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# Created by http://www.gitignore.io | ||
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### LaTeX ### | ||
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%!TEX root = thesis.tex | ||
%---------------------------------------------------------------------- | ||
\chapter{Conclusions and open problems} | ||
\label{chap:conclusions} | ||
%---------------------------------------------------------------------- | ||
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In this thesis we have used techniques from convex optimization to study | ||
the limitations of LOCC, separable, and PPT measurements for the task of | ||
distinguishing sets of bipartite states. Compared to previous approaches, | ||
our techniques turned out to be effective in providing precise bounds on | ||
the maximum probability of locally distinguishing some interesting sets | ||
of maximally entangled states and unextendable product sets. | ||
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Several specific questions regarding the local distinguishability of sets of | ||
bipartite states remain unsolved. | ||
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In Chapter \ref{chap:mes} we proved a tight bound on the entanglement | ||
cost of discriminating sets of Bell states by means of LOCC protocols. | ||
One could ask the following more general question. | ||
\begin{question} | ||
How much entanglement does it cost to distinguish | ||
maximally entangled states in $\complex^{n}\otimes\complex^{n}$? | ||
\end{question} | ||
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Ghosh et al.~\cite{Ghosh04} have shown that orthogonal maximally | ||
entangled states, which are in canonical form, can always be discriminated, | ||
by means of LOCC protocols, if two copies of each of the states are provided. | ||
One could ask if two copies are always sufficient. In fact, this question is open | ||
even for separable and PPT measurement. | ||
\begin{question} | ||
Are two copies sufficient to discriminate any set of orthogonal pure states | ||
by PPT measurements? | ||
\end{question} | ||
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The techniques presented in the paper are not intrinsically limited | ||
to the setting of bipartite pure states, and applications of these techniques to | ||
the \emph{multipartite} setting are topics for possible future work. | ||
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Global distinguishability of random states was studied by A. Montanaro \cite{Montanaro07}. | ||
More precisely, he put a lower bound on the probability of distinguishing | ||
(by global measurements) an ensembles of $n$ random quantum states in $\complex^{d}$, | ||
in the asymptotic regime where $n/d$ approaches a constant. A similar question | ||
on the distinguishability of random states by PPT and separable measurements | ||
could be investigated by using the convex optimization approach developed in the thesis. | ||
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Apart from quantum states, one could also study the LOCC distinguishability of | ||
quantum operations \cite{Matthews10}. It is a topic that has not been studied as throughly | ||
as in the case of states, but once again, one could approach it through | ||
the lens of convex optimization. | ||
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A more speculative project, yet very exciting, is to give a somewhat useful | ||
characterization of the set dual to the set of LOCC measurement and its corresponding | ||
set of linear mappings (both labeled by a question mark in the diagrams of Figure \ref{fig:measurements-dual}). | ||
As we do not have a nice characterization of the LOCC set itself, we suppose | ||
this is a difficult project. One direction to approach this problem | ||
can be to consider smaller sets that are contained in the LOCC set, such as | ||
one-way LOCC, where the communication is only in one direction, say from | ||
Alice to Bob, or LOCC-$r$ , in which the communication is limited to $r$ rounds. | ||
Let us fantasize we had a characterization of the set dual to the set of LOCC measurement | ||
and let us denote it as $\Meas_{\LOCC}^{\ast}(N, \X:\Y)$. Then we could plug it in the | ||
following cone program and proceed as we did for all the cone programs analyzed | ||
on this thesis. | ||
\begin{center} | ||
\underline{Dual (LOCC measurements)} | ||
\begin{equation} | ||
\label{eq:locc-dual-problem} | ||
\begin{split} | ||
\text{minimize:} \quad & \tr(H)\\ | ||
\text{subject to:} | ||
\quad & | ||
\begin{pmatrix} | ||
H - p_{1}\rho_{1} & & \\ | ||
& \ddots & \\ | ||
& & H - p_{N}\rho_{N} | ||
\end{pmatrix}\in \Meas_{\LOCC}^{\ast}(N, \X:\Y),\\ | ||
\quad & H \in \Herm(\X\otimes\Y). | ||
\end{split} | ||
\end{equation} | ||
\end{center} | ||
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Finally, apart from \cite{Gharibian13}, I am not aware of any results where | ||
cone programming is explicitly used in quantum computing for any cones different than the | ||
cone of semidefinite operators. | ||
I hope this work helps toward the rise of more applications of cone programming | ||
in quantum information theory. |
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