The most interesting “photo” I’ve ever taken

This week I’ve started my Study Leave with a short visit to the University of Otago in Dunedin. Today, amongst other things,  I had a quick tour of one of the atomic physics labs there. Recently, Mikkel Andersen’s group has managed to trap a single atom of rubidium. It’s quite a complicated process – you can read about it in their Nature Physics article here. The main ideas are to grab a small number of atoms, trap them by holding them in place with beams of light, cool them down (again with light), and then, using some rather clever methods involving resonance (with, you guessed it, more light), remove one atom at a time until only one is left. Just one, single, solitary atom. Wow.

How do you know for sure? Take a photo of it, silly. And yes, that means yet more light. With a single atom, you can’t take a conventional photograph, because the wavelength of visible light is around 500 nanometres compared to the atom size of around a nanometre. It’s impossible to focus that small. But what you can do is measure exactly how much light is being scattered by the atom, and from that know for sure that there is only one atom there.

I had a go at this experiment. Admittedly, it wasn’t desperately taxing on my abilities as a physicist – in fact all I had to do was push a button on a computer window and in half a second the machine did its thing, completely silently, and produced a nice looking photo with a splurge of light in the centre. And that light was being scattered by just a single atom.

So, you CAN see single atoms, after all. I have a picture to prove it. (If you want to see one, have a look on the group’s website here).

 Postscript. I’m staying with my sister-in-law and her family. When I got back to their house this afternoon, I showed the picture to my eight year old niece and asked her if she could guess what it was. She gazed at it for a few seconds, then said "An atom?"


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