Bose–Einstein condensation and superfluidity

Priyanka Verma

Anand K. Ramanathan

Superfluidity is a remarkable phenomenon. Superfluidity was initially characterized by flow without friction, first seen in liquid helium in 1938, and has been studied extensively since. Superfluidity is believed to be related to, but not identical to Bose-Einstein condensation, a statistical mechanical phenomena predicted by Albert Einstein in 1924 based on the statistics of Satyendra Nath Bose, where bosonic atoms make a phase transition to form a Bose-Einstein condensate (BEC), a gas which has macroscopic occupation of a single quantum state. Developments in laser cooling of neutral atoms and the subsequent realization of Bose-Einstein condensates in ultracold gases have opened a new window into the study of superfluidity and its relation to Bose-Einstein condensation. In our atomic sodium BEC experiment, we studied superfluidity and dissipationless flow in an all-optical toroidal trap, constructed using the combination of a horizontal ``sheet''-like beam and vertical ``ring''-like beam, which, like a circuit loop, allows flow around the ring. On inducing a single quantum of circulation in the condensate, the smoothness and uniformity of the toroidal BEC enabled the sustaining of a persistent current lasting 40 seconds, limited by the lifetime of the BEC due to background gas pressure. This success set the stage for further experiments studying superfluidity. In a first set of experiments, we studied the stability of the persistent current by inserting a barrier in the flow path of the ring. The superflow stopped abruptly at a barrier strength such that the local flow velocity at the barrier exceeded a critical velocity, which supported decay via the creation of a vortex-antivortex pair. Our precise control in inducing and arresting superflow in the BEC is a first step toward studying other aspects of superfluidity, such as the effect of temperature and dimensionality. This thesis discusses these experiments and also details partial-transfer absorption imaging, an imaging technique developed in the course of this work.

Matthew J. Davis, Tod M. Wright, Tapio Simula +2 more

Dilute gas Bose-Einstein condensates (BECs) are an example of a superfluid, and exhibit many of the same phenomena as the strongly interacting superfluid helium. However, their isolation from the environment combined with their inhomogeneous density profiles make it difficult to observe some phenomena, such as the thermo-mechanical effect, in experiments. Here we perform dynamical simulations of a finite temperature Bose-Einstein condensate coupled to spatially separated heat and particle reservoirs to observe such non-equilibrium superfluid phenomena. We analyse the resulting steady-state non-equilibrium flows in the BEC, and consider the prospect of observing sustained superfluid turbulence.

Wolfgang Ketterle

Matthew J. Davis, Tod M. Wright, Tapio Simula +2 more

Dilute gas Bose-Einstein condensates (BECs) are an example of a superfluid, and exhibit many of the same phenomena as the strongly interacting superfluid helium. However, their isolation from the environment combined with their inhomogeneous density profiles make it difficult to observe some phenomena, such as the thermo-mechanical effect, in experiments. Here we perform dynamical simulations of a finite temperature Bose-Einstein condensate coupled to spatially separated heat and particle reservoirs to observe such non-equilibrium superfluid phenomena. We analyse the resulting steady-state non-equilibrium flows in the BEC, and consider the prospect of observing sustained superfluid turbulence.

Ali A. Shams, H. R. Glyde