Is fluorescence from a single atom/ion visible with the naked eye (e.g. in a strongly coupled trap or cavity)

Experiments with trapped ions generally use fluorescence for detecting the ions. This means that they use a strongish pump to take the ion from its ground state to a dipole-allowed excited state and wait for the ion to decay by emitting a photon in a random direction, and then re-run the cycle over and over. This means that each ion essentially emits one photon per natural lifetime of the line, which can be quite often (in atomic terms) for dipole-allowed lines.

One example, taken from this paper, is an $S$-$P$ line in $\mathrm{Ca}^+$ with a linewidth of 21 MHz, at 397 nm. This means that each individual ion will emit about 20 million photons per second or so, evenly spread about $4\pi$ solid angle. This is in general just below the limit of human sensitivity but if you had a high enough numerical aperture and a dark enough background you could in principle do it. I asked the authors and they said that they can't do it because of lack of sufficient optical access (they have huge magnets in the way) and they don't know anyone who does but in principle it's just about doable.