What Shape is a Photon

I remember once, a long time ago, wondering what a photon would look like if I could freeze one, or better yet, if I could catch up to one and travel with it. I also recall being completely flummoxed by the question and the answers my professors gave me (when they didn't roll their eyes and kick me out of their offices.)

Stylized map showing the probaility that a photon will be detected at various locations. Image courtesy of M. Bellini/National Inst. of Optics

Our good friends over at Physics (the publication, not the subject) have finally explained it in a way that I can understand - a photon doesn't have a shape. However, you can map out a region where you could find the photon if you placed a detector at various places and took a measurement. That is, you can map out the probability that a photon would appear at any given place. It's a lot like the orbital maps of electrons around the Hydrogen atom.

Hydrogen orbitals. Image courtesy of FlorianMarquardt

What's really cool is that a group of Italian physicists have figured out a way to actually measure the probability map of a photon. Of course, you can only detect a photon once, so they actually make many measurements with many photons, each of which have been prepared in the same way, so they are essentially identical. Here's what one of the maps looks like.

C. Polycarpou, K. N. Cassemiro, G. Venturi, A. Zavatta, and M. Bellini, Physical Review Letters

The formal article describing the research was published last week. It's a bit dense (hey, its' the Physical Review). But the Focus article in Physics is a pretty clear piece.