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Comment by bishalb | Hacker News Reader

I thought atoms were invisible even with advanced microscopes.

Small objects are usually hard to see because they scatter small amounts of light. In this case, the ion is illuminated with a laser close to a strong resonance which makes it emit a surprisingly large amount of light (tens of million photons per second). This makes it relatively easy for the camera to detect.

Atoms are about three orders of magnitude smaller than the wavelength of visible light. So for any optical microscope they are just point sources. The finite size spot a camera sees is only due to diffraction from the finite size of the objective. This is also why stars don't appear infinitely small in images.

For those wondering about the word “objective” as it is used above;

> In optical engineering, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. [1]

I found this [2] article to be pretty approachable without the jargon overload like Wikipedia often suffers from.

[1] - https://en.m.wikipedia.org/wiki/Objective_(optics)

[2] - https://en.m.wikipedia.org/wiki/Diffraction-limited_system

FTA: "The Strontium atom appears larger than its true size because it was emitting light, and was oscillating slightly, over the course of the long exposure."

Thanks for that, (I missed the comment in the article), because my skeptic-o-meter was maxing out. You can see the machining marks on the electrodes. There's even screws to be seen in the photo, which makes the 'atom' about the size of a largish dust particle, using a rough guess.

I believe the journalist misunderstood this point. Trapped ions do oscillate somewhat, but the amplitude should be way below the resolution of this imaging system. Most likely the finite spot size is due to the finite resolution and maybe some blooming from over-exposure.

Exactly. It's really imaging the point-spread function of the camera + window.

It's a very accurate way of doing it though. You know exactly where the point source is and ... that it is as close to a point as you're going to get. I wonder if this has an application in optics?

Hehe yeah, but probably hideously expensive, and there are cheaper ways to get real point sources.

Quantum dots are a few nanometers in size (so << the wavelength that they emit) and you can get a solution of them from Sigma Aldrich for ~$500. People mix them into their samples and then use that to deconvolve the image/volume that they acquire.

Advanced microscopes can distinguish individual atoms. This is relatively trivial if you have the correct setup. Search atomic force microscope and HOPG (highly ordered pyrolytic graphene).

This wasn’t the correct setup though to do that. Presumably the reason you can see a single atom is because there is only one present, ie if there was a second one next to it you wouldn’t necessarily be able to see the difference in the photograph.

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