A source S of entangled photon pairs is placed between two polarised detectors A and B whose filters are placed at right angles. The source emits pairs of photons whose polarisation is the same but whose orientation is random. The detectors ring a bell whenever a photon is detected.(adsbygoogle = window.adsbygoogle || []).push({});

Now my understanding of the situation is as follows:

Under classical rules, if the emitted photon has its polarisation parallel to one of the detectors, that detector will ring its bell and the other will remain silent. If the emitted photon is polarised at 45deg to the detectors, then there is a 50% probability that each photon will pass through either filter and therefore a 25% probability that the two photons will pass through both filters. I haven't done the integration but my intuition tells me that, on average, with random polarisation, you would expect both bells to ring 12.5% of the time. You would certainly expect both bells to ring some of the time.

Under QT (as I understand it), the two photons are entangled and if one photon passes through filter A and rings the bell, it is impossible for photon B to pass through the other filter. In other words, QT forbids both bells from ringing at the same time.

Now this is such a simple experiment to describe (and do) and gives such an unequivocal difference in predicted behaviour that, if it were correct, it would be described in every book and be in every physics undergraduate practical course. The fact that it is not implies that I have misunderstood something.

I would be extremely grateful to you if someone could enlighten me!

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# A simple quantum experiment: what have I got wrong?

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