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> We use a logic primitive called the adiabatic quantum-flux-parametron (AQFP), which has a switching energy of 1.4 zJ per JJ when driven by a four-phase 5-GHz sinusoidal ac-clockat 4.2 K.

The landauer limit at 4.2K is 4.019×10^-23 J (joules). So this is only a factor of 38x away from the landauer limit.

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I'm curious about how the Landauer limit relates to Bremermann's Limit: https://en.wikipedia.org/wiki/Bremermann%27s_limit

Admittedely, I haven't done much reading, but I see it is a linked page from Bremermann's Limit: https://en.wikipedia.org/wiki/Landauer%27s_principle

Mind expanding on this a bit more? What is that that limit and how does it relate to the clock speed?

It's not about clock speed per se. It's about the lowest possible energy expenditure to erase one bit of information (or irreversibly destroy it by performing a logical operation). The principle comes about from reasoning about entropy loss in said situations. There's a hypothesized fundamental connection between information and entropy manifest in physical law. The idea is that if you destroy one possible state of a system, you have reduced the entropy of that system, so the 2nd law of thermodynamics implies that you must increase the entropy of the universe somewhere else by at least that amount. This can be used to say how much energy the process must take as soon as you choose a particular temperature.

This applies to any irreversible computation.

IMO, The fact that it's only 38x the minimum is MIND BLOWING.

> IMO, The fact that it's only 38x the minimum is MIND BLOWING.

It's like if someone made a car that drives at 1/38th the light speed.

For anyone too lazy to math, its a car that can go:

28.4 million km per hour (i.e. 17.6 million miles per hour)

I wonder how much that speeding ticket would cost.

Disclaimer: Assuming the one-way speed of light is 300k km/s

> I wonder how much that speeding ticket would cost.

Depends on the frame the officer is viewing you from, I suppose.

> I wonder how much that speeding ticket would cost.

I once got one for going at about 3x the speed limit (very nice road in Brazil, broad daylight, nobody in sight, short run, and very unfortunate radar gun positioning). The policeman was impressed and joked that he would like to, but couldn't give me 3 speeding tickets instead of one.

There are some places where the cost of the ticket goes up the higher you are above the limit. Assuming it’s a school zone in one of those areas (pick whichever one you can answer for), what would the cost be then for going 28.4 mkm/hr?

Fortunately you could quickly get to a jurisdiction that didn't extradite.

Hahahaha! Well played

The sonic boom would destroy anything in a few mile radius

The heated plasma from compressing the atmosphere as well. I guess the progression of events and the carthage is similar to the relativistic baseball.

Relevant xkcd: "What would happen if you tried to hit a baseball pitched at 90% the speed of light?"

https://what-if.xkcd.com/1/

No need to assume when we can define ;p

I think they were alluding to the fact that it is impossible to measure the one-way speed of light, and even the definition is an assumption based on the two-way speed

Since the one-way speed is impossible to measure can we just define it to be half of the two-way speed? There's no way to tell if that's correct or not but it makes no difference in any measurements you could make. So maybe for convenience we can just choose the one-way speed to be half the two way speed to simplify the calculations.

That's exactly what's done most of the time, but it should be noted that it's convention and assumption.

This comment took effort and adds to the discussion; what's with the downvotes?

Is there an idea of entropic potential energy/gradient/pressure? Could you differentiate encrypted data from noise by testing how much energy it requires to flip a bit?

No, because the energy of the system isn't related to the order of the underlying data, it's related to the changes that happen to the underlying data. If you have 5 bits and flip 3, it takes the same energy regardless of if the 5 bits have meaning or not. This is speaking in terms of physics. There obviously could be some sort of practical side channel attack based on error checking times if this was an actual processor.

Only if you were measuring a system with access to the key material and doing something with the plaintext, in which case this would be a side-channel attack (and an already-studied one). The whole point of encryption is that the data output is indistinguishable from noise without knowing the key.

> The fact that it's only 38x the minimum is MIND BLOWING.

It doesn't run at room temp. The difference is much larger than 38x. It's closer to 1000x.

The landauer limit is temperature-dependent. The difference was calculated for the system's operating temperature.

> The landauer limit is temperature-dependent.

No it's not. It's linearly dependent on temperature. The figure commonly stated is arbitrarily chosen to be at room temperature.

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

Note that the gates themselves used here are reversible, so the limit shouldn't apply to them. But the circuits built from them them aren't reversible as far as I can see in the paper, so it would still apply to the overall computation.

This book goes into some great detail about this. :) https://www.amazon.com/dp/B004ZLS3TU/ref=dp-kindle-redirect?... I will always recommend it!

The interesting thing about this is that if we get close to the Landauer limit we may have to seriously start thinking of using reversible computing[1] paradigms and languages to get optimal performance.

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

> adiabatic quantum-flux-parametron

https://youtube.com/watch?v=BBqIlBs51M8

Reminded me of the Rockwell Retro Encabulator

https://www.youtube.com/watch?v=RXJKdh1KZ0w

> The landauer limit at 4.2K is 4.019×10^-23 J (joules).

At room temperature.

What does the 4.2K mean then? I assumed that was the temperature in kelvin.

Not sure what you're trying to say. Room temperature is 293K, and at temperature the Laundauer limit is 2.8E-21 J. The chips in the article were cooled to 4.2 K, and at that temperature the Laundauer limit is 4.0E-23 J.

That number checks out to be accurate to just over 4 significant digits. Divide the 4.019×10^-23 J by 4.2 K and ln(2), and you get a result just a hair smaller than the Boltzmann constant.

Unless the formula at [0] is wrong, in which case my calculations would also be wrong.

[0]: https://en.wikipedia.org/wiki/Landauer%27s_principle#Equatio...

Hmm that's interesting. At lower temperatures, would the limit also be lower? Or what's the relationship between temp and Landauer?

It's linear, so there's an almost 100 times decrease.

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