  Generating nonclassical states of photons such as polarization entangled
states on a monolithic chip is a crucial step towards practical applications of
optical quantum information processing such as quantum computing and quantum
key distribution. Here we demonstrate two polarization entangled photon sources
in a single monolithic semiconductor waveguide. The first source is achieved
through the concurrent utilization of two spontaneous parametric
down-conversion (SPDC) processes, namely type-0 and type-I SPDC processes. The
chip can also generate polarization entangled photons via the type-II SPDC
process, enabling the generation of both co-polarized and cross-polarized
entangled photons in the same device. In both cases, polarization entanglement
is generated directly on the chip without the use of any off-chip compensation,
interferometry or bandpass filtering. This enables direct, chip-based
generation of both Bell states $(|H,H\rangle+|V,V\rangle)/\sqrt{2}$ and
$(|H,V\rangle+|V,H\rangle)/\sqrt{2}$ simultaneously utilizing the same pump
source. In addition, based on compound semiconductors, this chip can be
directly integrated with it own pump laser. This technique ushers an era of
self-contained, integrated, electrically pumped, room-temperature polarization
entangled photon sources.
