well, i can see best around 540 terahertz, so i think you'd need digital state transition rates around a couple petahertz for digital visible light interferometry, about six orders of magnitude faster than this cellphone. that's clearly physically possible, but one or another technology might be able to compute much faster than this cellphone while topping out in the low terahertz

such speeds would likely require that most bit operations be reversible to avoid vaporizing due to landauer's limit (perhaps coincidentally also first proposed by von neumann)