# PlanetPhysics/Canonical Quantization

Canonical quantization is a method of relating, or associating, a classical system of the form ${\displaystyle (T^{*}X,\omega ,H)}$, where ${\displaystyle X}$ is a manifold, ${\displaystyle \omega }$ is the canonical symplectic form on ${\displaystyle T^{*}X}$, with a (more complex) quantum system represented by ${\displaystyle H\in C^{\infty }(X)}$, where ${\displaystyle H}$ is the Hamiltonian operator. Some of the early formulations of quantum mechanics used such quantization methods under the umbrella of the \htmladdnormallink{correspondence principle {http://planetphysics.us/encyclopedia/PrincipleOfCorrespondingStates.html} or postulate}. The latter states that a correspondence exists between certain classical and quantum operators, (such as the Hamiltonian operators) or algebras (such as Lie or Poisson (brackets)), with the classical ones being in the real (${\displaystyle \mathbb {R} }$) domain, and the quantum ones being in the complex (${\displaystyle \mathbb {C} }$) domain. Whereas all classical Observables and States are specified only by real numbers, the 'wave' amplitudes in quantum theories are represented by complex functions.

Let ${\displaystyle (x^{i},p_{i})}$ be a set of Darboux coordinates on ${\displaystyle T^{*}X}$. Then we may obtain from each coordinate function an operator on the Hilbert space ${\displaystyle {\mathcal {H}}=L^{2}(X,\mu )}$, consisting of functions on ${\displaystyle X}$ that are square-integrable with respect to some measure ${\displaystyle \mu }$, by the operator substitution rule:

$\displaystyle \begin{matrix} x^i \mapsto \hat{x}^i &= x^i \cdot, (1)\\ p_i \mapsto \hat{p}_i &= -i \hbar \pdiff{}{x^i} (2), \end{matrix}$

where ${\displaystyle x^{i}\cdot }$ is the "multiplication by ${\displaystyle x^{i}}$" operator. Using this rule, we may obtain operators from a larger class of functions. For example,

1. ${\displaystyle x^{i}x^{j}\mapsto {\hat {x}}^{i}{\hat {x}}^{j}=x^{i}x^{j}\cdot }$,
2. $\displaystyle p_i p_j \mapsto \hat{p}_i \hat{p}_j = -\hbar^2 \pdiff{^2}{x^i x^j}$ ,
3. if ${\displaystyle i\neq j}$ then $\displaystyle x^i p_j \mapsto \hat{x}^i \hat{p}_j = -i \hbar x^i \pdiff{}{x^j}$ .

\begin{rmk} The substitution rule creates an ambiguity for the function ${\displaystyle x^{i}p_{j}}$ when ${\displaystyle i=j}$, since ${\displaystyle x^{i}p_{j}=p_{j}x^{i}}$, whereas ${\displaystyle {\hat {x}}^{i}{\hat {p}}_{j}\neq {\hat {p}}_{j}{\hat {x}}^{i}}$. This is the operator ordering problem. One possible solution is to choose

${\displaystyle x^{i}p_{j}\mapsto {\frac {1}{2}}\left({\hat {x}}^{i}{\hat {p}}_{j}+{\hat {p}}_{j}{\hat {x}}^{i}\right),}$

since this choice produces an operator that is self-adjoint and therefore corresponds to a physical observable. More generally, there is a construction known as Weyl quantization that uses Fourier transforms to extend the substitution rules (2)-(3) to a map

$\displaystyle \begin{matrix} C^\infty(T^*X) &\to \Op (\mathcal{H}) \\ f &\mapsto \hat{f}. \end{matrix}$

\end{rmk}

\begin{rmk} This procedure is called "canonical" because it preserves the canonical Poisson brackets. In particular, we have that

${\displaystyle {\frac {-i}{\hbar }}[{\hat {x}}^{i},{\hat {p}}_{j}]:={\frac {-i}{\hbar }}\left({\hat {x}}^{i}{\hat {p}}_{j}-{\hat {p}}_{j}{\hat {x}}^{i}\right)=\delta _{j}^{i},}$

which agrees with the Poisson bracket ${\displaystyle \{x^{i},p_{j}\}=\delta _{j}^{i}}$. \end{rmk}

\begin{ex} Let ${\displaystyle X=\mathbb {R} }$. The Hamiltonian function for a one-dimensional point particle with mass ${\displaystyle m}$ is

${\displaystyle H={\frac {p^{2}}{2m}}+V(x),}$

where ${\displaystyle V(x)}$ is the potential energy. Then, by operator substitution, we obtain the Hamiltonian operator

${\displaystyle {\hat {H}}={\frac {-\hbar ^{2}}{2m}}{\frac {d^{2}}{dx^{2}}}+V(x).}$

\end{ex}