by Artur Widera, Individual Quantum Systems, Technische Universität Kaiserslautern
Technological advances in recent years to cool, manipulate, and detect neutral atoms open new avenues in studying diffusion phenomena at the nexus of quantum and classical physics. On the one hand, experimenters can trace individual atoms’ trajectories, where the particles can show a broad range of states governed either by classical or by quantum mechanical laws. On the other hand, baths can be tailored, either by light fields, where the absorption and emission processes emulate the collisional events or tightly controlled ultracold gases.
I will report on our recent results to study the diffusion of single atoms in a periodic potential, where a light field realizing laser cooling of the atoms forms the bath. While the ensemble-averaged mean-squared displacement values indicate Brownian motion, the hopping-distance distribution extracted from a single particle’s trajectories shows pronounced non-Gaussian distributions. We explain this observation by a continuous-time random walk, where the microscopic properties of the experimental system allow predicting the characteristic length and time scales.
Second, I will report on the diffusion of single atoms in a dilute, ultracold gas. Here, collisional events are rare, thus sampling the discrete nature of the bath particles. Simultaneously, every collision has a significant impact, as the tracer particle has approximately the same mass as a bath particle. In this regime, a Langevin description is expected to fail. Accounting for the close-to-unity mass ratio, we show that a modified Langevin equation describes the system’s dynamics surprisingly well.
Finally, I will report recent experiments studying single atoms’ diffusion in tilted, or accelerated, lattice potentials. We find that by changing the acceleration, the diffusion constant can be modified.
Our work points toward elucidating diffusion in unconventional regimes, where quantum effects might play a role, or where additional external drive or dissipation can be precisely tuned via the atomic system.