A new light-spin interface with Europium(III) molecule advances the development of quantum computers - KIT researchers report in Nature Communications
Light can be used to operate quantum information processing systems, such as quantum computers, quickly and efficiently. Researchers at the Karlsruhe Institute of Technology (KIT) and at Chimie ParisTech - CNRS have now significantly advanced the development of molecule-based materials that are suitable as light-addressable fundamental quantum units: As they report in the journal Nature Communications, they have demonstrated for the first time the possibility of addressing nuclear spin levels of a molecular complex of europium(III) rare-earth ions with light. (DOI: 10.1038/s41467-021-22383-x)
Whether in drug development, in communication or for climate predictions: Processing information quickly and efficiently is crucial in many fields. Digital computers, which work with so-called bits, currently serve this purpose. The state of a bit is either 0 or 1 - there is nothing in between. This severely limits the power of digital computers, and it is becoming increasingly difficult and time-consuming to handle complex problems related to real-world tasks. Quantum computers, on the other hand, use quantum bits to process information. A quantum bit (qubit) can be in many different states between 0 and 1 at the same time due to a special quantum mechanical property, the quantum superposition. This makes it possible to process data in parallel. This increases the computing power of quantum computers exponentially compared to digital computers.
Superposition states of a qubit must exist long enough
"In order to develop quantum computers that can be used practically, the superposition states of a qubit should exist for a sufficiently long time. Researchers refer to this as coherence lifetime," explains Professor Mario Ruben, head of the research group "Molecular Quantum Materials" at the Institute for Quantum Materials and Technologies (IQMT) of KIT and at the European Center for Quantum Sciences - CESQ at the University of Strasbourg. "However, the superposition states of a qubit are fragile and are disturbed by fluctuations in the environment, which leads to decoherence, i.e. shortening of the coherence lifetime." To preserve the superposition state long enough for computational operations, isolating a qubit from the noisy environment is conceivable. Nuclear spin levels in molecules can be used to create superposition states with long coherence lifetimes, as nuclear spins are well shielded from the environment by external electronic orbitals, protecting the superposition states of a qubit from interfering external influences.
Molecules are ideal qubit systems
However, a single qubit is not enough to build a quantum computer. It requires many qubits that have to be organised and addressed. Molecules are ideal qubit systems because they can be arranged in sufficiently large numbers as identical scalable units and addressed with light to perform qubit operations. Furthermore, the physical properties of molecules, such as emission and/or magnetic properties, can be tailored by changing their structure using chemical design principles. In their publication, which has now appeared in the journal Nature Communications, researchers led by Professor Mario Ruben at KIT's INT and Dr Philippe Goldner at the École nationale supérieure de chimie de Paris (Chimie ParisTech - CNRS) present a dimeric europium(III) molecule containing nuclear spin as a light-addressable qubit.
The molecule, which belongs to the rare earth metals, is designed to show luminescence when excited by ultraviolet light-absorbing ligands surrounding the centre, i.e. a europium(III)-centred sensitised emission. After light absorption, the ligands transfer the light energy to the europium(III) centre, thereby exciting it. The relaxation of the excited centre to the ground state leads to light emission. The whole process is called sensitised luminescence. Spectral hole burning - special experiments with lasers - detects the polarisation of the nuclear spin levels, indicating the generation of an effective light-nuclear spin interface. This enables the generation of light-addressable hyperfine qubits based on nuclear spin levels. "By demonstrating for the first time polarisation at a light-nuclear spin interface associated with the nuclear spin of the europium(III) ion in a molecule, we have succeeded in taking a promising step towards the development of quantum computing architectures based on rare-earth ion-containing complexes," explains Philippe Goldner.
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