On-demand generation of all four Bell states using a single PPKTP entangled photon source

Abstract

We present a compact, automated, high-brightness entangled photon source capable of generating all four Bell states with high fidelity. The system utilizes a type-0 quasi-phase-matched PPKTP crystal embedded within a polarization Sagnac interferometer. We introduce a switching scheme based on the controlled, motorized translation of the nonlinear crystal. This device is capable of generating any one of the Bell states on-demand. Experimentally, we demonstrate that translating the crystal from the interferometer's balanced position repeatedly toggles the state between $|\phi^+ \rangle$ and $|\phi^- \rangle$ (as well as $|\psi^+ \rangle$ and $|\psi^- \rangle$) at regular intervals of $122 \pm 14 ~\mu m$. Subsequently, a half-wave plate (HWP) in the idler arm transitions between the quantum states $|\phi^{\pm}\rangle$ and $|\psi^{\pm}\rangle$. While the non-collinear geometry imposes an upper limit on the translation range as verified via EMCCD imaging, the source however, displays very little change of intensity in the operational window. State purity and entanglement are certified through quantum state tomography (QST), visibility measurements, Bell state measurements (BSM), and CHSH inequality violations, confirming that the source is robust and provides a repeatable, high-fidelity output.