A bit of information can be kept secure for 24 hours before being revealed — more than 5 million times longer than the previous record.

Quantum cryptography guards against eavesdroppers, but in secure voting and sealed-bid auctions, a message must remain unread and protected for a certain period of time. Routing the message through a pair of trusted 'friends' that are between the sender and receiver can delay and secure the message, but the friends would need to be located extremely far away from the sender and receiver to achieve a delay of more than a few milliseconds.

Anthony Martin and his colleagues at the University of Geneva in Switzerland developed a protocol in which this kind of exchange happened 5 billion times, with encryption occurring at each round that built on that created previously. This allowed the authors to separate the sender and receiver computers from their 'friends' by just 7 kilometres while securing the bit for 24 hours.

Phys. Rev. Lett. 117, 140506 (2016)