How is squeezed light detected? And how does photodiode efficiency affect it?'m just curious I'm just curious, but in general, how is squeezed light measured and calculated in the lab?  
I know that it has a strong relationship with the quantum efficiency of your photodiode, but is there anyway to calculate how much squeezing you could measure if you knew the efficiency of your diode? Or vice verse?
 A: There are several things you mixed together here.


*

*Measuring squeezing (i.e. quantum fluctuations of an electromagnetic mode) is often done via a homodyne measurement. Here, you send your squeezed beam under test and a (strong) local oscillator beam onto a beam splitter to superimpose these. Then you take the two outputs, send each on a photodiode and subtract the photocurrents. Looking at it in frequency space (e.g. with a signal analyser) you will see that for certain frequencies the noise spectral density lies below the noise spectral density you get if your squeezed beam is switched off.

*Calculating the degradation due to losses is a different topic. The photodiodes in general are imperfect and act as a loss channel (just as every other loss channel, e.g. propagation losses, imperfect homodyne overlap, ...). A loss channel can be modelled as a beam splitter where one input is your squeezed beam and the other input is vacuum fluctuations. The squeezing is coupled to the vacuum fluctuations and degrades/decoheres. A simple formula gives you the squeezing after such a process if you know the squeezing beforehand and the losses (and vice versa):
$$
V^{\pm}_\text{after}=\eta V^\pm_\text{before}+(1-\eta),
$$
where $V^\pm$ are your squeezed/anti-squeezed variances normalised to vacuum fluctuations and $\eta$ is the efficiency ($=1-\text{losses}$) of the process.

*The amount of squeezing you could measure thus depends on the losses on the way from creating squeezed light until (and including) detection, but also on the creation process. In intracavity parametric down conversion which is often used for creating squeezed light, this includes how strong your pump beam is, or more exactly, how closely you approach the system's oscillation threshold.
