And people like you are often stunned by what gets settled in and out of court . good luck indeed . <| sep |> What brand is that sweater ? <| sep |> I am a postdoc in a lab specialised in multimode quantum optics , I am actually quite aware of the scientific consensus regarding this field . Well , I ' m a professor and I think if you asked a dozen physicists how to exactly define the division between classical and quantum effects you would probably get a dozen answers . My working definition of defining when an effect is classical instead of quantum is to ask if you are in some limit where the interesting quantum properties go away . So for example a single pressure is the classical limit , but fluctuations in pressure can be measured on small scales . On the other hand , if a known quantum effect at one scale is reproduced at larger scales , that still strikes me as a quantum effect . Interference , which emphasizes waveparticle duality , is a quantum effect . The fact that we identified this effect for some particles photons before quantum mechanics was discovered doesn ' t change that . Young ' s double slit experiment was thought to have proven photons were not particles and that turned out to be an incorrect conclusion precisely because we didn ' t realize it was a quantum effect at the time . As far as the coherent laser source being important , I don ' t think a distinction without a difference should produce a shift in our interpretation of when something is quantum or not . Poisson statistics are common to many random processes . If I detect dimmed sunlight with single photon counter it will exhibit Poisson statistics . The output from a SPDC crystal has Poisson statistics too if I showed you the counts coming from one channel how would you distinguish them from another dim light source ? Again , the important thing is a difference you can measure with an experiment . Theories can produce conceptual distinctions but without measurable differences we should be wary of assigning them too much importance . The SPDC gives us the knowledge of when individual photons have been created . But that knowledge isn ' t needed to create the interference pattern or remove it . By invoking the states of the light field , maybe your argument is something like With SPDC we can talk about what a single photon does , but without SPDC then we can ' t say how many photons exist so it is a field theory problem instead . I disagree with that distinction . I would say that the SPDC instead proves that interpreting the results with other light sources as the sum of a Poisson distribution of single photon events gives the exact same physical answer and is therefore a valid interpretation . But I don ' t pretend that my opinions here represent a scientific consensus , because I think people will continue to argue about that consensus for a while .