  Virtual potentials are an elegant, precise and flexible tool to manipulate
small systems and explore fundamental questions in stochastic thermodynamics.
In particular double-well potentials have applications in information
processing, such as the demonstration of Landauer's principle. In this chapter,
we detail the implementation of a feedback loop for an underdamped system, in
order to build a tunable virtual double-well potential. This feedback behaves
as a demon acting on the system depending on the outcome of a continuously
running measurement. It can thus modify the energy exchanges with the
thermostat and create an out-of-equilibrium state. To create a bi-stable
potential, the feedback consists only in switching an external force between
two steady values when the measured position crosses a threshold. We show that
a small delay of the feedback loop in the switches between the two wells
results in a modified velocity distribution. The latter can be interpreted as a
cooling of the kinetic temperature of the system. Using a fast digital
feedback, we successfully address all experimental issues to create a virtual
potential that is statistically indistinguishable from a physical one, with a
tunable barrier height and energy step between the two wells.
