  The generation and manipulation of entanglement between isolated particles
has precipitated rapid progress in quantum information processing. Entanglement
is also known to play an essential role in the optical properties of atomic
ensembles, but fundamental effects in the controlled emission and absorption
from small, well-defined numbers of entangled emitters in free space have
remained unobserved. Here we present the control of the spontaneous emission
rate of a single photon from a pair of distant, entangled atoms into a
free-space optical mode. Changing the length of the optical path connecting the
atoms modulates the emission rate with a visibility $V = 0.27 \pm 0.03$
determined by the degree of entanglement shared between the atoms,
corresponding directly to the concurrence $\mathcal{C_{\rho}}= 0.31 \pm 0.10$
of the prepared state. This scheme, together with population measurements,
provides a fully optical determination of the amount of entanglement.
Furthermore, large sensitivity of the interference phase evolution points to
applications of the presented scheme in high-precision gradient sensing.
