Fiber source of entangled photons with giant tunable frequency separation

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Nonclassical states of light are an indispensable tool in many quantum technologies. Most demanded are pairs of entangled photons (biphotons), which are used for the heralded generation of single photons for quantum communication and quantum computation, and for quantum imaging, sensing, and spectroscopy. The diverse applications of entangled photon pairs lead to challenging requirements to their production. Highly demanded is the tunability of both the frequency range and the mode content. In particular, quantum imaging requires reliable multimode sources; on the contrary, heralded production of pure single photons dictates a need for single-mode biphotons. Recently a lot of interest is attracted to imaging and spectroscopy ‘with undetected photons’, based on the ‘induced coherence’ effect with entangled photons. Due to ‘induced coherence’, one can perform imaging or spectroscopy of any material at the frequency of one photon by looking at the photon entangled to it, which can be at a very different frequency. These methods give access to ‘difficult’ spectral ranges like mid-infrared (MIR) and terahertz.Imaging, sensing, and spectroscopy with undetected photons require sources of entangled photons with large spectral separation within the pairs. In this proposal, we aim at the generation of entangled photon pairs with the signal and idler photons separated in frequency by more than three octaves, one of them being in the UV range and the other in the IR range of spectrum. Here we plan to exploit third-order nonlinearity in gas-filled hollow-core photonic crystal fibers. These offer several advantages over the sources relying on second-order nonlinearity. First of all, the pump does not need to have a shorter wavelength than the daughter photons. We expect to achieve phase-matched narrow sidebands with a very large separation from the pump wavelength. Additionally, the dispersion of the system can be finely adjusted through the pressure of the filling gas, which makes such a system extremely versatile. This will allow not only tuning of the wavelength of the generated sidebands, but also a full control of the frequency mode content of the generated entangles states. As a test, we will carry out time ghost imaging and induced coherence experiments using the obtained fiber source. Such a project is at the frontier of two disciplines: nonlinear fiber optics and quantum optics.