diff --git a/lectures/phase-ii/lecture9.tex b/lectures/phase-ii/lecture9.tex index 32e4043..eea4388 100644 --- a/lectures/phase-ii/lecture9.tex +++ b/lectures/phase-ii/lecture9.tex @@ -24,7 +24,7 @@ \subsection*{Problem Statement} and only one item is “special.” Classically, you must check each item one by one (on average, \( O(2^n) \) trials) to find the special item. Grover's algorithm, however, uses quantum amplitude amplification to solve this -problem in only \( O(\sqrt{2^n}) \) iterations- a quadratic speedup over +problem in only \( O(\sqrt{2^n}) \) iterations—a quadratic speedup over classical search. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% @@ -57,8 +57,8 @@ \subsection*{Grover's Algorithm Circuit} O \ket{x} = (-1)^{f(x)} \ket{x}. \] - In our example, if the winning state is \(\ket{w}\), then \( O\ket{w} - = -\ket{w} \). + For example, if the winning state is \(\ket{w}\), then + \( O\ket{w} = -\ket{w} \). \item \textbf{Diffusion Operator \(D\):} Reflect all amplitudes about the average amplitude. This operator is given by @@ -67,18 +67,20 @@ \subsection*{Grover's Algorithm Circuit} D = 2\ket{s}\bra{s} - I. \] - In practice, \( D \) is implemented by the sequence: + In practice, \( D \) is implemented as \[ - H^{\otimes n} \; X^{\otimes n} \; (CZ) \; X^{\otimes n} \; - H^{\otimes n}. + D = H^{\otimes n} \; X^{\otimes n} \; (CZ) \; X^{\otimes n} \; + H^{\otimes n}, \] + where the \(CZ\) gate here represents a controlled phase flip on + \(\ket{1}^{\otimes n}\) (for \(n>2\), this is a multi-controlled \(Z\) + gate). \end{itemize} \end{enumerate} -The following diagram shows the high-level circuit for an \( n \)-qubit -Grover algorithm: +The high-level circuit for an \( n \)-qubit Grover algorithm is illustrated as: \[ \begin{quantikz} @@ -100,14 +102,13 @@ \subsection*{Grover's Algorithm Circuit} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsubsection*{2-Qubit Example} -To build intuition, let us consider the case \( n=2 \) (i.e. \( N=4 \)) with -the winning state chosen as \(\ket{11}\). +To build intuition, consider the case \( n=2 \) (i.e., \( N=4 \)) with the winning +state chosen as \(\ket{11}\). \vspace{0.3cm} -\textbf{Oracle:} For a 2-qubit system, the Oracle \(O\) can be implemented as -a controlled-Z (CZ) gate since - +\textbf{Oracle:} For a 2-qubit system, the Oracle \(O\) can be implemented as a +controlled-\(Z\) (CZ) gate: \[ CZ = \begin{pmatrix} @@ -128,10 +129,9 @@ \subsubsection*{2-Qubit Example} D = H^{\otimes 2}\; X^{\otimes 2}\; CZ\; X^{\otimes 2}\; H^{\otimes 2}. \] -\textbf{2-Qubit Grover Circuit:} - +\vspace{0.3cm} -The following circuit implements Grover's algorithm for 2 qubits: +\textbf{2-Qubit Grover Circuit:} \[ \begin{quantikz} @@ -146,13 +146,11 @@ \subsubsection*{2-Qubit Example} \textbf{Explanation:} \begin{itemize} - \item \textbf{Oracle:} The Oracle adds a phase of \(-1\) to the winner + \item \textbf{Oracle:} The Oracle adds a phase of \(-1\) to the winning state \(\ket{11}\), effectively marking it. - - \item \textbf{Diffusion Operator:} The diffusion circuit reflects all - amplitudes about the average. This inversion about the mean amplifies the - amplitude of the marked state while reducing that of all others. - + \item \textbf{Diffusion Operator:} The diffusion circuit (implemented as + \(H\;X\;CZ\;X\;H\)) reflects all amplitudes about the average. This + inversion about the mean amplifies the amplitude of the marked state. \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% @@ -188,10 +186,10 @@ \subsection*{Walkthrough of the Algorithm and Generalization to \(n\) Qubits} \[ O\ket{x} = (-1)^{f(x)}\ket{x}, \] + (i.e., flip the phase of the marked state).\; - \quad (i.e., flip the phase of the marked state).\; - Apply the Diffusion Operator \( D = 2\ket{s}\bra{s} - I \) (which - reflects amplitudes about the average).\; + Apply the Diffusion Operator \( D = 2\ket{s}\bra{s} - I \) + (which reflects amplitudes about the average).\; } Measure the state in the computational basis to obtain \( x_0 \).\; @@ -211,15 +209,13 @@ \subsection*{Walkthrough of the Algorithm and Generalization to \(n\) Qubits} \(O\ket{w} = -\ket{w}\)). \item \textbf{Diffusion (Inversion about the Mean):} This operator reflects - the state vector about the average amplitude. Geometrically, if you - visualize the amplitudes as vectors in the complex plane, the Oracle - rotates the winning state's amplitude by 180°, and the diffusion operator - then “pushes” this amplitude further away from the average, thereby - amplifying it. + the state vector about the average amplitude. Geometrically, one can + view the process as a rotation in a two-dimensional subspace (see + below). \item \textbf{Iteration:} Repeating the Oracle and Diffusion steps - approximately \( \sqrt{2^n} \) times ensures that the probability - amplitude of the marked state is maximized. + approximately \( \sqrt{2^n} \) times amplifies the probability amplitude of + the marked state. \item \textbf{Measurement:} Finally, a measurement in the computational basis yields the marked element with high probability.