A state is called '''pure''' if it cannot be represented as a mixture (convex combination) of other states, i.e., if it is an extreme point of the convex set of states.
This is equivalent to the density matrix being a one dimensional projector, i.e., of the form
$\backslash rho=\backslash vert\backslash psi\backslash rangle\backslash langle\backslash psi\backslash vert$
for some unit vector $\backslash vert\backslash psi\backslash rangle$. In this case the formula for the expectation value of an operator
$A$ in the state simplifies as
$tr(\backslash rho\; A)=\backslash langle\backslash psi,A\backslash psi\backslash rangle$. Equivalently, a state $\backslash rho$ is pure if $tr(\backslash rho^2)=1$.
In the Bloch sphere (the state space of a qubit), the pure states exactly form the surface of the sphere. For Hilbert space dimension larger than 2, however, the topological boundary (consisting of all density operators with at least one zero eigenvalue) is much larger than the set of extreme points (which have all but one eigenvalue zero).
Caratheodory's Theorem guarantees that every point in a compact convex set of dimension $D$ can be represented as a mixture of at most $D+1$ points. The state space of a a system with $d$-dimensional Hilbert space is $d^2-1$ dimensional, so from this we would expect $d^2$ pure states to be necessary to represent a general mixed state. However, as the spectral theorem shows, the geometry of state spaces is such that $d$ pure states always suffice.
Category:Handbook of Quantum Information
Category:Quantum States

## Last modified:

Monday, October 26, 2015 - 17:56