Nature 477, 73–77 (2011)

Credit: © GETTY

“What's that? Voodoo!” “No, it's feedback.” Such was the conversation between John Lennon and his fellow Beatles when Lennon first produced, accidentally, audio feedback with his acoustic-electric guitar. The year was 1964 and the pronounced note eventually made it into the intro to 'I Feel Fine', now known as the first example of feedback sound used on a rock 'n' roll record.

Clément Sayrin and colleagues have explored feedback to quite a different end. They have implemented a real-time quantum feedback scheme in which a superconducting microwave cavity is prepared and stabilized in a quantum state of a well-defined (and user-defined) number of photons.

Controlling quantum systems is, if not voodoo, at least a tricky business; measurement processes typically provoke random back-action on the system. To tame this randomness, Sayrin et al. deploy a stream of Rydberg atoms through the cavity to gather information about the photon-number distribution therein. This information is fed back to adjust the cavity field. In this way, the target number of photons is reached five times quicker, compared with preparing the state using an optimized trial-and-error method. Through further monitoring of the cavity field, decoherence-induced quantum jumps can also be detected and corrected.