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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Generation of non-degenerated polarization entangled photon pairs in periodically poled Ti:LiNbO3 waveguides with interlaced domains

Abu Thomas, Harald Herrmann, W. Sohler

Published May 1, 2011• 1 citation
Publisher

Abstract

Entangled photon pair sources are of key importance in the emerging field of quantum information technology. A common method to generate entangled pairs is based on type II phase-matched spontaneous parametric down conversion (SPDC) followed by a spatial separation of the pair using a conventional beam splitter (see e.g.). This scheme can be used to generate entangled pairs at degeneracy with the drawback that half of the generated pairs are “lost” if both photons leave the beam splitter at the same output port. Recently, we have demonstrated a new scheme providing non-degenerated pairs by employing a sophisticated poling pattern. In this contribution we report a source of non-degenerated polarization entangled photon pairs using such a PPLN waveguide together with a polarization maintaining fiber as quantum eraser and a commercially available fiber-based coarse WDM coupler.

DOI
10.1109/cleoe.2011.5943449

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Entangled photon pair sources are of key importance in the emerging field of quantum information technology. A common method to generate entangled pairs is based on type II phase-matched spontaneous parametric down conversion (SPDC) followed by a spatial separation of the pair using a conventional beam splitter (see e.g.). This scheme can be used to generate entangled pairs at degeneracy with the drawback that half of the generated pairs are “lost” if both photons leave the beam splitter at the same output port. Recently, we have demonstrated a new scheme providing non-degenerated pairs by employing a sophisticated poling pattern. In this contribution we report a source of non-degenerated polarization entangled photon pairs using such a PPLN waveguide together with a polarization maintaining fiber as quantum eraser and a commercially available fiber-based coarse WDM coupler.
We present an integrated source of polarization entangled photon pairs in the telecom regime, which is based on type II-phasematched parametric down-conversion (PDC) in a Ti-indiffused waveguide in periodically poled lithium niobate. The domain grating - consisting of an interlaced bi-periodic structure - is engineered to provide simultaneous phase-matching of two PDC processes, and enables the direct generation of non-degenerate, polarization entangled photon pairs with a brightness of B = 7 × 10(3) pairs/(s×mW×GHz). The spatial separation of the photon pairs is accomplished by a fiber-optical multiplexer facilitating a high compactness of the overall source. Visibilities exceeding 95 % and a violation of the Bell inequality with S = 2.57±0.06 could be demonstrated.
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.
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.
Generation of entangled photon pairs at telecom wavelength has been garnering a growing interest in the context of quantum communications and quantum computing. Several generation techniques have been proposed, including spontaneous Four-Wave Mixing in resonators or Spontaneous Parametric Down Conversion (SPDC) in Periodically Polled Lithium Niobate (PPLN) waveguides. Large photon pair efficiencies have been demonstrated in particular in PPLN on insulator [3]. However, polarization entangled photon pair generation (Type II phase matching configuration) suffers from a less favourable non-linear coefficient and optical mode birefringence in PPLN.
We report the realization of a fiber-coupled time-bin entangled photon-pair source at 1538 nm. Unlike previous time-bin sources based on type-0 spontaneous parametric down conversion, our approach takes advantage of cross-polarized paired photons generated in a type-II periodically poled lithium niobate waveguide (PPLN/W). Here, we use a birefringent delay line (BDL) in a Michelson configuration to introduce a controllable delay between the paired photons. After the BDL, these photons are separated by a polarizing beam splitter (PBS) at 45° and sent to Alice and Bob. This way we create the non common time-bin entangled Bell state |Ψ ⟩ = 1/√2 [|s,l⟩ + |l,s⟩], where s and l represent the short and long time-bins, respectively.
As optical quantum information processing protocols and experiments become increasingly more complex, integrated optics provide a small and robust alternative to traditional bulk optics. Specifically, waveguide technology allows for the creation of bright single-photon sources based on the fact that photon pairs can be created at any location along the waveguide. For our goals, we are working on the characterization of a highly nondegenerate Spontaneous Parametric Down-Conversion (SPDC) waveguide source on a periodically poled KTP (PPKTP) crystal. Our current waveguide source uses type-II phase-matching to create collinear signal and idler photons at 1550 nm and 810 nm, respectively, with the promise of generating simultaneous time-bin and polarization entanglement in future iterations. Our intended source application is for use in quantum key distribution and superdense teleportation protocols between a space platform and collection telescopes on Earth.