The photons are produced at telecom wavelength (1550 nm), which is optimal for bridging long distances via readily deployed glass fibres used for high-speed internet. At this wavelength, absorption losses in fibres are extremely low, with only 2% of photons being lost over one kilometre. Since these photons are entangled, they cannot be considered as individual systems, no matter how far they are apart. In other words: When one measures e.g. the polarisation of one of the photons, one knows that a polarisation measurement of its partner photon will yield a correlated outcome. This strange “spooky action at a distance”, how Albert Einstein called it, enables two communication partners (traditionally called Alice and Bob) to create an unconditionally secure means of communication. Alice and Bob are each connected to the source of entangled photons via their own glass fibre, therefore getting one photon out of an entangled pair each.