Ultracold atoms flow on a chip
Hydrodynamics usefully describes the behavior of many-body systems, from molecules in a cup of tea to cars on a highway. The framework treats a collection of particles as a continuous fluid whose total mass, momentum, and energy are conserved, and it offers the advantage of describing a system in terms of local characteristics rather than individual particle movements. But in some systems that do not reach thermal equilibrium, hydrodynamics fails because many more quantities are conserved. Now Max Schemmer, Isabelle Bouchoule, and their colleagues have provided the first direct evidence in support of new hydrodynamic models that are sufficiently general to describe the nonequilibrium behavior of ultracold atoms in one dimension.
The new work tested a version of hydrodynamics proposed in 2016 by research groups in the UK, Italy, and France to model the large-scale nonequilibrium dynamics of 1D quantum systems. Conventional hydrodynamics describes the evolution of 1D quantum systems that reach thermal equilibrium. Generalized hydrodynamics extends to a special class of systems known as integrable systems, in which atoms do not reach equilibrium but instead maintain constant predictable motion. Whereas a conventional system has a finite number of local quantities (mass, energy, and momentum densities) that correspond to conserved global quantities, an integrable system has an infinite number of local momentum-like densities. Each must be conserved for the entire system, which leads to a continuity equation for each local quantity. Generalized hydrodynamics identifies a set of equations that represent the entire set of conserved quantities.
To test the theory, Schemmer and colleagues confined several thousand cold rubidium atoms in a magnetic trap (shown in the figure) comprising conducting wires several millimeters long deposited on a chip. Turning off the current through some of the wires partially cancelled the confinement and freed the gas to flow either left or right. The atoms spread into density distributions predicted by generalized hydrodynamics for several different initial configurations. The researchers plan next to test the theory against other 1D gases, including ones composed of strongly repulsive particles. (M. Schemmer et al., Phys. Rev. Lett. 122, 090601, 2019 .)
