  The demonstration of optical multipath interference from a large number of
quantum emitters is essential for the realization of many paradigmatic
experiments in quantum optics. However, such interference remains still
unexplored as it crucially depends on the sub-wavelength positioning accuracy
and stability of all emitters. We present the observation of controlled
interference of light scattered from strings of up to 53~trapped ions. The
light scattered from ions localized in a harmonic trapping potential is
collected along the ion crystal symmetry axis, which guarantees the spatial
indistinguishability and allows for an efficient scaling of the contributing
ion number. We achieve the preservation of the coherence of scattered light
observable for all the measured string sizes and nearly-optimal enhancement of
phase sensitivity. The presented results will enable realization and control of
directional photon emission, direct detection of enhanced quadrature squeezing
of atomic resonance fluorescence, or optical generation of genuine
multi-partite entanglement of atoms.
