High-Brightness Semiconductor Laser-Pumped $1.56~\mu$ m Polarization-Entangled Photon Pairs

Abstract

In this paper, a semiconductor laser-pumped high-brightness $1.56~\mu \text{m}$ polarization-entangled photon pairs with a 30-mm-length PPKTP down-converter inside a Sagnac-type interferometer is firstly demonstrate. Large active-area InGaAs/InP-based single photon detectors and step-index multi-mode fiber collection are attempted to evaluate the brightness and entanglement quality. The maximum spectral brightness is about $1.33\times 10^{5}$ pairs( $\text{s}\ast $ mW $\ast $ nm) $^{-1}$ with a pump power of 200 mW and a spectral bandwidth of 1.34 nm. Under the condition of maximum entanglement brightness, polarization visibility of 96.08%±1.62% is obtained in the diagonal orthogonal basis. The quantum state tomography also shows a high fidelity of 97% for the ideal Bell states. Meanwhile, the CHSH-type Bell inequality is violated with a value of 2.8196±0.0098 ( $96~\sigma$ ). Both the performance in brightness and entanglement quality indicate that the current $1.56~\mu \text{m}$ polarization-entangled source has great potential in free-space quantum key distribution or quantum communication.