Light bends around massive bodies: this has been known since Einstein's time. But what if gravity itself is not a smooth fabric of space but a stream of tiny carrier particles? Then a beam passing near a particle should not just bend but enter into a special quantum relationship with it.
This relationship is called entanglement. Two pianos in different rooms: you press a key on one, and a string on the other responds instantly, without any wires. Similarly, a particle's spin and a photon's polarization become a single whole if quantum gravity acts between them. The authors of the study have figured out how to reliably detect such a connection. They propose measuring Stokes parameters — something like "fingerprints" of a light wave that reveal exactly how it oscillates.
To see the weak signal, the photon is mixed with a reference beam, like amplifying a quiet voice by directing it into the same microphone as a loud tuning fork. By varying the phase of this tuning fork, even barely noticeable quantum correlations can be pulled out of the noise. Calculations show that for a non-ideally entangled state born from gravity, the proposed criterion yields a value of minus 0.052. This is like a negative bank balance that clearly says: "Yes, there is non-classical correlation here."
The criterion works when the overlap of the two photon states lies in the range from 0.71 to 1. If an experiment confirms these numbers, it would be the first laboratory proof that gravity obeys the laws of the quantum world.
The Stokes parameters used for detection were invented back in the 19th century to describe ordinary light. Now they might help uncover quantum gravity.
In Hannu Rajaniemi's novel The Quantum Thief, gravity and quantum information are woven into a technology that allows control of spacetime.