  Tunable biphotons are highly important for a wide range of quantum
applications. For some applications, especially interesting are cases where two
photons of a pair are far apart in frequency. Here, we report a tunable
biphoton source based on a xenon-filled hollow-core photonic crystal fiber.
Tunability is achieved by adjusting the pressure of the gas inside the fiber.
This allows us to tailor the dispersion landscape of the fiber, overcoming the
principal limitations of solid-core fiber-based biphoton sources. We report a
maximum tunability of 120 THz for a pressure range of 4 bar with a continuous
shift of 30 THz/bar. At 21 bar, the photons of a pair are separated by more
than one octave. Despite the large separation, both photons have large
bandwidths. At 17 bar, they form a very broad (110 THz) band around the
frequency of the pump.
