> it's more about wire length and capacitance At the fastest speeds, interconnect on crucial lines is engineered as a transmission line, where the inductance b

> it's more about wire length and capacitance At the fastest speeds, interconnect on crucial lines is engineered as a transmission line, where the inductance balances out the capacitance. When you look at the propagation in the transmission line, the energy of the traveling wave is dominated by the electric and magnetic fields outside the metal. So it is not generally appreciated that transmission-line interconnect is already photonic, and moving at photonic speeds. As you might expect, for most transmission-line geometries, the speed of the signal is not terribly far away from the speed-of-light in the material. Moreover, the intuition that "metal wires" are slowing down the signal is not taking into account that an "all photonic transmission" still requires some sort of confinement. If you were to eliminate the metal, you would still need some sort of waveguide to confine the photonic energy. Inevitably, the waveguide will have some region of a higher dielectric, which as you know will slow down the propagation and also have some loss. As it turns out, the final propagation speed is close to a transmission line. So again, it's important to quantify exactly how the speedup occurs by switching to "photonic interconnect", because the reality is that it is already photonic. Where photonic interconnect might have had a use is where the lines are very long, and the loss associated with a transmission line gets impractically large due to the confining metals. At that point, it's an engineering tradeoff: Converting to and from photons at each end of the line is non-trivial, and not all materials lend themselves to converting photons to electrons. There have been efforts since the 80s to put GaAs on silicon just to address this issue. (I was part of a team that got to 100MHz for GaAs-on-Si. The fact that GaAs-on-Si photonic transmission is still pretty much confined to the lab tells you everything you need to know about the manufacturability.)