diff --git a/thesis/Makefile b/thesis/Makefile index 60e5943..d90be32 100644 --- a/thesis/Makefile +++ b/thesis/Makefile @@ -5,6 +5,7 @@ bibtex=bibtex chapters = chapters/introduction.tex \ chapters/naive_simulator.tex \ chapters/introduction_qc.tex \ + chapters/stabilizer.tex \ chapters/graph_simulator.tex all: main.pdf diff --git a/thesis/chapters/graph_simulator.tex b/thesis/chapters/graph_simulator.tex index e426add..f1cbf37 100644 --- a/thesis/chapters/graph_simulator.tex +++ b/thesis/chapters/graph_simulator.tex @@ -7,14 +7,6 @@ graph formalism works and how the simulator handles gates. %A $n$ qbit graph or stabilizer state is a $+1$ eigenstate of some $ p \in P_n$ where $P_n$ is the Pauli group\cite{andersbriegel2005}. -\begin{definition} - \begin{equation} - C_n := \{U \in SU(2) | UpU^\dagger \in P_n \forall p \in P_n\} - \end{equation} - is called the Clifford group on $n$ qbits. - $C_1$ is called the local Clifford group. -\end{definition} - One can show that the Clifford group $C_n$ can be generated using the elements of $C_1$ acting on all qbits and the controlled phase gate $CZ$ between all qbits\cite{andersbriegel2005}. It is worth noting that the $CX$ gate can be generated using $CZ$ and $C_1$ gates. diff --git a/thesis/chapters/stabilizer.tex b/thesis/chapters/stabilizer.tex index a1ed329..1b84384 100644 --- a/thesis/chapters/stabilizer.tex +++ b/thesis/chapters/stabilizer.tex @@ -19,7 +19,7 @@ Where $X = \left(\begin{array}{cc} 0 & 1 \\ 1 & 0\end{array}\right)$, $Z = \left(\begin{array}{cc} 1 & 0 \\ 0 & -1\end{array}\right)$ are the Pauli matrices and $I$ is the identity. - $p \in = P_n$ is called a multi-local Pauli operator. + $p \in P_n$ is called a multi-local Pauli operator. \end{definition} \begin{definition} @@ -41,16 +41,46 @@ Where $X = \left(\begin{array}{cc} 0 & 1 \\ 1 & 0\end{array}\right)$, \begin{lemma} For every $\langle S_i \rangle_i$ fulfilling the first two conditions in definition \ref{def:stabilizer} there exists - a (up to a global phase) unique state $\ket{\psi}$ fulfilling the third condition. + a (up to a global phase) unique state $\ket{\psi}$ fulfilling the third condition. This state is called stabilizer state. \end{lemma} \begin{proof} - All multi-local Pauli operators are hermitian (observables in terms of quantum mechanics), as they + All multi-local Pauli operators are hermitian (observables in terms of quantum mechanics), as the $S_i$ commute they have a common set of eigenstates. Because each $S_i$ has eigenvalues $+1, -1$, there exist $2^n$ eigenstates, one state $\ket{\psi}$ with eigenvalue $+1$ for all $S_i$. As the dimension of $n$ qbits is $2^n$ the state $\ket{psi}$ is unique up to a global phase. \end{proof} +One can study the dynamics of stabilizer states using only the stabilizers\cite{nielsen_chuang_2010}. Two important +cases are the unitary transformation of a state and the measurement of a qbit. When applying a unitary gate to a stabilizer +state $\ket{\psi}$ the resulting state will in general be no stabilizer state anymore, however there exists a group of +transformations that map stabilizers to other stabilizers: the Clifford group. + +\begin{definition} + \begin{equation} + C_n := \{U \in SU(2) | UpU^\dagger \in P_n \forall p \in P_n\} + \end{equation} + is called the Clifford group on $n$ qbits. + $C_1$ is called the local Clifford group. +\end{definition} + +The properties of this group will be discussed later, for the time being is existence is enough. + +\begin{lemma} + Let $\ket{\psi}$ be stabilized by $\langle S_i \rangle_i$, then $U\ket{\psi}$ is stabilized + by $\langle US_iU^\dagger \rangle_i$. +\end{lemma} + +\begin{proof} +$$ U\ket{\psi} = US_i\ket{\psi} = US_iU^\dagger U\ket{\psi}$$ +So $U\ket{\psi}$ is a $+1$ eigenstate of $US_iU^\dagger$. +\end{proof} + +This is an important insight that is used for simulations\cite{gottesman_aaronson2008}, as +updating the $n$ stabilizers that are a tensor product of $n$ Pauli matrices scales with roughly +$\mathcal{O}(n^2)$ instead of $\mathcal{O}(2^n)$ for the state vector approach. + + \subsection{The Vertex Operator-Free Graph States} In order to understand some essential transformations of graph states it is necessary