  Pairs of entangled photons -- biphotons -- are indispensable in quantum
applications. However, some important spectral ranges, like 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 235 nm and their entangled
partners, from 764 nm to 1342 nm, respectively. The tunability up to 170 THz is
achieved by adjusting the gas pressure by only 0.57 bar. At 1.32 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.
