Comment by mikepfrank

Comment by mikepfrank 5 days ago

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It's been a while since I looked at it, but I believe PFAL is one of the not-fully-adiabatic techniques that I have a lot of critiques of.

There have been studies showing that a truly, fully adiabatic technique in the sense I'm talking about (2LAL was the one they checked) does about 10x better than any of the other "adiabatic" techniques. In particular, 2LAL does a lot better than PFAL.

> reversibility isn't actually necessary

That isn't true in the sense of "reversible" that I use. Look at the structure of the word -- reverse-able. Able to be reversed. It isn't essential that the very same computation that computed some given data is actually applied in reverse, only that no information is obliviously discarded, implying that the computation always could be reversed. Unwanted information still needs to be decomputed, but in general, it's quite possible to de-compute garbage data using a different process than the reverse of the process that computed it. In fact, this is frequently done in practice in typical pipelined reversible logic styles. But they still count as reversible even though the forwards and reverse computations aren't identical. So, I think we agree here and it's just a question of terminology.

Lower bounds on clock speed are indeed important; generally this arises in the form of maximum latency constraints. Fortunately, many workloads today (such as AI) are limited more by bandwidth/throughput than by latency.

I'd be interested to know if you can get energy savings factors on the order of 100x or 1000x with the capacitive switching techniques you're looking at. So far, I haven't seen that that's possible. Of course, we have a long way to go to prove out those kinds of numbers in practice using resonant charge transfer as well. Cheers...

colanderman 5 days ago

PFAL has both a fully adiabatic and quasi-adiabatic configuration. (Essentially, the "reverse" half of a PFAL gate can just be tied to the outputs for quasi-adiabatic mode.) I've focused my own research on PFAL because it is (to my knowledge) one of the few fully adiabatic families, and of those, I found it easy to understand.

I'll have to check out 2LAL. I haven't heard of it before.

No, even with a fully adiabatic switched-capacitance driver I don't think those figures are possible. The maximum efficiency I believe is 1-1/n, n being the number of steps (and requiring n-1 capacitors). But the capacitors themselves must each be an order of magnitude larger than the adiabatic circuit itself. So it's a reasonable performance match for an adiabatic circuit running at "max" frequency, with e.g. 8 steps/7 capacitors, but 100x power reduction necessary to match a "slowed" adiabatic circuit would require 99 capacitors... which quickly becomes infeasible!

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  • mikepfrank 4 days ago

    Yeah, 2LAL (and its successor S2LAL) uses a very strict switching discipline to achieve truly, fully adiabatic switching. I haven't studied PFAL carefully but I doubt it's as good as 2LAL even in its more-adiabatic version.

    For a relatively up-to-date tutorial on what we believe is the "right" way to do adiabatic logic (i.e., capable of far more efficiency than competing adiabatic logic families from other research groups), see the below talk which I gave at UTK in 2021. We really do find in our simulations that we can achieve 4 or more orders of magnitude of energy savings in our logic compared to conventional, given ideal waveforms and power-clock delivery. (But of course, the whole challenge in actually getting close to that in practice is doing the resonant energy recovery efficiently enough.)

    https://www.sandia.gov/app/uploads/sites/210/2022/06/UKy-tal... https://tinyurl.com/Frank-UKy-2021

    The simulation results were first presented (in an invited talk to the SRC Decadal Plan committee) a little later that year in this talk (no video of that one, unfortunately):

    https://www.sandia.gov/app/uploads/sites/210/2022/06/SRC-tal...

    However, the ComET talk I linked earlier in the thread does review that result also, and has video.

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    • westurner 4 days ago

      How do the efficiency gains compare to speedups from photonic computing, superconductive computing, and maybe fractional Quantum Hall effect at room temperature computing? Given rough or stated production timelines, for how long will investments in reversible computing justify the relative returns?

      Also, FWIU from "Quantum knowledge cools computers", if the deleted data is still known, deleting bits can effectively thermally cool, bypassing the Landauer limit of electronic computers? Is that reversible or reversibly-knotted or?

      "The thermodynamic meaning of negative entropy" (2011) https://www.nature.com/articles/nature10123 ... https://www.sciencedaily.com/releases/2011/06/110601134300.h... ;

      > Abstract: ... Here we show that the standard formulation and implications of Landauer’s principle are no longer valid in the presence of quantum information. Our main result is that the work cost of erasure is determined by the entropy of the system, conditioned on the quantum information an observer has about it. In other words, the more an observer knows about the system, the less it costs to erase it. This result gives a direct thermodynamic significance to conditional entropies, originally introduced in information theory. Furthermore, it provides new bounds on the heat generation of computations: because conditional entropies can become negative in the quantum case, an observer who is strongly correlated with a system may gain work while erasing it, thereby cooling the environment.

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      • mikepfrank 4 days ago

        I have concerns about density & cost for both photonic & superconductive computing. Not sure what one can do with quantum Hall effect.

        Regarding long-term returns, my view is that reversible computing is really the only way forward for continuing to radically improve the energy efficiency of digital compute, whereas conventional (non-reversible) digital tech will plateau within about a decade. Because of this, within two decades, nearly all digital compute will need to be reversible.

        Regarding bypassing the Landauer limit, theoretically yes, reversible computing can do this, but not by thermally cooling anything really, but rather by avoiding the conversion of known bits to entropy (and their energy to heat) in the first place. This must be done by "decomputing" the known bits, which is a fundamentally different process from just erasing them obliviously (without reference to the known value).

        For the quantum case, I haven't closely studied the result in the second paper you cited, but it sounds possible.

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        • westurner 3 days ago

          /? How can fractional quantum hall effect be used for quantum computing https://www.google.com/search?q=How+can+a+fractional+quantum...

          > Non-Abelian Anyons, Majorana Fermions are their own anti particles, Topologically protected entanglement

          > In some FQHE states, quasiparticles exhibit non-Abelian statistics, meaning that the order in which they are braided affects the final quantum state. This property can be used to perform universal quantum computation

          Anyon > Abelian, Non Abelian Anyons, Toffoli (CCNOT gate) https://en.wikipedia.org/wiki/Anyon#Abelian_anyons

          Hopefully there's a classical analogue of a quantum delete operation that cools the computer.

          There's no resistance for electrons in superconductors, so there's far less waste heat. But - other than recent advances with rhombohedral trilayer graphene and pentalayer graphene (which isn't really "graphene") - superconductivity requires super-chilling which is too expensive and inefficient.

          Photons are not subject to the Landauer limit and are faster than electrons.

          In the Standard Model of particle physics, Photons are Bosons, and Electrons are Leptons are Fermions.

          Electrons behave like fluids in superconductors.

          Photons behave like fluids in superfluids (Bose-Einstein condensates) which are more common in space.

          And now they're saying there's a particle that only has mass when moving in certain directions; a semi-Dirac fermion: https://en.wikipedia.org/wiki/Semi-Dirac_fermion

          > Because of this, within two decades, nearly all digital compute will need to be reversible.

          Reversible computing: https://en.wikipedia.org/wiki/Reversible_computing

          Reverse computation: https://en.wikipedia.org/wiki/Reverse_computation

          Time crystals demonstrate retrocausality.

          Is Hawking radiation from a black hole or from all things reversible?

          What are the possible efficiency gains?

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