Generation of photon triplets via three-photon parametric down-conversion

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Nonclassical states of light are indispensable in many quantum technologies. Currently available are on-demand single photons, squeezed states, and pairs of entangled photons. In the previous proposal we aimed at expanding this list of available quantum states by generating, for the first time, triplets of entangled photons through third-order spontaneous parametric down conversion (TOSPDC). This can be viewed as the reversal of third harmonic generation (THG) but it is considerably more challenging experimentally. Although TOSPDC, as well as its potential uses, have been extensively studied theoretically, its experimental implementation in optics remains a challenge. It would enable heralded generation of photon pairs, direct generation a non-Gaussian states and substantially increase the toolbox of quantum information.During the work on the project, we explored, theoretically and experimentally, various potential sources of TOPDC, including bulk crystals, gas-filled photonic crystal fibres and specially designed hybrid fibre. We performed THG for all these systems and identified the most suitable ones. For bulk crystals we also implemented seeded TOPDC. The results showed that unseeded TOPDC is too weak to observe in all investigated systems. Meanwhile, we identified two promising new systems: highly nonlinear microlayers and tapered optical fibres embedded in gas-pressure cells. For both systems, we achieved first results: pressure-tunable THG in tapered fibers and generation of entangled photons in microlayers. We now apply for the extension of the project aiming at unseeded TOPDC in these new systems. In tapered fibres, we also plan an additional new step, namely the use of alkali-vapor in the gas environment. This will enable resonant control over dispersion, allow phase-matching fusing only the lowest-order modes and increase the efficiency. The same method will allow for enhancing THG in such systems. Finally, during the extension we plan to investigate TOPDC using stimulated emission tomography.