Standard optical fiber is like a glass thread that carries light. But when you send weak quantum signals (individual photons) and powerful classical laser light through it at the same time, trouble arises. The glass slightly scatters light: this is called Raman scattering. It creates noise that overwhelms quantum signals, much like loud music drowns out a whisper.
Scientists found a workaround: use hollow-core fiber. Inside such a fiber, there's air instead of glass, so light barely scatters. In the experiment, two signals traveled simultaneously through a 122 km fiber: a quantum one for time synchronization and a classical one for transmitting a reference frequency. They occupied the same wavelength band, just 10 nanometers apart.
When the classical power was raised to 3 milliwatts, the quantum signal barely suffered. Calculations show that with new, even quieter fibers, signals could travel 500 km and withstand powers above 10 milliwatts. This means quantum and classical data could peacefully share the same cables, like different instruments in one orchestra, without drowning each other out.
Hollow-core fiber guides light through an air channel inside a glass tube. Light travels faster than in standard fiber because air slows it down less than glass does.