- #1

Paul159

- 17

- 4

I have a question about the creation of the Bell's entanglement state ##1/\sqrt{2} (|HH> + |VV>)##using type I BBO crystals (https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion).

Two crystals are put orthogonal to each other and each of them emits a photon pair (##|HH>## or ##|VV>##). Then at the intersection of the two emitted cones we have the Bell's state. But I don't understand why.

Indeed, if I do measurement of the photons with two polarizers, one at 90° and the other 0°, I don't understand why I will have 0 correlation. For me I could detect for example the signal photon of the ##|HH>## state and the idler photon of the ##|VV>##.

I hope my "question" is clear.