High-sensitivity magnetometry with a single atom in a superposition of two circular Rydberg states

Superpositions of states with macroscopically different properties, named 'cats' after Schrödinger's Gedanken experiment, are extraordinarily sensitive probes of their environment. They can be used to investigate the decoherence mechanism and the quantum-to-classical transition1–5, as well as to realize quantum-enabled sensors6 with promising applications. We report here the creation of a 'circular cat', namely an atom in a superposition of two circular Rydberg states with huge opposite magnetic momenta. It is an exquisite probe of the magnetic field, able to perform a single-shot detection of a 13 nT field in only 20 μs. This single-atom cat is as sensitive as a set of 1,800 ordinary atoms, demonstrating the usefulness of Rydberg state engineering for quantum-enabled technologies. An atom in a superposition of two circular Rydberg states with huge opposite magnetic momenta is reported and demonstrated to be an extremely sensitive probe of the magnetic field.