quantum noise vs. thermal radiation?

[sbourdeauducq]

"the dramatic change in the signal despite the same oscilloscope settings means that we are surely seeing some kind of quantum interference resulting from the vacuum fluctuations"

How can you be so sure, and how do you tell quantum vacuum fluctuations from thermal photons seeding the laser cavity?

Blackbody spectrum at room temperature isn't zero at 1550nm. Of course, for most practical purposes it would still make a good RNG.

[cryptocrack4011]

For this question and support, I created a version that uses a second balanced detection path.

Dual-path places a receiver on both output ports of the MZI simultaneously. Constructive interference appears on one port while destructive interference appears on the other, so the two ports are anticorrelated by design. We then subtract the two receiver channels.

That subtraction is the important part. Classical common-mode effects — thermal drift, electrical noise, ADC offset, power supply movement, EMI — tend to move both channels in the same direction and are suppressed in the differential signal. The optical interference term appears oppositely across the two ports, so it survives the subtraction.

So when the differential channel consistently shows higher H_min than the single-ended channel on the same hardware, same fiber, and same time window, that is strong evidence that the added entropy is coming from the MZI interference term rather than ordinary shared electronics or environmental noise.

For extra support to the oscilloscope trace, I am measuring both output ports at the same time and looking at the differential result directly.

[sbourdeauducq]

that is strong evidence that the added entropy is coming from the MZI interference term rather than ordinary shared electronics or environmental noise.

Sure, but thermal (blackbody) photons seeding the laser cavity would show up just the same.

[cryptocrack4011]

Technically, the thermal population is not exactly zero. But at 1550 nm and room temperature it is so small that it is not a meaningful competing mechanism.

A tiny nonzero thermal occupation does not explain the observed entropy or replace the phase-diffusion mechanism. Spontaneous emission from the gain medium dominates by many orders of magnitude, and that is the quantum process the QRNG is using.

So I agree that thermal photons are physically possible in the abstract, but at this wavelength and temperature they are not large enough to be the source of the measured effect.