  Joint analysis of CMB and large-scale structure at high redshifts provide new
and unique windows into unexplored epochs of early structure formation. Here,
we demonstrate how cosmic infrared background and high-redshift galaxies can be
jointly analysed with CMB to probe the epoch of helium reionization ($2<z<4$)
on the light cone using kinetic Sunyaev Zel'dovich tomography. Characterising
this epoch has great potential significance for understanding astrophysics of
galaxy formation, quasar activity and formation of the super-massive black
holes. We find a detection at $8-10\sigma$ can be expected from combinations of
data from CCAT-prime, Vera Rubin Observatory and CMB-S4 in the upcoming years.
