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AIP conference proceedings

Characterization of our source of polarization-entangled photons

Guillaume Adenier

Published Jan 1, 2012• 6 citations
Publisher

Abstract

We present our source of polarization entangled photons, which consist of orthogonally polarized and collinear parametric down converted photons sent to the same input of a nonpolarizing beam splitter. We show that a too straightforward characterization of the quantum state cannot account for all the experimental observations, in particular for the behavior of the doublecounts, which are the coincidences produced whenever both photons are dispatched by the beam splitter to the same measuring station (either Alice or Bob). We argue that in order to account for all observations, the state has to be entangled in polarization before the non-polarizing beam splitter, and we discuss the intriguing and nevertheless essential role of the time-compensation required to obtain such a polarization entanglement.

DOI
10.1063/1.4773123

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We present our source of polarization entangled photons, which consist of orthogonally polarized and collinear parametric down converted photons sent to the same input of a nonpolarizing beam splitter. We show that a too straightforward characterization of the quantum state cannot account for all the experimental observations, in particular for the behavior of the doublecounts, which are the coincidences produced whenever both photons are dispatched by the beam splitter to the same measuring station (either Alice or Bob). We argue that in order to account for all observations, the state has to be entangled in polarization before the non-polarizing beam splitter, and we discuss the intriguing and nevertheless essential role of the time-compensation required to obtain such a polarization entanglement.
Many quantum information protocols require a Bell-state measurement of entangled systems. Most optical Bell-state measurements utilize two-photon interference at a beam splitter. By creating polarization-entangled photons with spontaneous parametric down-conversion using a first-order Hermite-Gaussian pump beam, we invert the usual interference behavior and perform an incomplete Bell-state measurement in the coincidence basis. We discuss the possibility of a complete Bell-state measurement in the coincidence basis using hyperentangled states ( Phys. Rev. A, 58 (1998) R2623).
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
Summary form only given. Photon pairs entangled in different degrees of freedom are of great interest for quantum communication. In long distance fiber based communications, time-energy entanglement [1] is preferred over polarization entanglement due to its robustness against polarization mode dispersion due to the fiber propagation. On the other hand, the measurement of time-energy entanglement is more complex than that of polarization. In this work we convert the quantum correlation of time -energy entangled photons into polarization entanglement using unbalanced Michelson interferometers. Time correlated photons are generated thorough spontaneous parametric down conversion of a laser of wavelength 405nm in a second order nonlinear crystal (β-barium borate). Correlated photons carrying horizontal polarization are converted to diagonal polarization and send to a Michelson interferometer (Fig. la) with a polarizing beam splitter. In both arms of the interferometer, one quarter -wave plate oriented at 45° is kept so that the returning photons are converted from horizontal to vertical and vice -versa. The output of the interferometer yields co -polarized photons (HUI and VV) with the same relative time of arrival. The anti -correlated photons (HV and VH) exit the interferometer with different relative arrival time due to the unbalanced interferometer arms. A coincidence window less than the photon travel time for the path length difference will fi lter out the anti -correlated photons. We ensure that the coherence length of the pump laser is larger than the path length difference such that the co -polarized bi-photon state coherently adds up to the so called Bell state.
We present an experimental study of polarization-entangled photon pairs, investigating the dependence of the measurement outcomes on different polarization rotation angles and the slow temporal fluctuations of the generated quantum state. Entangled photon pairs are produced and analyzed using single-photon detectors while systematically varying polarization settings on both sides. Bell tests yield an S parameter of approximately 2.52, confirming strong entanglement. The measured coincidence counts between identical measurement outcomes exhibit the expected sinusoidal dependence on the combined rotation angles, but systematic deviations appear as pronounced peaks and variations in certain configurations. Long-term monitoring over twenty minutes shows correlated fluctuations on both sides in individual detection outcomes of up to 25%, while the total photon flux remains constant, indicating that these deviations originate from slow intrinsic changes in the generated quantum state. These temporal fluctuations can reduce the effective time window over which optimal polarization and phase alignment is maintained, highlighting the importance of implementing real-time compensation or feedback mechanisms in experiments requiring high-fidelity entanglement over extended durations. Our results provide insight into the stability limits of entangled photon sources and underscore the importance of accounting for temporal fluctuations in precision entanglement experiments.
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.