Entanglement?

Question

Im not quite sure about this, when an entangled particle is observed does that break the entanglement?

Answer 1

Yes.  I'm a bit fuzzy on the details myself, but you only get one measurement before the entanglement breaks.  Otherwise you could instantly transmit information by repeatedly measuring an entangled particle to change the spin of its distant partner, and that would violate the cosmic speed limit.

Answer 2

Not necessarily.  Entanglement is a fancy form of multi-particle superposition.  When you observe a system, physicists call that taking a measurement.  When a superposition is measured, it only collapses If you measure it in such a way that the you can tell the difference between the states of the superposition.

If you make a measurement that can not tell the difference between the states of the superposition, then the superposition will survive the measurement.  And since a superposition can survive certain measurements, so can entanglement.  In fact, there is a very common way to entangle between two systems by doing the reverse process.  You make a measurement on the systems that can differentiate between two possible entangled states of the systems.  Since you are measuring which of two entangled states the systems are in, the systems will be projected into one of the entangled states and you will be left with entangled systems after the measurement.  This is called a Bell measurement.