In the rapidly evolving landscape of information technology, digital systems today are fundamentally built upon the binary digit, or bit. This seemingly simple unit of information exists in one of two states—0 or 1—and underlies the vast majority of data processing and storage technologies in classical computing architectures. Bits are typically represented in electronic circuits as distinct voltage levels, facilitating reliable encoding and manipulation of digital information. Over the decades, this binary foundation has enabled the emergence of a highly interconnected world, driving advancements ranging from complex simulations to everyday communication and media consumption.
As we venture further into the quantum era, the classical bit faces a revolutionary counterpart: the quantum bit, or qubit. Unlike classical bits, qubits exploit the principles of quantum mechanics, allowing them to exist simultaneously in a superposition of states 0 and 1. This quantum superposition opens an entirely new realm of possibilities for computing and information processing, promising unprecedented capabilities in speed and problem-solving potential. Nevertheless, harnessing qubits’ power remains a challenge due to their fragile nature and the difficulty inherent in their reliable storage and retrieval.
Emerging quantum technologies such as quantum computers and the developing quantum internet rely on sophisticated quantum memory systems to hold and manipulate these qubits. Quantum memories serve as critical components that temporarily store quantum information, enabling synchronization and scalability in quantum networks and computational schemes. Among the diverse implementations explored, solid-state quantum memories stand out as particularly promising, offering the advantages of robustness, scalability, and integration potential with existing photonic platforms.
Researchers at ICFO—The Institute of Photonic Sciences—have recently achieved a groundbreaking advance in this realm. The team, led by ICREA Professor Hugues de Riedmatten and including scientists Dr. Markus Teller, Susana Plascencia, Cristina Sastre Jachimska, and Dr. Samuele Grandi, have developed an innovative array comprising ten individually controllable solid-state quantum memory cells embedded within a single praseodymium-doped crystal. This platform heralds a significant leap forward in quantum memory technology, particularly because it allows the storage and on-demand retrieval of qubits across multiple cells with high fidelity.
Their revolutionary approach is detailed in a forthcoming publication in the journal Physical Review X, where the researchers demonstrate the ability to store qubits encoded both in spatial modes—known as path encoding—and temporal modes, or time-bin encoding, within the quantum memory array. Time-bin encoding exploits the photon’s arrival time to represent qubit states, enabling the storage of multiple photons in distinct temporal slots per memory cell. This temporal multiplexing substantially increases the quantum memory’s storage capacity and operational flexibility.
The heart of their apparatus is a praseodymium-doped crystal cooled to cryogenic temperatures near 3 Kelvin. Within this crystal lattice, the researchers effectively engineer 250 distinguishable storage “slots” or spatio-temporal modes, setting a new world record for solid-state devices capable of on-demand qubit retrieval. Achieving on-demand access to stored quantum information is a formidable technical challenge, yet is indispensable for practical quantum networks where data synchronization and adaptive control determine overall system performance.
Intelligent use of acousto-optical deflectors allowed the team to direct writing and reading laser pulses selectively to any of these ten memory cells, enabling unprecedented control over the spatial distribution of stored qubits. This capability to address memory cells independently and retrieve photons exactly when required elevates the platform from a passive memory bank to a dynamic, random-access quantum memory system. Subsequent measurements confirmed that the quantum states preserved within the array retain high fidelity upon retrieval, a strong indication of the system’s potential for reliable quantum information processing.
By simultaneously recalling two time-bin qubits stored in separate cells, the researchers showcased the versatility and scalability of their approach. Such capability moves closer to the quantum equivalent of classical random-access memory (RAM), a cornerstone for the advancement of scalable quantum computing architectures. Dr. Markus Teller envisions integrating this solid-state memory array with sources of photonic cluster states, enabling the generation and storage of large entangled states essential for measurement-based quantum computing paradigms.
Beyond computing, this technology promises substantial benefits for quantum communication networks. Quantum repeaters, the critical devices enabling long-distance quantum entanglement distribution, stand to gain notably from this research. Traditional solid-state quantum memories struggled with the operational bottleneck posed by waiting for entanglement success signals before progressing. The multiplexed memory array allows the system to circumvent these delays by dynamically switching across memory cells to attempt entanglement distribution without idling, thereby increasing the entanglement distribution rate and enhancing overall quantum network throughput.
Looking ahead, the team acknowledges that challenges remain before fully scalable quantum memory arrays can be realized. Enhancements in storage efficiency, coherence time extension, and the number of controllable memory cells are active areas of investigation. Equally important is achieving the storage and manipulation of entangled states between spatially separated memory cells, a key requirement for complex quantum network topologies and error-corrected quantum computing schemes.
This work represents a decisive stride toward bridging the gap between quantum information theory and practical hardware capable of supporting robust, high-capacity quantum storage. By harnessing the unique properties of rare-earth-doped crystals and sophisticated optical control mechanisms, the ICFO team has paved the way for quantum memories that approach the functionality and flexibility of classical RAM but operate under fundamentally different quantum mechanical principles.
Future quantum processors and communication infrastructures will depend heavily on such advances to realize their full potential. As the quantum information field moves from isolated proof-of-concept demonstrations toward integrated, scalable systems, devices like the solid-state quantum memory array described here will be fundamental enablers. The era of quantum-enhanced technologies inches closer, promising to revolutionize computing, secure communications, and beyond, with the quantum memory array standing out as a crucial piece of this transformative puzzle.
Subject of Research: Development of a solid-state quantum memory array for storage and on-demand retrieval of qubits.
Article Title: Quantum Storage of Qubits in an Array of Independently Controllable Solid-State Quantum Memories
News Publication Date: 25-Aug-2025
References:
M. Teller, S. Plascencia, C. Sastre Jachimska, S. Grandi, and H. de Riedmatten. et al. A solid-state temporally multiplexed quantum memory array at the single-photon level. npj Quantum Inf 11, 92 (2025). DOI: [not provided]
M. Teller, S. Plascencia, S. Grandi, and H. de Riedmatten. Quantum storage of qubits in an array of independently controllable solid-state quantum memories. Phys. Rev. X (2025). DOI: [not provided]
Image Credits: ICFO
Keywords: Quantum memory, Communications
