# Pure/mixed state

1. Oct 9, 2007

### Physicist

Hello

I read the following statement somewhere:

"... we study the entaglement of the ground state and the mixed state at finite temperatures ..."

Does this mean that the ground state of a system is always pure?

Thanks

2. Oct 9, 2007

### meopemuk

Any state that can be represented by a vector in the Hilbert space (e.g., any solution of the stationary Schroedinger equation, including the ground state solution) is a pure quantum state.

Eugene.

3. Oct 9, 2007

### f95toli

Mixed state are important when you study statistical properties of a system. I.e. the density matrix describes the property of an ensemble (which is why the . Hence, a mixed state does not describe a "proper" state as such.
You can also get what is sometimes known as an "improper" mixture if you study a specific subsystem of a composite system by tracing out all other degrees of freedom.

4. Oct 9, 2007

### f95toli

Double post

Last edited: Oct 9, 2007