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arXiv:2012.05134 · Applied Physics Letters

Temperature insensitive type II quasi-phasematched spontaneous parametric downconversion

Xin-Yi Pan, Christian Kurtsiefer, Alexander Ling, James A. Grieve

Published Jul 12, 2021• 7 citations
arXivHTMLPDF

Abstract

The temperature dependence of the refractive indices of potassium titanyl phosphate (KTP) is shown to enable quasi-phasematched type II spontaneous parametric downconversion (SPDC) with low temperature sensitivity. Calculations show the effect to be maximized for emission of photons at around 1165 nm, as well as producing similar plateaus for wavelengths throughout the telecommunications bands. We experimentally demonstrate the effect, observing temperature-insensitive degenerate emission between 20 °C and 100 °C at 1327 nm, within the telecommunications O band. This result has practical implications for the development of entangled photon sources for resource-constrained environments, and we demonstrate a simple polarization entangled source as a proof of concept.

DOI
10.1063/5.0052239

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The temperature dependence of the refractive indices of potassium titanyl phosphate (KTP) is shown to enable quasi-phasematched type II spontaneous parametric downconversion (SPDC) with low temperature sensitivity. Calculations show the effect to be maximized for emission of photons at around 1165 nm, as well as producing similar plateaus for wavelengths throughout the telecommunications bands. We experimentally demonstrate the effect, observing temperature-insensitive degenerate emission between 20 °C and 100 °C at 1327 nm, within the telecommunications O band. This result has practical implications for the development of entangled photon sources for resource-constrained environments, and we demonstrate a simple polarization entangled source as a proof of concept.
Noncritical phasematched (NCPM) Type-II spontaneous parametric downconversion (SPDC) was used to produce highly nondegenerate polarization-entangled photon pairs in a 50mm single-domain potassium titanyl phosphate (KTP) crystal. The center wavelength of the idler photon and high entanglement quality of the source were observed to have a low temperature dependence. The high temperature stability makes this approach of generating and distributing polarization-entangled photon pairs suitable for field deployment without temperature control. In addition to the wavelengths of the signal and idler photons being in the visible and telecom bands respectively, the photons have narrow linewidths, making the NCPM approach of generating entangled photon pairs suitable for multi-channel entanglement distribution where the signal photon can be transmitted through free space and the idler photon can be transmitted through fiber.
High Resolution Image Download MS PowerPoint Slide Photon-pair sources are useful for entanglement distribution. The most mature of these are spontaneous parametric downconversion (SPDC) sources, most of which achieve phasematching via engineering the domains in poled crystals or the angle between the optic axis and the pump beam. For multichannel entanglement distribution of photon pairs, where one photon is transmitted through free-space and the other photon is transmitted through fiber, it is beneficial to use highly nondegenerate photon-pair sources. The currently accepted approach in such sources is quasi-phasematching. In this paper, a simpler, more stable alternative is presented for producing highly nondegenerate photon pairs. A source of polarization-entangled photon pairs with low temperature sensitivity based on noncritical phasematched SPDC in single-domain potassium titanyl phosphate was demonstrated. Over a crystal temperature range of 75 °C, the center wavelength of the idler photons was observed to change by 10.8(20) nm while the average entanglement visibility was maintained above 98%. With the signal photons detected locally, the idler photons were transmitted through 62 km and 93 km of deployed telecom fibers with average raw visibilities of 98.2(1)% and 95.6(3)% respectively.
We theoretically investigate the properties of extended phase-matching (EPM) for spontaneous parametric down-conversion (SPDC) in four kinds of potassium titanyl phosphate isomorphs ( i.e. , KTiOPO 4 , KTiOAsO 4 , RbTiOPO 4 , and RbTiOAsO 4 crystals). The technique is based on Type II 2nd-order nonlinear optic interaction in periodically poled ferroelectric domain structures, where a single photon with a frequency of 2 ω generates a pair of photons with frequencies of ω that are orthogonally polarized with respect to each other. Under EPM, both quasi-phase matching (QPM) and group velocity (GV) matching between interacting waves are efficiently satisfied for the generation of a pair of polarization-entangled bi-photon states with frequencies of ω . Our simulation results show that the generated photon pairs have broad spectral bandwidths of over 69 - 95 nm in tele-communication bands, which are much broader than those of the typical non-EPM case ( e.g. , ~ sub-nm bandwidth when only the QPM, not the GV matching, is achieved). We will describe the EPM properties of the four kinds of KTP isomorphs in terms of interaction type, GV matching wavelength, domain poling period, and spectral bandwidth. We highlight that the KTP isomorphs have nice potential as suitable components for constituting new quantum information processing systems.
Spontaneous parametric down-conversion (SPDC) is a reliable and robust source of photons for quantum information applications. For applications that involve operations such as entanglement swapping or single-photon heralding, two-photon states are required to be factorable (uncorrelated) in their spectral and spatial degrees of freedom. We report the design and experimental characterization of an SPDC source that has been optimized for high spectral and spatial purity. The source is pumped by the 776 nm output of a mode-locked Ti:Sapphire laser and consists of a periodically-poled Potassium Titanyl Phosphate (PPKTP) crystal phase-matched for collinear type-II SPDC. The dispersive properties of PPKTP at these wavelengths is such that it is possible to minimize the spectral entanglement by matching the widths of the pump to the spectral phase-matching function. The spatial entanglement is minimized through careful control of the pump focus, yielding nearly single-mode emission. An advantage of this approach is that the emission rate into the collection modes is very high, resulting in a very bright SPDC source. We also report a scheme that employs the output of collinear sources such as these to produce polarization-entangled photon pairs. The scheme, which requires only simple polarization elements, can be scaled to N-photon GHZ states.
Entangled photon pairs are crucial resources for quantum information processing protocols.Via the process of spontaneous parametric downconversion (SPDC), we can generate these photon pairs using bulk nonlinear crystals.Traditionally, the crystal is designed to satisfy a specific type of phase-matching condition.Here, we report controllable transitions among different types of phase matching in a single periodically poled potassium titanyl phosphate crystal.By carefully selecting pump conditions, we can satisfy different phase-matching conditions.This allows us to observe first-order Type-II, fifth-order Type-I, third-order Type-0, and fifth-order Type-II SPDCs.The temperature-dependent spectra of our source were also analyzed in detail.Finally, we discussed the possibility of observing more than nine SPDCs in this crystal.Our work not only deepens the understanding of the physics behind phase-matching conditions, but also offers the potential for a highly versatile entangled biphoton source for quantum information research.