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Optics Letters

Generation and transformation of spatial mode entanglement by using a mode-maintaining few-mode fiber

Shengjie Zhu, Liang Cui, Jian Guo, qi mo, Michael Vasilyev, Xiaoying Li

Published Mar 9, 2026• 1 citation
Publisher

Abstract

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.

DOI
10.1364/ol.588440

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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.
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.
Abstract Spin and orbital angular momenta of light are attractive resources to harness for encoding, and manipulating information, with applications in various quantum photonic technologies. However, to fully harness that potential, we require robust sources of high-order angular momentum photons exhibiting nonclassical correlations. Here, we propose a fiber-based source of polarization-entangled photons in high-order orbital angular momentum (OAM) modes. In our setup the pairs of photons are generated in a cylindrical fiber through a four-wave mixing process, which induces polarization, or spin entanglement. The photons are then converted to modes exhibiting large OAM by two helical gratings inscribed in the core of the fiber. We present a complete theoretical framework used to consistently describe this process, and demonstrate a robust control over the joint spectral amplitude of the generated photons.
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.
The global quantum network requires the distribution of entangled states over long distances, with significant advances already demonstrated using polarization, reaching approximately 1200 km in free space and 100 km in optical fiber. While Hilbert spaces with higher dimensionality, e.g., spatial modes of light, allows higher information capacity per photon, such spatial mode entanglement transport requires custom multimode fiber and is limited by decoherence induced mode coupling. Here we circumvent this by transporting multi-dimensional spatial entangled states down conventional single-mode fiber (SMF). We achieve this by entangling the spin-orbit degrees of freedom of a bi-photon pair, passing the polarization (spin) photon down the SMF while accessing multiple orbital angular momentum (orbital) sub-spaces with the other, thereby realizing multi-dimensional spatial entanglement transport. We show high fidelity hybrid entanglement preservation down 250 m of SMF across multiple 2 x 2 dimensions, which we confirm by quantum state tomography and Bell violation measures. This work offers an alternative approach to spatial mode entanglement transport that facilitates deployment in legacy networks across conventional fiber optic links.
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.