Comment by wolfi1
5 hours ago
ok, the Landauer limit defines the minimum energy for a bit flip but I don't see how a Toffoli gate would require less energy for a bit flip let alone come into the region of the Landauer limit. Could someone with more knowledge enlighten us (or at least me)?
The Landauer limit defines minimum energy for a bit *erasure*.
A reversible gate doesn't involve any such erasure and therefore Landauer's principle doesn't apply to it.
What will happen in practice if you do an entirely reversible computation is that you end up with the data you care about and a giant pile of scratch memory that you're going to need to zero out if you ever want to reuse it. Or perhaps you rewind the computation all the way back to the beginning to unscratch the scratch memory but you're going to at least need to pay to copy the output somewhere.
IANAP, but my understanding is that the Landauer limit defines the minimum energy of forcing a unknown bit into a known state. Physics as we know it is fully reversible at the microscale - every possible state have exactly one ancestor state. An irreversible process (that is, one that would force to macroscopically distinguishable states into a single one) is only possible if we conduct the "unknowness" aka entropy away from our computer - i. e. generate heat. Toffoli gate are reversible, and therefore in theory you can implement it in a way that is not subject to the Landauer limit.
Obviously, implementing one as a CMOS gate wouldn't be enough. Reversible gates would be very different. AFAIR they need to have a fan-out of one - you can't just wire an output to two inputs without losing reversibility.
For example all the quantum computing is reversible and really doesn't want qbits to interact (hence get any energy) with the outside. So if you ignore all the supporting apparatus in theory it could work without spending energy. Toffoli gates can be used/realized in quantum computes.