  The measurement of circular dichroism (CD) has widely been exploited to
distinguish the different enantiomers of chiral structures. It has been applied
to natural materials (e.g. molecules) as well as to artificial materials (e.g.
nanophotonic structures). However, especially for chiral molecules the signal
level is very low and increasing the signal-to-noise ratio is of paramount
importance to either shorten the necessary measurement time or to lower the
minimum detectable molecule concentration. As one solution to this problem, we
propose here to use quantum states of light in CD sensing to reduce the noise
below the shot noise limit that is encountered when using coherent states of
light. Through a multi-parameter estimation approach, we identify the ultimate
quantum limit to precision of CD sensing, allowing for general schemes
including additional ancillary modes. We show that the ultimate quantum limit
can be achieved by various optimal schemes. It includes not only Fock state
input in direct sensing configuration but also twin-beam input in
ancilla-assisted sensing configuration, for both of which photon number
resolving detection needs to be performed as the optimal measurement setting.
These optimal schemes offer a significant quantum enhancement even in the
presence of additional system loss. The optimality of a practical scheme using
a twin-beam state in direct sensing configuration is also investigated in
details as a nearly optimal scheme for CD sensing when the actual CD signal is
very small. Alternative schemes involving single-photon sources and detectors
are also proposed. This work paves the way for further investigations of
quantum metrological techniques in chirality sensing.
