Beta-Barium Borate Non-Linear Optical Crystal for Entangled photon generation (405nm to 810nm but can be used for novel 425nm to 850nm). The crystal is 3.04mm wide and 4.55mm long (a custom boule for integrated fiber optics source).

Length: 4.55mm, width 3.04mm

Spontaneous Parametric Down-Conversion (SPDC) using this crystal are designed for SHG but is tested for generating Type-I 810nm to 1550 nm SPDC emission when pumped with a 405nm to 775 nm laser when using a seed laser. This can also generate a broadband light source for continuous-variable (CV) entanglement resources - Ref [3]

Indeed, using a 775nm to 850nm seed laser can allow for simultaneous broad-band PSDC photon generation, with a signal at 810nm and an idler up to 1550nm allowing for very interesting communications protocols over fiber infrastructure and free-space. 

Please view picture for details and to study how to open the crystal as refunds cannot be accepted. This is a rare and fragile item so please consider.

Used with a slight crack on the bottom facet but otherwise clear. Has been tested extensively and is used as a bright light source for entangled 810nm photons using a 405nm source. A one of a kind crystal used in quantum optics labs using a previous testbench (Not included but refer to last 2 pictures to judge) 

Again see pictures for details of its use in the experiment. Used for direct SPDC and also 810nm seed-assisted. Can be used for frequency doubling also but untested by me. Given the size of the crystal can make for a good bright source in my opinion. 

The experimental results demonstrate the potential of non-linear optical SPDC sourced polarization states of quantum entanglement distributed as a resource in fibered networks 

as seen here: https://www.youtube.com/watch?v=wFGMRyKXDy0&feature=youtu.be

References 

[1] Polarization Characterization of BBO and ppKTP SPDC-based Entanglement Light Sources John Campbell John Dooley, Bryan Hennelly https://phantomsfoundation.com/QUANTUM/2024/Abstracts/2024_Campbell_John_29.pdf

[2] Kwiat, P. G., Mattle, K., Weinfurter, H., Zeilinger, A., Sergienko, A. V., & Shih, Y. (1995). New high-intensity source of polarization-entangled photon pairs. Physical Review Letters, 75(24), 4337-4341. 

[3] Valentina Parigi, "Continuous variables quantum complex networks," Laboratoire Kastler Brossel – Sorbonne Université, Paris, 2023. 

[4] . Xavier GB, De Faria GV, Temporão GP, Von der Weid JP. Full polarization control for fiber optical quantum communication systems using polarization encoding. Opt. Express. 2008