# B Quantum Entanglement. Is spin conserved?

1. Jul 21, 2016

### Clever Penguin

I have two photons that are entangled and exist in all possible states simultaneously. By measuring photon A, I cause the superposition to collapse, and i can see that it has a certain spin (say left). This measurement also causes photon B's superposition to collapse, but will it be spinning left or right?

Is spin conserved? If so, then should photon B be spinning right? In some articles i have read, the photons both spin in the same direction, and in others it is in opposite directions.

2. Jul 21, 2016

### Truecrimson

The difference of the measured angles can be any value (of course within $0$ and $2\pi$). The mathematical definition of entanglement doesn't limit this. Conservation of angular momentum (spin is a kind of angular momentum) just means that the entangled pair that is created has to have the same total angular momentum as that of what was used to create it.

3. Jul 21, 2016

### DrChinese

It depends on how they are created. For example, Type I PDC usually produces pairs that have the same spin. Type II PDC usually produces pairs that have opposite spin. And there are many other techniques too, so it varies.