If photons and electrons travel at the same speed am I right in thinking that the benefits of optical computing would be limited to parallel processing?
"If" - they do not! https://en.wikipedia.org/wiki/Electron_mobility
Quoting from the above source...
>Typical electron mobility for Si at room temperature (300 K) is 1400 cm2/ (V·s) and the hole mobility is around 450 cm2/ (V·s).[2]
However this is besides the point since electric fields in a conductor do move at the speed of light.
TL;DR: "The speed at which energy or signals travel down a cable is actually the speed of the electromagnetic wave, not the movement of electrons. Electromagnetic wave propagation is fast and depends on the dielectric constant of the material. In a vacuum the wave travels at the speed of light and almost that fast in air."
Especially since there are some semi-conductors which actually require you to move electrons, like e.g. flash memory ;)
While this is technically possible with electronics as well by setting a different voltage limit it's much more effective with photonic computing and doesn't not increase the complexity of your base components as much.
The current designs for a photonic computer are also much more parallel most of them basically layers of LED's and detectors with a very fast LCD matrix which serves as a mask between them. if you have a 256x256 pixel screen you can perform an operation on 65536 bits in a single clock, if you stack them up you basically getting 1 order of magnitude with each layer this isn't something you could ever achieve with current solid state electronics.
You don't have to wait with processing for the electron from the beggining of the wire to get to the end?
So, the difference between photons and electrons speed as measured by the time it takes electronic and optical signal to move through the same distance - is insignificant.
You also need to remember that when we talking about waves then the wavelength ties directly to your data throughput, and with electrons the wavelength in conductors at say a frequency of 1GHZ is still around 3 kilometers, visible light has much much shorter wavelengths which allows you to pass more data per given amount of time.
Regarding the wavelength - it's another of these things I never quite understood about physics. I've been told that every particle can be thought of as a wave with a frequency, and that it corresponds to the energy of the particle. Photons have lower energies than electrons or protons, so the wavelength of electrons should be shorter, not longer?
They don't move at a constant speed like say in a conductor (which isn't the case either because electric field causes resistance e.g. eddy currents, but lets say a super-conductor)
You have quite a few concepts with semiconductors primarily saturation velocity (which in most cases is the peak velocity, but not necessarily attained in actual operation) which is also affected by the drift velocity due to any electric fields in your components.
https://en.wikipedia.org/wiki/Saturation_velocity
Light speed in vacuum is 29979245800 cm/s Saturation velocity in Si based SC is 10000000 cm/s
So there's quite a bit of difference there ;)
Also because light has much more shorter wavelength than "electricity" electromagnetic waves in conductors (kilometers vs nano-meters) you can have a much higher throughput with light even if you use it to replace the carrier wires in electronics (which is what Intel is working on they want to keep electronic switching but make all carriers photonic).