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arXiv:2109.00690 · Quantum Science and Technology

Broadband diffraction of correlated photons from crystal superlattices

Zi S. D. Toa, Anna V. Paterova, Leonid A. Krivitsky

Published Oct 27, 2021• 2 citations
arXivHTMLPDF

Abstract

Abstract Sources of broadband quantum correlated photons present a valuable resource for quantum metrology, sensing, and communication. Here, we report the generation of spectrally broadband correlated photons from frequency nondegenerate spontaneous parametric down-conversion in a custom-designed lithium niobate superlattice. The superlattice induces a nonlinear interference between the pump, signal and idler, resulting in an experimentally observed comb-like emission spanning 0.060 μm and 1.4 μm of spectral bandwidth at 0.647 μm and 3.0 μm wavelengths, respectively. While this broad mid-infrared bandwidth is attractive to quantum metrology and sensing due to the enablement of fast spectral multiplexing for data acquisition, the comb-like structure, achieved without an input frequency comb, offers targeted frequencies for quantum communication applications. In addition to useful technological applications, our concept offers an interesting analogy between optical diffraction in quantum and classical optics.

DOI
10.1088/2058-9565/ac33b9

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Abstract Sources of broadband quantum correlated photons present a valuable resource for quantum metrology, sensing, and communication. Here, we report the generation of spectrally broadband correlated photons from frequency nondegenerate spontaneous parametric down-conversion in a custom-designed lithium niobate superlattice. The superlattice induces a nonlinear interference between the pump, signal and idler, resulting in an experimentally observed comb-like emission spanning 0.060 μm and 1.4 μm of spectral bandwidth at 0.647 μm and 3.0 μm wavelengths, respectively. While this broad mid-infrared bandwidth is attractive to quantum metrology and sensing due to the enablement of fast spectral multiplexing for data acquisition, the comb-like structure, achieved without an input frequency comb, offers targeted frequencies for quantum communication applications. In addition to useful technological applications, our concept offers an interesting analogy between optical diffraction in quantum and classical optics.
Broadband quantum light is a vital resource for quantum metrology and spectroscopy applications such as quantum optical coherence tomography or entangled two photon absorption. For entangled two photon absorption in particular, very high photon flux combined with high time-frequency entanglement is crucial for observing a signal. So far these conditions could be met by using high power lasers driving degenerate, type 0 bulk-crystal spontaneous parametric down conversion (SPDC) sources. This naturally limits the available wavelength ranges and precludes deterministic splitting of the generated output photons. In this work we demonstrate an integrated two-colour SPDC source utilising a group-velocity matched lithium niobate waveguide, reaching both exceptional brightness 1.52⋅10 6 pairssmWGHz and large bandwidth (7.8 THz FWHM) while pumped with a few mW of continuous wave (CW) laser light. By converting a narrow band pump to broadband pulses the created photon pairs show correlation times of Δ τ ≈ 120 fs while maintaining the narrow bandwidth Δ ω p ≪ 1 MHz of the CW pump light, yielding strong time-frequency entanglement. Furthermore our process can be adapted to a wide range of central wavelengths.
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Quantum-enhanced optical systems operating within the 2- to 2.5-μm spectral region have the potential to revolutionize emerging applications in communications, sensing, and metrology. However, to date, sources of entangled photons have been realized mainly in the near-infrared 700- to 1550-nm spectral window. Here, using custom-designed lithium niobate crystals for spontaneous parametric down-conversion and tailored superconducting nanowire single-photon detectors, we demonstrate two-photon interference and polarization-entangled photon pairs at 2090 nm. These results open the 2- to 2.5-μm mid-infrared window for the development of optical quantum technologies such as quantum key distribution in next-generation mid-infrared fiber communication systems and future Earth-to-satellite communications.
Photon pairs from spontaneous parametric down-conversion (SPDC) are important for a wide range of quantum optics experiments with spectral properties such as their bandwidths often being a crucial concern. Here we show the generic existence of particular phase-matching conditions in quasi-phase-matched KTP, MgO:LN, and SLT crystals that lead to ultra-broadband, widely nondegenerate photon pairs. It is based on the existence of group-velocity-matched, far apart wavelength pairs and, for 2 mm long crystals, results in SPDC bandwidths between 15 and 25 THz for photon pairs with the idler photon in the technologically relevant mid-IR range of 3-5 μm and the signal photon in the near-IR below 1100 nm. We experimentally demonstrate this type of broadband phase matching in ppKTP crystals for photon pairs centered at 800 and 3800 nm and measure a bandwidth of 15 THz. This novel method of generating broadband photon pairs will be highly beneficial for SPDC-based imaging, spectroscopy, refractometry, and optical coherence tomography with undetected mid-IR photons.
Broadband nonlinear optical devices play a critical role in both classical and quantum optics. Here, we design and fabricate a 6.82-mm-long step-chirped periodically poled lithium niobate (CPPLN) waveguide on lithium niobate on insulator, which enables quasi-phase matching over a broad bandwidth for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC). The SHG achieves an average efficiency of 91%/W/cm 2 over the first-harmonic wavelength range of 1510 nm–1620 nm, paving the way for realizing SPDC across a wide range of pump wavelengths. For SPDC, by tuning the pump wavelength to 775 nm, 780 nm, and 785 nm, we achieve broadband photon-pair generation with a maximum full bandwidth and brightness up to 99 THz (846 nm) and 21 MHz/mW/nm, respectively. Our findings provide an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum metrology.