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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Characterization of type-II spontaneous parametric down-conversion in domain-engineered PPLN

Paulina S. Kuo, Thomas Gerrits, Varun Boehm Verma, Sae Woo Nam, Oliver Slattery, Lijun Ma, Xiao Hong Tang

Published Mar 16, 2016• 10 citations
Publisher

Abstract

We characterize spontaneous parametric downconversion in a domain-engineered, type-II periodically poled lithium niobate (PPLN) crystal using seeded emission and single-photon techniques. Using continuous-wave (CW) pumping at 775 nm wavelength, the signal and idler are at 1532.5 nm and 1567.5 nm, respectively. The domain-engineered crystal simultaneously phasematches signal and idler pairs: [H(1532.5 nm), V(1567.5 nm)] and [V(1532.5 nm), H(1567.5 nm)]. We observe the tuning curves of these processes through difference-frequency generation and through CW fiber-assisted, single-photon spectroscopy. These measurements indicate good matching in amplitude and bandwidth of the two processes and that the crystal can in principle be used effectively to generate polarization-entangled photon pairs.

DOI
10.1117/12.2218535

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We characterize spontaneous parametric downconversion in a domain-engineered, type-II periodically poled lithium niobate (PPLN) crystal using seeded emission and single-photon techniques. Using continuous-wave (CW) pumping at 775 nm wavelength, the signal and idler are at 1532.5 nm and 1567.5 nm, respectively. The domain-engineered crystal simultaneously phasematches signal and idler pairs: [H(1532.5 nm), V(1567.5 nm)] and [V(1532.5 nm), H(1567.5 nm)]. We observe the tuning curves of these processes through difference-frequency generation and through CW fiber-assisted, single-photon spectroscopy. These measurements indicate good matching in amplitude and bandwidth of the two processes and that the crystal can in principle be used effectively to generate polarization-entangled photon pairs.
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.
We describe the design and application of domain-engineered, periodically poled lithium niobate (PPLN) for use to produce entangled photons and for other tools in quantum information and communications. By specially designing and controlling the PPLN poling pattern, multiple nonlinear optical processes can be simultaneously phasematched. This capability can be used to generate polarization-entangled photon pairs through type-II spontaneous parametric downconversion. The single PPLN crystal is designed to produce both the |HV〉 and |VH〉 states where the downconverted photons are distinguishable by wavelengths, which enables generation of post-selection-free, polarization-entangled twin photons. We describe the design and fabrication of the PPLN crystal, and initial experimental results for downconversion of a 775 nm pump to 1532 nm and 1567 nm orthogonally polarized photons. We also discuss other applications of engineered optical frequency conversion for quantum information including the use of dual-wavelength upconversion as a beamsplitter to route or analyze photons.
In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.
A periodically poled lithium niobate (PPLN) crystal has been used as an efficient source of non-collinearly generated polarization-entangled photon pairs at 810 and 1550 nm. The PPLN crystal was endowed with a specially designed poling pattern and the entangled photons were generated via the nonlinear optical process of spontaneous parametric down conversion (SPDC). A novel design based on overlapping two concurrent type-I quasi-phase-matching structures in a single PPLN crystals produced correlated pairs of alternatively polarized photons in largely separated spectral regions. The phase of the resulting two-photon state is directly linked to parameters of the nonlinear grating. Continuous tunability of the generated Bell state, from Phi(+) to Phi(-), has been demonstrated by translating a slightly wedged crystal perpendicular to the pump beam.
We have developed an entangled photon pair source based on a domain-engineered, type-II periodically poled lithium niobate crystal that produces signal and idler photons at 1533 nm and 1567 nm. We characterized the spectral correlations of the generated entangled photons using fiber-assisted signal-photon spectroscopy. We observed interference between the two down-conversion paths after erasing polarization distinguishability of the down-converted photons. The observed interference signature is closely related to the spectral correlations between photons in a Hong- Ou-Mandel interferometer. These measurements suggest good indistinguishability between the two downconversion paths, which is required for high entanglement visibility.
A method is presented for generating multiple-channel color-entangled photon pairs simultaneously based on type I quasi-phase-matched spontaneous parametric downconversion pumped in a single periodically poled lithium niobate (PPLN) crystal. It is predicted that one-, and two-channel color-entangled photon pairs can be simultaneously generated just by suitably choosing the PPLN grating period and the pump frequency. We point out that the full width at half maximum (FWHM) of idler and signal light are broadened dramatically due to spontaneous parametric downconversion in case of a pulsed pump.