  Partial polarization is the manifestation of the correlation between two
mutually orthogonal transverse field components associated with a light beam.
We show both theoretically and experimentally that the origin of this
correlation can be purely quantum mechanical. We perform a two-path first-order
(single photon) interference experiment and demonstrate that the degree of
polarization of the light emerging from the output of the interferometer
depends on path distinguishability. We use two independent methods to control
the distinguishability of the photon paths. While the distinguishability
introduced in one of the methods can be erased by performing a suitable
measurement on the superposed beam, the distinguishability introduced in the
other method cannot be erased. We show that the beam is partially polarized
only when both types of distinguishability exist. Our main result is the
dependence of the degree of polarization on the inerasable distinguishability,
which cannot be explained by the classical (non-quantum) theory of light.
