Nat. Commun. 9, 4320 (2018)

Dispersive measurement is the central idea behind modern sensor devices: the presence of a perturbation changes the original resonant frequency of the electromagnetic mode, which can be detected at very high precision. Recent investigations have suggested that the sensitivity could be further improved in a non-Hermitian system, making use of its spectral degeneracies known as exceptional points. Now Hoi-Kwan Lau and Aashish Clerk have identified the fundamental limit of this approach.

For a system to be non-Hermitian, coupling with an external reservoir is an essential ingredient. The dissipative dynamics is inevitably accompanied by noise, which was neglected in previous studies. By mapping the non-Hermitian sensing set-up to an open quantum system, Lau et al. were able to fully account for the noise effect, and derive the limit that constrains sensing protocols based on non-Hermitian Hamiltonians. Within their model, the improvement arising from exceptional points can be achieved by simply adding gain to conventional set-up without any such points in the system.