Quantum key distribution is a cryptographic method that can guarantee secure communication. A satellite-based experiment has shown that this technique can be applied over long distances without the need for trusted relays.
Modern society is driven by the large-scale exchange of information. As a result, secure communication of sensitive data around the world is an increasingly valuable asset. The mathematical toolbox that is widely used for this task can be complemented by applying the principles of quantum physics to enhance the security of the communication link.
This approach has highly desirable features, such as protection of the encrypted information from threats that might arise as a consequence of future advances in computational power. However, it also comes with substantial technological challenges in terms of the range of communication possible and the degree of trust in the devices used. Writing in Nature, Yin et al. demonstrate that such cryptographic solutions can be deployed over distances exceeding 1,000 kilometres, without compromising the security promised by the underlying quantum technology.
The flagship application of quantum communication is known as quantum key distribution (QKD). This process enables two parties located at a distance from each other to share a secret string of bits (units of information) called a key, which they can use to encrypt and decrypt secret messages, without making assumptions about the computational power of a potential eavesdropper. Although the principle of such absolute security is based solidly on fundamental laws of nature, practical implementations come in different configurations.
For example, it is possible for one of the two parties to prepare quantum states of light — the natural physical carrier of information in quantum communication — and to send them to the second party, who measures them. By processing these data using standard classical communication, the two parties can then extract the secret key.
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