Experimental demonstration of the fluctuation theorem in expansion and compression process of a single-particle gas Hyuk Kyu Pak (UNIST) The fluctuation theorem is a newly established principle that provides a rigorous statistical rule for thermally fluctuating quantities such as work, heat, and entropy production, and considers the probability of all possible fluctuations including rare events that violate the second law of thermodynamics. However, testing the theorem is difficult owing to the experimental difficulty in the measurements in small systems. In this work, we investigate the motion of a colloidal particle trapped in a harmonic potential with time-varying stiffness. This system has been studied theoretically, but has not been followed by any experiment. Here, we estimate the work done on the particle during compression and expansion by measuring its particle position in the first time. The resultant probability distributions of the work in both processes satisfy very well the Jarzynski equality and the Crooks fluctuation theorem. Because this isothermal expansion and compression process in a soft wall qualitatively mimics that in a rigid wall, it offers valuable insight into potential micromechanical heat engines.