So let's say I have a single ($N=1$) quantum harmonic oscillator and the energy is determined by $E_n = (n + 1/2) \cdot \hbar \omega$ (where $n$ is the quantum number and $n$ = $0, 1, 2, \ldots$)
What's the probability that the oscillator is in the state labeled $n$ at temperature $T$?
So according to my calculation, $Z$, the partition function, is
$Z = 1 / (1 – x)$ where $x = e^{-\beta \hbar \omega} \Rightarrow P = x ^ n (1 – x)$.
Is it correct?
Also, how do I go about calculating the probability of finding the oscillator in a state with odd quantum number?
Best Answer
Your calculation looks fine to me (technically, your partition function should have an extra factor of $e^{-\frac 12 \beta\hbar\omega}$, but this is unimportant, as it cancels in all observables).
Edit: As in the comment by abcXYZ, the probability of finding the system in a state corresponding to any odd value of $n$ is $$ P(n~\text{is odd}) = (1-x)(x + x^3 + \ldots + x^{2k-1} + \ldots) = \frac{(1-x)x}{1-x^2} = \frac{x}{1+x} $$ where $x = e^{-\beta\hbar\omega}$. To give some confidence that this is indeed the right answer, we can check some limits:
It's a good habit to do these sorts of simple checks on any formulae you derive.