[sgdpk]

Inspired by this, in my PhD we actually used a quantum computer to do classical logic to investigate if that can lead to energy savings [1]. Quantum machines are (in principle) reversible, so they may avoid Landauer's principle.

However, there are subtle energy costs that you can hit before getting to Landauer's. The most interesting to me is that the qubits can become entangled with the wires that control them! This reduces the quality of information, and one way around it is to use a lot of energy [2].

[1] https://arxiv.org/abs/2210.10470

[2] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.89...

[gnramires]

Interesting, I suppose you'd use reversible logic for the computation? I think non-reversibility is relatively common in computing, but surely a significant fraction of operations could be made using reversible logic. Fourier transforms are a good example. Sorting a simple example of an operation destroying information (log2(n!) roughly ~ n log2(n) bits). Algorithms would need to be redesigned significantly I suppose if it ever came to reversible computing being energy advantageous.

[bee_rider]

You could also think of reversible blocks. For example, if you had an half-adder, you each constituent gate could be thought of as doing something like erasing in normal operation. If instead the half-adder was reversible, and you perform the operation, save the outputs, reverse it, and then erase the inputs, at least you only have to pay the Landauer tax on the input bits (rather than each intermediary bit). The bigger the block, the more you save.

[orlp]

All of quantum computing is done with strictly reversible logic. But this is not a reduction of power. The Toffoli gate is an example of a reversible 3-bit logic gate that is universal. You can transform any classical circuit into a reversible one using this gate.

The trick? You have some input/output wires in your circuit that start with initial value 0 or 1, and some that end up in a 'garbage' state. You don't care about what they end up with, they strictly exist to maintain reversibility.

As an concrete example, the binary operator a OR b is not reversible - it destroys information. But if we make it a three-in three-out operator f(a, b, c) -> (a, b, c XOR (a OR b)), then it is reversible. Here f is it's own inverse, because applying it twice you'd end up with c XOR (a or b) XOR (a or b) = c.

Yet if you start with c in an initial 0 state, you can now compute the binary OR of two variables a, b in a reversible manner.

[_a_a_a_]

Is there any reason to think that quantum computing can fundamentally do more computation than classical computers? Consider this a leading question rather than a stupid one.

[dskrvk]

Here’s a recent quote from Scott Aaronson:

> In truth, anything that a QC can calculate, a classical computer can calculate as well, given exponentially more time: for example, by representing the entire wavefunction, all 2n amplitudes, to whatever accuracy is needed. That’s why it was understood from the very beginning that quantum computers can’t change what’s computable, but only how efficiently things can be computed.

https://scottaaronson.blog/?p=7321

[dreamcompiler]

No. Scott Aaronson makes this point repeatedly in his blog. Quantum computers are much faster than conventional computers on a few problems, and practically useless for all the others. That's the whole story.

Quantum computers are not hyper-turing.

[lmm]

There are many problems thought to be hard for which efficient quantum algorithms are known.

[cookieperson]

How much energy do you spend cooling your quantum computer?