Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
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Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#2An interesting paper in somewhat the opposite direction, but still building on Landauer's work is "Ultimate physical limits to computation":
https://arxiv.org/abs/quant-ph/9908043
The end of the abstract:
> [...] quantitative bounds are put to the computational power of an `ultimate laptop' with a mass of one kilogram confined to a volume of one liter.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#3There's a plethora of air-gap malware studies from Ben-Gurion university [1], abused channels range from thermal, acoustic, optical, to classic TEMPEST.
The interesting thing is that there's no escape from such covert attacks, since machines are bound by the laws of physics that demand energy dissipation and generation of noise, heat, etc. If it leaks, then it computes.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#4So, here's a question:
If heat dissipation is so fundamental to computation, why aren't we using the amount of required heat dissipation as the fundamental measure of complexity in quantum computing?
In particular, say I construct a box with a uniform mixture of all quantum states on n qubits, and all the unitaries that can operate on those n qubits. how much heat do I have to dissipate in order to refine the state in my box to a "particular" quantum state and a "particular" unitary? There are some interesting, and recent, results about how quickly I can dissipate heat. In particular, T(t) is bounded from below by k*T(0)/t^7, where k is some constant.
Since the number of quantum computers you could want to build in that box is growing very fast with the size of your input, I suspect that your inability to throw away states in the process of constructing your computer very rapidly becomes the dominant effect in how long it takes to go from "which number do I want to factor" to actually getting the factors of that number.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#5A lighter, non-quantitative starting point https://en.wikipedia.org/wiki/Reversible_computing
I thought of this when I learned about Rust 'moving' values could--in reversible computing hardware--have a zero thermodynamic lower bound.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#6Sorry for offtopicness: submitter, could you please email hn@ycombinator.com? I would like to send you a repost invite for another submission.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#7So, here's a question: If heat dissipation is so fundamental to computation, why aren't we using the amount of required heat dissipation as the fundamental measure of complexity in quantum computing? In particular, say I construct a box with a uniform mixture of all quantum states on n qubits, and all the unitaries that can operate on those n qubits. how much heat do I have to dissipate in order to refine the state i…
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.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#8An interesting paper in somewhat the opposite direction, but still building on Landauer's work is "Ultimate physical limits to computation": https://arxiv.org/abs/quant-ph/9908043 The end of the abstract: > [...] quantitative bounds are put to the computational power of an `ultimate laptop' with a mass of one kilogram confined to a volume of one liter.
That along with "on the computational capacity of the universe" was the paper that stopped me from ever taking Seth Loyd seriously again. Later finding out he was a creepy Epstein croney... well sometimes the universe makes a lot of sense.
Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#9An interesting paper in somewhat the opposite direction, but still building on Landauer's work is "Ultimate physical limits to computation": https://arxiv.org/abs/quant-ph/9908043 The end of the abstract: > [...] quantitative bounds are put to the computational power of an `ultimate laptop' with a mass of one kilogram confined to a volume of one liter.
That along with "on the computational capacity of the universe" was the paper that stopped me from ever taking Seth Loyd seriously again. Later finding out he was a creepy Epstein croney... well sometimes the universe makes a lot of sense.
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Re: Irreversibility and Heat Generation in the Computing Process (1961) [pdf]
#10So, here's a question: If heat dissipation is so fundamental to computation, why aren't we using the amount of required heat dissipation as the fundamental measure of complexity in quantum computing? In particular, say I construct a box with a uniform mixture of all quantum states on n qubits, and all the unitaries that can operate on those n qubits. how much heat do I have to dissipate in order to refine the state i…
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'…
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.