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\begin{document}

\title{\vspace{-10mm}Quantum Complexity Theory, UPB\\Winter 2020, Assignment 4\\{\large To be completed by: Tuesday, December 15, start of tutorial}}
\date{}
\maketitle
\noindent \textbf{This assignment assumes the notation and terminology from Lectures 4 and 5.}

\section{Exercises}
\begin{enumerate}
    \item
        \textbf{A BQP-complete problem: Matrix inversion.} In this question, we assume the terminology from Section 3 of Lecture 4 of the course notes.
        \begin{enumerate}
            \item  Recall from Lecture 4 the equation
        \[
            \ket{\widetilde{x}}=\frac{1}{\sqrt{m+1}}\left(\ket{0}\ket{0^n}+\ket{1}V_1\ket{0^n}+\cdots+\ket{m}V_m\cdots V_1\ket{0^n}\right).
        \]
            Assume, without loss of generality, that in all but the last time step, $m$, $V$ sets the output qubit to $\ket{0}$, and only in time step $m$ does $V$ perform a CNOT to copy over its answer to the output qubit. Prove that (where recall $\Pi=\ketbra{1}{1}$ is a single qubit projector onto the output qubit of the second register):
    \begin{itemize}
        \item If $V$ denotes a YES instance, then $\bra{\widetilde{x}}\Pi\ket{\widetilde{x}}\geq \frac{2}{3(m+1)}$.
        \item If $V$ denotes a NO instance, then $\bra{\widetilde{x}}\Pi\ket{\widetilde{x}}\leq \frac{1}{3(m+1)}$.
    \end{itemize}

        \item How can we modify $V$ in a seemingly trivial manner in order to boost the completeness/soundness bounds of the previous exercise to constants (i.e. independent of $m$)?

            \item Choose $A=I-e^{-1/m}U$ for scalar $e^{-1/m}$. Prove that now $\kappa(A)\in O(m)$ for the new definition of $A$. (Hint: You only need to use the fact that $U$ is unitary, not the specific definition of $U$. Also, use the fact that for normal operators $A$, the singular values of $A$ are $\set{\abs{\lambda(A)}}$ (why?).)
        \end{enumerate}

    \item
        \textbf{Weak error reduction for QMA.} Suppose the QMA prover sends $k$ copies of its proof, $\ket{\psi}$, instead of a single copy. On the $j$th copy of the proof, the verifier runs the verification circuit $Q_n$. Finally, the verifier measures the output qubits of all runs of $Q_n$, takes a majority vote of the resulting bits, and accepts if and only if the majority function yields $1$. Prove that this procedure indeed amplifies the completeness and soundness parameters for QMA. (Hint: In the NO case, a cheating prover is \emph{not} obligated to send $k$ copies of some state $\ket{\psi}$ in tensor product, but rather can cheat by sending a large entangled state $\ket{\phi}\in\B{k\cdot p(n)}$ across all $k$ proof registers. Why does entanglement across proofs not help the prover in the NO case?)

    \item \textbf{Strong error reduction for QMA.} All references below are to the Lecture 5 course notes.
    \begin{enumerate}
        \item Prove Equations $(8)$-$(11)$.
        \item What is $\enorm{S_0Q^\dagger E_1Q\ket{\phi}}$?
        \item Prove that after we measure the right hand side of Equation~(6) with $\set{S_0,S_1}$, we obtain $S_1$ with probability $p$ and $S_0$ with probability $1-p$. Similarly, measuring the right hand side of Equation~(7) with ${S_0,S_1}$ yields $S_0$ with probability $p$ and $S_1$ with probability $1-p$.

        \item  Conclude from the last exercise that after the first iteration of the while loop, $y_1=y_2$ with probability $p$. Accordingly, $y_1\neq y_2$ with probability $1-p$.\\
    \end{enumerate}
\end{enumerate}

\end{document}
