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.

Coherent light−matter interfaces are key elements for many quantum information processing architectures. Rare-earth ion (REI)-containing systems featuring narrow inter-configurational f−f transitions are suitable for creating such interfaces. Recently, narrow homogeneous linewidths (Γ h ) and long nuclear spin lifetimes (T 1spin ) associated with two Eu 3+ complexes featuring the 5 D 0 → 7 F 0 f−f transition have been reported, elucidating that REI-molecule-based coherent light−matter interfaces can be obtained. In this study, we report a homogeneous linewidth of 2.8 MHz at 4.2 K, corresponding to an optical coherence lifetime (T 2opt ) of 114 ns (0.11 μs), associated with the 5 D 0 → 7 F 0 transition of the prototypical charge neutral Eu 3+ complex�[Eu(trensal)]. Moreover, we have observed long nuclear spin lifetimes (T 1spin ) up to 460 ± 80 s at 4.2 K for the complex and efficient optical pumping of hyperfine levels of the 5 D 0 ground state. The results presented in this study indicate that narrow homogeneous linewidths and long nuclear spin lifetimes could be a generic property of molecular Eu 3+ complexes featuring the 5 D 0 → 7 F 0 transition.