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Conference on Lasers and Electro-Optics

Non-phase matched spontaneous parametric down conversion in ultra-thin lithium niobate

Cameron Okoth, Tomás Santiago, Andrea Cavanna, Maria V. Chekhova

Published Jan 1, 2019• 0 citations
Publisher

Abstract

We report the generation of entangled photon pairs via type-0 spontaneous parametric down conversion in ultra-thin lithium niobate in which momentum between the pump photon and daughter photons is not conserved.
DOI
10.1364/cleo_at.2019.jtu2a.44

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Entangled photon pairs play a major role in various modern technologies such as quantum imaging, communication, and computing. Conventional photon-pair sources are often based on spontaneous parametric down-conversion in bulk nonlinear crystals. Recent advances have also shown entangled photon-pairs from transition metal dichalcogenide thin-films, however, these materials are not widely available and are not compatible with existing fabrication capabilities. We present a new thin-film lithium niobate source of polarization-entangled photon pairs at the telecom wavelength that requires no additional optical elements for entanglement generation and allows for easy application using the existing lithium niobate fabrication technologies. We demonstrate tunable entanglement generation using the three-fold rotational crystal symmetry of lithium niobate, allowing the generation of different maximally entangled Bell states or completely separable states depending on the polarization of the pump beam.
Spontaneous parametric down-conversion (SPDC) is one of the most efficient sources for quantum emitters, which holds promise for numerous applications in quantum key distribution [1] , quantum computing [2] , and quantum networks [3] . Despite its vast potential yet simple measurement setup, the applicability of the SPDC sources is still limited due to their modest conversion efficiency. Therefore, many attempts have been made to improve the efficiency of the process, but mainly focused on phase-matched regime which relied upon the use of macroscopic birefringent crystals or material engineering. Here, we study the SPDC process at the telecom wavelength range in 200 nm thick thin-film lithium niobate (TFLN). This study not only provides an insight into the two-photon light generation process in the new non-phase-matched regime [4] , [5] also paves the way for the monolithic integration of available SPDC sources in chip-based devices [6] and metasurfaces [7] from nonlinear materials such as barium titanate, and gallium arsenide.
We report, for the first time, the observation of spontaneous parametric down-conversion (SPDC) free of phase matching (momentum conservation). We alleviate the need to conserve momentum by exploiting the position-momentum uncertainty relation and using a planar geometry source, a 6 μm thick layer of lithium niobate. Nonphase-matched SPDC opens up a new platform on which to investigate fundamental quantum effects but it also has practical applications. The ultrasmall thickness leads to a frequency spectrum an order of magnitude broader than that of phase-matched SPDC. The strong two-photon correlations are still preserved due to energy conservation. This results in ultrashort temporal correlation widths and huge frequency entanglement. The studies we make here can be considered as the initial steps into the emerging field of nonlinear quantum optics on the microscale and nanoscale.