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arXiv:quant-ph/0312118 · Physical Review Letters

Efficient Conditional Preparation of High-Fidelity Single Photon States for Fiber-Optic Quantum Networks

Alfred B. U’Ren, Christine Silberhorn, Konrad Banaszek, Ian A. Walmsley

Published Aug 25, 2004• 169 citations

Abstract

Highly correlated photons or, accordingly, high-fidelity single-photon states are a prerequisite for closing detection loopholes in experimental tests of local realism and implementing scalable linear optical quantum computation. We demonstrate a parametric down-conversion source exhibiting a conditional detection efficiency of $51%$ (with corresponding preparation efficiency of $85%$) and extraordinarily high detection rates of up to $8.5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\mathrm{c}\mathrm{o}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{s}/(\mathrm{s}\text{ }\mathrm{m}\mathrm{W})$. We exploit a novel type-II phase matching configuration in a microstructured waveguide in conjunction with an ultrashort pump.

DOI: 10.1103/physrevlett.93.093601

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Highly correlated photons or, accordingly, high-fidelity single-photon states are a prerequisite for closing detection loopholes in experimental tests of local realism and implementing scalable linear optical quantum computation. We demonstrate a parametric down-conversion source exhibiting a conditional detection efficiency of

51%

(with corresponding preparation efficiency of

85%

) and extraordinarily high detection rates of up to

8.5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\mathrm{c}\mathrm{o}\mathrm{i}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{s}/(\mathrm{s}\text{ }\mathrm{m}\mathrm{W})

. We exploit a novel type-II phase matching configuration in a microstructured waveguide in conjunction with an ultrashort pump.

The reliable generation of true single‐photon wavepackets with well‐defined modal structure is a crucial ingredient for the realization of linear optical quantum computing. We have implemented a highly efficient waveguided parametric downconversion source pumped by ultrashort pulses, which is characterized by the spatial decoupling of the signal and idler photon while exhibiting a conditional detection efficiency of 51%, corresponding to a preparation efficiency of 85%. We have performed various tests to confirm the single‐photon character of the conditionally prepared state.

Parametric down-conversion (PDC) offers the possibility to control the fabrication of non-Gaussian states such as Fock states. However, in conventional PDC sources energy and momentum conservation introduce strict frequency and photon number correlations, which impact the fidelity of the prepared state. In our work we optimize the preparation of single-photon Fock states from the emission of waveguided PDC via spectral filtering. We study the effect of correlations via photon number resolving detection and quantum interference. Our measurements show how the reduction of mixedness due to filtering can be evaluated. Interfering the prepared photon with a coherent state we establish an experimentally measured fidelity of the produced target state of 78%. (C) 2009 Optical Society of America

The reliable generation of true single-photon wavepackets with well-defined modal structure is a crucial ingredient for the realization of linear optical quantum computing. In this paper we present experimental results indicating the efficient generation of conditionally prepared single photons from parametric downconversion in a KTP nonlinear waveguide. In addition we present, theoretically and experimentally, a novel criterion for the assessment of conditional single photon sources which takes into account the contributions of heralded vacuum due to optical losses, of higher photon numbers and of the binary response of typical single photon detectors. Utilizing this criterion, we show that our KTP nonlinear waveguide represents a high fidelity source of conditionally prepared single photons.

Parametric down-conversion (PDC) offers the possibility to control the fabrication of non-Gaussian states such as Fock states. However, in conventional PDC sources energy and momentum conservation introduce strict frequency and photon number correlations, which impact the fidelity of the prepared state. In our work we optimize the preparation of single-photon Fock states from the emission of waveguided PDC via spectral filtering. We study the effect of correlations via photon number resolving detection and quantum interference. Our measurements show how the reduction of mixedness due to filtering can be evaluated. Interfering the prepared photon with a coherent state we establish an experimentally measured fidelity of the produced target state of 78%.

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.