I was reading the book \"The singularity is near\" written by Kurzweil and he me

Question

I was reading the book "The singularity is near" written by Kurzweil and he mentioned the reversible gates like for example the Fredkin gate. The advantage using such gates is that we could get rid of the thermal waste related to computation where bits just disappear into heat, and computation won't need any energy input. Those assumptions make these gates sound like a miracle solution. So the question is what technical hurdles are still preventing their large scale usage.

I also think it is a shame that I never heard about those gates in my electrical engineering bachelor and master studies at a top German university...

Explanation / Answer

The problem with practical reversible gates (gates that can (and have been) fabricated in silicon) is that the actual energy savings are linearly proportional to how slowly you run them.

I know that Tom 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.

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