We demonstrate that after distributing in few mode fibers, the measured visibility of entanglement depends on the degree of freedom of entanglement and the spatial mode of photon pairs in transmission 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 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.
Abstract The spatial degree of freedom has enabled a wide range of applications in quantum information processing (QIP), such as increasing the capacity of quantum channels and realizing novel QIP protocols. In such applications, generating and manipulating quantum states in different spatial modes is one of the primary tasks. In this paper, by utilizing a telecom-band panda-type mode-maintaining few-mode fiber (MM-FMF) with a Sagnac loop configuration, we generate polarization-entangled photon pairs in higher-order spatial modes, namely the LP 11 modes, via intermodal spontaneous four-wave mixing process. The polarization- and spatial-mode-maintaining capabilities of the MM-FMF, combined with the automatic phase-stabilization of the Sagnac loop, provide a robust architecture for entanglement generation. We verify the polarization entanglement by two-photon interference measurements, yielding raw visibilities exceeding 76%. The visibilities are mainly limited by spontaneous Raman scattering noise, which could be reduced by cooling the fiber. The photon pairs directly emerging from MM-FMF are in a well-defined spatial mode, which greatly facilitates further manipulation and transmission of the entanglement. Our work demonstrates that the MM-FMF is a robust platform for developing devices and protocols exploiting the spatial degree of freedom in QIP applications.
The spatial degree of freedom (DOF) has significant applications in the emerging high-dimensional quantum information processing. Using a mode-maintaining few-mode fiber (FMF) and a dual-Sagnac-loop configuration, we generate polarization-entangled photon pairs and transform the entanglement into spatial DOF. We obtain Bell states encoded in the linearly polarized (LP) and orbital angular momentum (OAM) mode bases, respectively. The entangled photons from the FMF are in well-defined spatial modes, providing great advantages for the entanglement generation, manipulation, and transmission. The measured fidelities of the obtained entangled states are more than 97%. Our generation-and-transformation protocol can also be adopted on other integrated waveguide platforms to develop robust and miniaturized sources of spatial entanglement.