The quantum network works like a series of turnstiles that open only for a pair of perfectly synchronized photons. The central node tells them which turnstile to go to, but cannot forge the synchronization. 200 users can communicate with each other over distances up to 200 km — about the size of a large city.
To do this, two photons are sent to a half-silvered mirror. If they are identical, they always exit from the same side, like two people stepping into a turnstile in perfect unison. Any attempt to eavesdrop disturbs their synchronicity and is detected. From these joint passages, a key is born with complete randomness.
To create hundreds of identical pairs, they use a microscopic comb made of silicon nitride: it generates a rainbow of colors with precise spacing, like in a laboratory spectrometer. Photons travel at the speed of light and keep the secret even from the provider, who merely services the turnstiles.
The effect that makes photons 'march in step' was first demonstrated in 1987 by Leonard Mandel and colleagues. It works only for fully indistinguishable particles — the slightest imperfection breaks the synchronization. Today, this same trick underpins quantum teleportation and computing.