Phase-stable generation of polarization-entangled photon triplets by cascaded down conversion

Cascaded down conversion has been demonstrated as an effective method of directly producing three-photon entanglement. Until now, these sources have relied on intricate active phase stabilization schemes. In this work, we present a completely phase-stable source of three-photon entanglement in the polarization degree of freedom. We produce states with over 96% fidelity with an ideal Greenberger-Horn-Zeilinger state. Moreover, we demonstrate the stability of the source over several days without any ongoing optimization.