  Incoherent scattering of photons off two remote atoms with a Lambda-level
structure is used as a basic Young-type interferometer to herald long-lived
entanglement of an arbitrary degree. The degree of entanglement, as measured by
the concurrence, is found to be tunable by two easily accessible experimental
parameters. Fixing one of them to certain values unveils an analog to the
Malus' law. An estimate of the variation in the degree of entanglement due to
uncertainties in an experimental realization is given.
