  Quantum metasurfaces, i.e., two-dimensional subwavelength arrays of quantum
emitters, can be employed as mirrors towards the design of hybrid cavities,
where the optical response is given by the interplay of a cavity-confined field
and the surface modes supported by the arrays. We show that, under external
magnetic field control, stacked layers of quantum metasurfaces can serve as
helicity-preserving cavities. These structures exhibit ultranarrow resonances
and can enhance the intensity of the incoming field by orders of magnitude,
while simultaneously preserving the handedness of the field circulating inside
the resonator, as opposed to conventional cavities. The rapid phase shift in
the cavity transmission around the resonance can be exploited for the sensitive
detection of chiral scatterers passing through the cavity. We discuss possible
applications of these resonators as sensors for the discrimination of chiral
molecules.
