Distribution of Entangled Photon Pairs over Few Mode Fibers

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 LP 01 mode but significantly declines when photon pairs are distributed in LP 11 mode due to a mode coupling effect in LP 11 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.Quantum communication has experienced rapid progress in recent years, and is developing towards a practical and mature technology 1 . Transferring high volume of quantum information between distanced places is a paramount task of quantum communication. An effective solution for the task is to encode quantum information on single photons or entangled photons, and distribute the photons over quantum channels of low losses such as optical fiber links. Previously, several examples of entangled photon pair distribution over single-mode fiber (SMF) with distances up to 200 kilometers were successfully demonstrated 2-6 . More recently, quantum key distribution and quantum teleportation have been achieved over distances of several to tens of kilometers by using metropolitan SMF networks [7][8][9] , which further confirms the capability of SMF network as quantum channel. In the past decades, multiplexing technologies in different degrees of freedom, such as time, wavelength, polarization, and phase, have been exploited to increase the information carrying capacity of SMF [10][11][12][13] . However, due to the fast growing internet traffic, the current fiber communication network based on SMFs is reaching its capacity limit 14 . Recent studies have shown that the space-division multiplexing (SDM) based on few-mode fibers (FMFs) is a promising solution to break through such a limit 15,16 . The core diameter of an FMF is slightly larger than that of an SMF, so FMFs can support a few higher-order spatial modes such as LP 11 modes in addition to the fundamental mode LP 01 (see inset of Fig. 1 for the mode structures). Thus the capacity of fiber network can be increased by using the multiplicity of space channels. Since practical quantum communication needs to employ fiber networks as the quantum channels, it is of great importance to evaluate whether the FMF network is capable of carrying quantum information. In a recent work, single photons and classical light were simultaneously transmitted over a 2-m-long FMF in different spatial and polarization mode...