A thin film of a special crystal (for instance, molybdenum disulfide) sits on a microscope stage. The temperature is just above the point where the material becomes a superconductor. At this moment, a hidden life begins inside it: pairs of electrons flicker into and out of existence — fluctuating Cooper pairs (brief couplings of two electrons). They act like invisible conductors that momentarily change the rules for light.
Light passing through such a film behaves strangely. Its waves can be absorbed differently depending on direction: in one direction the beam fades more than in the other. This resembles a street where cars are allowed to drive only eastward, while the westward route is closed. Physicists call this nonreciprocal dichroism (direction-dependent absorption of light). Another effect, gyrotropic birefringence, is similar to how light splits into two beams in some crystals, but here the splitting depends on the wave's rotation direction.
The biggest surprise is how absorption behaves as light frequency changes. It isn't monotonic; instead, it peaks when the frequency matches the decay rate of those short-lived pairs. It's like a pendulum swinging most strongly when pushes match its natural rhythm. As the temperature approaches the critical point of the superconductor, the effect sharply intensifies, like an echo growing louder at the very edge of silence.
Such sensitivity of light to the hidden life of electrons opens a tantalizing prospect. We could create ultra-sensitive sensors that detect the slightest changes in a material by how light passes through it. Or help develop superconducting diodes, where current flows in only one direction. The thin film on the stage is no longer just a sample: it becomes a window into the world of quantum fluctuations (rapid random changes at the microscopic level).
The effect where light is absorbed differently depending on direction is similar to how a siren's sound changes as a car passes by — except here it's not the pitch that changes, but the brightness of the beam.
In Peter Watts's novel Blindsight, alien creatures use asymmetry in physical laws to build biological structures — something like that might underlie future superconducting diodes.