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Quantum Electronics and Laser Science Conference

Polarization-entangled states: the next generation

Paul G. Kwiat, Harald Weinfurter, Anton Zeilinger, Alexander V. Sergienko, Yanhua H. Shih

Published May 22, 1995• 0 citations
Publisher

Abstract

We have discovered theoretically and confirmed experimentally that a vast improvement over the traditional (type-I) downconversion sources (in which both correlated photons have the same polarization) is possible by using type-II downconversion (in which the correlated photons are orthogonally polarized).1 In previous systems it was necessary to use a number of very carefully aligned extra optical elements, beam splitters, mirrors, etc., to obtain and measure the quantum-mechanical entangled states required for many fundamental experiments.2,3

DOI
10.13016/m2tajx-qxkv

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We have discovered theoretically and confirmed experimentally that a vast improvement over the traditional (type-I) downconversion sources (in which both correlated photons have the same polarization) is possible by using type-II downconversion (in which the correlated photons are orthogonally polarized).1 In previous systems it was necessary to use a number of very carefully aligned extra optical elements, beam splitters, mirrors, etc., to obtain and measure the quantum-mechanical entangled states required for many fundamental experiments.2,3
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Type-II parametric down conversion in nonlinear optical crystals has turned out to be one of the most powerful methods to generate entangled photon pairs, and even allowed a large set of experiments on foundations of quantum mechanics and recent applications in quantum communication. In this paper, we present an experimental investigation of the emission characteristics for down-converted photons in order to investigate the possibility of high brightness sources for correlated photon pairs. Furthermore, we discuss possible ways to increase the collection efficiency for polarization entangled photon pairs.
Polarization correlation measurement of two-photon state generated in type-II spontaneous parametric downconversion (SPDC) has been performed in a single input port beam splitter configuration. Two orthogonally polarized entangled photons with degenerate k vectors were directed to the single input port 50%-50% beam splitter. Polarization were measured with linear and circular polarization entangled state by the use of two polarization analyzers followed by two photon counting detectors. A strong correlation of polarization has been observed. The type of two-photon state generated in Type-II SPDC was tested. A strong dependence of the polarization correlation upon the spectral bandwidth of the detector spectral filter was found and a different correlation between linear and circular polarization entangled state were observed. It seems that the standard theoretical models for SPDC need to be improved in order to provide a comprehensive treatment for the result of the experiment.
We present integrated sources of polarization entanglement built on portable platforms for quantum communication. We employ type-II degenerate spontaneous parametric down conversion processes in PPKTP waveguides to generate pairs of orthogonally polarized photons at near-infrared wavelengths. The photons are sent to a 50/50 non-polarizing beam splitter (NPBS) to generate post-selected polarization-entangled states via coincidence measurements. The small size of the waveguides allows the installation of several units on the same platform, and thereby the generation of multiple entangled states simultaneously. We use a computer-controlled tomography system to characterize the produced states and combine the photons from adjacent waveguides on another NPBS to perform Bell-state measurements, a prerequisite to entanglement swapping. The overall experiment is compact and requires little alignment when set, making it an attractive option for local quantum networking using mobile platforms, e.g., drones or satellites.