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arXiv:1002.4226 · Applied Physics Letters

Performance of hybrid entanglement photon pair source for quantum key distribution

Mikio Fujiwara, Morio Toyoshima, Masahide Sasaki, K. Yoshino, Y. Nambu, Akihisa Tomita

Published Dec 28, 2009• 26 citations
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Abstract

We report on the experimental demonstration of a source of hybrid entanglement pairs between two different degrees of freedom, a 1550 nm time-bin qubit and an 810 nm polarization qubit. The polarization qubit at 810 nm is transformed by an asymmetric Mach–Zehnder interferometer consisted of a Glan laser prism and a polarization-maintaining fiber. We obtained visibilities of 95.8% and 88% with tolerance ±0.2% and ±1% along Z-Z and X-X axes on the Poincare sphere, respectively, with a coincidence count rate of more than 800 c/s after entanglement format transformation. These values are well above the threshold of 70.7% needed to violate a Bell inequality and allow distilling a secure key in the quantum key distribution.

DOI
10.1063/1.3276559

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We demonstrate for the first time hybrid entanglement of photon pairs generated via the experimental violation of a Bell inequality with two different degrees of freedom (DOF), namely the path/linear momentum of one photon and the polarization of the other photon. Polarization entangled photon pairs are created by Spontaneous Parametric Down Conversion. Then, a modified Mach-Zehnder interferometer is used to convert the polarization DOF of one photon to a path DOF. For that photon, path superposition is analyzed and polarization superposition for its twin photon.
Entanglement distribution based on time-bin qubits is an attractive option for emerging quantum networks. We demonstrate a 4.09 GHz repetition rate source of photon pairs entangled across early and late time bins separated by 80 ps. Simultaneous high rates and high visibilities are achieved through frequency multiplexing the spontaneous parametric down conversion output into 8 time-bin entangled pairs. We demonstrate entanglement visibilities as high as 99.4%, total entanglement rates up to 3.55e6 coincidences/s, and predict a straightforward path towards achieving up to an order of magnitude improvement in rates without compromising visibility. Finally, we resolve the density matrices of the entangled states for each multiplexed channel and express distillable entanglement rates in ebit/s, thereby quantifying the tradeoff between visibility and coincidence rates that contributes to useful entanglement distribution. This source is a fundamental building block for high-rate entanglement-based quantum key distribution systems or advanced quantum networks.