nLab
quantum harmonic oscillator

Idea

“Harmonic oscillator” is a fancy name for a rock on a spring:

• in classical mechanics it is the physical system given by a point mass in a parabolic potential, feeling forces driving it back to a specified origin that are propertional to the distance of the mass from that origin.

• in quantum mechanics and in particular quantum field theory the quantum harmonic oscillator governs not just the dynamics of idealized point masses but crucially appears in the dynamics of all free massive quantum fields.

To quote the field theorist Sidney Coleman,

The career of a young theoretical physicist consists of treating the harmonic oscillator in ever-increasing levels of abstraction.

Classical oscillator

First the harmonic oscillator in classical mechanics.

The force exerted by a spring is proportional to how far you stretch it:

(1)$$F=kx.$$F = k x.

The potential energy? stored in a stretched spring is the integral of that:

(2)$$V_0=\frac{1}{2}k{x}^2+C,$$V_0 = \frac{1}{2}k x^2 + C,

and to make things work out nicely, we’re going to choose $C=-1/2.$ The total energy $H_0$ is the sum of the potential and the kinetic energy:

(3)$$H_0=V_0+T=\frac{1}{2}k{x}^2+\frac{1}{2}m{v}^2-\frac{1}{2}.$$H_0 = V_0 + T = \frac{1}{2}k x^2 + \frac{1}{2}m v^2 - \frac{1}{2}.

By choosing units so that $k=m=1,$ we get

(4)$$H_0=\frac{x^2}{2}+\frac{p^2}{2}-\frac{1}{2},$$H_0 = \frac{x^2}{2} + \frac{p^2}{2} - \frac{1}{2},

where $p$ is momentum.

Quantum harmonic oscillator

Now the harmonic oscillator in quantum mechanics.

We quantize, getting a quantum harmonic oscillator, or QHO. We set $p=-i\frac{\partial }{\partial x},$ taking units where $\hslash =1.$ Now

(5)$$\begin{array}{ccc}[x,p]x^n& =& xp-px\\ & =& (-xi\frac{\partial }{\partial x}+i\frac{\partial }{\partial x}x)x^n\\ & =& -i(n{x}^n-(n+1)x^n)\\ & =& i{x}^n.\end{array}$$\array{ [x, p]x^n & = & x p - p x \\ & = &(- x i \frac{\partial}{\partial x} + i \frac{\partial}{\partial x} x)x^n \\ & = & -i(n x^n - (n+1)x^n) \\ & = & i x^n. }

If we define a new observable $z=\frac{p+\mathrm{ix}}{\sqrt{2}},$ then

(6)$$\begin{array}{ccc}z{z}^{*}& =& \frac{(p+ix)}{\sqrt{2}}\frac{(p-ix)}{\sqrt{2}}\\ & =& \frac{1}{2}(p^2+i(xp-px)+x^2)\\ & =& \frac{1}{2}(p^2-1+x^2)\\ & =& H_0.\end{array}$$\array{ z z^* & = & \frac{(p + i x)}{\sqrt{2}} \frac{(p - i x)}{\sqrt{2}} \\ & = & \frac{1}{2}(p^2 + i(x p - p x) + x^2) \\ & = & \frac{1}{2}(p^2 -1 + x^2) \\ & = & H_0. }

We can think of $z^{*}$ as $\frac{d}{\mathrm{dz}}$ and write the energy eigenvector?s as polynomials in $z:$

(7)$$H_0{z}^n=z\frac{d}{\mathrm{dz}}{z}^n=n{z}^n.$$H_0 z^n = z \frac{d}{dz} z^n = n z^n.

The creation operator $z$ adds a photon to the mix; there’s only one way to do that, so $z\cdot z^n=1z^{n+1}.$ The annihilation operator $\frac{d}{\mathrm{dz}}$ destroys one of the photons; in the state $z^n$, there are $n$ photons to choose from, so $\frac{d}{\mathrm{dz}}{z}^n=n{z}^{n-1}.$

Schrödinger's equation? says $i\frac{d}{\mathrm{dt}}\psi =H_0\psi ,$ so

(8)$$\psi (t)=\sum _{n=0}^{\mathrm{\infty }}{e}^{-\mathrm{itn}}{a}_n{z}^n.$$\psi(t) = \sum_{n=0}^{\infty} e^{-itn} a_n z^n.

This way of representing the state of a QHO is known as the Fock basis.

References

Categorification

A program initiated by John Baez aims to identify a categorification of sorts of the quantum harmonic oscillator

The notes

• John Baez’s Fall ‘03 Quantum gravity seminar

relate wavefunctions expressed in the Fock basis to structure types.

This originates in

• John Baez, James Dolan, From finite sets to Feynman diagrams (arXiv).

For more along these lines see

• Jeffrey Morton, Categorified Algebra and Quantum Mechanics (arXiv)