The electron spins localized at colour centres in diamond can be used as magnetic field sensors, in part because the almost-atomic size of the defect allows them to be packed at a high density that increases sensitivity. However, there is a limit to this optimization because the interactions between closely spaced spins cause decoherence of the ensemble.
Hengyun Zhou and collaborators have now found a way to push the sensitivity past this fundamental limit. They applied a carefully chosen sequence of laser pulses to the spin system, which extended the coherence time beyond the limit set by the interactions. The sensitivity for the magnetic field could be enhanced by 40%, so that weaker fields can be detected.
This work shows how delicately-controlled high-density spin ensembles can be used in quantum metrology and suggests that they can also be employed as sensors of observables other than magnetic fields in the future.