[krastanov]

The nonlinear optics is what enables optical signal modulators. For modulation of classical optical signals it is not necessary to have a "strong nonlinearity at the single photon level". However, that is being developed for quantum computing applications and as a side effect it makes today's optical modulators much better.

[fsh]

Telecom signal modulators are almost exclusively based on the Kerr effect or acousto-optics. Both effects have been known for at least 100 years and have nothing to do with nonlinear optics. The development of modern modulators was always driven by telecom and then applied to scientific experiments, not the other way round.

[krastanov]

There might be some semantic misunderstanding here. "Kerr effect" is "nonlinear optics" in my book - it works both at the low GHz frequency of electro-optics and at the high THz frequencies of pure optics. I agree with the rest of your statement (and it does not seem to be in conflict with the initial comparison: vacuum tubes were being developed for 50 years due to the needs of the radio industry before they became viable for computation).

And the direction symbiotic improvements is not necessary as clear: there are academic groups solely focused on quantum optical effects, whose research then gets reused by the telecom industry and vice versa. These quantum research groups would be dead in the water if it was not for the fab capabilities initially developed by telecom folks, but they have certainly surpassed them by far now (in one-off "hero" devices).

P.S. Same with acousto-optic devices. What you would find in a quantum computing lab is far more impressive than what is being deployed today. Even if the quantum computing field is a bust, the tech they developed would improve telecom state of the art by orders of magnitude.