Mid-circuit measurements are one of the biggest practical hurdles in quantum error correction on encoded qubits. Researchers in Innsbruck and Aachen have now proposed and experimentally demonstrated that a universal fault-tolerant quantum algorithm can be executed without such measurements. Using a trapped-ion quantum processor, the team successfully ran Grover's quantum search algorithm on three logical qubits.
A key bottleneck in today's leading approaches to quantum error correction is the need to repeatedly pause and measure the quantum processor mid-computation, a process that is slow, technically demanding, and itself a significant source of errors. Now, a joint team from the University of Innsbruck, RWTH Aachen University, Forschungszentrum Jülich and spin-off Alpine Quantum Technologies (AQT) has demonstrated fault-tolerant quantum computation without any such interruptions.
Faster and less error-prone
In a study published in Nature Communications, the team presents a complete toolbox of fault-tolerant quantum operations that eliminates so-called mid-circuit measurements and feed-forward control entirely. Rather than stopping the computation to read out error information and classically deciding on a correction, the new approach processes error information coherently. "That happens entirely within the quantum computation itself, using only standard quantum gate operations", says Friederike Butt. "This makes the method faster and potentially less error-prone than conventional schemes, and particularly well-suited to hardware platforms where measurements are especially costly."
First proof-of-concept
To put their approach to the test, the researchers implemented Grover's quantum search algorithm fault-tolerantly on three logical qubits encoded across eight physical qubits of a trapped-ion quantum processor. The experiment clearly identified the correct solutions, providing a compelling proof-of-concept. "For the first time, we have shown that a complete fault-tolerant quantum algorithm can be executed without mid-circuit measurements with feed-forward control," says Ivan Pogorelov from the Department of Experimental Physics at the University of Innsbruck. "This is a new paradigm for quantum error correction, and this experiment is a first, important step toward realizing its full potential", adds team leader Thomas Monz.
Measurement-free quantum computation
The theoretical framework was developed by Friederike Butt and Markus Müller at RWTH Aachen University and Forschungszentrum Jülich, while the experimental implementation was carried out by Ivan Pogorelov and others at the University of Innsbruck. Their findings demonstrate the practical feasibility of measurement-free protocols and mark an important first step toward exploring this largely uncharted direction in quantum computation.
The work was supported by the European Union, the Austrian Science Fund FWF, the Austrian Research Promotion Agency FFG, the Federation of Austrian Industries Tyrol and other funding bodies.
Publication: Demonstration of measurement-free universal logical quantum computation. Friederike Butt, Ivan Pogorelov, Robert Freund, Alex Steiner, Marcel Meyer, Thomas Monz & Markus Müller. Nature Communications (2026) 17:995. DOI: 10.1038/s41467-026-68533-x
