Mechanistic Explanation of Circular Dichroism
Some part of a mechanical macroscopic explanation, acoustical activity where mechanical transverse oscillation direction is being rotated, is described in

Frenzel, T., Köpfler, J., Jung, E. et al. Ultrasound experiments on acoustical activity in chiral mechanical metamaterials. Nat Commun 10, 3384 (2019).

The notion “activity” refers the rotation of the linear polarization axis of a transversely polarized mechanical wave. The rotation angle is proportional to the propagation distance and does not depend on the orientation of the incident linear polarization. It has nanometer precision under time-harmonic excitation at ultrasound frequencies in the range from 20 to 180 kHz. Polarization rotations are as large as 22° per unit cell.
Add a net neutral electrical charge distribution to the components of the metamaterial where the negative charge distribution has a different elastic or inertial nature than the positive charge distribution to get a circularly polarized electromagnetic wave.
A quantum mechanical explanation was requested. Just add stochastic bias to the mechanical input so that only inputs of at least a minimum value get statistical significance. This makes the setup equivalent to quantum mechanics.