Quantum computers based on spin photons and diamond promise significant advantages over competing quantum computing technologies, such as lower cooling requirements, longer operating times and lower error rates. The consortium of the BMBF project SPINNING coordinated by Fraunhofer IAF has succeeded in advancing the development of spin-photon-based quantum computers. The partners recently presented the interim project results at the mid-term meeting of the BMBF funding measure Quantum Computer Demonstration Setups in Berlin.
There are still several competing approaches to realizing quantum computers, each with specific advantages and disadvantages in terms of hardware and software, ranging from reliability and energy consumption to compatibility with conventional systems. Under the coordination of the Fraunhofer Institute for Applied Solid State Physics IAF, a consortium of 28 partners is working on the project "SPINNING - Diamond spin-photon-based quantum computer" to develop a quantum computer based on spin photons and diamond. This should be characterized by lower cooling requirements, longer operating times and lower error rates than other quantum computing approaches. The hybrid concept of the spin-photon-based quantum computer also provides for greater scalability and connectivity, which enables flexible connection with conventional computers.
"In the SPINNING project, we want to make an important contribution to the German quantum technology ecosystem. To this end, we are using the material properties of diamond to develop a quantum computing technology that can be just as powerful as the other technologies but has none of their specific weaknesses. We create qubits using color centers in the diamond lattice by trapping an electron in one of four artificially created lattice defects (vacancy centers) doped with nitrogen (NV), silicon and nitrogen (SiNV), germanium (GeV) or tin (SnV). The electron spin couples through magnetic interaction with five nuclear spins of neighboring 13C carbon isotopes. The central electron spin can then be used as an addressable qubit," explains Prof. Dr. Rüdiger Quay, coordinator of the SPINNING network and institute director at Fraunhofer IAF.
"The individual qubits form a matrix structure, the qubit register. The SPINNING quantum computer will consist of at least two and later up to four of these registers, which in turn will be optically coupled over long distances of 20 m, for example, so that a comprehensive exchange of information can take place," Quay continues. The optical coupling between the central electron spins and registers is realized by an optical router in combination with a light source and a detector for readout. The individual states of the nuclear spins are controlled by high-frequency pulses.
On the occasion of the mid-term meeting of the funding measure Quantum Computer Demonstration Setups of the Federal Ministry of Education and Research (BMBF), under which SPINNING is funded, the consortium presented the interim project results on October 22 and 23, 2024, in Berlin. They are characterized by remarkable successes. For the first time, the project team successfully demonstrated the entanglement of two registers of six qubits each over a distance of 20 m and achieved a high mean fidelity (in the sense of the similarity of the entangled states).
Further project successes include significant improvements in the central hardware and software as well as the peripherals for the spin-photon-based quantum computer: The basic material and its processing, the realization of color centers in diamond for the generation of qubits, could be improved as well as the technology of the photonic resonators. The basis for this was a better understanding of the four types of defects in the diamond lattice and the error mitigation of diamond-based qubits. The consortium also succeeded in developing the electronics required to operate the quantum computer and demonstrating the first applications of the quantum computer for artificial intelligence.
The exemplary comparison of the interim results of SPINNING with the key indicators of quantum computers based on superconducting Josephson junctions (SJJs) underlines the value of the work done in the project as, to date, many times more resources have been invested worldwide into the latter's development. With an error rate of < 0.5%, the spin-photon-based quantum computer comprising twelve qubits to date achieves the same result in the one-qubit gate as the prominent SJJ models Eagle (127 qubits) and Heron (154 qubits).
In terms of coherence time, the spin-photon-based quantum computer with a length of over 10 ms clearly outperforms the SSJ models (> 50 µs), although the distance for entanglement is many times greater at 20 m compared to a few millimeters.
The remaining technical challenges until the end of the project include the further development of the resonator design towards improved reproducibility and more precise alignment. On the other hand, the researchers are working on further improving the software for automatic control of the spin-photon-based quantum computer's routing.
SPINNING is funded by the Federal Ministry of Education and Research BMBF with the funding measure Quantum Computer Demonstration Setups within the framework program of the Federal Government Quantum Technologies - from Fundamentals to Market. Fraunhofer IAF leads the SPINNING consortium of six universities, two non-profit research institutions, five industrial companies (SMEs and spin-offs) and 14 associated partners: Fraunhofer Institute for Applied Solid State Physics IAF (Coordinator), Fraunhofer Institute for Integrated Systems and Device Technology IISB, Research Center Jülich GmbH, Karlsruhe Institute of Technology (KIT), University of Constance, University of Heidelberg, Technical University of Munich, University of Ulm, Diamond Materials GmbH, Freiburg im Breisgau, NVision Imaging Technologies GmbH, Ulm, Qinu GmbH, Karlsruhe, University of Stuttgart, Quantum Brilliance GmbH, Stuttgart, Swabian Instruments GmbH, Stuttgart and 14 associated partners from science and industry.