diff --git a/thesis/chapters/stabilizer.tex b/thesis/chapters/stabilizer.tex index 797cc08..2a08437 100644 --- a/thesis/chapters/stabilizer.tex +++ b/thesis/chapters/stabilizer.tex @@ -281,6 +281,7 @@ This section will discuss the vertex operator(VOP)-free graph states. Why they a vertex operator-free will be clear in the following section about graph states. \begin{definition} + \label{def:graph} The tuple $(V, E)$ is called a graph iff $V$ is a set of vertices with $|V| = n \in \mathbb{N}$. In the following $V = \{0, ..., n-1\}$ will be used. $E$ is the set of edges $E = \left\{\{i, j\} | i,i \in V, i \neq j\right\}$. @@ -452,5 +453,55 @@ just constructive. As seen in theorem \ref{thm:clifford_group_approx} any $c \in can be constructed from $CZ$ and $C_L$ and in the following discussion it will become clear that both $C_L$ and $CZ$ can be applied to a general graph state. -\subsubsection{Graph States and Vertex operators} +\subsubsection{Graph States and Vertex Operators} + +\begin{definition} + \label{def:g_state} + A tuple $(V, E, O)$ is called the graphical representation of a stabilizer state + if $(V, E)$ is a graph as in Definition \ref{def:graph} and $O = \{o_1, ..., o_n\}$ where $o_i \in C_L$. + + The state $\ket{G}$ is defined by the eigenvalue relation + + \begin{equation} + +1 \ket{G} = \left(\prod\limits_{j=1}^no_j\right) K_G^{(i)} \left(\prod\limits_{j=1}^no_j\right)^\dagger \ket{G} + \end{equation} + + $o_i$ are called the vertex operators of $\ket{G}$. +\end{definition} + +Recalling the dynamics of stabilizer states the following relation follows immediately: + +\begin{equation} + \ket{G} = \left(\prod\limits_{j=1}^no_j\right) \ket{\bar{G}} +\end{equation} + +The great advantage of this representation of a stabilizer state is its space requirement: +Instead of storing $n^2$ $P_1$ matrices only some vertices (which often are implicit), +the edges and some vertex operators ($n$ matrices) have to be stored. The following theorem +will improve this even further: instead of $n$ matrices it is enough to store $n$ integers +representing the vertex operators is enough: + +\begin{theorem} + $C_L$ has $24$ degrees of freedom. +\end{theorem} +\begin{proof} + It is clear that $\forall a \in C_L$ a is a group isomorphism $P_1 \circlearrowleft$: $apa^\dagger a p' a^\dagger = a pp'a^\dagger$. + Therefore $a$ will preserve the (anti-)commutator relations of $P$. + Further note that $Y = iXZ$, so one has to consider the anti-commutator relations + of $X,Z$ only. + + As the transformations are unitary they preserve eigenvalues, so $X$ can be mapped + to $\pm X, \pm Y, \pm Z$ which gives $6$ degrees of freedom, the image of $Z$ has to + anti-commute with the image of $X$ so $Z$ has four possible images under the transformation. + This gives another $4$ degrees of freedom and a total of $24$. +\end{proof} + +From now on $C_L = \langle H, S \rangle$ (disregarding a global phase) will be used, +one can show (by construction) that $H, S$ generate a possible choice of $C_L$, as is +$C_L = \langle \sqrt{-iX}, \sqrt{-iZ}\rangle$ which will be used in one specific operation on graph states. + +$$ S = \left(\begin{array}{cc} 1 & 0 \\ 0 & i \end{array}\right)$$ +$$ \sqrt{-iX} = \frac{1}{\sqrt{2}}\left(\begin{array}{cc} 1 & i \\ i & 1 \end{array}\right)$$ +$$ \sqrt{-iZ} = \exp(-i\frac{\pi}{4})\left(\begin{array}{cc} 1 & 0 \\ 0 & -i \end{array}\right)$$ +