Colloquium : Quantum limits to the energy resolution of magnetic field sensors

Magnetometry, that is, the measurement of magnetic fields, has applications that range from brain imaging to exploration of the outer Solar System. The best magnetometry technologies achieve a sensitivity close to Planck's constant, the number that appears in the Heisenberg uncertainty relation. This Colloquium reviews what is known about quantum mechanical limits on field sensing, and identifies new sensing approaches that may break the current impasse in magnetic sensitivity.