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arXiv:2101.04877 · Physical Review Letters

Ultrabroadband Entangled Photons on a Nanophotonic Chip

Usman A. Javid, Jingwei Ling, Jeremy Staffa, Mingxiao Li, Yang He, Qiang Lin

Published Oct 27, 2021• 142 citations
arXivHTMLPDF

Abstract

The development of quantum technologies on nanophotonic platforms has seen momentous progress in the past decade. Despite that, a demonstration of time-frequency entanglement over a broad spectral width is still lacking. Here we present an efficient source of ultrabroadband entangled photon pairs on a periodically poled lithium niobate nanophotonic waveguide. Employing dispersion engineering, we demonstrate a record-high 100 THz (1.2 μm-2 μm) generation bandwidth with a high efficiency of 13 GHz/mW and excellent noise performance with the coincidence-to-accidental ratio exceeding 10^{5}. We also measure strong time-frequency entanglement with over 98% two-photon interference visibility.

DOI
10.1103/physrevlett.127.183601

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The development of quantum technologies on nanophotonic platforms has seen momentous progress in the past decade. Despite that, a demonstration of time-frequency entanglement over a broad spectral width is still lacking. Here we present an efficient source of ultrabroadband entangled photon pairs on a periodically poled lithium niobate nanophotonic waveguide. Employing dispersion engineering, we demonstrate a record-high 100 THz (1.2 μm-2 μm) generation bandwidth with a high efficiency of 13 GHz/mW and excellent noise performance with the coincidence-to-accidental ratio exceeding 10^{5}. We also measure strong time-frequency entanglement with over 98% two-photon interference visibility.
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Entangled photon pairs generated by a low-power continuous-wave laser could replicate pulsed laser-based tabletop spectroscopy by taking advantage of the inherent quantum correlations between the photons. Much of the current work in the thin-film lithium niobate platform has focused on infrared wavelengths, leaving shorter wavelengths still a largely unexplored space, particularly for spectroscopy. In this work, we have fabricated periodically poled lithium niobate nanophotonic waveguides for entangled photon generation through spontaneous parametric downconversion with a visible pump (406 nm). Characterization of the waveguided pair source confirms the spectral and temporal correlations of energy-time entangled photons with an on-chip pair generation efficiency of (2.3 ± 0.5) × 1011 pairs/s/mW, brightness of (1.6 ± 0.3) × 109 pairs/s/mW/nm, and two-photon interference visibility greater than 99%. With the same material platform, we have also demonstrated second harmonic generation with on-chip powers up to 30 μW and wavelengths as low as 355 nm, demonstrating lithium niobate's potential for ultraviolet nonlinear photonics and frequency doubling in the UV-A spectral region. Through design of larger cross-section waveguides, we have also explored how variations in the lithium niobate thin film thickness can affect quasi-phase matching. To date, this is the first reported demonstration of periodically poled lithium niobate nanophotonic waveguides for spontaneous parametric downconversion at a fully visible pump wavelength (406 nm) as well as second harmonic generation in the UV (355 nm). Future work towards fully on-chip spectroscopy will explore integrating an on-chip Mach-Zehnder interferometer with the entangled photon source.
Photonics offers unique capabilities for quantum information processing (QIP) such as room-temperature operation, the scalability of nanophotonics, and access to ultrabroad bandwidths and consequently ultrafast operation. Ultrashort pulse sources of quantum states in nanophotonics are an important building block for achieving scalable ultrafast QIP; however, their demonstrations so far have been sparse. Here, we demonstrate a femtosecond biphoton source in dispersion-engineered periodically poled lithium niobate nanophotonics. We measure 17 THz of bandwidth for the source centered at 2.09 µm, corresponding to a few optical cycles, with a brightness of 8.8 GHz/mW. Our results open new paths toward realization of ultrafast nanophotonic QIP.