The following is from https://www.feynmanlectures.caltech.edu/I_32.html#Ch32-S3
Now let us actually calculate the Q of an atom that is emitting light—let us say a sodium atom. For a sodium atom, the wavelength is roughly 6000 angstroms, in the yellow part of the visible spectrum, and this is a typical wavelength. Thus \begin{equation} Q=\frac{3\lambda}{4\pi r_0}\approx 5\times10^7, \end{equation} so the Q of an atom is of the order $10^8$. This means that an atomic oscillator will oscillate for $10^8$ radians or about $10^7$ oscillations, before its energy falls by a factor $1/e$.
The above statement seems to have no correlation to the concept of photon emission by an atom. Is it just a presentation of classical theory for the sake of the exercise, or does it fit into quantum theory? If it does have a QM interpretation, what would that be?
