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Scientific Reports

Distribution of entangled photon pairs over few-mode fibers

Liang Cui, Jie Su, Xiaoying Li, Z. Y. Ou

Published Oct 27, 2017• 19 citations

Abstract

Abstract Few-mode fibers (FMFs) have been recently employed in classical optical communication to increase the data transmission capacity. Here we explore the capability of employing FMF for long distance quantum communication. We experimentally distribute photon pairs in the forms of time-bin and polarization entanglement over a 1-km-long FMF. We find the time-bin entangled photon pairs maintain their high degree of entanglement, no matter what type of spatial modes they are distributed in. For the polarization entangled photon pairs, however, the degree of entanglement is maintained when photon pairs are distributed in LP01 mode but significantly declines when photon pairs are distributed in LP11 mode due to a mode coupling effect in LP11 mode group. We propose and test a remedy to recover the high degree of entanglement. Our study shows, when FMFs are employed as quantum channels, selection of spatial channels and degrees of freedom of entanglement should be carefully considered.

DOI: 10.1038/s41598-017-14955-z

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In the absence of quantum repeaters, the only way to increase the quantum communication capacity at a given reach is to employ more modes for encoding and transmitting information. While frequency, temporal, and polarization modes have already been exploited for this purpose, the use of many spatial modes over long distances has only recently become possible owing to the development of low-loss few-mode fibers (FMFs). We will discuss the development of two key enablers of quantum communication over FMF spatial modes: 1) spatiallyentangled photon-pair generator and 2) dynamically-reconfigurable spatial-mode de-multiplexer that can perform projective measurements alternating between two sets of mutually unbiased bases in a given spatial mode space. Both devices are based on the spatial-mode-selective quantum frequency conversion process, implemented in either χ(2) (multimode LiNbO3 waveguide) or χ(3) (custom-made FMF) nonlinear medium.

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