Monolithic Entangled Photon Sources Using Second Order Optical Nonlinearities

Entangled photon sources play a pivotal role in most of the optical systems that utilize quantum effects. Those include quantum key distribution, quantum computing, metrology, sensing and imaging. Photon pair generation using spontaneous parametric down conversion (SPDC) is one of the most popular routes for producing entangled photons. While SPDC has become more popular, the creation of entangled photon pairs in a degree of freedom, such as polarization, is more challenging. The difficulty is related in no small part to the birefringence of the structure where the generation takes place. Birefringence results in distinguishing information that hinders the production of entanglement in the platforms where SPDC has been most popular including ferroelectric crystals.

Despite this limitation, entangled sources have been realized in bulk crystals via well designed interference techniques, which often include additional compensating optics. As the boundaries of optical quantum information science continue to be extended, multi-photon entanglement is now required. Solutions based on SPDC have been attempted, and while some outstanding results have been achieved, the interferometers and compensation procedures become increasingly complex as the size of the entangled state grows. Such scalability and stability issues are directly addressed by integrated entangled photon sources, and while still in their infancy, they promise to help a great deal towards the practical utilization and the production of entanglement on a larger scale.

Semiconductors are an ideal platform to fabricate such integrated entangled photon sources. Recently, it was demonstrated that the gallium-arsenide (GaAs) based Bragg refection waveguide (BRW), could efficiently produce photon pairs via SPDC. This platfrom was shown to have a distinct advantage over other semiconductor sources, due largely to its monolithic architecture and its layered epitaxy, which underpins many photonic devices. In this work we demonstrate yet another advantage: the intrinsic capability of the BRW to directly produce polarization entangled photon pairs, without any additional interferometry, spectral filtering, compensation or post-selection. Not only do we show that the BRW can produce entangled photons, but we wish to emphasize that the traditional compensation and interferometric methods used to create entanglement, which presumably would have occurred on chip, may no longer be necessary. Our experimental results confirm that the BRW has the potential to be one of the very first self-contained integrated room temperature resources of entanglement.

In this talk various approaches for utilizing SPDC processes in various integration platforms will be discussed. The root cause of entanglement generation is the lack of birefringence in these material systems, which otherwise makes exact phase matching impossible in bulk crystals. Because of this, cross-polarized photons propagate at almost identical group velocities, making off-chip path compensation unnecessary.