Fig. 1: Schematic diagram of the optical setup.

a The optical processes utilized in the experiment. Left (1): Photoluminescence (PL) with the formation of neutral excitations. An electron from the valence band is excited into the higher spin sublevel of the zero Landau level (0LL) of the conductance band; then the resulting hole relaxes into the state of minimal energy—0LL of heavy holes (hh); an equilibrium electron from the lower spin sublevel of 0LL of the conductance band recombines with this hole; The excited electron on higher spin sublevel and a hole left at the lower spin sublevel after the recombination form a spin exciton (SE). If during the recombination process a part of the energy is redistributed into an excitation in the lower spin state the resulting excitation is spin-magneto-graviton (SMG). Middle (2): Resonant reflection from the unexcited system. From 0LL of hh, an electron is excited into the higher spin sublevel of the conductance band and immediately (phase conserving process) returns back to the valence band. Right (3): Photoinduced resonant reflection. The process is similar to (2), but the electron is transferred in presence of a spin-zero excitation in the ground state. b The scheme of the experimental setup is shown. Pump (LS 2) and probe (LS 1) signals go through the optical scheme into the cryostat and excite the sample. The reflected signal is decomposed into a spectrum on a diffraction grating. For the extended description of the experimental setup and the processes, see in “Methods” section and Supplementary Materials.