## Third update of aggregate notes for phy1520, Graduate Quantum Mechanics.

I’ve posted a third update of my aggregate notes for PHY1520H Graduate Quantum Mechanics, taught by Prof. Arun Paramekanti. In addition to what was noted previously, this contains lecture notes up to lecture 13, my solutions for the third problem set, and some additional worked practice problems.

Most of the content was posted individually in the following locations, but those original documents will not be maintained individually any further.

## More on (SHO) coherent states

### [1] pr. 2.19(c)

Show that $$\Abs{f(n)}^2$$ for a coherent state written as

\ket{z} = \sum_{n=0}^\infty f(n) \ket{n}

has the form of a Poisson distribution, and find the most probable value of $$n$$, and thus the most probable energy.

### A:

The Poisson distribution has the form

P(n) = \frac{\mu^{n} e^{-\mu}}{n!}.

Here $$\mu$$ is the mean of the distribution

\begin{aligned}
\expectation{n}
&= \sum_{n=0}^\infty n P(n) \\
&= \sum_{n=1}^\infty n \frac{\mu^{n} e^{-\mu}}{n!} \\
&= \mu e^{-\mu} \sum_{n=1}^\infty \frac{\mu^{n-1}}{(n-1)!} \\
&= \mu e^{-\mu} e^{\mu} \\
&= \mu.
\end{aligned}

We found that the coherent state had the form

\ket{z} = c_0 \sum_{n=0} \frac{z^n}{\sqrt{n!}} \ket{n},

so the probability coefficients for $$\ket{n}$$ are

\begin{aligned}
P(n)
&= c_0^2 \frac{\Abs{z^n}^2}{n!} \\
&= e^{-\Abs{z}^2} \frac{\Abs{z^n}^2}{n!}.
\end{aligned}

This has the structure of the Poisson distribution with mean $$\mu = \Abs{z}^2$$. The most probable value of $$n$$ is that for which $$\Abs{f(n)}^2$$ is the largest. This is, in general, hard to compute, since we have a maximization problem in the integer domain that falls outside the normal toolbox. If we assume that $$n$$ is large, so that Stirling’s approximation can be used to approximate the factorial, and also seek a non-integer value that maximizes the distribution, the most probable value will be the closest integer to that, and this can be computed. Let

\begin{aligned}
g(n)
&= \Abs{f(n)}^2 \\
&= \frac{e^{-\mu} \mu^n}{n!} \\
&= \frac{e^{-\mu} \mu^n}{e^{\ln n!}} \\
&\approx e^{-\mu – n \ln n + n } \mu^n \\
&= e^{-\mu – n \ln n + n + n \ln \mu }
\end{aligned}

This is maximized when

0
= \frac{dg}{dn}
= \lr{ – \ln n – 1 + 1 + \ln \mu } g(n),

which is maximized at $$n = \mu$$. One of the integers $$n = \lfloor \mu \rfloor$$ or $$n = \lceil \mu \rceil$$ that brackets this value $$\mu = \Abs{z}^2$$ is the most probable. So, if an energy measurement is made of a coherent state $$\ket{z}$$, the most probable value will be one of

E = \Hbar \lr{
\lceil\Abs{z}^2\rceil
+ \inv{2} },

or