  Polarized electron-deuteron DIS with spectator proton tagging offers a way of
measuring the neutron spin structure functions with maximal theoretical control
of nuclear effects. We calculate the nuclear structure factors in the
longitudinal double-spin asymmetries using methods of light-front nuclear
structure. A unique feature of the spin-1 system is that spin asymmetries can
be formed either relative to the cross section in all three spin states
($\lambda_d = \pm 1, 0$) or in the two maximum-spin states only ($\pm 1$,
involving tensor polarization). We find that the two-state deuteron spin
asymmetry at small spectator proton momenta permits accurate extraction of the
neutron structure function $g_{1n}$. Such measurements could be performed at a
future electron-ion collider (EIC) with polarized deuteron beams and suitable
forward detectors.
