[scottlocklin]

One of the things they don't tell you about quantum computing is it's supposed to be reversible. Which, ceteris paribus, probably means they're not physical. Nobody likes to talk about that as it's terrible for funding.

You can build totally reversible computers using ordinary classical physics which ... in principle can be arranged to dissipate no heat (in practice they'll always dissipate heat). The problem is you're basically effectively dissipating the "heat" into a memory system which rapidly becomes practically infinite [1]. Imagine keeping around all the bits that got AND-gated away from .... I dunno, fitting GPT-3. Or even just inverting some big matrix. That's what you got to do for reversible computing: at the individual bit level mind you -many of the fundamental floating point operations are not themselves reversible, so they throw off more "heat" aka fill up memory cells with bits which allow you to reverse them.

Landauer, who is an underappreciated genius, wasn't aware of the reversible computing idea, or had too much sense to bother with it. There are others who attempt to defeat his very common sense idea with hand wavey adiabatic relaxation ideas, but I think they're all baloney. All of this is a barrel of monkeys to think about (not QC, which is dumb; the general reversible computing stuff); I recommend the seminal papers listed in the below wiki article if you have an afternoon to burn. Bennett, Toffoli and Vitanyi in particular are real fun to read.

[1] https://en.wikipedia.org/wiki/Reversible_computing

[ouid]

These are interesting reads, and it seems that reversible computers suffer the same problem, physically. If I want to actually make a reversible computer, I still have to make the blank tape that it uses.

[scottlocklin]

Precisely: and that issue (among many others) is the one ding dongs in "quantum information" sweep under the carpets and hope you don't notice. It's HUGELY OBVIOUS if you try to do it for an HP42 calculator tier computer. Somehow the mystification of adding entanglement to the mess pushes it off into "Hilbert space" and nobody notices.