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Journal of the Optical Society of America B

Direct generation of hybrid entangled photon pairs in waveguides

Divya Bharadwaj, Krishnan Thyagarajan

Published Mar 13, 2018• 6 citations
Publisher

Abstract

In this paper, we propose a scheme for direct generation of hybrid spatial modal–polarization entangled photon pairs using a type-II spontaneous parametric down-conversion process and the electro-optic effect in a domain engineered potassium titanyl phosphate nonlinear waveguide. Such hybrid entangled states must find applications in quantum information processing using integrated photonic circuits.

DOI
10.1364/josab.35.000790

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In this paper, we propose a scheme for direct generation of hybrid spatial modal–polarization entangled photon pairs using a type-II spontaneous parametric down-conversion process and the electro-optic effect in a domain engineered potassium titanyl phosphate nonlinear waveguide. Such hybrid entangled states must find applications in quantum information processing using integrated photonic circuits.
We present experimental realization of type-II spontaneous parametric down-conversion in a periodically poled potassium titanyl phosphate (KTiOPO4) nonlinear waveguide. We demonstrate that by careful exploitation of intermodal dispersion in the waveguide it is feasible to produce photon pairs in well defined transverse modes without any additional spatial filtering at the output. Spatial characteristics is verified by measurements of the M2 beam quality factors. We also prepared a postselected polarization-entangled two-photon state shown to violate Bell’s inequality. Similar techniques based on intermodal dispersion can be used to generate spatial entanglement and hyperentanglement.
Integrated optical devices are expected to play a promising role in the field of quantum information science and technology. In this paper we propose a scheme for the generation of nondegenerate, copolarized, modal, and path-entangled photons using a directional coupler and an asymmetric Y\mathsf{Y}Y-coupler geometry in type-0 phase-matched domain-engineered lithium niobate (LN) waveguide. The nonlinearity in LN is tailored in such a way that quasi-phase-matching conditions for two different spontaneous parametric down-conversion processes are obeyed simultaneously, leading to a modal and path-entangled state at the output. Assuming typical values of various parameters, we show, through numerical simulations, that an almost maximally entangled state is achievable over a wide range of waveguide parameters. For the degenerate case, the proposed scheme gives a NOON state for NNN === 2. The generated entangled photon pairs should have potential applications in quantum information schemes and also in quantum metrology. By appropriate domain engineering and component designing, the idea can be further extended to generate hyperentangled and two-photon multipath-entangled states, which may have further applications in quantum computation protocols.
We report on fiber-coupled nonlinear optical waveguides in periodically poled potassium titanyl phosphate (PPKTP), fabricated via femtosecond laser direct writing for efficient quantum light generation through spontaneous parametric down-conversion. The platform achieves high nonlinear conversion efficiency with fiber integration for compact, alignment-free operation. We discuss applications including enhanced performance for low-efficiency parametric processes and high-visibility polarization-entangled photon pair generation. This work demonstrates laser-written PPKTP waveguides as promising building blocks for in-fiber integrated quantum photonic circuits and quantum technologies.
Abstract Polarisation-entangled photon pairs generated through nonlinear optical processes are crucial ingredients for a range of quantum information processing applications, such as quantum sensing and communication. In this work, we report an optimised design of a lithium niobate on insulator waveguide that is capable of directly generating polarisation-entangled photon pairs enabled through two different spontaneous parametric down-conversion processes simultaneously in the telecommunication wavelength band with a single poling period, based on Type II phase matching. The generated photon pairs cover wavelengths in telecom O (1310 nm) and telecom C (1550 nm) bands with the computed spectral brightness of ≈ 2 × 10 9 pairs sec − 1 mW − 1 . We also study the effect of temporal delay on the quality of entanglement, which is minimised by selecting an appropriate length of the waveguide and pump bandwidth. The optimised integrated quantum light source, presented here, eliminates the need for complex domain engineering or intermodal dispersion and will serve as a valuable resource for various quantum-optical experiments.