Dual wavelength source of entanglement for space quantum communication

TL;DR

This paper presents a stable, dual-wavelength entangled photon source at 810nm and 1550nm, optimized for hybrid quantum communication networks combining fiber and free-space links.

Contribution

The authors demonstrate a compact, phase-stable entangled photon source with high brightness and efficiency, suitable for space quantum communication.

Findings

Spectral brightness of 4800 pairs/s/mW/GHz

Fiber coupling efficiencies over 0.48 at both wavelengths

High entanglement visibilities of 0.995 and 0.991

Abstract

We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement. We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.