Tunable fiber source of entangled UV-C and infrared photons.

Pairs of entangled photons-biphotons-are indispensable in quantum applications. However, some important spectral ranges, like the ultraviolet, have been inaccessible to them so far. Here, we use four-wave mixing in a xenon-filled single-ring photonic crystal fiber to generate biphotons with one of the photons in the ultraviolet and its entangled partner in the infrared spectral range. We tune the biphotons in frequency by varying the gas pressure inside the fiber and thus tailoring the fiber dispersion landscape. The ultraviolet photons are tunable from 271 nm to 231 nm and their entangled partners, from 764 nm to 1500 nm, respectively. Tunability up to 192 THz is achieved by adjusting the gas pressure by only 0.68 bar. At 1.43 bar, the photons of a pair are separated by more than 2 octaves. The access to ultraviolet wavelengths opens the possibility for spectroscopy and sensing with undetected photons in this spectral range.