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arXiv:quant-ph/0409093 · Physical Review A

Long-distance entanglement swapping with photons from separated sources

Hugues de Riedmatten, I. Marcikic, J. A. W. van Houwelingen, Wolfgang Tittel, Hugo Zbinden, Nicolas Gisin

Published May 31, 2005• 180 citations
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Abstract

We report the experimental realization of entanglement swapping over long distances in optical fibers. Two photons separated by more than $2\phantom{\rule{0.3em}{0ex}}\mathrm{km}$ of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell-state measurement is performed with one photon from each pair, which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to infer a violation of a Bell inequality is reported, after both photons have each traveled through $1.1\phantom{\rule{0.3em}{0ex}}\mathrm{km}$ of optical fiber.

DOI
10.1103/physreva.71.050302

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We report the experimental realization of entanglement swapping over long distances in optical fibers. Two photons separated by more than 2km2\phantom{\rule{0.3em}{0ex}}\mathrm{km}2km of optical fibers are entangled, although they never directly interacted. We use two pairs of time-bin entangled qubits created in spatially separated sources and carried by photons at telecommunication wavelengths. A partial Bell-state measurement is performed with one photon from each pair, which projects the two remaining photons, formerly independent onto an entangled state. A visibility high enough to infer a violation of a Bell inequality is reported, after both photons have each traveled through 1.1km1.1\phantom{\rule{0.3em}{0ex}}\mathrm{km}1.1km of optical fiber.
Establishing entanglement between two remote systems by the method of entanglement swapping is an essential step for a long-distance quantum network. Here we report a field-test entanglement swapping experiment with two independent telecommunication band entangled photon-pair sources over an optical fiber network in Hefei. The two sources are located at two nodes that are 12.5km12.5\phantom{\rule{4pt}{0ex}}\mathrm{km}12.5km apart and the Bell-state measurement is performed at a third location which is connected to the two source nodes with 14.7-km and 10.6-km optical fibers, respectively. The observed average visibility is $79.9\ifmmode\pm\else\textpm\fi{}4.8%$, which is sufficient for the violation of Bell inequalities. Furthermore, with the swapped entanglement, we demonstrate a source-independent quantum key distribution, which is also immune to any detection attacks at the measurement site.
Realizing long-distance entanglement swapping with independent sources in the real-world condition is important for both future quantum networks and fundamental study of quantum theory. Currently, entanglement swapping over a few tens of kilometers of underground optical fiber has been achieved. However, future applications demand entanglement swapping over longer distances in more complicated environments. We exploit two independent 1-GHz-clock sequential time-bin entangled photon-pair sources; develop several automatic stability controls; and successfully implement a field test of entanglement swapping over an optical fiber link of more than 100 km, including coiled, underground, and suspended optical fibers. Our result verifies the feasibility of such technologies for long-distance quantum networks and for many interesting quantum information experiments.
Realizing long distance entanglement swapping with independent sources in the real-world condition is important for both future quantum network and fundamental study of quantum theory. Currently, demonstration over a few of tens kilometer underground optical fiber has been achieved. However, future applications demand entanglement swapping over longer distance with more complicated environment. We exploit two independent 1-GHz-clock sequential time-bin entangled photon-pair sources, develop several automatic stability controls, and successfully implement a field test of entanglement swapping over more than 100-km optical fiber link including coiled, underground and suspended optical fibers. Our result verifies the feasibility of such technologies for long distance quantum network and for many interesting quantum information experiments.
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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.