# What is the fundamental differences between bound and entangled states

Specifically, are all entangled states considered bound?

-

These are two unrelated concepts. Certainly not all entangled states are bound states.

A configuration of objects is called a bound state when it has less energy than the configuration in which the objects are kept far away from each other (or from any external potentials that you apply). You can use this definition in both classical and quantum-mechanical systems.

An entangled state is a purely quantum-mechanical concept which has nothing to do with energy. Separated particles are said to be entangled when measurements performed on different particles are correlated. Namely, if measuring a property of particle 1 teaches us something about a separated particle 2, then the particles are in an entangled state.

-
What occurs to establish this correlation. Do these entangled particles need to exchange energy to achieve entanglement? –  Argus Aug 8 '12 at 12:26
Here is a toy example. Let's say we have a massive particle with spin 0, at rest, that decays into two photons. The two photons fly out back-to-back due to momentum conservation. Due to angular momentum conservation, the total angular momentum in the photon axis is zero. So if we measure the helicity of photon 1, the helicity of photon 2 must be opposite. Thus the state after the decay is an entangled state. I am not sure what you mean by the second question, since in this example (and in many others) the entangled particles do not exist before the decay. –  Guy Gur-Ari Aug 8 '12 at 16:24
The decay transfers energy to each so the particle and the energy are bound to the process of entanglement? Is this correct because each opposite energy is determined to stay bound by the original division of energy. –  Argus Aug 8 '12 at 20:38
I am working on the assumption that both photons exist in the aforementioned massive particle in the form of electron energy. –  Argus Aug 8 '12 at 20:43
Sorry, but I do not understand what you are asking. Specifically the phrases "the particle and the energy are bound to the process of entanglement" and "energy is determined to stay bound ..." make no sense to me. The photons do not exist before the decay. They are created when the particle decays. –  Guy Gur-Ari Aug 8 '12 at 23:57