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Japanese Journal of Applied Physics

Accidental Coincidence Counts Observed in Mandel Dip Measurement Using Independently Produced Photon Pairs at 1550 nm

Akio Yoshizawa, Yinghong Xue, Hidemi Tsuchida

Published Dec 1, 2010• 2 citations
Publisher

Abstract

Mandel dip measurement was performed at 1550 nm using photon pairs created via spontaneous parametric down conversion in two separately located periodically poled lithium niobate waveguides. A visibility of 77.6% was obtained in fourfold coincidence counting. We experimentally clarified that the simultaneous generation of three pairs (one pair created in one waveguide and two pairs created in another waveguide) is the main source of accidental coincidence counts. Propagation loss before mixing two photons at a beamsplitter for observing photon bunching plays an important role in the determination of visibility. A large loss improves visibility, but also reduces the number of fourfold coincidence counts.

DOI
10.1143/jjap.49.122802

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Mandel dip measurement was performed at 1550 nm using photon pairs created via spontaneous parametric down conversion in two separately located periodically poled lithium niobate waveguides. A visibility of 77.6% was obtained in fourfold coincidence counting. We experimentally clarified that the simultaneous generation of three pairs (one pair created in one waveguide and two pairs created in another waveguide) is the main source of accidental coincidence counts. Propagation loss before mixing two photons at a beamsplitter for observing photon bunching plays an important role in the determination of visibility. A large loss improves visibility, but also reduces the number of fourfold coincidence counts.
To show the feasibility of a long-distance partial Bell-state measurement, a Hong-Ou-Mandel experiment with coherent photons is reported. Pairs of degenerate photons at telecommunication wavelength are created by parametric down-conversion in a periodically poled lithium niobate waveguide. The photon pairs are separated in a beam splitter and transmitted via two fibers of 25km25\phantom{\rule{0.3em}{0ex}}\mathrm{km}25km. The wave packets are relatively delayed and recombined on a second beam splitter, forming a large Mach-Zehnder interferometer. Coincidence counts between the photons at the two output modes are registered. The main challenge consists in the trade-off between low count rates due to narrow filtering and length fluctuations of the $25\text{\ensuremath{-}}\mathrm{km}$-long arms during the measurement. For balanced paths a Hong-Ou-Mandel dip with a net visibility of 47.3% is observed, which is close to the maximal theoretical value of 50% developed here. This proves the practicability of a long-distance Bell-state measurement with two independent sources, as, e.g., required in an entanglement swapping configuration in the scale of tens of kilometers.