A tuning fork rings with a single note until a tremor of the floor reaches it. A photon in empty space is like such a tuning fork: its polarization can be strictly vertical or horizontal, and that is a pure quantum state. But the space around it is not empty. It is threaded by gravitational waves that stretch and squeeze everything in their path. Decoherence is exactly that moment when the floor tremor starts throwing the tuning fork off pitch.
In earlier work, this process was calculated for a scalar field, that is, for an object that has no direction. A photon is trickier: it has polarization, and you have to make sure the calculation does not depend on how we look at the system. The authors of the new paper got around this difficulty by choosing the gravitational field itself as the clock. This trick is called a relational clock, but the gist is simple: time is set not by an external observer but by the course of events themselves.
The result is an equation that describes how a photon gradually loses its quantum purity. It contains none of the simplifications usually made to keep the calculation manageable: no Markovian approximation, no rotating-wave approximation. This means the equation also applies when the gravitational noise changes rapidly.
Practical benefits will not arrive tomorrow, but they are there. If we manage to measure how the gravitational background erases the quantum purity of a single photon, that will be a test of ideas in quantum gravity. For now, the model is a reminder: even in the emptiest room, the floor always trembles a little.
In cosmological models the temperature of the thermal gravitational background is estimated at 0.9 K, only slightly above the temperature of the cosmic microwave background.