Unforgeable quantum money can be stored in an ultracold ‘debit card’

A rudimentary quantum flow card that can be loaded with ruthless quantum money has been made from extremely cold atoms and particles of light.

In ordinary banks, the discovery of a forged bank often depends on the competence of the forger, but in a quantum bank, a law of physics called the theorem without cloning would make a successful counterfeit impossible. This law indicates that identical copies of quantum information cannot simply be done and, in 1983, the physicist Stephen Wiesner designed a protocol that exploits the theorem without cloning to create a ruthless currency. Julien Laurat at the Kastler Brossel Laboratory in France and his colleagues have now implemented the idea in the most advanced experience to date.

In this protocol, a bank issues banknotes in quantum particles which have a special set of properties – a specific quantum state – and are protected against counterfeiting by the non -cloning theorem. Laurat says that the protocol itself is a founding work in quantum cryptography, but it had never been implemented in such a way that the user could store quantum money – the fragility of quantum states means that a user should spend it immediately.

His team made such storage possible by integrating memory devices similar to hard drives in their configuration. In their experience, the user communicates with a quantum system playing the role of the bank by exchanging light particles or photons. The state of each photon can be filed in memory, similar to the loading of a debit card.

The team’s memory system was made from several hundred millions of cesium atoms that the researchers cooled to only a few million degrees above Absolute Zero by hitting them with lasers. At this extreme temperature, quantum states of atoms could be controlled very precisely with light, but Laurat says that it took years to determine how to do well enough for the cold atomic memory to work within the framework of a quantum flow card. Thanks to repeated tests, he and his colleagues have shown that photons can be recovered effectively atoms when the user wants to spend his quantum money without these states being corrupt in the process.

Christoph Simon at the University of Calgary in Canada says that the new experience is a step in the direction of quantum money in its own right, but the storage time for quantum memory, which is around 6 million seconds, is still too short for the protocol to be practical. “Another [future step] is to increase portability. I think that the long -term objective, especially in the context of quantum money, would be a quantum memory that you can put in your pocket. But we are certainly not yet there, ”he says.

The team aims to increase this storage time – if it was a thousand times more, the protocol could be used in metropolitan quantum networks that already exist in the cities of the world, explains Laurat. In addition, cutting-edge quantum memories could allow long-term ultra-secure quantum communication as well as helping to connect several quantum computers to a more powerful device, he said.