Experimental studies of Bose–Einstein condensates in disorder

Abstract

We review recent studies of the effects of disorder on an atomic Bose-Einstein condensate (BEC). We focus particularly on our own experiments with 7 Li BECs in laser speckle. Both the interaction, which gives rise to the nonlinearity in a BEC, and the disorder can be tuned experimentally. This opens many opportunities to study the interplay of interaction and disorder in both condensed matter physics and nonlinear science.

Density modulations in an elongated Bose-Einstein condensate released from a disordered potential

“…It has been suggested [1,13], by analogy with elongated (non-disordered) quasi-BECs, that the observation of large fringes in time-offlight (TOF) images of disordered BECs (see Fig. 1 and [1,14,15]) may signal strong initial phase fluctuations [16,17,18]. For quasi-BECs, such fringes in TOF images are indeed a signature of initial phase fluctuations [17].…”

“…Our main experimental result is that the fringes in the TOF images are reproduced from shot to shot when using the same realization of the disorder. This excludes disorder-induced phase fluctuations (thermal or quantum) in the trapped BEC as the origin of the fringes observed after TOF, for the parameter range of the experiment (relevant also for [1,14,15]). Using analytical and numerical calculations, which do not include initial phase fluctuations, we show that the fringes actually develop during the TOF according to the following scenario.…”

“…Last but not least, we have observed identical density modulations in successive experiments performed with the same realization of the disorder (see also Refs. [14,15]). Hence, averaging over various images taken with the same realization of the disorder does not wash out the fringes [23] and this excludes initial random fluctuations, quantum or thermal [19].…”

“…In conclusion, we have shown that the large fringes observed in this work and Refs. [1,14,15] in TOF images of disordered BECs do not rely on initial disorder-induced phase fluctuations. They actually result from the TOF process of a BEC with small initial density modulations but without phase fluctuations.…”

Two-dimensional optical quasicrystal potentials for ultracold atom experiments

“…Systems with disorder, however, exhibit Anderson localization [30,31] caused by the destructive interference of single-particle wavefunctions over multiple random paths. The experimental signature of Anderson localization is the exponential decay of particle momentum distributions [32][33][34]. Some quasi-periodic systems exhibit weaker localization than disordered systems [35][36][37], and quasicrystals are also expected to show this behavior.…”