Light can be used to distribute quantum information rapidly, efficiently and in a secure, tamper-proof manner. Researchers at the Karlsruhe Institute of Technology (KIT) in Germany, and at Strasbourg University, Chimie ParisTech and the French national research center CNRS, all in France, have now achieved major progress in the development of materials for processing quantum information with light. In a paper in Nature, they present a europium molecule that possesses nuclear spins, allowing the creation of an effective photon-spin interface.

Quantum information will revolutionize not only research and industry, but also our everyday lives. Among other benefits, it promises enormous progress in the simulation of materials and processes, furthering the development of new medical substances and the improvement of batteries, transport planning, and secure information and communication.

Unlike a conventional bit, a quantum bit (qubit) can assume many different states between 0 and 1 at the same time. This so-called quantum superposition allows the massive parallel processing of data. As a result, the computing capacity of quantum computers will increase exponentially compared to digital computers.

To carry out quantum computing operations, however, the superposition states of a qubit have to persist for a certain time. In quantum research, this is referred to as coherence lifetime. Nuclear spins, i.e. angular momentums of atomic nuclei, in molecules can produce superposition states with long coherence lifetimes, because nuclear spins are well shielded from the environment and can protect qubits against external impacts.

“For practical applications, we have to be able to store, process and distribute quantum states,” says Mario Ruben, head of the Molecular Quantum Materials Group at KIT’s Institute for Quantum Materials and Technologies (IQMT) and of the European Center for Quantum Sciences – CESQ of Strasbourg University. “For this, we have now identified a promising novel type of material: a europium molecule containing nuclear spins. Europium belongs to the rare-earth metals.”

The molecule is structured such that it exhibits luminescence when excited by a laser, emitting photons that carry nuclear spin information. By means of specific laser experiments, an effective light/nuclear spin interface can be produced. This current work covers the addressing of nuclear spin levels with the help of photons, coherent storage of photons and the execution of first quantum operations.

To execute useful quantum operations, many qubits entangled by quantum mechanics are required. For this, the qubits must interact with each other. The researchers from KIT, Strasbourg and Paris have now proved that europium ions in molecules can couple via electric fields, potentially leading to entanglement and, hence, quantum information processing. As the molecules are structured with atomic precision and arrange in exact crystals, a high qubit density can be reached.

Another aspect relevant to practical applications is the addressability of individual qubits. Optical addressing increases the readout speed and helps prevent interfering electrical feeds, while frequency separation allows a number of molecules to be addressed individually. Compared to previous projects, the researchers were able to achieve around 1000 times better optical coherence in a molecular material. This means nuclear spin states can be manipulated optically in a specific way.

Light is also suited for distributing quantum information over larger distances, to connect quantum computers or to securely transmit information. This might be achieved by future integration of the novel europium molecule in photonic structures to enhance transitions. “Our work represents an important step towards quantum communication architectures with rare-earth molecules as a basis for a quantum internet,” says David Hunger at IQMT.

This story is adapted from material from Karlsruhe Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

Researchers have shown how the photon-spin interface in a europium molecular crystal can be used to entangle nuclear spin qubits (arrows) with the help of photons (yellow). Image: Christian Grupe, KIT.
Researchers have shown how the photon-spin interface in a europium molecular crystal can be used to entangle nuclear spin qubits (arrows) with the help of photons (yellow). Image: Christian Grupe, KIT.