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An Ultrafast Source of Polarization Entangled Photon Pairs based on a Sagnac Interferometer

Devin H. Smith

Published Jan 26, 2009• 1 citation
OpenAlex

Abstract

This thesis describes the design, development, and implementation of a pulsed source of polarization-entangled photons using spontaneous parametric down-conversion in a Sagnac interferometer. A tangle of 0.9286 ± 0.0015, fidelity to the state (|10〉 + |01〉)/√2 of \n0.9798 ± 0.0004 and a brightness of 597 pairs/s/mW were demonstrated. \n \nSpontaneous parametric down-conversion is a nonlinear optical process in which one photon is split into two lower-frequency photons while conserving momentum and energy, in this \nexperiment nearly degenerate photons are produced. These photons are then interfered at the output beamsplitter of the interferometer, exchanging path entanglement for polarization entanglement and generating the desired polarization-entangled photon pairs.

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This thesis describes the design, development, and implementation of a pulsed source of polarization-entangled photons using spontaneous parametric down-conversion in a Sagnac interferometer. A tangle of 0.9286 ± 0.0015, fidelity to the state (|10〉 + |01〉)/√2 of \n0.9798 ± 0.0004 and a brightness of 597 pairs/s/mW were demonstrated. \n \nSpontaneous parametric down-conversion is a nonlinear optical process in which one photon is split into two lower-frequency photons while conserving momentum and energy, in this \nexperiment nearly degenerate photons are produced. These photons are then interfered at the output beamsplitter of the interferometer, exchanging path entanglement for polarization entanglement and generating the desired polarization-entangled photon pairs.
We demonstrate a simple, robust, and ultrabright parametric down-conversion source of polarization-entangled photons based on a polarization Sagnac interferometer. Bidirectional pumping in type-II phase-matched periodically poled KTiOPO4\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}{\mathrm{O}}_{4}KTiOPO4​ yields a measured flux of 5000 polarization-entangled pairs/s/mW of pump power in a $1\text{\ensuremath{-}}\mathrm{nm}$ bandwidth at 96.8% quantum-interference visibility. The common-path arrangement of the Sagnac interferometer eliminates the need for phase stabilization for the biphoton output state.
Photonic quantum information experiments demand bright and highly entangled photon pair sources. The combination of periodic poling and collinear excitation geometry allows the use of considerably longer crystals for parametric down-conversion. We demonstrate a picosecond-pulsed laser pumped source of high quality polarization entangled photon pairs. The phase of the output biphoton state is affected by the relative phase of the two-color interferometer and the phase of the nonlinearly interacting Gaussian beams. We measure the influence of these onto the phase of the output state. The presented source is a promising candidate for a compact, semiconductor laser driven source of entangled photon pairs.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2^22) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. A lumped device brightness of about 104 pairs s−1 nm−1 mW−2 is reported over several standard 100-GHz ITU channel pairs, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96% over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
This thesis deals with the production of entangled photons using spontaneous parametric down-conversion (SPDC). We start with a short overview of some important theoretical concepts. First we provide a brief reminder of the theory of entanglement. We then discuss how the state of quantum systems can be determined using quantum state tomography. We also explain SPDC, the physical process which we use to produce entangled photons. Finally, we give an overview of the methods which have been used to produce entangled photons in the past, both for two- and three-photon entanglement. \n \nThe first experiment is the design of an efficient source of entangled photon pairs based on a polarizing Sagnac interferometer configuration. With this configuration, we can use quasi-phasematched materials which allow for higher efficiencies than standard bulk nonlinear materials. The source is pumped by a low-power continuous-wave laser diode, and produces degenerate photon pairs at 809nm. It has a spectral brightness of 87,500 pairs/(s mW nm), and the fidelity of the produced quantum states with a Bell state is 98.9%. The source is used for experiments in quantum key distribution, cluster state quantum computing, remote state preparation, state discrimination, and entanglement-enhanced classical communication. \n \nThe second experiment discussed in this thesis is the generation of photon triplets using cascaded SPDC. In this experiment, a primary SPDC source is pumped with a low-power, continuous-wave laser diode, producing photon pairs. Single photons from these pairs serve as the pump for a second down-conversion, resulting in photon triplets. This is the first demonstration of the direct production of photon triplets, and the first observation of SPDC at the single photon level. This method could potentially be used to produce entangled photon triplets without post-selection, and as a source of triggered Bell pairs.
Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs (around 810 and 1550 nm), the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of Clauser-Horne-Shimony-Holt and Leggett inequalities, as well as an instance of the ``beautiful'' Bell inequality, which has not previously been tested experimentally.