Marino, GiuseppeSolntsev, AlexanderXu, LeiGili, Valerio F.Carletti, LucaPoddubny, AlexanderRahmani, MohsenSmirnova, DariaChen, HaitaoLemaitre, AristideZhang, GuoquanZayats, Anatoly V.De Angelis, ConstantinoLeo, GiuseppeSukhorukov, AndreyNeshev, Dragomir2020-11-062020-11-062334-2536http://hdl.handle.net/1885/214114Optical nanoantennas have shown a great capacity for efficient extraction of photons from the near to the far field, enabling directional emission from nanoscale single-photon sources. However, their potential for the generation and extraction of multi-photon quantum states remains unexplored. Here we experimentally demonstrate the nanoscale generation of two-photon quantum states at telecommunication wavelengths based on spontaneous parametric down-conversion in an optical nanoantenna. The antenna is a crystalline AlGaAs nanocylinder, possessing Mie-type resonances at both the pump and the bi-photon wavelengths, and when excited by a pump beam it generates photon pairs with a rate of 35 Hz. Normalized to the pump energy stored by the nanoantenna, this rate corresponds to 1.4 GHz/Wm, being 1 order of magnitude higher than conventional on-chip or bulk photon-pair sources. Our experiments open the way for multiplexing several antennas for coherent generation of multi-photon quantum states with complex spatial-mode entanglement and applications in free-space quantum communications and sensing.Australian Research Council (DP150103733, DP160100619, DE180100070, DP190101559, DE170100250, DE190100430; UK Engineering and Physical Sciences Research Council (EP/M013812/1); PANAMA; NOMOS (ANR-18-CE24-0026); EU MULTIPLY program; Royal Society; Erasmus Mundus NANOPHI (5659/002-001); Russian President (MD-5791.2018); ERC iCOMM project (789340); Wolfson Foundation; National Natural Science Foundation of China (11774182, 91750204); Russian Foundation for Basic Research (18-29-20037, 18-32-20065).application/pdfen-AU© 2019 Optical Society of Americahttps://www.osapublishing.org/submit/review/pdf/OSACopyTransferOAAgrmnt(2017-09-05).pdf2019-11-0410.1364/OPTICA.6.0014162020-07-06OSA Open Access Publishing Agreement