In the movies, rewinding the film magically mends a shattered cup. Physicists have made a qubit — a quantum switch — live through its evolution the same way, from end to start. This 'anti-qubit' behaves like a positron, which Paul Dirac predicted and Richard Feynman described as an electron moving backward in time. The foundation is the standard model of particles and the all-pervading role of the speed of light.

Reverse evolution erases any random environmental nudges — like a quantum eraser. If you entangle the anti-qubit with an ordinary one (entanglement is an invisible bond where partners instantly sense each other), you get a sensor. It captures not only the strength but also the hidden direction of the weakest magnetic field, squeezing out all possible information down to the last drop. The most unexpected part: the idea of reverse time, once thought of as just a mathematical trick, here works physically for the first time.