What is IIT-Delhi's quantum communications breakthrough? | Explained

The story so far: On June 16, the Ministry of Defence said in a statement that IIT-Delhi scientists together with the Defence Research and Development Organisation (DRDO) demonstrated quantum communication over a distance of more than 1 km in free space. The development was hailed as a breakthrough in quantum cybersecurity.

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What is quantum communication?

When two or more photons, the subatomic particles of light, are created in just the right way, measurements made on one photon will instantly determine the result for the partner photon, too — even if the photons are far apart. This phenomenon is called quantum entanglement. Ordinary or classical physics can't explain this correlation, it's a purely quantum phenomenon. Quantum communication is an umbrella term for any scheme that uses the concepts of quantum physics, but especially entanglement, to make a given communication channel leak-proof. In one scheme, like the one the IIT-Delhi team demonstrated, entangled photons carry information from a source to two stations. If any third party intercepts one of the photons, the other photon will immediately be disturbed as well and the channel will be revealed as insecure.In short, quantum communication can be used to create communication channels that are protected against computational attacks since any attempt to tap the quantum channel will itself be revealed. Thus they have great value in defence settings. An important method in quantum communication is quantum key distribution (QKD).

How does QKD work?