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arXiv:1906.10344

Joint spectral amplitude analysis of SPDC photon pairs in a multimode ppLN ridge waveguide

Ramesh Kumar, Joyee Ghosh

Published Jun 25, 2019• 2 citations
arXivHTMLPDF

Abstract

In this paper, we study the possible parametric down conversion processes in a periodically poled customized Lithium Niobate (LiNbO3) ridge waveguide. Our analysis of spontaneous parametric down-conversion (SPDC), first, with a Gaussian pump beam mode and second, with an anti-symmetric Hermite-Gaussian HG (1,0) pump beam mode predict the possible down conversion processes in each case. From our JSA analysis, it is evident that the generated photons pairs in all these cases are negatively correlated and have orthogonal polarizations. In case of the former, degenerate photon pairs are emitted at 1550 nm with the highest efficiency in the fundamental waveguide mode. While, in case of the latter, non-degenerate photon pairs in different higher order spatial modes are generated. Such photons, thus, have multiple degrees of freedom, like polarization and spatial modes, which can be further harnessed towards hyper-entangled photons for quantum information applications.

DOI
10.48550/arxiv.1906.10344

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In this paper, we study the possible parametric down conversion processes in a periodically poled customized Lithium Niobate (LiNbO3) ridge waveguide. Our analysis of spontaneous parametric down-conversion (SPDC), first, with a Gaussian pump beam mode and second, with an anti-symmetric Hermite-Gaussian HG (1,0) pump beam mode predict the possible down conversion processes in each case. From our JSA analysis, it is evident that the generated photons pairs in all these cases are negatively correlated and have orthogonal polarizations. In case of the former, degenerate photon pairs are emitted at 1550 nm with the highest efficiency in the fundamental waveguide mode. While, in case of the latter, non-degenerate photon pairs in different higher order spatial modes are generated. Such photons, thus, have multiple degrees of freedom, like polarization and spatial modes, which can be further harnessed towards hyper-entangled photons for quantum information applications.
In this paper, we propose the generation of hyperentangled photon pairs using type-II spontaneous parametric down-conversion in a biperiod, 5% MgO-doped lithium-niobate ridge waveguide. The photon pairs are entangled in spatial mode and polarization degrees of freedom. A pulsed laser source at 687 nm, having an antisymmetric Hermite-Gaussian HG (1,0) spatial mode, is considered as the pump. Down-converted photon pairs, with the signal being at the telecommunication wavelength of 1550 nm, are characterized through a joint spectral amplitude analysis of their biphoton state.
Abstract We study modal characteristics of a customized ridge waveguide in lithium niobate designed to generate twin photons at telecom wavelength. A quantum analysis of SPDC predicts the possible down conversion processes and optimizes the input beam parameters for fundamental mode emission. Further, a joint spectral amplitude analysis ensures that the parameters are ambient for generating frequency-correlated photons throughout the phase-matching spectrum. Moreover, negatively correlated degenerate signal/idler photon pairs in the fundamental mode emission occurs at 1550 nm. A key result of our paper is an explicit expression and plot for comparing the generated signal power in case of waveguides as compared to bulk crystals. Our calculations clearly show a dependence of ℒ 3/2 for a waveguide of length ℒ , compared to a linear dependence in case of a bulk crystal.
We study modal characteristics of a customized ridge waveguide in Lithium Niobate designed to generate twin photons at telecom wavelength. A quantum analysis of SPDC predicts the possible down conversion processes and optimizes the input beam parameters for fundamental mode emission. Further, a Joint Spectral Amplitude (JSA) analysis ensures the generation of signal/idler at 1550 nm in the customized LN waveguide. A calculation of the parametric down conversion (PDC) signal power shows a dependence of L^(3/2) for a waveguide compared to a linear dependence in case of bulk crystal.
Generation of hyperentangled photon pairs is investigated based on the lithium niobate straight waveguide. We propose to use the nonlinear optical process of spontaneous parametric down-conversion (SPDC) and a well-designed lithium niobate waveguide structure to generate a hyperentangled (in the polarization dimension and the energy-time dimension) two-photon state. By performing numerical simulations of the waveguide structure and calculating the possible polarization states, joint spectral amplitudes (JSA), and joint temporal amplitudes (JTA) of the generated photon pair, we show that the generated photon pair is indeed hyperentangled in both the polarization dimension and the energy-time dimension.
We report the generation of high-purity correlated photon-pairs and polarization entanglement in a 1.5 μm telecommunication wavelength-band using cascaded χ((2)):χ((2)) processes, second-harmonic generation (SHG) and the following spontaneous parametric down conversion (SPDC), in a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. By using a PPLN module with 600%/W of the SHG efficiency, we have achieved a coincidence-to-accidental ratio (CAR) higher than 4000 at 7.45×10(-5) of the mean number of the photon-pair per pulse. We also demonstrated that the maximum reach of the CAR was truly dark-count-limited by the single-photon detectors used here. This indicates that the fake (noise) photons were negligibly small in this system, even though the photon-pairs, the Raman noise photons, and the pump photons were in the same wavelength band. Polarization entangled photon pairs were also generated by constructing a Sagnac-loop-type interferometer which included the PPLN module and an optical phase-difference compensator to observe maximum entanglement. We achieved two-photon interference visibilities of 99.6% in the H/V basis and 98.7% in the diagonal basis. The peak coincidence count rate was approximately 50 counts per second at 10(-3) of the mean number of the photon-pair per pulse.