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Optics Express

Quantum state tomography of a fiber-based source of polarization-entangled photon pairs

Jingyun Fan, Matthew D. Eisaman, Alan L. Migdall

Published Jan 1, 2007• 18 citations
Publisher

Abstract

We report an experimental demonstration of a bright high-fidelity single-mode-optical-fiber source of polarization-entangled photon pairs. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 microW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm. To characterize the purity of the states produced by this source, we use quantum-state tomography to reconstruct the corresponding density matrices, with fidelities of 95 % or more for each Bell state.

DOI
10.1364/oe.15.018339

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We report an experimental demonstration of a bright high-fidelity single-mode-optical-fiber source of polarization-entangled photon pairs. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 microW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm. To characterize the purity of the states produced by this source, we use quantum-state tomography to reconstruct the corresponding density matrices, with fidelities of 95 % or more for each Bell state.
We describe the preparation of a high spectral brightness, broad wavelength coverage, single-spatial mode source of polarization-entangled photon pairs operated at room temperature. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 μW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm.
We describe the preparation of a high spectral brightness, broad wavelength coverage, single-spatial mode source of polarization-entangled photon pairs operated at room temperature. The source takes advantage of single-mode fiber optics, highly nonlinear microstructure fiber, judicious phase-matching, and the inherent stability provided by a Sagnac interferometer. With a modest average pump power (300 μW), we create all four Bell states with a detected two-photon coincidence rate of 7 kHz per bandwidth of 0.9 nm, in a spectral range of more than 20 nm.
We describe an achromatic, phase-stable, broadband source of polarization-entangled photon pairs with high spectral brightness that uses four-wave mixing in a fiber Sagnac interferometer. We achieved a polarization-entangled two-photon coincidence rate of 7kHz7\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}7kHz per 0.5THz0.5\phantom{\rule{0.3em}{0ex}}\mathrm{THz}0.5THz (0.9nm)(0.9\phantom{\rule{0.3em}{0ex}}\mathrm{nm})(0.9nm) of bandwidth per $300\phantom{\rule{0.3em}{0ex}}\mathrm{\ensuremath{\mu}}\mathrm{W}$ of average pump power. At this rate, we observed two-photon fringe interference visibilities greater than 97%, over a 10THz10\phantom{\rule{0.3em}{0ex}}\mathrm{THz}10THz $(\ensuremath{\approx}21\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ spectral range. We measured violations of Bell's inequality by more than 22 standard deviations for each of the four Bell states in less than 3minutes3\phantom{\rule{0.3em}{0ex}}\text{minutes}3minutes per state. The high spectral brightness $(26\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}\phantom{\rule{0.2em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{mW}}^{\ensuremath{-}1})$, large tunable wavelength range, single spatial mode, and phase stability make this source a promising candidate for a wide range of quantum-information applications.
We describe the generation of entangled and hyperentangled photon pairs using a microstructure-fiber Sagnac interferometer, which is formed by a polarizing beam splitter and a highly nonlinear microstructure fiber twisted by 90° from end to end. This interferometer allows two identical four-wave mixing processes to occur on the same fiber principal axis, ensuring perfect spatial and temporal mode matching of the two four-wave mixing outputs on the polarizing beam splitter to create entanglement over the entire four-wave mixing phase-matching spectral range. With an average pump power of 220 μW, we measure a two-photon coincidence rate of 1 kHz with ▵λ = 0.9 nm. Two-photon interference visibilities exceed 91% for polarization-entangled photon pairs generated from this source, and are > 84% for both time-bin and polarization degrees of freedom for hyperentangled photons, all without subtracting accidental coincidences.
We demonstrate the capability of standard, commercially available polarization-maintaining fibers (PMFs) to generate polarization-entangled photon-pairs by inserting a PMF source into a Sagnac interferometer. We perform a quantum state tomography to reconstruct the density matrix, yielding, without background subtraction or spectral filtering, a polarization-entangled photon-pair state with 92.37 ± 0.14% fidelity to the maximally entangled Bell state. With its high coupling efficiency into SMFs and ability to produce spectrally uncorrelated photons, we expect this source to be useful for free-space and fiber-based quantum information protocols.