Figure 2: Experimental setup and level schemes.

(a) (Top left) Four-wave mixing part, providing heralded single photons: pump 1 (795 nm) and pump 2 (762 nm) are overlapped in a copropagating geometry inside the cold cloud of ⁸⁷Rb atoms in a magneto-optical trap (MOT), generating pairs of herald (776 nm) and probe (780 nm) photons. The detection of a photon at D_h heralds a probe photon. (Top right) Tuning the resonance of a bandwidth-matched cavity with respect to the heralding photon frequency controls the temporal envelope. (Bottom) Single atom part: A ⁸⁷Rb atom is trapped at the focus of a confocal aspheric lens pair (AL; numerical aperture 0.55) with a far-off-resonant optical dipole trap (980 nm). The probe photons are guided to the single atom part by a single mode fibre and focused onto the atom by the first AL. Avalanche photodetectors D_f and D_b detect photons collected in forward and backward directions. An acousto-optic modulator (AOM) shifts the probe photon frequency to compensate for the shift of the atomic resonance frequency caused by the bias magnetic field and the dipole trap. λ/2, λ/4, half- and quarter-wave plates; D_h, D_f, D_b, avalanche photodetectors (APDs); DM, dichroic mirror; F, interference filter; PDH lock, Pound–Drever–Hall frequency lock electronics; P, polarizer; (P)BS, (polarizing) beam splitter. (b) Relevant level scheme of the four-wave mixing process in a cloud of ⁸⁷Rb atoms. (c) Relevant level scheme of the single ⁸⁷Rb atom in the dipole trap. The probe photons are resonant with the closed transition |g〉=5 S_1/2, F=2, m_F=−2 to |e〉=5 P_3/2, F=3, m_F=−3.