Spatial mode conversion of single photons at the C-band using in fiber long-period gratings

The ability to convert the spatial mode of single photons opens up a promising path to enhancing quantum communication protocols by enabling high-dimensional encoding and efficient multiplexing. In this work, we demonstrate spatial mode conversion of single photons at 1550.6 nm using a fiber long-period grating (LPG). The fundamental [Formula: see text] mode was converted to higher-order modes [Formula: see text] and [Formula: see text], with quantum mode conversion efficiencies of 87.5 ± 1.4% and 96.1 ± 1.6%, respectively. The characterization of the converted single photons was carried out using a time-of-flight technique and coincidence measurements, by taking advantage of the differences in group velocity between the modes. We also performed loss measurements at the single-photon level and demonstrated mode re-conversion by using a second LPG to restore the photons back to the fundamental mode. These results highlight the potential of LPGs as a versatile tool for spatial mode manipulation at the single-photon level, with applications in high-dimensional quantum communication and nonlinear optical interactions.