Question about quantum entanglement

[smithnya]

Hello everyone,

I have a basic understanding of quantum entanglement. One thing I don't understand is the following: For particles to be entangled, is it necessary to produce the entanglement via whichever means, or are all particles entangled to another particle to begin with? By the latter, I mean, were all particles, at the moment of creation, entangled to a second particle, or there is no initial entanglement, and it is something we must produce.

 

[questionpost]

I think particles just become entangled when they both share the same quantum state. They don't have to be entangled prior to that.

 

[Wolfgang2b]

I would say if you have two particles (say photons) that interact (say via atoms) and after the interactions if we cannot write the quantum state of the 2 photons as product state, we have entanglement.

In quantum information theory, a gate called CNOT is used to entangle two qubits. They need not be created in an entangled state.