An all-optical general-purpose CPU and optical computer architecture
arxiv.org
arxiv.org
What they actually built was a 2-bit wide machine with one instruction. No, they can't run Doom, which they mention a lot.
There's a lot of hand-waving about memory, around page 10. They seem to have used a delay line, which is very slow; you have to wait for the bits you want to come around. That's been a classic problem with photonics. You can build gates, which is nice for switching packets, but how do you store data?
Much of the architectural discussion is about what you can do if memory is mostly ROM. They talk about fast-read, really slow write memory. Here's an article about building something like that.[2] It's a clunky technology. Writing involves on-chip heaters and switching memory cells back and forth from amorphous to crystalline. There's a long history of forgotten devices like that - photochromic memory, UV-erasable EEPROMS, rewritable DVDs, Ovonics, etc. All were superseded by something with better read-write properties.
The underlying device technology is not theirs. It's from the Cornerstone project.[1]
I agree though, I've never seen anything explain how to make this work well as a general computer with contemporary tech.
Logic gates can be the fundamental building block. So if you can build logic gates, then you can use those to build flip flops (the basis of static RAM) which store data. Might not be the most efficient way (depending on your requirements) but it can be done.
https://en.wikipedia.org/wiki/Flip-flop_(electronics)#D_flip...
The SRAM memory cells are simpler than flip flops, they are just S-R latches (i.e. equivalent with a half of an M-S flip-flop) made from two gates (either NAND or NOR).
In any technology where static gates are possible SRAM memories are also possible.
However there are technologies where only dynamic gates are possible, i.e. gates that provide an output that is valid only during a clock pulse and which cannot remain valid indefinitely. Only in such technologies you cannot make SRAMs, but you can still make dynamic memories, which must consume energy all the time, for refreshing their content.
All the "analog magic" that you mention has only the purpose of making an SRAM array much denser than when implemented with the standard gates of a technology, but an implementation with standard gates is always possible and it may be chosen for certain register files, where high speed may be more important than the occupied area.
Macroscopic delay lines sure, but microscopic ones presumably can be on the order of a wavelength, if that's all that's needed. Not much time needed to come around in that case.
Obviously this is only an easy option if the production is automated via lithography or something similar. If the process involves tweezering parts into place by hand, then a 2 bit SUBLEQ CPU is what you're gonna get...
or what are you referring to?
Warning.
There is NO opportunity for large scale integration, the MOST IMPORTANT ASPECT in computing. This is because the de Broglie wavelength of the information carriers, typically 1.5um, is so HUGE.
If your gate gets 50x50x50 times bigger, you need some pretty extreme savings per area/volume of circuit if you want to reduce the per-gate usage. Can they save that much?
Light would reduce the time cost of distance, and increase the density of connections (optical signals can pass through each other) so this could actually work.
For classics optics. Exists superlens optics, which using metamaterials and monochromatic light source, and could "see" artifacts of size much less then wave length.
But unfortunately, for small N, like the N = 2 bits here, the additional complexity of pure optical + quantum computing just doesn’t pay off!
The first is that light beams can cross paths without interfering with each other, allowing for a level of parallelism and density of signal paths without the concern of crosstalk/interference or shorting. Additionally, the information density of an optical signal is vastly higher than an electrical signal, and multiple optical signals can share the same pathways simultaneously. Also, energy usage is greatly reduced, so the constraints due to heat waste are much less.
Having said all that, the idea of optical circuits in a VLSI is still a very foreign and exotic concept for us, so it's hard to say how far we can take it if we invest at the level we have for electrical ICs. It's naive though to say it's not feasible due to some oversimplification of feature size limitations.
So there is a likely opportunity for large-scale integrated photonics, as long as you have enough parallelism.
Some potential benefits of optics include high data rate, parallel processing of multiple streams, transmission over longer distances, and lower heat dissipation.
What about vertically?
CMOS logic is still "mostly planar". With sufficiently low heat dissipation, you could make a cube and easily overcome the planar density problem.
The main challenge seems like it would be lithography cost for each of the many layers, but if the minimum feature size is 1.5um, there might be a clever way to make this work cheaply (DLP projection + gradual extrusion?)
I’m worried over this trend of private companies putting press releases into LaTeX templates.
People need to realize once and for all that templates no longer represent quality or truthfulness, if they ever did. Maybe that lesson has to hurt a bit.
Or maybe “peer or open review,” or something like that.
And we are decades away from modulatable miniaturized 250nm laser sources. It is typically 1.5um with today’s devices.
Not in this case, no. But in cases where I do have more knowledge, the additional detail makes it much easier to tell if there's anything of substance there, compared to traditional press releases which just make superficial marketing claims with minimal technical detail.
And if this were something more relevant to me, but where I didn't have expertise necessary to look at it, I could reach out to someone with the expertise needed to take a look. The point here is that it's very difficult talk at great length and in great detail about BS without making it apparent to experts that you're talking BS, whereas with a more traditional PR, the best you can often say is "well, if this is anything, these are very big claims."
It’s pretty easy to con investors if you have the same “look” as a real lab.
I think you are talking about an extremely small segment of the population, so I don't think we're talking about a very large social impact. I'm also unconvinced that that segment doesn't generally take ordinary tech press releases at face value anyway.
> I trust citations from private companies without strong academic pedigree
OK, but the first two authors on this have doctorates in ML and applied photonics respectively. They don't have peer review on this paper, but I don't think you can say they're lacking in academic pedigree.
> It’s pretty easy to con investors if you have the same “look” as a real lab.
I don't know. My feeling is that the "conning investors who are terrible at due diligence" game is largely unavoidable and mostly a zero-sum competition between con artists. So while it's obviously bad, I'm not convinced that the specifics matter all that much. Fools and their money, and all that.
Citation extortion rings are part of every journal. I had a reviewer from Nature give feedback that I should cite her co-authors work on a topic that had nothing to do with my paper. It got rejected because I wouldn't. It went into archive and has been cited nearly a hundred times now. To add insult to injury Nature News asked to interview me about my work.
Some more info on the subject, and a vast underestimation of how prevalent it is: https://www.science.org/content/blog-post/cite-my-papers-els...
At this point if you can figure out how to make a pdf paper using latex I consider your work to be on par with anything in a journal.
The head of the jury for my thesis defense had no shame in opening complaining in the public defense session:
"I am THE authority in the field [never had heard of the guy before], why the hell you didn't come to me!?"
The project had already been international award-winning, but then the thesis about it (from the single author) received a bad grade.
I have updated my contact info if you still want to chat.
Email servers do reject messages quite often, for example when they look like spam.
This discussion started in 1959 when Feynman pointed out we eventually will create things at atomic sizes with elementary particles [2].
[2] There is Plenty of Room at the Bottom -Richard P. Feynman (Dated: Dec. 1959).
https://cdn1.richplanet.net/pdf/0099.pdf
[15] D. A. B. Miller, “Are optical transistors the logical next step?” Nature Photonics, vol. 4, pp. 3–5, 2010.
https://www.researchgate.net/profile/David-Miller-65/publica...
I cannot recall ever having seen Power Dissipation versus Compute Performance together with Total global power generation, Total global data center electricity consumption, Electronic thermal noise limit and the Landauer Limit all graphed together before. Presenting the data in this fashion provides a stark and very clear overview of what's actually possible together with the theoretical limits. Graphing the Landauer Limit is a masterstroke because we can instantly see computation vs power efficiency for any given tech.
I think the visual impact of this chart is important enough to see it expanded further and the authors and/or others should think about doing so. It would make an excellent poster-sized lab wall chart if the graticule lines were subdivided from 10³ to 10 (leaving 10³ lines bold) to provide finer granularly and allow more detail of the tech together with the dates of their introduction and phase out, etc.
You've raised a good point through, tables/charts of this type should be footnoted with definitions/conditions etc., and of necessity should only be considered high-level overviews—a bit like the Periodic Table which contains only key information about the elements.
That said, I'd like to see a more precise version of this table. There are good examples to follow, one often comes across really good lab posters like this where the main chart is actually footnoted with smaller charts detailing the specifics of objects to avoid clutter and or to represent info that would otherwise have had to be projected in 3D views.
Edit: incidentally, I find it annoying that so many authors of scientific papers fail to define and label graph axes properly, same with equation terms. It's all very well for professionals to resort to jargon and shorthand when talking amongst themselves (I even do it myself), as they are dealing with their subjects on a daily basis but it's a different matter in publications where papers are read by a wider audience.
Really interesting paper!
Edit: Advantages of multiplexing are very real too!
After wading through the paper for 10mn, I still haven't found the answer. If someone spotted it, please point where they talk about it, I would be grateful.
Or I could go ask an AI to find the answer for me I guess.
The almost canonical way of performing all-optical switching and logic is to use semiconductor optical amplifiers (SOA) and exploit their cross-gain modulation (XGM) or cross-phase modulation (XPM) capabilities[21]. With very reliable devices having been shown over the past 20 years[22], SOAs have proven useful for various types of all-optical operation, including decoder logic[23, 24] and signal regeneration[25, 26, 27]. The recovery time of the SOA limits its performance, but it has been shown that more than 320 Gbit/s[28]all-optical switching is possible, with some implementations enabling even the Tbit/s domain[29].
I once created a Zork clone on a 4096 byte Tandy TRS-80 Pocket Computer, with a one line text display (and graphics using 64 3x2 pixel “bitmap” characters).
My Zork had a small map, a simple parser and some of the starting objects and puzzles from the original. It is incredible how many efficiencies you can find when you have no alternative and think you are building something really great at the time
And if you think that’s something, now imagine playing pong against it across a 3 x 48 pixel screen (1x24 characters, of 3x2 pixels each). Every volley a straight shot, or lots of rebounds, as the ball made its way across the “table”.
So yes I was an early game developer and I am so prepared for 4-bit optical so my special skills can be appreciated once more!
The authors mainly address the issue of integration density (which is also an issue), but not in sufficient detail the problem of efficiency. They handwavy this away by referring to 2d materials, but 2d are not a pancea. It's true that they exhibit very strong nonlinear coefficients (although I'm unsure if even that would be sufficient to overcome the efficiency challenges), however the overlap between the optical field and the 2d material is fundamentally very small (a single sheet of 2d material in the plane of propagation), so the observed enhancements have been very modest.
>Photons are bosons and therefore are very reluctant to interact, essentially requiring nonlinearities.
Please don't throw out random sciency terms. First of all, interaction is pretty much by definition nonlinear. Second, photons are not reluctant to interact. Photon-photon scattering is negligible (which has nothing to do with them being bosons, as gluons and mesons readily demonstrate), but nonlinear optics doesn't rely on photon-photon scattering.
What magical device do they use to take TWO separate 1-hot encoded optical signals and produce a single 1-hot output? Figure 6 just shows a black box labeled "decoder" which is never explained, anywhere.
I think this word salad^H^H^H^H^H paper might have been produced by an LLM.
• Gravitationally.
• Conversion to matter-antimatter pairs.
• In non-linear optical media, which is technically all substances, but effect strength varies.
Of course electrons are involved in the last of those, but not in the way you mean. Non-linear optical media electrons remain bound to their atoms, and act by orbital resonance effects.
The last of those is a realistic path to a general-purpose photonic computer. I worked on a design for one, and I was surprised to find our design would run not much faster than a good electronic computer, while being larger, due to optical wavelengths. But it may have been more energy efficient if we'd built it. Or rather, the calculation part may have been more efficient, with memory being less efficient than transistors. Also we didn't spend much time optimising it.
Small photonic machines are good for particular calculations such as energy-efficient FFTs, but that is smaller scale than a general purpose processor.
(If anyone is interested in photonic computer design, feel free to get in touch!)
And, nonlinear optical media do involve electrons -- that was precisely my point. And that's why those "optical" computers perform no better than electrical computers: because they are in fact electrical computers that use photons for communication, not for computation.
But using photons for communication is nothing new, we've been doing that since the late 1970s.
No, in this case photons would be doing computation across a substrate with electrons in it.
Similarly, in electronics the electrons do the computation on a doped substrate. We don’t say the doping is doing the computation, although it defines what the computation will be (I.e. the logical behavior).
(To PaulHoule: Another truly excellent post of yours to HN, thank you very much, the HN community and myself appreciate it greatly!)
Anyway, let's delve into it -- here's the key quote, IMHO:
>"As the previous discussion showed, SUBLEQ is, of course, not the target realization for optical computing. Its purpose is to showcase the simplest form a general-purpose optical computer could take and an intermediary step we take.
It can be implemented with less than 100 logic gates and, given enough memory, able to emulate a full x86
with a graphics card running Windows and Doom™ loaded, while crunching AI models as a background task (admittedly all extremely slowly)."
Now that is truly awesome!
Also, it should be pointed out that if SUBLEQ could be implemented optically, it could also be implemented digitally, say, on the smallest of small gate count FPGA's...
While such a FPGA Soft CPU would not be fast -- it would definitely be interesting, and probably very simple (comparatively!) to implement!
(Also, it might be implementable on a tiny IC, for example, Sam Zeloof's "Z2" 1,000 gate IC: https://www.youtube.com/watch?v=IS5ycm7VfXg)
Anyway, 5+ Stars for this excellent paper!
Upvoted and favorited!
Which is maybe fine, but its not still not clear how to implement these components.
"With our research, however, we are focused on the phase thereafter. Once optical interconnects and interposers have been fully established and are the main mode of inter-chip and intra-chip communication, solving the 6× inefficiencies... "
Therefore this paper is more about a computer architecture that will be in place AFTER general purpose hardware swaps from electrical to photonic communication, but we aren't there yet. Also seems the paper is more about tackling the next phase of `energy efficiency` problems that will arise after the swap.
Still useful info to consider but i agree with most here that these click-bait titles in research are abused. But I can't really argue it got me to click ;)