Harmonic Oscillator Solution using Operators

Operator methods are very useful both for solving the Harmonic Oscillator problem and for any type of computation for the HO potential. The operators we develop will also be useful in quantizing the electromagnetic field.

The Hamiltonian for the 1D Harmonic Oscillator

\begin{displaymath}\bgroup\color{black} H={p^2\over 2m}+{1\over 2}m\omega^2x^2 \egroup\end{displaymath}

looks like it could be written as the square of a operator. It can be rewritten in terms of the operator \bgroup\color{black}$A$\egroup
\bgroup\color{black}$\displaystyle A\equiv \left(\sqrt{m\omega\over2\hbar}x+i{p\over\sqrt{2m\hbar\omega}}\right)$\egroup
and its Hermitian conjugate \bgroup\color{black}$A^\dagger$\egroup.
\bgroup\color{black}$\displaystyle H=\hbar\omega\left(A^\dagger A+{1\over 2}\right) $\egroup

We will use the commutators between \bgroup\color{black}$A$\egroup, \bgroup\color{black}$A^\dagger$\egroup and \bgroup\color{black}$H$\egroup to solve the HO problem.

\bgroup\color{black}$\displaystyle [A,A^\dagger]=1 $\egroup
The commutators with the Hamiltonian are easily computed.

\begin{eqnarray*}[H,A]&=&-\hbar\omega A \\
{[H,A^\dagger]}&=&\hbar\omega A^\dagger \\

From these commutators we can show that \bgroup\color{black}$A^\dagger$\egroup is a raising operator for Harmonic Oscillator states

\bgroup\color{black}$\displaystyle A^\dagger u_n=\sqrt{n+1}u_{n+1} $\egroup
and that \bgroup\color{black}$A$\egroup is a lowering operator.
\bgroup\color{black}$\displaystyle Au_n=\sqrt{n}u_{n-1}$\egroup
Because the lowering must stop at a ground state with positive energy, we can show that the allowed energies are

\begin{displaymath}\bgroup\color{black}E_n=\left(n+{1\over 2}\right)\hbar\omega.\egroup\end{displaymath}

The actual wavefunctions can be deduced by using the differential operators for \bgroup\color{black}$A$\egroup and \bgroup\color{black}$A^\dagger$\egroup, but often it is more useful to define the \bgroup\color{black}$n^{th}$\egroup eigenstate in terms of the ground state and raising operators.

\bgroup\color{black}$\displaystyle u_n={1\over\sqrt{n!}}(A^\dagger)^nu_0 $\egroup

Almost any calculation of interest can be done without actual functions since we can express the operators for position and momentum.

x&=&\sqrt{\hbar\over 2m\omega}(A+A^\dagger) \\
p&=&-i\sqrt{m\hbar\omega\over 2}(A-A^\dagger) \\

Jim Branson 2013-04-22