A wavelength-tunable fiber-coupled source of narrowband entangled photons

TL;DR

This paper presents a highly bright, tunable fiber-coupled source of polarization-entangled photons with excellent entanglement quality, achieved through optimized crystal and setup design, suitable for quantum communication applications.

Contribution

Introduces a wavelength-tunable, fiber-coupled entangled photon source with significantly improved spectral brightness and entanglement quality using a long PPKTP crystal and optimized coupling.

Findings

Spectral brightness of 273,000 pairs/(s mW nm)

Two-photon state tangle of T=0.987

Wide wavelength tunability without loss of entanglement

Abstract

We demonstrate a wavelength-tunable, fiber-coupled source of polarization-entangled photons with extremely high spectral brightness and quality of entanglement. Using a 25 mm PPKTP crystal inside a polarization Sagnac interferometer we detect a spectral brightness of 273000 pairs/(s mW nm), a factor of 28 better than comparable previous sources while state tomography showed the two-photon state to have a tangle of T=0.987. This improvement was achieved by use of a long crystal, careful selection of focusing parameters and single-mode fiber coupling. We demonstrate that, due to the particular geometry of the setup, the signal and idler wavelengths can be tuned over a wide range without loss of entanglement.