The pressure of a gas, the van der Waals attraction between molecules, and the Casimir force in quantum electrodynamics (QED) are classical examples of forces resulting from equilibrium (thermal or quantum) fluctuations. Current research on "Active Matter" studies collective behaviors of large groups of self-driven entities (living or artificial), whose random motions superficially resemble thermally fluctuating particles. However, the absence of time reversal symmetry leads to unusual phenomena such as
directed (ratchet) forces, and a pressure that depends on the shape and structure of the confining wall.
Some manifestations of QED fluctuations out of thermal equilibrium are well-known, as in the Stefan-Boltzmann laws of radiation pressure and heat transfer. These laws, however, acquire non-trivial twists in the near-field regime of sub-micron separations, and in the proximity of moving surfaces. Symmetry arguments suggest that lateral ratchet forces should emerge out of equilibrium and with broken spatial symmetry. We inquire if such forces can be used to construct a heat engine, and discuss
constraints on its operation.
In his seminal 1940 paper, H.A. Kramers proposed that the theory of Brownian motion, formulated originally by Einstein, Smoluchowski, and Langevin with the goal to describe motion of micrometer-sized particles in water, can also be applied to the internal motions of molecules (as well as to nuclear fission). Recently, advances in single-molecule experimental techniques have finally enabled direct experimental tests of Kramers’ model. Outcomes of such tests are however controversial, given that they probe molecular lengthscales (nanometers) and timescales (microseconds) that are far beyond observational limits of conventional microscopies yet they still lack temporal and spatial resolution to measure finer dynamical details such as microscopic velocities. In this talk, I will describe several approaches to testing Kramers’ model that work in the presence of experimental constrains. Those include testing for violation of recently proven inequalities that must be satisfied by Brownian motion as well as information-theory-based detection of memory in the observed trajectories. I will further discuss what such tests can tell us about hidden, unobserved molecular degrees of freedom.
Ultracold atoms at temperatures just above absolute zero have proven to be ideal systems for studying novel regimes of nonequilibrium physics that are not readily accessible with other implementations. In recent years, methods for preparing, manipulating, and detecting the properties of ultracold atoms have been refined to the level of single atoms. Beyond the physics of single quantum systems, the coupling of single atoms to an engineered bath paves the way to studying the physics of open systems, both quantum and classical.
In my talk, I will introduce the methods, tools, and particular properties of the ultracold atomic world and show the dynamics of single atoms in two different scenarios.
First, I will discuss the dynamics of single atoms diffusing in a light bath when the motion of the atom is confined to a periodic potential. We show that such an underlying periodic structure modifies the diffusion so that it deviates from the well-known Brownian motion. Such behavior is known from many systems diffusing on structured potentials, and we can relate it to a simple continuous-time random walk model.
Second, we immerse individual Cs atoms in an ultracold atomic Rb bath in which, in particular, spin-exchange collisions between the Cs atom and Rb atoms from the bath lead to spin relaxation of the Cs atom. The control over all quantum states of Cs atom and Rb bath allows to control the spin dynamics and make it useful for different scenarios, such as using single atoms as single-atom thermometers in a quantum bath, or operating a single atom machine with "spin fuel" at the level of single converted quanta.
