  Temporally varying electromagnetic media have been extensively investigated
recently to unveil new means for controlling light. However, spin-dependent
phenomena in such media have not been explored thoroughly. Here, we reveal the
existence of spin-dependent phenomena at a temporal interface between chiral
and dielectric media. In particular, we show theoretically and numerically that
due to the material discontinuity in time, linearly polarized light is split
into forward-propagating right-handed and left-handed circularly polarized
waves having different angular frequencies and it the same phase velocities.
This salient effect allows complete temporal separation of the two spin states
of light with high efficiency. In addition, a phenomenon of spin-dependent
gain/loss is observed. Furthermore, we show that when the dielectric medium is
switched again to the original chiral medium, the right- and left-handed
circularly polarized light waves (with different angular frequencies) merge to
form a linearly polarized wave. Our findings extend spin-dependent interactions
of light from space to space-time.
