Computers also make mistakes. These are usually suppressed by technical measures or detected and corrected during the calculation. In quantum computers, this involves some effort, as no copy can be made of an unknown quantum state. This means that the state cannot be saved multiple times during the calculation and an error cannot be detected by comparing these copies. Inspired by classical computer science, quantum physics has developed a different method in which the quantum information is distributed across several entangled quantum bits and stored redundantly in this way. How this is done is defined in so-called correction codes. In 2022, a team led by Thomas Monz from the Department of Experimental Physics at the University of Innsbruck and Markus Müller from the Department of Quantum Information at RWTH Aachen and the Peter Grünberg Institute at Forschungszentrum Jülich in Germany implemented a universal set of operations on fault-tolerant quantum bits, demonstrating how an algorithm can be programmed on a quantum computer so that errors can be corrected efficiently. However, different quantum error correction codes also come with different difficulties. A theorem states that no correction code can implement all the gate operations required for freely programmable computations with the logical quantum bits easily and protected against errors.
Quantum gates are realized with different correction codes
To circumvent this difficulty, Markus Müller's research group has established a method that allows the quantum computer to switch back and forth between two correction codes in an error-tolerant manner. "In this way, the quantum computer can switch to the second code whenever a logic gate that is difficult to realize appears in the first code. This makes it easier to implement all the gates required for computing," explains Friederike Butt, a doctoral student in Markus Müller's research group. She developed the quantum circuits on which the experiment is based and implemented them in close collaboration with Thomas Monz's research group in Innsbruck. "Together, we have succeeded for the first time in realizing a universal set of quantum gates on an ion trap quantum computer using two combined quantum error correction codes," says PhD student Ivan Pogorelov from the Innsbruck research group.
"This result is based on our many years of good collaboration with Markus Müller's team," says Thomas Monz, who knows the theoretical physicist from his doctoral studies at the University of Innsbruck.
The findings of the current study were published in the journal Nature Physics. The research was financially supported among others by the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG, the German DFG, the Bavarian State Government, the European Union and the Federation of Austrian Industries Tyrol.
Publication: Experimental fault-tolerant code switching. Ivan Pogorelov, Friederike Butt, Lukas Postler, Christian D. Marciniak, Philipp Schindler, Markus Müller, and Thomas Monz. Nature Physics 2025. DOI: 10.1038/s41567-024-02727-2 [arXiv: 2403.13732 ]
