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arXiv:1907.05959 · Optics Letters

Ultra-broadband SPDC for spectrally far separated photon pairs

Aron Vanselow, Paul Kaufmann, Helen M. Chrzanowski, Sven Ramelow

Published Sep 16, 2019• 59 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1364/ol.44.004638

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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 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.
Recent advances of high-quality lithium niobate (LN) on insulator technology have revitalized the progress of novel chip-integrated LN-based photonic devices and accelerated application research. One of the promising technologies of interest is the generation of entangled photon pairs based on spontaneous parametric down-conversion (SPDC) in LNs. In this paper, we investigated, theoretically and numerically, Type II SPDC in two kinds of LNs—undoped and 5-mol% MgO doped LNs. In each case, both non-poled and periodically poled crystals were considered. The technique is based on the SPDC under Type II extended phase matching, where the phase matching and the group velocity matching are simultaneously achieved between interacting photons. The proposed approach has not yet been reported for LNs. We discussed all factors required to generate photon pairs in LNs, in terms of the beam propagation direction, the spectral position of photons, and the corresponding effective nonlinearities and walk-offs. We showed that the spectral positions of the generated photon pairs fall into the mid-infrared region with high potential for free-space quantum communication, spectroscopy, and high-sensitivity metrology. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.999 with proper pump filtering.
Spontaneous Parametric Down-conversion (SPDC) is now routinely used to generate photon pairs for imaging, spectroscopy, and quantum information and metrology. However, the peculiar spectro-spatial properties of SPDC beams are not always known from the largest audience: they can give rise to ring-like beams, depending on the temperature, the wavelengths and pump focusing. This has to be considered for imaging or long-range applications. We will show examples of such beams created in 532-pumped PPLN or PPKTP and 1064-nm-pumped PPLN crystals. With a simple model of noncolinear phase matching we will give physical insight into the phenomena and explain the role of the signal/idler group delay. We will also explain how aperiodic crystals could be useful.
Spontaneous parametric downconversion (SPDC) using periodically poled nonlinear optical crystals under the quasiphase- matching condition has found wide use in quantum optics. High efficiencies and good coupling to single-mode fibers resulted from using channel waveguides in crystals. It is often desirable to have a very narrow bandwidth for the signal and idler photons, but under the typical operating conditions, phase matching dictates the bandwidth of the SPDC to be of the order of <1 nm. This occurs because the co-propagating signal and idler photons are entangled, and an increase of the signal wave-vector is compensated by a decrease of the idler wave-vector. One way to reduce the bandwidth is by forming either external or internal cavities. Additionally, bandwidth reduction is possible without cavities when the signal and idler are counter-propagating, and the changes in the wave-vector with frequency are additive. To accomplish this a domain inversion on the wavelength scale is required. In this work, we experimentally demonstrate SPDC in one-dimensional KTP-based waveguides with sub-micron poling for forward and backward interactions. Some of the spectral features of the generated light are accounted for by mode coupling theory in periodically poled waveguides but other features are as yet not explained.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.