Curator's Take
This theoretical breakthrough offers a clever way to finally settle one of the most contentious debates in molecular spintronics: whether chiral molecules act as coherent quantum spin rotators or simply filter spins incoherently. The researchers propose using the Dzyaloshinskii-Moriya interaction as a "smoking gun" test - it should be dramatically enhanced for coherent spin rotation but completely absent for incoherent filtering, creating a clear experimental signature that existing quantum dot spectroscopy can detect. What makes this particularly exciting is that coherent chirality-induced spin selectivity could open doors to chiral molecules serving as quantum information processing elements, potentially bridging the gap between biological systems and quantum technologies. The predicted sensitivity is well within reach of current experiments, suggesting we may soon have definitive answers about this fundamental quantum phenomenon that has puzzled scientists for over two decades.
— Mark Eatherly
Summary
Whether chirality-induced spin selectivity (CISS) reflects coherent SU(2) spin rotation or incoherent spin-dependent filtering is a central unresolved question in molecular spintronics, with implications ranging from asymmetric chemistry to quantum information. We show that these two scenarios are distinguishable by a sharp symmetry criterion on the superexchange interaction mediated by a chiral molecular bridge. Coherent CISS, implemented as a unitary spin rotation of the tunneling electron, generates a giant Dzyaloshinskii-Moriya (DM) interaction with ratio |D|/JH up to 3, which is two orders of magnitude beyond intrinsic Rashba spin-orbit coupling in Si/SiGe. Incoherent CISS, represented by any Hermitian (non-unitary but spin-diagonal) tunneling matrix, produces D = 0 identically; we prove this as a structural theorem, reinforced by a Lindblad argument that dissipative spin filtering cannot modify virtual-tunneling-mediated superexchange. The DM interaction thus serves as a coherence order parameter, nonzero only when quantum amplitudes for opposite-spin transmission maintain a fixed relative phase. We derive closed-form angular, enantiomeric, and sensitivity signatures and show that the critical coherent rotation angle lies two orders of magnitude below current transport-inferred values and is accessible to existing 10 kHz exchange spectroscopy in gate-defined quantum dots. Five candidate molecules are predicted to exceed this threshold by one to two orders of magnitude even in a conservative interface-amplification scenario. The proposed measurement converts a long-standing transport controversy into a binary spin-qubit experiment with quantum-amplitude resolution.