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Quantum information

A new experiment efficiently distributes the highly secure keys to four parties instead of the typical two.

The ultra-secure ‘quantum internet’ hasn’t arrived yet, but when it does, it looks like we’ll have quantum video conferences.

One well-established quantum communication technique uses photons to create a string of data, or key, that can encrypt and decrypt messages. Eavesdroppers can be detected easily because observation disturbs the states of the photons. But these photon-based ‘quantum keys’ are typically distributed to only two parties.

Alessandro Fedrizzi at Heriot-Watt University in Edinburgh, UK, and his colleagues showed a way to pass out quantum keys efficiently to groups of three or more. For the demonstration, the team used lasers to produce sets of four photons. Each of the four photons was ‘entangled’ with the others in its set: measuring it would affect the rest, making eavesdropping difficult.

The researchers could distribute their entangled quartets over a network of fibre-optic cable with a combined length of as much as 50 kilometres. From the quartets of entangled photons, the team built a cryptographic key about one million bits long. The researchers used this to encrypt an illustration of Lewis Carroll’s Cheshire Cat and share it securely between four parties during a short conference call.

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