Do exceptional points improve sensing once quantum noise is counted?
In plain words
At special points (exceptional points, where two modes of a lossy system merge) a sensor's response grows faster than normal with the signal, but its noise grows too. Whether there is a net gain is debated.
Precise statement
Sensor operated near an order-n exceptional point, where frequency splitting scales as $\epsilon^{1/n}$. Including quantum noise, Petermann excess noise and gain or loss, compare the quantum Fisher information per unit time and per photon with that of the best Hermitian sensor of equal loss. Answer: yes or no for a fundamental advantage, with conditions.
What would settle it
A general bound on Fisher information near exceptional points, tested by an atomic or optical experiment at the quantum noise limit.
Status in the literature
Theoretical analyses since 2018 (Lau and Clerk; Langbein) find no generic advantage for some schemes and possible gains for nonreciprocal ones.