The degree of optical 2D chirality is quantified by diagonalizing J $_{L}$th. and J $_{R}$th. . For J $_{L}$th. , the eigenstates are |± L 〉 = √$_{B}$ | L 〉 ± √$_{C}$ | R 〉 associated with the eigenvalues λ$_{L}$ ( ± ) = A ± √ ( B · C ). The eigenstates for J $_{R}$th. are obtained by permutation of B and C with consequently λ$_{L}$ ( ± ) = λ$_{R}$ ( ± ). The scalar product 〈 +$_{L}$ |− L 〉 th. = −〈 +$_{R}$ |− R 〉 th. = ( | B | − | C | ) / ( | B | + | C | ) is the eigenstates Stokes parameter S$_{3}$/S$_{0}$ and provides a direct mea- surement of the degree of optical chirality. It also evaluates losses since the non-orthogonality of these two states is related to the necessary non-unitarity of the Jones matrix for planar chirality. We have 〈 +$_{L}$ |− L 〉 fit ≃ 0 . 255 and 〈 +$_{R}$ |− R 〉 fit ≃ − 0 . 277, which, within experi- mental uncertainties, are in good agrement with the theoretical expectations. As shown in the insets of Fig. 2, both eigenstates of each structure (e.g., |± L , R 〉 ) can be represented by two ellipses having the same axis ratio and the same handedness, but rotated 90 ◦ relative to each other. In agreement with the theoretical predictions, these polarization ellipses for |± L 〉 and |± R 〉 are mirror reflections. This behaviour is significantly different from the results obtained with optically active media [12–14, 16–18] where the eigenstates associated with a given enantiomer have opposite handedness [4]. This point, which reflects itself in the symmetry property of chiral Jones matrices, namely J th. L , $_{R}$( L, L ) = J th. L , $_{R}$( R, R ) = A , has far reaching consequences, as pointed out in reference [5]. It implies that a 2D plasmonic spiral mimics a Faraday medium when we reverse the light path and this even if the system, unlike a true Faraday medium, obeys rigorously to the principle of reciprocity [4, 5] (inversely, one can show that equation (1) results from both this requirement and the absence of mirror symmetry). It means that a photon coming from the second side will probe a structure of opposite chirality. After going through the structure and retracing back the light path with a mirror normal to the axis, the polarization state will be different at the end of journey from the initial one. This would be impossible for an optically active medium and is solely due to planar chirality. To summarize, our results therefore demonstrate that 2D chirality is possible in the visible domain in the absence of optical activity and add another element to the promising plasmonic toolkit. We apply a procedure similar to the one considered in [26, 27] in order to record the Mueller matrix: a collimated laser beam at λ = 785 nm is focussed normally on the struc- IV. APPENDIX A: POLARIZATION TOMOGRAPHY SETUP. 8