  The Jarzynski equality relates the free energy difference between two
equilibrium states to the fluctuating irreversible work afforded to switch
between them. The prescribed fixed temperature for the equilibrium states
implicitly constrains the dissipative switching process that can take the
system far from equilibrium. Here, we demonstrate theoretically and
experimentally that such a relation also holds for the nonisothermal case,
where the initial stationary state is not in equilibrium and the switching is
effected by dynamically changing temperature gradients instead of a
conservative force. Our demonstration employs a single colloidal particle
trapped by optically induced thermophoretic drift currents. It relies on
identifying suitable equivalents of classical work and heat and our ability to
measure their distributions and express them in terms of a virtual potential.
