Developing first petahertz-speed phototransistor in ambient conditions

I wouldn't hold my breath for a petahertz one anytime soon.

That depends on what you mean by 'processor'. Optical systems are great at linear processing, but terrible at nonlinearity. You wouldn't want an optical processor based CPU to run Doom. Encoding something like an AI model or a codec into an optical processor would be great though.

One Google search later.. Turns out a team have made a photonic tensor processor. https://www.nature.com/articles/s41586-020-03070-1 So you could have an even fast one of those.

The speed limits are because of the latency of the wires and the issue of heat

Maybe the new ARM architectures will deliver.