Is the Ground State of a System Always Pure?

[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

 

[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.

 

[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.

 

[f95toli]

Double post