Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated.
- seems pretty vague
This leads to correlations between observable physical properties of the systems.
- need to distinguish from classical correlation
For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa, this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed. As a result, measurements performed on one system seem to be instantaneously influencing other systems entangled with it. However, at this time classical information cannot be transmitted through entanglement faster than the speed of light.
- preparing a mixed state 1/2( |ud><ud| - |du><du|) (bertlemans socks)
we also cannot predict which set of measurements will be observed need to distinguish from classical
If the composite system is in this state, it is impossible to attribute to either system A or system B a definite pure state. Instead, their states are superposed with one another. In this sense, the systems are "entangled".
- again, in the mixed state above, we cannot attribute a pure state to either object. this is not the meaning of entanglement
