Europium molecular crystals, a rare-earth-based material, could provide a robust platform for photonic quantum technologies. The work of a research team from the National Centre for Scientific Research (CNRS), the University of Strasbourg, Chimie ParisTech-PSL, and the Karlsruhe Institute of Technology (KIT) demonstrated the quantum capabilities of this material, the ultranarrow optical transitions of which support optimal interaction with light. The researchers believe that given the high number of different molecular compounds that can be synthesized, the results of the work could additionally broaden the prospects for research into molecular crystals for photonic quantum technologies. Rare-earth ions show promise as solid-state systems for building light-matter interfaces at the quantum level due to their ability to show long-lived quantum states. However, few crystalline materials have exhibited an environment that is quiet enough to fully exploit the quantum properties of rare-earth ions. Molecular systems can provide such an environment, but they generally lack spin states. Moreover, when molecular systems do have spin states, they show optical lines that are too broad to allow a reliable link to be established between spins and light. Europium molecular crystals are a combination of rare-earth ions and molecular systems. These crystals have linewidths in the tens of kilohertz range — orders of magnitude narrower than those of other molecular systems. The researchers took advantage of this property to demonstrate the potential of europium molecular crystals to provide efficient optical spin initialization, coherent storage of light using an atomic frequency comb, and optical control of ion-ion interactions for the implementation of quantum gates. The ultranarrow linewidths of europium molecular crystals translated into a long-lasting quantum state, and the researchers exploited this property to demonstrate the storage of a light pulse inside the crystals. They believe that the results of their experiments show the potential of these rare-earth molecular crystals to become a platform for photonic quantum technologies that would combine highly coherent emitters with the versatile capabilities of molecular materials in the areas of composition, structure, and integration. Communication between quantum systems over long distances depends on their ability to effectively interact with light. So far, it has been challenging to find a material that can fully exploit the quantum properties of light. Molecular crystals are of increasing interest as a means to develop quantum computers that can communicate with each other using fiber optic networks. An illustration representing a quantum computer using a europium molecular crystal. Courtesy of Christian Grupe. Quantum systems that can interact with light to create processing functionalities for information and communication, through fiber optics in particular, require not only an interface with light, but also information storage units, or memory. Information processing must be possible within these units, which take the form of spin. The development of materials that enable a link between spins and light on the quantum level has proven especially challenging. The research was published in Nature (www.doi.org/10.1038/s41586-021-04316-2).