Researchers at ICFO in Barcelona have successfully stored telecom-heralded single photons in a solid-state quantum memory for a record 180 microseconds while preserving nonclassical correlations. Published in Physical Review Letters, this breakthrough utilizes praseodymium-doped crystals and spin rephasing to tackle the coherence losses stalling practical quantum networks.
Today's classical internet relies on strong infrastructure to route data across vast distances. Building a quantum internet requires overcoming an entirely different physical constraint: quantum states, or qubits, cannot be copied or amplified using standard repeaters. Because photons degrade and get lost in optical fibers, long-distance quantum communication depends on quantum memories that can safely hold information while distant parts of a network catch up.
If a memory loses its quantum coherence before neighboring links finish executing their operations, the entire network synchronization fails. Storage time is the critical bottleneck.
Inside the Praseodymium Crystal Architecture
To tackle this synchronization challenge, the team led by Prof. Hugues de Riedmatten at ICFO constructed their memory using a praseodymium-doped yttrium orthosilicate crystal, chemically designated as Pr³⁺:Y₂SiO₅. Inside a cryostat, researchers cooled this solid-state medium down to roughly 3 kelvin.
The experimental setup utilizes a cavity-enhanced spontaneous parametric down-conversion source to generate photon pairs. One photon emerges at a wavelength of 606 nanometers to match the crystal’s optical transition. Its partner is emitted at 1,436 nanometers, sitting comfortably within the telecom range optimized for fiber-optic transmission.
When the 1,436-nanometer telecom photon is detected, it acts as a herald, signaling that its 606-nanometer partner is primed and available for storage inside the crystal.
Upon entry, the incoming photon encounters an atomic frequency comb, or AFC. The researchers prepare groups of praseodymium ions with a precisely spaced pattern of absorption frequencies. Rather than localizing the photon in a single atom, the material collectively absorbs it across many ions simultaneously.
Optical control pulses then shift this collective excitation into a long-lived hyperfine spin state, effectively pausing the standard optical re-emission process. Later, a subsequent control pulse converts the stored excitation back into light.
This architecture grants operators the ability to choose the retrieval time dynamically while the photon remains locked in storage, providing a vital tool for synchronizing distributed quantum-network links.
Overcoming Phase Decay with Radiofrequency Pulses
Pausing the optical process via a hyperfine spin state solves one major hurdle, but it immediately introduces another physical obstacle. Every individual praseodymium ion occupies a slightly unique microscopic environment within the crystal matrix.
These microscopic disparities cause the quantum phases of the ions to evolve at diverging rates. Over time, the ensemble falls entirely out of step.
This dephasing destroys the collective excitation, making it impossible for the ions to reconstruct the original photon efficiently. To counteract this degradation, the researchers apply carefully timed radiofrequency pulses to the crystal.
These pulses successfully reverse the spin dephasing. They allow the collective atomic excitation carrying the fragile quantum information to recover its operational coherence.
The system successfully preserved nonclassical correlations for up to 180 microseconds. In practical terms, that storage duration is enough time for light to travel an equivalent fiber distance of roughly 36 kilometers.
The Path Toward Practical Quantum Repeaters
While 180 microseconds marks a major milestone for this specific absorptive memory architecture using spin rephasing, it does not instantly instantiate a working quantum internet.
Classical light remained stored in the setup for roughly 3 milliseconds. That stark difference suggests that more refined magnetic-field control could eventually push single-photon storage durations much farther along.
Operational quantum networks will demand extended storage times, higher end-to-end efficiencies, and stable connections spanning numerous intermediate nodes.
By demonstrating that a solid-state memory architecture can successfully preserve quantum light while utilizing a technique capable of scaling to longer storage timelines, the ICFO team has cleared a critical hurdle on the roadmap toward global quantum communication.
