A bathroom mirror lasts for years, even though water and steam hit it every day. But a mirror designed to catch a single barium ion gets damaged by its own light — in complete darkness and deep vacuum.
In an ion trap, mirrors form a cavity: light bounces between them, enhancing the interaction with the ion. For blue light with a wavelength of 493 nanometers (the color of a sea wave), mirrors need dozens of ultra-thin oxide layers. Such coatings work great in air, but in vacuum they start to degrade. The authors measured how quickly losses in the cavity grow: the worse the mirror, the faster the light pulse inside dies out.
It turned out that with short laser pulses or at low power, the mirrors don't degrade. But if you run a powerful beam through the cavity for a long time, losses increase. The cause is not heating (ruled out by calculations) but the deposition on the mirror surface of particles that the laser light knocks off or carries. The good news: the degradation can be partially reversed simply by letting the mirrors rest or by heating them.
That very trap with a barium ion is like a tightrope walker balancing over an abyss. The mirrors are the balancing pole. If the pole starts to bend under its own weight, the act falls apart. Here, the light itself ruins its own tool, but physicists found a way to give the tightrope walker a safety net.
The coating of a mirror for blue light consists of layers tens of nanometers thick — thousands of times thinner than a human hair. If you stack all the layers, you get a film as thick as a soap bubble.