Quantum computers promise the ability to solve complex problems in seconds that would take decades for modern supercomputers. While the goal is clear, the path to achieve it remains unclear due to the quantity of potential approaches to realize it. Each one has specific advantages and disadvantages when it comes to 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. Schematic representation of a spin-photon-based quantum processor consisting of six optically coupled quantum registers. Courtesy of Fraunhofer. The researchers are using the materials properties of diamond to develop a quantum computing platform that can be just as powerful as other approaches, but without the drawbacks. Qubits are created using color centers in the diamond lattice by trapping an electron in one of the artificially created lattice defects, or vacancy centers, doped with nitrogen, silicon and nitrogen, germanium, or tin. The electron spin couples through magnetic interaction with five nuclear spins of neighboring carbon isotopes. The central electron spin can then be used as an addressable qubit, said 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 said. 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. Presenting at 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 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 along with the technology of the photonic resonators. The basis for this project 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 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 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.
