quantum fluid system. Our full-fiber setup used to observe this phenomenon in temporal domain proves to be a versatile alternative to other platforms currently employed to investigate the hydrodynamical properties of quantum fluids of light.