Abstract
We report the realization of a fiber-coupled polarization entangled photon-pair source at 1310 nm based on a birefringent titanium in-diffused waveguide integrated into periodically poled lithium niobate. By making use of a dedicated and high-performance setup, we characterized the quantum properties of the pairs by measuring two-photon interference in both Hong–Ou–Mandel and standard Bell inequality configurations. For the two sets of measurements we obtained interference net visibilities reaching nearly 100%, which represent important and competitive results compared to those for the similar waveguide-based configurations already reported. These results prove the relevance of our approach as an enabling technology for long-distance quantum communication.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Quantum information science often relies on the use of two-level quantum systems (photons, ions, electrons, atoms, etc) to carry the quantum analogue of bits, usually called qubits. In this context, pairs of entangled qubits serve as a major resource for many quantum applications, such as quantum key distribution, relays, processing and repeaters. Regarding entanglement-based quantum communication over long distance, spontaneous parametric down-conversion is the common way to produce polarization-encoded entangled photons. However, today's high-end experiments require brighter sources emitting perfectly entangled photons at a telecom wavelength, associated with narrower bandwidths, in a compact design.
Main results. The aim of this paper is to gather the above-mentioned features in a single source based on a non-linear titanium in-diffused periodically poled lithium niobate waveguide. Using a dedicated high-performance setup, we characterized the quantum properties of the emitted paired photons by measuring two-photon interference in both Hong–Ou–Mandel (HOM) and standard polarization entanglement analysis configurations. For both tests we obtained ultra-high interference pattern visibilities, demonstrating a near-perfect quality entanglement.
Wider implications. This very competitive result, associated with high brightness and practicality, makes this source a promising element for long-distance, telecom-network-based, quantum communication, and highlights the high potential of non-linear integrated optics in this field. An extension of this work would be to make this source compatible with state-of-the-art quantum memories.
Figure. (a) Degenerate spectra out of the source for the two cross-polarized paired photons (H and V). (b) HOM dips, measured at the two users' locations, show near-perfect indistinguishability. This is a key step towards achieving entanglement. (c) Entanglement measurements obtained following the four standard polarization analysis directions. Near-perfect quality entanglement is demonstrated.