Quantum computer completes verified task beyond practical reach of classical simulations

Quantum computer completes verified task beyond practical reach of classical simulations Gaby Clark Scientific Editor Robert Egan Senior Editor IBM and researchers from the University of Chicago announced a demonstration in quantum computing that meets the fundamental criteria for "quantum advantage"—the point where quantum computers can be confirmed to have outperformed classical computers on trusted computations. The collaboration said its system had performed computations beyond the reach of leading classical simulation methods while providing confidence that the computation returned accurate results. In their new paper, the researchers showed that these two goals could be achieved through a novel construction of encoded quantum circuits—one of the largest demonstrations of logical quantum computing to date. The paper is published on the arXiv preprint server. Building trust into quantum results For years, researchers have used a benchmark known as random circuit sampling, or RCS, to test whether quantum computers could outperform classical systems. In simple terms, RCS asks a quantum computer to generate patterns so complex that a classical computer cannot efficiently reproduce them. The challenge has been verification: As the problem becomes harder, it becomes increasingly difficult, then infeasible, to prove the quantum computer's answer is correct without making strong assumptions about the inner workings of the quantum computer. In their experiment, researchers addressed this obstacle with a structured alternative to RCS. The team was able to prove that this alternative retains the same hardness criteria as RCS, but crucially, the new structure can be used to detect errors during the computation. "Verification remains one of the biggest challenges in firmly establishing experimental quantum advantage," said Bill Fefferman, associate professor of computer science at UChicago and co-author of the paper. "This experiment develops techniques to better characterize the fidelity of hard quantum states under noise, increasing confidence that the quantum computer is solving a computationally hard problem." Co-author Soumik Ghosh, a graduate student in Fefferman's group at UChicago, added: "Beyond strengthening experimental validation, advances in verification have the potential to unlock practical applications for the next generation of quantum computers." In one of the world's largest error-correction demonstrations to date, the team successfully executed 70 logical qubits while shielding them from errors. They ran 2,415 logical two-qubit operations and 468 logical "T gates," both metrics that quantify the complexity of a quantum circuit. "We are now firmly in the quantum advantage era," said Jay Gambetta, director of IBM Research and IBM Fellow. "We have demonstrated a quantum computation beyond the practical reach of classical computers that establishes, with statistical confidence, a lower bound on how faithfully it was executed. This milestone gives scientists, developers and businesses a new foundation for trusting quantum computers as they scale to problems far beyond what we can achieve classically." The IBM quantum computer took approximately 15 minutes to accomplish the task; the team showed that many leading classical simulation approaches faced prohibitive runtimes. Publication details Simon Martiel et al, Sampling hard circuits with verifiably high fidelity, arXiv (2026). DOI: 10.48550/arxiv.2607.25941 Journal information: arXiv Key concepts Quantum algorithms & computationProvided by University of Chicago