Fig. 1: Experimental setup.

Light-matter entanglement is generated between a single trapped ⁴⁰Ca⁺ ion and the polarization state of an emitted photon at 854 nm. The photons are collected with a high-aperture laser objective (HALO), coupled to a single-mode fiber and guided to the QFC device. The latter features a PPLN waveguide embedded in a polarization Sagnac interferometer to guarantee polarization-preserving operation. The converted photons pass a series of spectral filters (band-pass filter (BPF), volume Bragg grating (VBG) and etalon) to suppress background stemming from the DFG process. The projection setup at 1550 nm consists of a motorized QWP and HWP, a Wollaston prism to split orthogonally-polarized photons, and two fiber-coupled superconducting-nanowire single-photon detectors (SNSPD). In the lower left part the level scheme of the ⁴⁰Ca⁺ ion including the most relevant states and transitions for entanglement generation and quantum state readout is shown. The atomic qubit is encoded in two Zeeman levels (m = − 3/2 and m = 1/2) of the metastable D_5/2-state.