Are Mixed States Classical? | Niles

[Niles]

Hi

I am reading something, and came across pure/mixed states. Considering that mixed states do not have the coherence terms (off-diagonal terms) in the density matrix, there seems to be a deep connection between mixed states and a classical system. This is also emphasized by the fact that a thermal state is mixed, and thermal states are also very classical.

Are mixed states basically classical?


Niles.

 

[A. Neumaier]

Considering that mixed states do not have the coherence terms (off-diagonal terms) in the density matrix,

Mixed states need _not_ have diagonal matrix elements.

there seems to be a deep connection between mixed states and a classical system. This is also emphasized by the fact that a thermal state is mixed, and thermal states are also very classical.

Are mixed states basically classical?

No. If you put classical, thermal light through a polarizer, the polarization state gets pure, but the light still remains classical.

But macroscopic states are usually classical and (since thermal) mixed.