  We report on the first measurement of the beam-spin asymmetry in the
exclusive process of coherent deeply virtual Compton scattering off a nucleus.
The experiment used the 6 GeV electron beam from the CEBAF accelerator at
Jefferson Lab incident on a pressurized $^4$He gaseous target placed in front
of the CEBAF Large Acceptance Spectrometer (CLAS). The scattered electron was
detected by CLAS and the photon by a dedicated electromagnetic calorimeter at
forward angles. To ensure the exclusivity of the process, a specially designed
radial time projection chamber was used to detect the recoiling $^4$He nuclei.
We measured beam-spin asymmetries larger than those observed on the free proton
in the same kinematic domain. From these, we were able to extract, in a
model-independent way, the real and imaginary parts of the only $^4$He Compton
form factor, $\cal H_A$. This first measurement of coherent deeply virtual
Compton scattering on the $^4$He nucleus, with a fully exclusive final state
via nuclear recoil tagging, leads the way toward 3D imaging of the partonic
structure of nuclei.
