  Lattice QCD calculations of two-nucleon systems are used to isolate the
short-distance two-body electromagnetic contributions to the radiative capture
process $np \to d\gamma$, and the photo-disintegration processes
$\gamma^{(\ast)} d \to np$. In nuclear potential models, such contributions are
described by phenomenological meson-exchange currents, while in the present
work, they are determined directly from the quark and gluon interactions of
QCD. Calculations of neutron-proton energy levels in multiple background
magnetic fields are performed at two values of the quark masses, corresponding
to pion masses of $m_\pi \sim 450$ and 806 MeV, and are combined with pionless
nuclear effective field theory to determine these low-energy inelastic
processes. Extrapolating to the physical pion mass, a cross section of
$\sigma^{lqcd}(np\to d\gamma)=332.4({\tiny \begin{array}{l}+5.4 \\ -
4.7\end{array}})\ mb$ is obtained at an incident neutron speed of $v=2,200\
m/s$, consistent with the experimental value of $\sigma^{expt}(np \to d\gamma)
= 334.2(0.5)\ mb$.
