Scientists have made a significant advancement in the ability to store single-photon states for extended periods, a crucial development in the pursuit of a quantum internet. A team led by ICREA professor Hugues de Riedmatten at ICFO, part of the Quantum Internet Alliance (QIA), implemented a technique called spin rephasing that enabled solid-state quantum memories to retain single photons longer than ever before. Their findings, published in Physical Review Letters, represent a major step toward creating a network that uses quantum mechanics for secure and ultra-fast communication. In the experiment, the researchers created a pair of entangled photons. One photon was tuned to a wavelength ideal for long-distance transmission through optical fibers, while the other was compatible with a quantum memory made of a crystal doped with praseodymium ions. This crystal was cooled to 3 kelvin (just above absolute zero) in a cryostat to maintain its quantum properties. The team used a technique known as the Atomic Frequency Comb (AFC) protocol, where the atoms in the crystal absorb the incoming photon, creating an excitation that is shared among all the atoms. An optical pulse then transfers this excitation into a spin state, which doesn’t emit light, effectively pausing the photon’s emission and storing its information. One of the main challenges in this process is dephasing, a phenomenon where the collective excitation stored in the spin state cannot remain stable indefinitely. Differences in the crystal environment for each ion cause the initial quantum state to lose coherence over time, which degrades the quality of the emitted photon. To address this, the researchers applied spin rephasing, a method involving a sequence of radiofrequency pulses that adjust the phase of each ion’s spin. This technique allows the spins to realign, preserving the quantum information for a longer time. Using this method, the team successfully stored single photons for up to 180 microseconds—the longest duration reported for this type of quantum memory. This corresponds to a fiber-link distance of over 30 kilometers, a significant milestone for quantum communication. The study also demonstrated that quantum correlations remained intact between the spin-rephased memory and the telecom photon. According to the research, even longer storage times could be achieved by applying small magnetic fields to the quantum memory. This work positions praseodymium-doped quantum memories as a promising platform for building scalable quantum networks.