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arXiv:2504.00094 · Science Advances

Quantum cryptography integrating an optical quantum memory

Hadriel Mamann, Thomas Nieddu, Félix Hoffet, Mathieu Bozzio, Félix Garreau de Loubresse, Iordanis Kerenidis, Eleni Diamanti, Alban Urvoy, Julien Laurat

Published Sep 19, 2025• 12 citations
arXivHTMLPDF

Abstract

Developments in scalable quantum networks rely critically on optical quantum memories, which are key components enabling the storage of quantum information. These memories play a pivotal role for entanglement distribution and long-distance quantum communication, with remarkable advances achieved in this context. However, optical memories have broader applications, and their storage and buffering capabilities can benefit a wide range of future quantum technologies. Here, we present the demonstration of a cryptography protocol incorporating an intermediate quantum memory layer. Specifically, we implement Wiesner's unforgeable quantum money primitive with a storage step, rather than as an on-the-fly procedure. This protocol imposes stringent requirements on storage efficiency and noise level to reach a secure regime. We demonstrate the implementation with polarization encoding of weak coherent states of light and a high-efficiency cold atom-based quantum memory and validate the full scheme. Our results showcase a major capability, opening broader avenues for quantum memory utilization and network functionalities.

DOI
10.1126/sciadv.adx3223

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Developments in scalable quantum networks rely critically on optical quantum memories, which are key components enabling the storage of quantum information. These memories play a pivotal role for entanglement distribution and long-distance quantum communication, with remarkable advances achieved in this context. However, optical memories have broader applications, and their storage and buffering capabilities can benefit a wide range of future quantum technologies. Here, we present the demonstration of a cryptography protocol incorporating an intermediate quantum memory layer. Specifically, we implement Wiesner's unforgeable quantum money primitive with a storage step, rather than as an on-the-fly procedure. This protocol imposes stringent requirements on storage efficiency and noise level to reach a secure regime. We demonstrate the implementation with polarization encoding of weak coherent states of light and a high-efficiency cold atom-based quantum memory and validate the full scheme. Our results showcase a major capability, opening broader avenues for quantum memory utilization and network functionalities.
Combining cryptographic protocols with quantum memories is an important step for quantum network development in order to establish secure communications where information can be stored and retrieved on demand. One possible use case of these networks is to perform authenticated transactions synchronized by the use of memories. However, the losses and noise added by storage devices can be exploited by dishonest agents to hide their cheating attempts. The constraints to operate in a secure regime are thus very demanding in terms of memory efficiency and fidelity. This thesis focuses on the implementation of a cold-atomic ensemble used as an EIT-based quantum memory in a cryptographic protocol. The key ingredients to optimize the storage-and-retrieval efficiency and the method employed to mitigate the decoherence sources are detailed. This work reports the first demonstration of the unforgeable quantum money including an intermediate quantum memory layer, taking advantage of our highly-efficient and low-noise storage platform. The next step would be to spatially multiplex the atomic cloud in order to store the whole sequence of random qubits at once. In this scenario, the multimode capacity of our memory has been numerically simulated using two different spatial multiplexing techniques.
Wiesner's unforgeable quantum money scheme is widely celebrated as the first quantum information application. Based on the no-cloning property of quantum mechanics, this scheme allows for the creation of credit cards used in authenticated transactions offering security guarantees impossible to achieve by classical means. However, despite its central role in quantum cryptography, its experimental implementation has remained elusive because of the lack of realistic protocols adapted to practical quantum storage devices and verification techniques. Here, we experimentally demonstrate a quantum money protocol that rigorously satisfies the security condition for unforgeability, using a practical system exploiting single-photon polarization encoding of highly attenuated coherent states of light for on-the-fly credit card state generation and readout. Our implementation includes classical verification and is designed to be compatible with state-of-the-art quantum memories, which have been taken into account in the security analysis, together with all system imperfections. Our results constitute a major step towards a real-world realization of this milestone protocol.
Abstract Wiesner’s unforgeable quantum money scheme is widely celebrated as the first quantum information application. Based on the no-cloning property of quantum mechanics, this scheme allows for the creation of credit cards used in authenticated transactions offering security guarantees impossible to achieve by classical means. However, despite its central role in quantum cryptography, its experimental implementation has remained elusive because of the lack of quantum memories and of practical verification techniques. Here, we experimentally implement a quantum money protocol relying on classical verification that rigorously satisfies the security condition for unforgeability. Our system exploits polarization encoding of weak coherent states of light and operates under conditions that ensure compatibility with state-of-the-art quantum memories. We derive working regimes for our system using a security analysis taking into account all practical imperfections. Our results constitute a major step towards a real-world realization of this milestone protocol.
In order to build long-distance quantum communications, quantum memories are required as a synchronization tool which allows entanglement to be stored and retrieved on-demand, when the system is ready to perform entanglement swapping [1]. The storage-and-retrieval efficiency is a key parameter: an increase from 60% to 90% drastically decreases, typically by 2 orders of magnitude, the average time for entanglement distribution over a distance of 600 kilometers. Within this context, we reported quantum transfer between light and cold-atom-based quantum memory featuring a single mode storage-and-retrieval efficiency as high as 87%(±5%), in the single excitation regime [2]. Moreover, we demonstrated the storage of single-photon entanglement into two memories, with a ratio of the input and output concurrences reaching 88%, a more than three-fold increase compared to prior works (Fig. 1). The demonstrated capability required operating at a very large optical depth (OD) of the atomic ensembles on theD1D_{1}D1​line of cesium, and with a strong and preserved suppression of the two-photon component.
Optical photons are powerful carriers of quantum information, which can be delivered in free space by satellites or in fibers on the ground over long distances. Entanglement of quantum states over long distances can empower quantum computing, quantum communications, and quantum sensing. Quantum optical memories are devices designed to store quantum information in the form of stationary excitations, such as atomic coherence, and are capable of coherently mapping these excitations to flying qubits. Quantum memories can effectively store and manipulate quantum states, making them indispensable elements in future long-distance quantum networks. Over the past two decades, quantum optical memories with high fidelities, high efficiencies, long storage times, and promising multiplexing capabilities have been developed, especially at the single-photon level. In this review, we introduce the working principles of commonly used quantum memory protocols and summarize the recent advances in quantum memory demonstrations. We also offer a vision for future quantum optical memory devices that may enable entanglement distribution over long distances.
Quantum memory for flying optical qubits is a key enabler for a wide range of applications in quantum information. A critical figure of merit is the overall storage and retrieval efficiency. So far, despite the recent achievements of efficient memories for light pulses, the storage of qubits has suffered from limited efficiency. Here we report on a quantum memory for polarization qubits that combines an average conditional fidelity above 99% and efficiency around 68%, thereby demonstrating a reversible qubit mapping where more information is retrieved than lost. The qubits are encoded with weak coherent states at the single-photon level and the memory is based on electromagnetically-induced transparency in an elongated laser-cooled ensemble of cesium atoms, spatially multiplexed for dual-rail storage. This implementation preserves high optical depth on both rails, without compromise between multiplexing and storage efficiency. Our work provides an efficient node for future tests of quantum network functionalities and advanced photonic circuits.