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: The research group in Japan sought to create a superconductor microprocessor that’s adiabatic, meaning that, in principle, energy is not gained or lost from the system during the computing process.

I thought that there was a law of information theory that requires expending energy, I think it's Landauer's principle. It seems to be disputed though.

https://en.wikipedia.org/wiki/Landauer%27s_principle

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Energy is expended when you zero or set a bit.

If you compute reversibly you need use special logic gates to not throw any bits away during the computation, like the Toffoli gate. All your operations need to have the same number of input and output bits and needs to be able to run forwards and backwards. Effectively you set or zero no bits during the entire computation that can't be losslessly reversed.

If you structure your computation this way you can do it adiabatically.

You still however need to expend energy when you set all the bits your program requires for execution when you start a computation.

I think maybe we need the idea of a "bit well" to match the "bit bucket".

Then it would be possible to just borrow bits when we set bits.

My analogy probably isn’t perfect but isn’t this how an abacus or a slide rule works? You expend energy to set the values for computation but the act of computing and then reading them expends no energy.

Yeah this is a good analogy

That's a bad analogy because moving stuff in abacus is the computation itself.

Suppose you put the abacus sideways, such that setting a bead requires working against gravity, but then returning the bead to the original position gives you that energy back?

Wait, so does this imply (ignoring the time requirements) that you could do NP calculations with a feasible amount of energy, because the inputs and outputs are small?

Combine that with something that uses time dilation to make it go fast (from our frame of reference) and you'd be giving even hypothetical quantum computers a silver medal.

If you're going to use time dilation so you can wait for the computer to calculate, it's hard to imagine a setup where the power needs of the computer are more than a rounding error in comparison.

If you're using time dilation the power needs of the computer (in terms of how much fuel you need) are proportional to the rate time passes inside. If the goal is to get the ratio way up there, the power consumption gets important.

Especially if you're using the velocity-based method and have to accelerate the fuel.

You'd be accelerating yourself, not the computer. If you launch the computer it just takes longer.

Not unless you can isolate the computer from environmental noise exponentially well. Otherwise you'll need to spend exponential energy on entropy removal / error correction (e.g. keeping the dilution fridge running).

I think a longer runtime for a terminating program will require more state to be kept. Every step needs to keep extra reversibility information.

So I don't think this gives any edge on NP.

I suspect that even if you waited for the universe to cool down a lot by waiting aeons and then performed computations arbitrarily slowly you'd still be limited by your starting energy (maximum bits you can write to start with).

Although maybe using random bits might help somehow?

The amount of energy it takes to write a bit depends on the temperature. But you'd still be limited by the amount of substrate available.

> time dilation to make it go fast (from our frame of reference)

The only way to do that is that we move to a place from which the computer processes appear accelerated, e.g. into a strong gravity well. That isn't very useful, because we do not have such a well nearby, as only very dense hypothetical objects can provide it (e.g. black holes), and it would not be compatible with life to move there.

Doesn’t have to be compatible with life, just compatible with computing.

You misunderstand. The problem with using time dilation is that you can only make the clock go slower, not faster. If you wanted to exploit time dilation to get a computer result faster (in subjective time), you don't send the computer to an exotic locale, you go there yourself, while you leave the computer to do work in normal space.

TIL that time dilation can only really be used to make something go slower than normal space, not faster.

Sci-fi authors owe some apologies. I'm looking at you, Star Trek.

Time dilation means the object in the gravity well is experiencing time at a slower rate than us humans outside. That is the opposite condition we want: us slow, computer fast.

Landauer's limit assumes that computation uses a thermodynamically irreversible process and erases bits of information. This is not necessarily true for all useful computers [0]. So theoretically speaking, it's not impossible to create an adiabatic microprocessor. Skepticism is definitely warranted though.

[0] https://en.m.wikipedia.org/wiki/Reversible_computing

If the superconducting microprocessor is 80x as efficient as a normal micro, that means that it uses about 100 million times as much energy as Landauer's principle. (A normal micro uses a billion times as much, and Landauer's energy is ~1/5th at superconducting temperature).

Yes, but the quote suggests no energy transfer at all. All you need then is a cold place (somewhere in remote space perhaps, or after the heat death of the Universe) and your computation can continue forever without power. It doesn't seem intuitive that that would be possible.

It is extremely likely that the quote is hyperbolic (wrong / inaccurate). If someone broke Landauers limit, that would be a massive headline in academia. Hell even reasonably approaching Landauer’s limit would be huge news.

Most likely this headline is a confusion from how signal/power transmission (rather than calculation/work) is truly lossless in superconductors.

Continue forever? no. Be performed once at no cost? maybe.

I'm kinda sceptical that a computation to which the 2nd law is indifferent would occur spontaneously without immediately reversing. The 2nd law is what determines the direction that things typically progress.

Per the wikipedia article: “Modern computers use about a billion times as much energy per operation.”

I’m guessing the efficiency gains can be considerable before hitting this limit.

That's actually fewer zeroes than I was expecting.

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