3 ms·

Do optical gates not switch a lot faster? I do not know whether it would be enough to offset the bigger size.

ben_w 2y ago
The switches themselves do (IIRC by a factor of about 1e4), but if you have to space them farther apart then the combined whole may not benefit from this. If you have a system clock running at 3 GHz, the speed of light limits your causal distance to just under 10cm per clock cycle. CPUs are already close to that for size and frequency, but let's say you're taking a 1 cm by 1cm silicon chip on a 2nm feature size process and replacing it like-for-like with a photonic chip with light that limits it to 200nm features — now it's 1m by 1m and can't go faster than 300 MHz, likely a lot less. This doesn't mean it is useless — for example, there's a hope that it will reduce energy use, which is directly useful all by itself, but also means it may be sensible to move to a fully 3D structure which silicon can't really manage because of the thermal issues. Going from 2D to 3D helps a lot, might allow that 1m by 1m by 200nm (*2 thickness for insulation) sheet to be compacted to a 7.4mm cube, which then doesn't need to be slowed all the way down to 300 MHz due to causality.
- bgnn 2y ago
  Very interesting. What about memory? Current problems are mainly memory bottleneck related. How can one solve that in photonic chips?
  - ben_w 2y ago
    Optical storage currently is things like BluRay (non-volatile), plasmons, delay lines (both volatile), or Bose-Einstein condensates (requiring extremely impractical cryogenics). All of these are much lower density than magnetic (hard drives) or electrical (RAM or flash). I've not heard of RAM or flash having thermal issues (though I'm sure it would happen eventually), but that suggests 3D stacking is easier for storage, which would remove that potential advantage of optical. One thing I've seen suggested for optical computing is to create optical elements (e.g. lenses, holograms) that represent the same transformations as a layer of a neural net; they're not at all space-efficient and any changes to the network requires basically replacing the whole thing, and you can do a similar thing with a suitable network of hard-wired resistors and transistors, but they're an interesting idea that I see come up very occasionally.
deleted 2y ago
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bgnn 2y ago
Intereting question. The answer to the second part: we have much faster switching transistors (GaAs, SiGe, InP, now GaN) already but they cannot be miniaturized easily and the production technology isn't as simple as CMOS. One can build computers with them, but due to physical size and large distances it wouldn't be performing good compared to a CMOS chip. So the answer is: size matters. Large devices cannot be used for building complex fast computers.
- namibj 2y ago
  SiGe scales down, but we can't afford the power density resulting from building logic with it, so it ends up loosing to CMOS.

The switches themselves do (IIRC by a factor of about 1e4), but if you have to space them farther apart then the combined whole may not benefit from this. If you have a system clock running at 3 GHz, the speed of light limits your causal distance to just under 10cm per clock cycle. CPUs are already close to that for size and frequency, but let's say you're taking a 1 cm by 1cm silicon chip on a 2nm feature size process and replacing it like-for-like with a photonic chip with light that limits it to 200nm features — now it's 1m by 1m and can't go faster than 300 MHz, likely a lot less. This doesn't mean it is useless — for example, there's a hope that it will reduce energy use, which is directly useful all by itself, but also means it may be sensible to move to a fully 3D structure which silicon can't really manage because of the thermal issues. Going from 2D to 3D helps a lot, might allow that 1m by 1m by 200nm (*2 thickness for insulation) sheet to be compacted to a 7.4mm cube, which then doesn't need to be slowed all the way down to 300 MHz due to causality.

bgnn 2y ago
Very interesting. What about memory? Current problems are mainly memory bottleneck related. How can one solve that in photonic chips?
- ben_w 2y ago
  Optical storage currently is things like BluRay (non-volatile), plasmons, delay lines (both volatile), or Bose-Einstein condensates (requiring extremely impractical cryogenics). All of these are much lower density than magnetic (hard drives) or electrical (RAM or flash). I've not heard of RAM or flash having thermal issues (though I'm sure it would happen eventually), but that suggests 3D stacking is easier for storage, which would remove that potential advantage of optical. One thing I've seen suggested for optical computing is to create optical elements (e.g. lenses, holograms) that represent the same transformations as a layer of a neural net; they're not at all space-efficient and any changes to the network requires basically replacing the whole thing, and you can do a similar thing with a suitable network of hard-wired resistors and transistors, but they're an interesting idea that I see come up very occasionally.

deleted 2y ago

[deleted]

bgnn 2y ago

Intereting question. The answer to the second part: we have much faster switching transistors (GaAs, SiGe, InP, now GaN) already but they cannot be miniaturized easily and the production technology isn't as simple as CMOS. One can build computers with them, but due to physical size and large distances it wouldn't be performing good compared to a CMOS chip. So the answer is: size matters. Large devices cannot be used for building complex fast computers.

namibj 2y ago
SiGe scales down, but we can't afford the power density resulting from building logic with it, so it ends up loosing to CMOS.