Expand knowledge base

1 files changed, 5 insertions, 1 deletions

diff --git a/content/know/concept/density-operator/index.pdc b/content/know/concept/density-operator/index.pdc
index 84c2d74..39c2e85 100644
--- a/content/know/concept/density-operator/index.pdc
+++ b/content/know/concept/density-operator/index.pdc
@@ -45,11 +45,15 @@ $$\begin{aligned}
 \end{aligned}$$
 
 However, from the special case where $\ket{\Psi_n}$ are indeed basis vectors,
-we can conclude that $\hat{\rho}$ is Hermitian,
+we can conclude that $\hat{\rho}$ is positive semidefinite and Hermitian,
 and that its trace (i.e. the total probability) is 100%:
 
 $$\begin{gathered}
     \boxed{
+        \hat{\rho} \ge 0
+    }
+    \qquad \qquad
+    \boxed{
         \hat{\rho}^\dagger = \hat{\rho}
     }
     \qquad \qquad