Chirality-dependent pathway interference in optimal enantioselective state transfer

We theoretically study enantioselective state transfer (ESST) in a reduced four-level rotational model of a chiral molecule. The cyclic three-level description is reliable only when the microwave bandwidth remains well below the detunings to nearby dipole-allowed rotational transitions. When the bandwidth becomes comparable to the detuning of the nearest relevant additional state, this state can participate in the dynamics, requiring a four-level description. Using a constrained multiobjective quantum optimal-control method, we design microwave control fields in the narrow-band three-level and broadband four-level regimes. The resulting optimized fields achieve final discrimination degrees above 0.999 under four-level propagation in both regimes, while direct four-level optimization permits shorter-duration control by explicitly accounting for transient fourth-state participation. To reveal the underlying coherent mechanism, the transition amplitudes are decomposed into even- and odd-a pathway groups. The even-a contribution is common to the two enantiomers, whereas the odd-a contribution changes sign, resulting in constructive interference for one enantiomer and destructive interference for the other. Lindblad simulations further show that the four-level optimized control fields exhibit enhanced robustness against rotational relaxation owing to their shorter control durations. These results show that broadband four-level optimization can enable faster ESST with improved robustness against dissipative relaxation while preserving the even/odd-a pathway-interference mechanism.