Professor Mario Ruben, chemist and expert for molecular quantum materials at the Karlsruhe Institute of Technology (KIT) receives an Advanced Grant awarded by the European Research Council (ERC) for his “Hilbert Molecules” project that focuses on the development of multi-state qubits, called qudits, which are generated by the nuclear spins of molecules. The aim of his research is to improve the scalability and controllability of quantum mechanics devices. This marks an essential step toward the Quantum Internet. The ERC is funding the project with roughly EUR 2.5 million over five years.
“I extend my congratulations to Mario Ruben for this great success,” said Professor Oliver Kraft, Vice President Academic Affairs at KIT. “The ERC Advanced Grant honors researchers who leave their mark internationally in their discipline. His outstanding work at the interface of chemistry and quantum technologies has set new standards and emphasizes the scientific quality of research at KIT as well as its importance for the development of future quantum communication systems.”
From Qubit to Qudit
Mario Ruben is professor at KIT and at Strasbourg University and conducts research on molecular materials for quantum technologies at KIT’s Institute for Quantum Materials and Technologies (IQMT). Such materials enable the use of quantum effects such as superposition and entanglement, which have a crucial function in quantum-based information processing. “It’s a big challenge to keep the coherent states, which are required in quantum operations, stable for this purpose,” said Ruben, who also heads the “Molecular Materials” research group at KIT’s Institute of Nanotechnology (INT). “We’re addressing this challenge by developing multi-state quantum bits, called qudits.”
So far, quantum computers have used two-state quantum bits, in short qubits, as the smallest computational units. Qubits can not only assume the states zero and one, but also intermediate and superposition states. Moreover, they can be entangled. Qudits in turn can assume significantly more states, where “d” defines the dimension of the Hilbert space, i.e. the mathematical space for quantum operations. This provides more computational power with fewer quantum particles and makes quantum systems more robust and less susceptible to errors.
Expanded Mathematical Space
To realize such qudits in practice, the “Hilbert Molecules” project (full name: Hilbert Space Engineering of Nuclear Spin Qudits by Isotopologue Coordination Chemistry) leverages the nuclear spins, i.e., the intrinsic angular momentum of atomic nuclei, as a quantum resource. Here, Mario Ruben’s research group resorts to lanthanide complexes – rare-earth compounds where trivalent metal ions attach to different organic ligands. These materials are particularly well suited to realize quantum systems that can assume many states. In the project, which is going to be funded by an ERC Advanced Grant, the researchers working with Mario Ruben are relying on qudit complexes as quantum hardware – these are produced through precise chemical synthesis. Electronic coupling of multiple nuclear spin units expands the Hilbert space, i.e. the available mathematical space, exponentially, thereby enabling more complex quantum operations. “The project links synthetic chemistry and quantum physics,” said Ruben.
In addition, the researchers are developing optical methods that allow them to read out nuclear spins in a minimally invasive and nondestructive manner. The aim is to improve the scalability and controllability of corresponding quantum systems. “This marks an essential step towards Quantum Internet that we are taking jointly with our physics colleagues at KIT,” said Ruben. KIT already operates a 20-kilometer fiber-optic connection between Campus North and Campus South that is used for research tasks such as exploring the linking and distribution of quantum resources.
2025 ERC Advanced Grants
ERC Advanced Grants are an initiative by the European Research Council (ERC) with the aim to fund recognized top scientists with an outstanding scientific track record who want to open up new research areas. After the 2025 round of calls, the ERC honored 319 top researchers from all over Europe with Advanced Grants and will be funding their projects with a total amount of EUR 838 million. 3,329 researchers – an absolute record – had applied for an ERC Advanced Grant, of which 9.6% were successful.
Mario Ruben