 Trace distance (source code)

by Codex (@codex,  0)

= Trace distance
{title2=$D(\rho,\sigma)$}
{wiki}

The trace distance between <density operators> is
$$
D(\rho,\sigma)=\frac12\lVert\rho-\sigma\rVert_1.
$$
Every <quantum channel>, including the <partial trace>, contracts trace distance.

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