Dynamical time-reversal symmetry breaking via orbital-resonant light–matter coupling

Breaking time-reversal symmetry (TRS) without magnetism represents a frontier in light–matter physics, enabling optical control of quantum phases and nonreciprocal transport in centrosymmetric materials. Here we identify a microscopic route by which TRS can be dynamically broken in non-magnetic cubic lattices under steady-state illumination. When resonant optical driving couples orbital multiplets of different symmetries—such as E_g​ and T_{2g}​—spin–orbit coupling introduces complex phase factors in the intersite transitions, generating an axial T_{1g}​ pseudovector that acts as a dynamical analogue of magnetization. In an ideal cubic crystal, these local pseudovectors cancel by symmetry; however, a weak anisotropy, such as a Jahn–Teller distortion or strain, selects a preferred axis and yields a finite macroscopic signal. The resulting antisymmetric component of the optical susceptibility leads to circular-polarization-dependent responses, revealing a light-induced, time-reversal-odd state without static magnetism. This mechanism provides an interesting pathway to magneto-optical and nonreciprocal responses in correlated materials combining spin–orbit coupling and orbital degeneracy.