  Nanophotonic chiral sensing has recently attracted a lot of attention. The
idea is to exploit the strong light-matter interaction in nanophotonic
resonators to determine the concentration of chiral molecules at ultra-low
thresholds, which is highly attractive for numerous applications in life
science and chemistry. However, a thorough understanding of the underlying
interactions is still missing. The theoretical description relies on either
simple approximations or on purely numerical approaches. We close this gap and
present a general theory of chiral light-matter interactions in arbitrary
resonators. Our theory describes the chiral interaction as a perturbation of
the resonator modes, also known as resonant states or quasi-normal modes. We
observe two dominant contributions: A chirality-induced resonance shift and
changes in the modes excitation and emission efficiencies. Our theory brings
new and deep insights for tailoring and enhancing chiral light-matter
interactions. Furthermore, it allows to predict spectra much more efficiently
in comparison to conventional approaches. This is particularly true as chiral
interactions are inherently weak and therefore perturbation theory fits
extremely well for this problem.
