This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: We are continuously sending information to each other, transmitting zeros and ones through a giant network of connected computers and devices. Scientists are now trying to extend this familiar concept of the internet to the quantum realm, looking for an efficient way to exchange quantum rather than classical information: qubits instead of bits.
The motivation is not just scientific curiosity. Qubits can be a 0, a 1 or any superposition of the two. They can also become entangled, showing a degree of correlation that is out of reach for classical bits.
A key element in enabling the quantum internet is the quantum repeater, an architecture aimed at distributing entanglement over long distances. Quantum repeaters, in turn, require quantum memories that can hold a quantum state long enough to synchronize measurements across different network segments and establish entanglement. ICFO researchers Alberto Rodríguez Moldes, Dr.
Félicien Appas, Jonathan Hänni, Dr. Samuele Grandi, led by ICREA professor Hugues de Riedmatten, have taken a significant step in this direction. By implementing the so-called spin rephasing protocol, they have demonstrated that solid-state quantum memories—promising candidates for building quantum networks because of their proven high efficiency, capacity to store entanglement and multiplexing features—can store single photons for longer than previously possible.
The results, published in Physical Review Letters and obtained within the Quantum Internet Alliance (QIA), bring us closer to the quantum internet. In the experiment, the team first generated a pair of entangled photons. One had a telecom wavelength, compatible with optical fiber for long-distance distribution; the other was compatible with the quantum memory, a crystal doped with praseodymium ions and cooled to 3 kelvin in a cryostat.
There, the well-established Atomic Frequency Comb (AFC) protocol took place. In AFC, the atoms in the crystal collectively absorb the incoming photon, and one of them is excited to a higher energy level, creating an atomic excitation delocalized among all the atoms. Before they can naturally re-emit the photon, an optical control pulse transfers the collective excitation into the spin state, a level that does not emit light, effectively pausing the emission and storing the photon.
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