# Entanglement for spin only?

Is spin the only physical parameter that can be entangled? If not, what else? If so, it seems there is something special about spin?

Nugatory
Mentor
Is spin the only physical parameter that can be entangled? If not, what else? If so, it seems there is something special about spin?
No, not at all. In principle just about any observable can be entangled, and in practice we're limited only by what states are easily prepared. Most of the Bell experiments have been done with polarized photons, because they are easier to produce and to measure than spin pairs. The original EPR paper considered position and momentum.

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tom.stoer
Is spin the only physical parameter that can be entangled? If not, what else? If so, it seems there is something special about spin?
I would not call it "parameter" but obervable.

An observable is a quantum mechanical operator with eigenvalues and eigenstates which can be used to label the results of measuments. In quantum mechanics we can have several obsvables, not all of them are mutually "compatible", but assume we have a maximal set of compatible observables {A, B, C, ...}. Then a quantum state is something like a set of labels |a, b, c, ...) with eigenvalues a, b, c, ... of the observables. Having such a state means that we can be sure that measuring A, B, C, ... will have the results a, b, c, ...

An entangled state w.r.t. an observable A is nothing else but a linear combination p|a, b, c, ...) + q|a', b, c, ...) where we have the probability p^2 to find a and the probability q^2 to find a' as result of the measurement on A.

So entanglement depends on the set of obsvables you chose to describe the system; and this depends e.g. on the experimental setup, what you want to measure. Observables can be energy, momentum, angular momentum, spin, ... (not all of them being mutually compatible!)

Khashishi