diff --git a/src/thesis/chapters/2_fundamentals.tex b/src/thesis/chapters/2_fundamentals.tex index 0882698..6326ed9 100644 --- a/src/thesis/chapters/2_fundamentals.tex +++ b/src/thesis/chapters/2_fundamentals.tex @@ -635,20 +635,6 @@ We will represent the state of a particle with wave function $\psi(x,t)$ using the vector $\ket{\psi}$ \cite[Sec.~3.3]{griffiths_introduction_1995}. -% We can model a wave function $\psi(x,t)$ as a linear combination of different -% \emph{basis functions} $e_n(x,t),~n\in \mathbb{N}$ as% -% \begin{align*} -% \psi(x,t) = \sum_{n=1}^{\infty} c_n \cdot e_n(x,t) -% .% -% \end{align*} -% To express this relation using linear algebra, we represent -% $\psi(x,t)$ and $e_n(x,t)$ as vectors $\ket{\psi}$ and $\ket{e_n}$. -% We write% -% \begin{align*} -% \ket{\psi} = \sum_{n=1}^{\infty} c_n \ket{e_n} -% .% -% \end{align*} - % Operators Another important notion is that of an \emph{operator}, a transformation @@ -785,7 +771,7 @@ We can decompose an arbitrary state as $\ket{\psi} = \sum_{n=1}^{\infty} c_n \ket{e_n}$, where $\lvert c_n \rvert ^2$ represents the probability of obtaining a certain measurement value. Note that when we speak of an \emph{observable}, we are usually -refering to the corresponding operator $\hat{Q}$. +refering to the operator $\hat{Q}$. %%%%%%%%%%%%%%%% \subsection{Projective Measurements} @@ -793,24 +779,23 @@ refering to the corresponding operator $\hat{Q}$. % Projective measurements -% TODO: Introduce concept of collapsing the wave function onto a basis state The measurements we considered in the previous section, for which \autoref{eq:gen_expr_Q_exp_lin} holds, belong to the category of \emph{projective measurements}. -For these, certain restrictions such as repeatability apply: after -measuring a quantum state and thus collapsing it onto one of the -determinate states, futher measurements should yield the same value. +For these, certain restrictions such as repeatability apply: the act +of measuring a quantum state should \emph{collapse} it onto one of +the determinate states. +Further measurements should then yield the same value. More general methods of modelling measurements exist, e.g., describing -destructive measurements, but they are not relevant to us here -\cite[Box~2.5]{nielsen_quantum_2010}. +destructive measurements \cite[Box~2.5]{nielsen_quantum_2010}, but +they are not relevant to us here. % Projection operators -% TODO: Fix notational issues related to e_n We can model the collapse of the original state onto one of the superimposed basis states as a \emph{projection}. -To see this, we insert \autoref{eq:determinate_basis} into -\autoref{eq:observable_eigenrelation}, obtaining% +To see this, we use Equations \ref{eq:determinate_basis} and +\ref{eq:observable_eigenrelation} to compute \begin{align*} \hat{Q}\ket{\psi} = \sum_{n=1}^{\infty} c_n \hat{Q} \ket{e_n} = \sum_{n=1}^{\infty} \lambda_n c_n \ket{e_n} @@ -823,7 +808,7 @@ the separate components as \begin{align*} \hat{Q} = \sum_{n=1}^{\infty} \lambda_n \hat{P}_n \end{align*} -using \emph{projection operators} +using \emph{projection operators} \cite[Eq.~3.160]{griffiths_introduction_1995} \begin{align*} \hat{P}_n := \ket{e_n}\bra{e_n}, \hspace{3mm} n\in \mathbb{N} . @@ -839,10 +824,12 @@ A particularly interesting property of projection operators is that = \hat{P}_n \ket{\psi}, \end{align*}% and the only way this can hold for any $\ket{\psi}$ is if $\hat{P}_n$ -only has the eigenvalues $0$ or $1$. -As explained in the previous section, the eigenvalues are the results -of performing a measurement. -We can thus use the projection operator as an observable and treat +only has the eigenvalues $0$ or $1$ +% tex-fmt: off +\cite[Prob.~3.57a)]{griffiths_introduction_1995}. +% tex-fmt: on +The eigenvalues can again be interpreted as possible measurement results. +We can thus use the $\hat{P}$ as an observable and treat the eigenvalue as an indicator of the state having a component along the related basis vector.