- #1
Emil_M
- 46
- 2
Homework Statement
Let [itex] \mathcal{E}[/itex] be a trace-preserving quantum operation. Let [itex] \rho [/itex] and [itex]\sigma [/itex]
be density operators. Show that [itex]
D(\mathcal{E}(\rho), \mathcal{E}(\sigma)) \leq D(\rho,\sigma)
[/itex]
Homework Equations
[itex]D(\rho, \sigma) := \frac{1}{2} Tr \lvert \rho-\sigma\rvert [/itex]
We can write [itex] \rho-\sigma=Q-S [/itex] where [itex] Q [/itex] and [itex] S[/itex] are positive matrices with orthogonal support. We choose a projector [itex] P [/itex], such that [itex]
D(\mathcal{E}(\rho), \mathcal{E}(\sigma))=Tr(P(\mathcal{E}(\rho)-\mathcal{E}(\sigma)))
[/itex]
[itex] [/itex]
The Attempt at a Solution
[itex]
\begin{align*}
D(\rho, \sigma) &=\frac{1}{2} Tr \lvert \rho-\sigma\rvert \\
&=\frac{1}{2} Tr \lvert Q-S\rvert \\
&=\frac{1}{2}(Tr(Q)+Tr(S))\\
&=\frac{1}{2}(Tr(\mathcal{E}(Q)+\mathcal{E}(S))\\
&=Tr(\mathcal{E}(Q))\;\; \Big(\text{since } Tr(Q)=Tr(S) \Big) \\
&\geq Tr(P\mathcal{E}(Q))
\end{align*}
[/itex]
Why is the last step valid? Why can a projector never increase the trace?
Thanks for you help!