>>3Tom Knight's research group at MIT fabricated a small adiabatic processor in the late 1990s. The practical logic family they developed is called split-level charge recovery logic, and can be implemented using standard (CMOS) fabrication techniques. I believe the work has been continued by Michael P Frank at Florida State University. An example of the work in Tom Knight's group is the following master's thesis (which has a pretty decent section on related work through the early 1990s.) Vieri, C J: Pendulum: A Reversible Computer Architecture, Master's Thesis, MIT EECS dept, 1995.
Reversible circuits need to be adiabatic (there can't be heat exchanges between the circuit and its environment), which means that they must be in equilibrium at all times. For any process that needs to change something you can only approximate equilibrium by making the change happen as slowly as possible.
If I remember my thermodynamics correctly, you can make the energy of a reversible computation arbitrarily small, but the minimum action (energy times time) must be a small constant.