Catching photons

The Germans have a great history of telling tales, most of which are supposed to teach some type of lesson. One series of such tales is about the citizens of Schilda, the "Schildbürger," who in each story excel in stupidity. One of the best known stories is the construction of a new city hall. Unfortunately, the Schildbürger forget the windows, and then try to carry light into the building with buckets. 


I forgot which lesson one was supposed to learn from that, maybe that the photon number is not a conserved quantity, more likely though that you better don't forget the windows if you build a house. I recall however that I was bugging my poor grandmother with that story over and over again because it wasn't really clear to me exactly why one can't catch light in a box. Surely you could just put mirrors on the inside and visible light would bounce around till you left it out again? 

Well, leaving aside that it's easier to bring light into a dark room by flipping a switch, the problem is that mirrors are imperfect, that is, they don't actually always bounce back photons. The typical mirror in your bathroom, glass with aluminium coating, only reflects about 90% of the infalling light in the visible spectrum, so this wouldn't help the citizens of Schilda very much. (This isn't obvious if you look into a mirror but if you hold two mirrors opposite to each other, you might notice the reflection getting weaker, also, probably getting a little blue/green tint which is from the glass not being perfectly transparent.)

The Schildbürger were on my mind when I came across this paper by a group of French physicists around Serge Haroche who is known for a series of quantum optics experiments. They developed "diamond-machined copper mirrors coated with superconducting niobium."

If you shape the mirrors suitably and arrange them opposite to each other, you can use them to capture photons. And the mind-boggling number that you should take away from here is that the typical decay time of light bouncing back between these mirrors is 0.129 seconds, which corresponds to about 39,000 km light travel time back and forth between the mirrors that have a distance of about 3 cm (a quality factor of more than 10¹⁰).

So they would have to run a little, the citizens of Schilda, but they might finally be able to carry light into the city hall. They would also have to cool their buckets to 0.8 K and it would only work in the infrared, but at least it's a start.

The two mirrors of the photon box at ENS
Photo Credits: Photothèque/LKB/Michel BRUNE
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