Heralded single-photon sources with on-demand readout are promising candidates for quantum repeaters enabling long-distance quantum communication. The need for scalability of such systems requires simple experimental solutions, thus favouring room-temperature systems. For quantum repeater applications, long delays between heralding and single-photon readout are crucial. Until now, this has been prevented in room-temperature atomic systems by fast decoherence due to thermal motion. Here we demonstrate efficient heralding and readout of single collective excitations created in warm caesium vapour. Using the principle of motional averaging we achieve a collective excitation lifetime of 0.27 ± 0.04 ms, two orders of magnitude larger than previously achieved for single excitations in room-temperature sources. We experimentally verify non-classicality of the light-matter correlations by observing a violation of the Cauchy-Schwarz inequality with R = 1.4 ± 0.1 > 1. Through spectral and temporal analysis we identify intrinsic four-wave mixing noise as the main contribution compromising single-photon operation of the source. 1 arXiv:1801.03286v1 [quant-ph] 10 Jan 2018Long-distance quantum communication beyond the limit of direct optical fiber transmission (∼400 km)[1] requires a network of quantum repeater (QR) nodes. A QR increases the distance over which entanglement can be efficiently distributed by means of entanglement swapping [2]. Many attempts to realize such nodes are based on the so-called DLCZ protocol for atomic ensembles [3].Key parameters for entanglement distribution in a QR network are the success rate of entanglement generation and the storage time in the elementary links of the network. For atomic ensemble-based QR nodes, this implies fast, high fidelity generation of heralded collective excitations and efficient retrieval after a controllable delay. The available storage time must exceed by far the average generation time as well as the end-to-end classical communication time of the network, although multiplexing of nodes can reduce the memory time requirement [4].Since the first experimental realizations of the DLCZ protocol [5,6] more than a decade ago, frequent improvements in cold atomic ensembles have been reported [7][8][9][10][11][12][13][14][15][16][17][18] with memory times reaching 0.22 s [17] and retrieval efficiencies up to 84 % [16]. Progress has recently also been shown in solid-state systems, particularly in rare-earth-doped crystals [19][20][21][22]. However, cryogenic cooling is required there. Room-temperature systems offer reliability and scalability, as they do not need cooling apparatus. Spin coherence with timescales of seconds in NV centers [23], and minutes with atomic vapour in anti-relaxation-coated glass containers [24], has been demonstrated at room temperature. Still, coherent optical interaction with NV centers at room temperature remains a challenge [25]. Broadband, short-lived quantum memories have been demonstrated in warm vapours [26,27], but thermal atomic motion impe...

Non-classical photon sources are a crucial resource for distributed quantum networks. Photons generated from matter systems with memory capability are particularly promising, as they can be integrated into a network where each source is used on-demand. Among all kinds of solid state and atomic quantum memories, room-temperature atomic vapours are especially attractive due to their robustness and potential scalability. To-date room-temperature photon sources have been limited either in their memory time or the purity of the photonic state. Here we demonstrate a single-photon source based on room-temperature memory. Following heralded loading of the memory, a single photon is retrieved from it after a variable storage time. The single-photon character of the retrieved field is validated by the strong suppression of the two-photon component with antibunching as low as $${g}_{{\rm{RR| W = 1}}}^{(2)}=0.20\pm 0.07$$ g RR∣W=1 ( 2 ) = 0.20 ± 0.07 . Non-classical correlations between the heralding and the retrieved photons are maintained for up to $${\tau }_{{\rm{NC}}}^{{\mathcal{R}}}=(0.68\pm 0.08)\ {\rm{ms}}$$ τ NC R = ( 0.68 ± 0.08 ) ms , more than two orders of magnitude longer than previously demonstrated with other room-temperature systems. Correlations sufficient for violating Bell inequalities exist for up to τBI = (0.15 ± 0.03) ms.

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