Demonstration of a polarization-entangled photon-pair source based on phase-modulated PPLN

We develop and demonstrate a source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in domain-engineered, periodically poled lithium niobate (PPLN) at telecom wavelengths. Pumped at 775 nm, this domain-engineered type-II SPDC source produces non-degenerate signal and idler pairs at 1530 nm and 1569 nm. Because of birefringence, the photon pair with horizontally polarized signal and vertically polarized idler has a different phasematching condition than the pair with vertically polarized signal and horizontally polarized idler. Using phase-modulation of the domain structure, we produced a crystal that can simultaneously generate both states in a distributed fashion throughout a single crystal. Performing SPDC using this aperiodically poled crystal, we observed polarization entanglement visibility above 93%. We compare the phase-modulated crystal to other aperiodic structures, including dual-periodically-poled and interlaced biperiodic structures. Entangled photon pairs are important for quantum information networks, where they are used to connect quantum nodes [1] and for quantum communications [2], including teleportation of quantum states [3–5], and quantum key distribution [6,7]. Entangled photons have also been used for tests of Bell’s theorem and local realism [8–10], as well as for generation of certified random numbers [11]. Spontaneous parametric down-conversion (SPDC) is often used to produce the entangled photon pairs in these applications [8]. In SPDC, a pump photon with frequency ωp propagating in a nonlinear optical crystal spontaneous splits into a pair of lower energy photons, signal (ωs) and idler (ωi), whose frequencies are related by ωp = ωs + ωi. The signal and idler photons are correlated in polarization, energy, time and other degrees of freedom, which serves as the basis for photon entanglement. Quasi-phasematching (QPM) [12] is a powerful technique that enables engineering of the down-conversion process to allow efficient pair production at the desired wavelengths. Aperiodic QPM or domain-engineered structures are additional tools that can enable further control of SPDC such as simultaneous multi-wavelength conversion [13–16] and apodization for side-lobe suppression [17,18] to shape the joint-spectral intensity and produce factorable photon states [19]. To produce polarization entanglement by SPDC, two consecutive crystals rotated by 90◦ relative to each other are often used [20,21]. The collinear configuration of these sources allows efficient collection into single-mode fibers. A similar scheme uses a single crystal with two sections in series having different QPM periods [22,23]. In these schemes, the first section produces |Hs〉|Vi〉 while the second section produces |Vs〉|Hi〉 (where the |H〉 and |V〉 refer to horizontally and vertically polarized states, respectively). When the signal and idler wavelengths are different, the two down-conversion processes are associated with two different phase-mismatches, ∆k = kp − ks − ki and hence, two different QPM periods. In our work, we use an aperiodic QPM crystal to generate both sets of signal and idler pairs simultaneously in a distributed fashion throughout the crystal. #387449 https://doi.org/10.1364/OSAC.387449 Journal © 2020 Received 15 Oct 2019; accepted 8 Jan 2020; published 29 Jan 2020 Research Article Vol. 3, No. 2 / 15 February 2020 / OSA Continuum 296 The concept of using domain-engineering to phasematch multiple SPDC processes has been previously explored. References [24,25] theoretically describe the dual-periodically-poled crystal structure, which is a special case of the phase-modulation procedure described in [14] and applied here. Reference [26] fabricated a phase-modulated crystal and observed interference during SPDC related to high indistinguishability between the two down-conversion paths. Recently, [27] produced a dual-periodically-poled waveguide crystal and observed excellent polarization entanglement. A closely related method of multi-process SPDC is the use of an interlaced biperiodic structure [28], where two different QPM periods alternate along the length of a QPM crystal. Here, we compare phase-modulation to other domain-engineering techniques. We also follow-up on studies presented in [26] to show that such a phase-modulated crystal can indeed be used to generate high-visibility polarization entanglement. In a phase-modulated QPM crystal, the device can be designed to phasematch multiple simultaneous processes, each with associated phase-mismatch ∆km, such that [14,29,30]