Quantum Limits to the Energy Resolution of Field Sensors

We describe a class of quantum sensing limits that – unlike the standard quantum limit and Heisenberg limit – make no reference to particle number. Rather, these “energy resolution limits” constrain the energy resolution per bandwidth, a figure of merit that combines the spatial, temporal, and field resolution in a single number with units of action. Interestingly, known energy resolution limits for dc-SQUIDS, NV centers in diamond, and rubidium atomic-vapor magnetometers converge on the Planck constant, suggesting a more universal limit. We evaluate possible origins of such a technology-spanning energy resolution limit, for example quantum speed limits. Finally, we describe a few newer technologies that are expected to give magnetic energy resolution beyond what is possible with today's highest-performing magnetic sensors.