Fig. 1

Experimental setup. Atom–photon entanglement is generated between a single trapped ⁴⁰Ca⁺-ion, confined and laser-cooled in a linear Paul trap, and a single photon at 854 nm. The photons are collected with a HALO (“High-numerical-Aperture Laser Objective”) and coupled to a single-mode fiber. A combination of two quarter waveplates (QWP) and one half wave plate (HWP) is inserted behind the fiber to compensate all unitary rotations of the polarization state caused by the single-mode fiber and several dichroic mirrors between the fiber and the HALO. A flip mirror can be inserted behind the fiber to send the photons either to the converter lab via a 90 m long single-mode fiber or to the projection setup for 854 nm. The polarization-preserving frequency converter is realized with a nonlinear waveguide crystal in a single-crystal Mach–Zehnder configuration (for details see main text and Methods). The converted photons are detected with superconducting single-photon detectors. PBS polarizing beam splitter, DM dichroic mirror, BS beam splitter, BPF band pass filter, FBG fiber Bragg grating