Fig. 1: Set-up for probing the heating–cooling asymmetry.
From: Heating and cooling are fundamentally asymmetric and evolve along distinct pathways
a, Schematic representation of the experiment. A charged dielectric microparticle dispersed in water is confined in a parabolic trap generated by a tightly focused infrared laser. Its effective temperature is controlled by an electric field that shakes the particle, mimicking a thermal bath at a higher temperature than the water. An arbitrary signal generator feeds a noisy signal with a white Gaussian spectrum into a pair of gold microelectrodes immersed in the liquid, thus producing the required electric field. Therefore, the particle exhibits Brownian motion inside the trap. This motion has a Gaussian distribution whose variance is determined by the effective temperature. b, In the experiments, we tracked the evolution of the position distribution upon quenches of the effective thermal bath during heating (red arrows) or cooling (blue arrows). c, Schematic representation of the respective protocols. In the forward protocol, the system is initially prepared at equilibrium with the thermal bath at a temperature higher (Th) or lower (Tc) than the target (Tw) temperature. Th and Tc are chosen to be TE from Tw with Th > Tw > Tc. During the backward protocol, the system relaxes at the respective TE temperatures Th and Tc, starting from a common initial condition that is the equilibrium at Tw. In a third situation, only two temperatures are compared, and the evolution of the system upon heating and cooling between them is assessed. In b and c, solid and dashed arrows stand for the forward and backward process, respectively, and thick lines indicate faster evolution than thin ones.
