Highly nondegenerate polarization-entangled photon pairs produced through noncritical phasematching in single-domain KTiOPO$_4$

TL;DR

This paper demonstrates a stable, low-temperature-sensitivity source of highly nondegenerate polarization-entangled photon pairs using noncritical phase matching in single-domain KTP, suitable for long-distance quantum communication.

Contribution

It introduces a simpler, more stable method for generating highly nondegenerate entangled photons via noncritical phase matching in single-domain KTP, reducing temperature sensitivity.

Findings

Achieved high entanglement visibility (>98%) over a 75°C temperature range.

Successfully transmitted idler photons through 62km and 93km telecom fibers with high raw visibility.

Maintained entanglement quality during long-distance fiber transmission.

Abstract

Photon-pair sources are useful for entanglement distribution. The most mature of these are spontaneous parametric downconversion (SPDC) sources, most of which achieve phasematching via engineering the domains in poled crystals or the angle between the optic axis and the pump beam. For multi-channel entanglement distribution of photon pairs, where one photon is transmitted through free-space and the other photon is transmitted through fiber, it is beneficial to use highly nondegenerate photon-pair sources. The currently accepted approach in such sources is quasi-phasematching. In this paper, a simpler, more stable alternative is presented for producing highly nondegenerate photon pairs. A source of polarization-entangled photon pairs with low temperature sensitivity based on noncritical phasematched (NCPM) SPDC in single-domain potassium titanyl phosphate (KTP) was demonstrated. Over a crystal temperature range of $75^\circ$C, the center wavelength of the idler photons was observed to change by $10.8$nm while the average entanglement visibility was maintained above $98\%$. With the signal photons detected locally, the idler photons were transmitted through 62km and 93km of deployed telecom fibers with average raw visibilities of $98.2(1)\%$ and $95.6(3)\%$ respectively.