In quantum mechanics, entangled particles are described using different states depending on the context. The density operator is used for mixed states, which applies to the individual particles in an entangled pair. However, the singlet state of two spins is a pure state characterized by a wavefunction, representing the system as a whole rather than the individual particles. Therefore, not all entangled states are mixed; the overall system can be in a pure state while individual measurements yield mixed states.
PREREQUISITESStudents of quantum mechanics, physicists specializing in quantum theory, and researchers exploring quantum entanglement and measurement techniques.
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?