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Integration into a quantum money protocol shows that memories can now handle very demanding applications for quantum networking.

Researchers at the Kastler Brossel Laboratory (Sorbonne Université, CNRS, ENS-Université PSL, Collège de France), together with colleagues from LIP6 (Sorbonne Université, CNRS), have taken a major step forward in quantum technology: for the first time, they have integrated an optical quantum memory into a cryptographic protocol. This achievement, based on Wiesner's unforgeable quantum money scheme, demonstrates that quantum memories are now mature enough to operate under very demanding conditions for networking.

In a study on September 19 in Science Advances, the Paris team implemented Wiesner's quantum money, a foundational idea in quantum cryptography that relies on the no-cloning theorem to prevent counterfeiting. Unlike previous demonstrations that bypassed storage, this experiment incorporated an intermediate memory step—an essential capability for real-world applications where quantum data must be held and released on demand.

The concept of quantum money dates back to the 1980s, when physicist Stephen Wiesner proposed using the laws of quantum mechanics to create unforgeable banknotes. Because unknown quantum states cannot be copied without being disturbed, such "quantum tokens" could be used for authenticated transactions, providing security guarantees far beyond what classical methods can achieve.

In the experiment, the researchers used weak pulses of light whose polarization encoded the information. These pulses were stored in a large ensemble of laser-cooled neutral atoms—a quantum memory platform that has recently reached record performance, combining near-unity efficiency with extremely low noise. After storage, the states were retrieved and passed through the rest of the protocol, where they had to be validated under strict security thresholds.

The results showed that the memory was able to meet the stringent requirements of the transaction, successfully enabling the creation and verification of "quantum money" tokens. "This is the first time a quantum memory has been integrated into a complete cryptography protocol," says Hadriel Mamann, former Ph.D. student at LKB and first author of the study. "The experiment combined several key advances in both the photonic implementation and the storage step. Reaching the high efficiency and low noise required for the protocol really shows how far quantum memories have come."

Quantum memories are best known as key building blocks for quantum repeaters, which would enable entanglement distribution across long distances, crucial for a future quantum internet. But their usefulness goes far beyond: they can synchronize quantum processors, allocate entanglement across a network, and -as shown here- enable tasks that were previously thought out of reach.

"This demonstration shows that quantum memories can now handle one of the most demanding tests," says Prof. Eleni Diamanti, co-lead of the study and director of the Paris Center for Quantum Technologies. "It opens the way to a much wider range of applications, from secure multiparty protocols to anonymous communication. Beyond cryptography, it also paves the road for advanced quantum networking functionalities, where memories act as essential buffers and synchronizers, key elements for distributed quantum computing and for building truly scalable, interconnected quantum systems."

This demonstration builds on earlier advances from Prof. Laurat's group, such as the demonstration of highly-efficient quantum memory for entanglement, and from , such as the optical implementation of verifiable quantum money.