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Molecular crystals for quantum memories and computers

Published on March 9th, 2022
Illustration representing a quantum computer using a europium molecular crystal. © Christian Grupe

A team of researchers, partners of the SQUARE project, reports in Nature on a novel material that exhibits promising features for quantum computers and communications.

Efficient communication between quantum systems mainly depends on their ability to effectively interact with light, which provides relevant functionalities such as communicating over large distances. Furthermore, performing computing operations requires that the superposition states of a qubit remain stable for long periods. However, effective quantum systems that combine both are rather scarce. There is a need so they can interact with light to create processing functionalities for information and communication.

Ideally, such a platform includes an interface with light and also information storage units, memories. Since information processing must be plausible within these memory units, we need new materials that enable the link between nuclear spins and light on the quantum level, allowing them to communicate properly. Developing these materials is a current challenge and an especially difficult one.

Now, a team of researchers from the Centre National de la Recherche Scientifique/Chimie ParisTech/PSL, the Karlsruhe Institute of Technology and Université de Strasbourg has demonstrated the potential of a new material doped with rare-earth ions as a reliable photonic quantum system. This work took place in the framework of the Quantum Flagship Square project, coordinated by KIT and in which CNRS/Chimie ParisTech/PSL is also a partner.

The results, published today in Nature, show the interest in molecular crystals containing europium – a rare earth element – for effective photon-spin interfaces.

Rare-earth molecular crystals for light/nuclear spin interfaces

“For building quantum devices, we need solid-state systems in which quantum states can be stored, efficiently recovered, and where they can interact with each other”, says Dr. Diana Serrano, first author of the publication. “Here, we propose a novel material platform combining rare-earth ions and molecules, able to support both photonic and spin qubits, and presenting great potential for integration with photonic structures.”

These materials are crystals that combine two systems already used in quantum technologies: rare-earth ions and molecular systems. On the one hand, rare-earth crystals are known for their excellent optical and spin properties, but their integration in photonic devices is complex. On the other hand, molecular systems generally lack spins -a storage or computing unit-, or on the contrary, present optical lines that are too broad to establish a reliable link between spins and light.

In that sense, the europium molecular crystals represent a great advantage, as they feature ultra-narrow linewidths and thus, can maintain long-lived quantum states. This makes it possible to store a light pulse inside these molecular crystals, as demonstrated in this work.

Promising properties for quantum computers

The study also highlights that to execute useful quantum operations, many entangled qubits need to interact with each other to produce valuable outcomes. For this, europium ions in molecules have shown that they can couple between each other using electric-stray fields, enabling future entanglement and, hence, quantum information processing. As the molecules are structured with atomic precision and arranged in exact crystals, a high density of qubits can be possibly reached.

This new material for quantum technologies offers properties not seen so far as any other material, paving the way for new architectures for computers and quantum memories in which light will play a central role. These results also open new paths for research in this field, as many new molecular compounds can be synthesized.


More information

Cited article: Diana Serrano, Kuppusamy Senthil Kumar, Benoît Heinrich, Olaf Fuhr, David Hunger, Mario Ruben, Philippe Goldner: Rare-earth Molecular Crystals with Ultra-Narrow Optical Linewidths for Photonic Quantum Technologies. Nature, 2022.



KIT institute Press release

SQUARE project

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