Figure 1: Experimental setup.

(a) Heralded single-photon source. Narrow linewidth continuous-wave (CW) light at 766.35 nm wavelength and with 100 μW power is sent to a fibre-pigtailed, periodically poled lithium niobate (PPLN) waveguide that is heated to 52.8 °C. A quarter-wave-plate (QWP) and a half-wave-plate (HWP) match the polarization of the light to the crystal’s C axis to maximize the nonlinear interaction. Spontaneous parametric down conversion (SPDC) in the PPLN crystal results in frequency degenerate photon pairs with 40 nm bandwidth, centred at 1,532 nm wavelength. The residual pump light at 766.35 nm is suppressed by 50 dB by a filter (F) and the bandwidth of the created photons is filtered down to 50 GHz using a dense-wavelength-division-multiplexer (DWDM). The filtered photon pairs are probabilistically split using a beam-splitter (BS2). The detection of one member (the ‘idler’ photon) heralds the presence of the other (the ‘signal’ photon), which is directed to the input of the AFC memory. (b) Quantum memory. The quantum memory is based on an erbium-doped fibre that is exposed to a magnetic field of 600 G and cooled to a temperature below 1 K. Light from two independent CW lasers with wavelengths of 1,532.5 and 1,532.7 nm, respectively, is used to spectrally tailor the inhomogeneously broadened 1,532 nm absorption line of erbium through frequency-selective optical pumping into one or several atomic frequency combs. Towards this end, phase-modulators (PM1 and PM2) followed by an acousto-optic modulator are used to generate chirped pulses with the required frequency spectrum. The optical pumping light from the two lasers is merged on a beam-splitter (BS1) and enters the erbium fibre from the back via an optical circulator. Polarization controllers (PM1 and PM2) match the polarization to the phase-modulators' active axes, and the polarization scrambler ensures uniform optical pumping of all erbium ions in the fibre¹³. (c) Measurement unit. The detection of the heralding photon (‘idler’) and subsequently the ‘signal’ photon is performed by two superconducting nano-wire single-photon detectors (SNSPD1 and SNSPD2) maintained at the same temperature as the memory. The coincidence analysis of the detection events is performed by a time-to-digital converter (TDC).