  It was theoretically proposed that a forward-biased p-n junction with a
superconducting layer (P-N-S), would produce pure, polarization entangled
photons due to inherent spin-singlet pairing of electrons. However, any
heterostructure interface generically induces Rashba spin-orbit coupling, which
in turn generates a mixed singlet-triplet superconducting order parameter. Here
we study the effect of triplet pairing on the purity of photons produced
through Cooper pair recombination. A unique directional dependence of the state
purity is found for a triplet superconductor with fixed $\vec{d}$: pure,
entangled photons are produced when the photon polarization axis is parallel to
$\vec{d}$. Induced triplet pairing in a singlet superconductor is shown to
degrade the state purity, while induced singlet pairing in a triplet
superconductor is shown to enhance the production of entangled pairs. These
considerations may aid the design of functional devices to produce entangled
photons.

Superconductivity;Mesoscale and Nanoscale Physics