**1) What properties of an entangled pair of particles are correlated?**

Are both particles' spins always in the same direction? Always in the opposite direction? Are their momenta correlated? Their direction of motion through space? Anything else?

Most entanglement that I'm aware of uses photons and spin because photons are easy to generate and handle, and photon spin only has 2 possible values. However, particles could be entangled through correlations in any measurable quantity: momenta, direction of motion, angular momentum, frequency, etc.

**2) How do we know that a particular pair of particles are entangled with one-another?**

I've read that in order to prevent a violation of causality, information cannot be sent instantaneously via a pair of entangled particles. Apparently, if you try to send a signal to particle B by manipulating particle A's properties, the message that anyone looking at particle B will see will be random or garbled. If this is the case, how can we know that particles A and B are even entangled? In order for us to tell that they are entangled, we have to be able to measure that there is a correlation between the two particles' properties in the first place. If particle B's properties seem random when compared to particle A's when measured, then how can we know there is a correlation?

I believe it comes down to knowing that the source of entangled particles is working properly. You can do this by measuring a lot of the entangled particle pairs coming from it. If they're entangled, you should start to notice correlations, and at some point you can be statistically confident that you have a source that's generating entangled particles. Once you're confident your particles are entangled, you can exploit that entanglement.

**3) How does particle 'splitting' affect entanglement?**

If I have two photons A and B that are entangled, and I send A through an extremely powerful magnetic field so that it splits into two weaker photons A1 and A2, will both of these new photons be entangled with photon B? Will only A1 or A2 be entangled with B but not both A1 and A2? Will photon B split as well?

I don't know the answer, but here's my intuition: You have to know what variable you're entangled with respect to, and you have to treat your particles as a system. For example, let's say your particles are entangled so that the spin of B is always opposite to the spin of A. Then let's say that you split A into A1 and A2, both of which have the same spin as A. Now, if you measure A1 or A2, you still have 100% correlation with B (since A1 or A2 tells you the spin that A had and that tells you the spin B must have). However, if you split A into A1 and A2 which have

**opposite** spin, then you lose your correlations. If you measure A1 or A2, you learn nothing about the spin of B. Losing entanglement through interaction with the environment is called

*decoherence*, and is a major obstacle in designing entanglement systems.

**4) Can two dissimilar particles be entangled?**

Can a photon be entangled with an electron? Can a quark in a proton be entangled with a quark in a neutron?

I don't see why not.