1D Bosons

A story on: Equations of state from individual one-dimensional Bose gases (New Journal of Physics 20 (11), 113032)

Background

Contact interacting bosons in the one-dimensional (1D) regime, where all dynamics happen preferentially along one dimension, “repel each other” like fermions at low enough densities and temperatures. Such a strongly anti-correlated bosonic gas may be used to emulate some of the properties of non-interacting fermions.

Experiment

We start with an ultracold, three-dimensional (3D) Bose gas, e.g. a Rb-87 Bose–Einstein condensate (BEC) where three characteristic energy scales exist; 1) the thermal energy scale associated with the temperature of the gas, 2) the harmonic level energy spacing associated with our trap, and 3) the mean-field energy scale associated with the short-range interactions. We freeze two degrees of freedom with a repulsive optical dipole trap (tweezer) shaped like a donut (LG-01 mode). The resulting trumpet-shaped potential squeezes atoms along two dimensions, leaving only the longitudinal one free. Then, we determine the thermodynamic state of our 1D gas by taking images of the resulting density distributions. Because we know the shape of the trapping potential, we can connect the observed local density with a local chemical potential, and a global unknown temperature. Relating such “state variables” comprises an equation of state.

Key finding

We discover that loading the same 1D traps with warmer 3D gases results in colder 1D gases! Furthermore, by waiting for some time we see that the most energetic bosons leave the 1D trap, resulting in a net evaporative cooling. While the 1D temperature drops, the once degenerate bosons actually escape degeneracy! We speculate this is due to the inefficient chasing of the critical temperature in 1D vs 3D, where evaporative cooling results in an efficient approach trajectory to degeneracy.