Photonics plays a significant role in the emerging quantum information industry, and entangled photon pairs are the workhorse of many key applications in quantum optics, where each application utilizes a certain property of the quantum state.
The development of new effective sources of entangled photon pairs (EPS) is crucial for practical applications in the ever-growing field of quantum technologies, spanning from fundamental research in quantum information science, quantum communications, cryptography, sensing, imaging, and more.
There are several types on entangled photon sources which are based on different quantum states and one important source is based on polarization-entangled photon pairs, which is composed of a superposition of perpendicularly-polarized signal and idler.
Unlike squeezed light, which is in principle achieved by a single-pass in a nonlinear gain medium (such as a ppKTP crystal), polarization-entanglement requires the addition and subtraction of photons, for example by passing through more than one crystal. A recent review paper by the group of Alexander Ling  surveys the evolution of sources of polarization-entangled photon pairs throughout the years, as well as their possible applications in quantum communications and cryptography.
Many of the different sources shown in this review paper rely on the ppKTP crystals that we produce at Raicol – mainly type-0 and type-II phase-matched crystals. However, the sources differ from one another in the way the crystal is utilized.
For example, in a recent quantum cryptography experiment by the University of Science and Technology of China (USTC) , a polarization-entangled photon source, based on type-II ppKTP crystal placed inside a Sagnac interferometer, was used to demonstrate quantum cryptography over a distance of 1,120km. Similarly, a different bell-state can also be generated using the same Sagnac configuration, with a type-0 crystal placed instead of the type-II.
Polarization-entanglement from a Sagnac interferometer, adapted from Ref .
A different approach for a source of polarization entanglement, utilized by the group of Ling , is birefringent splitting and recombination of photon pairs. In this scheme, photon pairs are generated from two spatially-separated pump beams (where one pair experiences polarization rotation) and the beams are recombined using a birefringent material. Compensation crystals are also used to match the temporal delay between the perpendicular pairs.
Birefringent splitting and recombination of beams, adapted from Ref .
A third approach, which is currently being explored in Raicol, is the generation of polarization entanglement through domain engineering of the nonlinear crystal, as was shown by Kuo . In this work, the researchers have designed a nonlinear crystal that can perform two simultaneous type-II processes, and showed the polarization-dependent coincidence between the two photons.
Domain-engineered source of orthogonal pairs, adapted from Ref .
All of these novel methods are part of a general effort to improve sources of polarization-entangled photons, which were initially demonstrated at the intersection of two polarization cones in a type-II BBO crystal . This process relies on a non-colinear interaction, therefore rendering only a small portion of the down-converted light to be used.
As the world leader in Quasi-phase matching solutions, Raicol quantum is proud to be part of the next quantum revolution with our advanced periodically polled crystals. For more information: www.raicol-quantum.com
 A. Anwar et al. “Entangled photon-pair sources based on three-wave mixing in bulk crystals”, Review of Scientific Instruments 92, 041101 (2021).
 J. Yin et al. “Entanglement-based secure quantum cryptography over 1,120 kilometres”, Nature 582, 501-505 (2020).
 A. Lohrmann et al. “Broadband pumped polarization entangled photon-pair source in a linear beam displacement interferometer”, Appl. Phys. Lett. 116, 021101 (2020).
 P.S Kuo et al. “Demonstration of a polarization-entangled photon-pair source based on phase-modulated PPLN”, OSA Continuum 3, 2, 295-304 (2020).
 P.G Kwiat et al. “New High-Intensity Source of Polarization-Entangled Photon Pairs”, Phys. Rev. Lett. 75, 4337 (1995).