Constraint dependence of average potential energy of a passive particle in an active bath

We quantify the mean potential energy of a passive colloidal particle harmonically confined in a bacterial solution using optical traps. We find that the average potential energy of the passive particle depends on the trap stiffness, in contrast to the equilibrium case where energy partition is independent of the external constraints. The constraint dependence of the mean potential energy originates from the fact that the persistent collisions between the passive particle and the active bacteria are influenced by the particle relaxation dynamics. Our experimental results are consistent with the Brownian dynamics simulations, and confirm the recent theoretical prediction.