> When illuminated by a laser of the right blue-violet color, the atom absorbs and re-emits light particles sufficiently quickly for an ordinary camera to capture it in a long exposure photograph.
That explanation doesn't really satisfy. Perhaps, based on a large body of experimental evidence, it is known that the number of atoms present can be determined by the apparent brightness. Perhaps it's something totally different. Regardless, it would be helpful if the author/photographer/somebody explained the rationale that makes them confident that this is only a single atom.
> the rationale that makes them confident that this is only a single atom
Nothing photographic, as I understand it. The atom is positively charged, and is suspended in ultra-high vacuum by electric fields. Effectively they're "weighing" it, by measuring the total charge of whatever is suspended, and knowing (from the ionization energy?) the charge on one atom.
It should be noted that this is similar to how Millikan first measured the electron charge back in 1909.
I'm guessing there are at least two layers of glass between that atom and the sensor. Not really specific to this (excellent) picture, but I imagine that avoiding distortion due to the glass or smudges on the glass must be a huge part of high magnification photography. The whole light from a single atom part just makes it more amazing how clean and clear that window is...
The article makes it sounds like the photographer already knew he could trap one atom, before taking the picture.
My guess would be, as this is an electromagnetic trap, that two such ions would repel each other and the well is set to be shallow enough that at most one ion can remain inside.
> When illuminated by a laser of the right blue-violet color, the atom absorbs and re-emits light particles sufficiently quickly for an ordinary camera to capture it in a long exposure photograph.