So this would have been OT except for the spate of noise stuff. Beam splitter noise is, just shot noise cut in half. And really nothing new. Except the source of the noise was first reported/ explained by Caves (1980,81)* And I just read about it.
So you take a beam with N photons per second. The noise is sqrt(N). You split the beam in half and each arm gets N/2 photons with sqrt(N/2) noise. And that's more noise than you started with! by sqrt(2).. And Caves said, yeah the extra noise is quantum vacuum fluctuations that come in through the 'other' port of the beam splitter.
Which is just cool, and given me pause.
George H.
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Sorry paywalls, but you can read the abstracts. I've done a few searches but found no good links.
This is some how related to Casimir effect,
dang hard to measure. But maybe easier with a big scary laser making lots of photons.