The discussion revolves around the nature of entangled states in quantum mechanics, specifically whether all entangled states are mixed states or if some can be described as pure states. The conversation touches on the use of density operators and wavefunctions in describing these states.
Participants do not appear to reach a consensus, as there are competing views on whether all entangled states are mixed states or if some can be pure states.
The discussion highlights potential confusion regarding the definitions of mixed and pure states, as well as the implications of using density operators versus wavefunctions. There are unresolved aspects regarding the interpretation of entangled states in different contexts.
When two particles are entangled the state of either one is a mixed state that can only be described with a density operator. (in the most convenient basis the density matrix for either particle is ##diag(1/2,1/2)##, equal probability of measuring spin-up or spin down).stephen8686 said:when two particles are entangled, we must describe them using the density operator because it is a mixed state.
The singlet state is the state of a single quantum system that will produce measurement results at two spatially separated detectors. It is a pure state with a wave function - but it is not the state of either particle considered in isolation.But I have another source that says that the singlet state of two spins is an entangled state, but that has a wavefunction.
To rephrase what @Nugatory said, the state that simultaneously describes both particles together is pure, while the state that describes any of the particles alone is mixed.stephen8686 said:I have a source that says when two particles are entangled, we must describe them using the density operator because it is a mixed state. But I have another source that says that the singlet state of two spins is an entangled state, but that has a wavefunction. So could someone explain what I am misunderstanding? Are all entangled states mixed states or only some?