From 540d23bff03bedbc8f68287d71c8b5e7dc54b054 Mon Sep 17 00:00:00 2001
From: Prefetch
Date: Mon, 8 Mar 2021 15:04:06 +0100
Subject: Expand knowledge base

---
 content/know/concept/density-operator/index.pdc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

(limited to 'content/know/concept/density-operator/index.pdc')

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
@@ -45,10 +45,14 @@ $$\begin{aligned}
 \end{aligned}$$
 
 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}
     }
-- 
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