  Small solid state qubits, most prominently single spins in solids, can be
remarkable sensors for various physical quantities ranging from magnetic fields
to temperature. They package the performance of their bulk semiconductor
counterparts into a nanoscale device, sometimes as small as a single atom. This
review is a minimalist introduction into this concept. It gives a brief summary
of quantum coherence, Ramsey spectroscopy and a derivation of the "standard
quantum limit" of the sensitivity that a single-qubit sensor can reach. It goes
on to discuss the surprising improvement that dynamical decoupling has brought
about and concludes with an outlook to the major frontiers of the field.
