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.
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Li, Y. Be no exception. Nature Phys 14, 1071 (2018). https://doi.org/10.1038/s41567-018-0354-y
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DOI: https://doi.org/10.1038/s41567-018-0354-y