Entanglement-based single-shot detection of a single magnon with a superconducting qubit.

The recent development of hybrid systems based on superconducting circuits provides the possibility of engineering quantum sensors that exploit different degrees of freedom. Quantum magnonics, which aims to control and read out quanta of collective spin excitations in magnetically ordered systems, provides opportunities for advances in both the study of magnetism and the development of quantum technologies. Using a superconducting qubit as a quantum sensor, we report the detection of a single magnon in a millimeter-sized ferrimagnetic crystal with a quantum efficiency of up to 0.