Scientists used a new error correction strategy to encode 70 logical qubits and tackle a problem considered classically intractable.
The quantum computation was completed in about 15 minutes, while leading classical computing methods would require an impractical amount of time to perform the same task.
IBM and researchers at the University of Chicago have announced a quantum computing demonstration that meets key criteria for quantum advantage. The experiment performed a calculation beyond the practical reach of leading classical simulation techniques while also providing evidence that the quantum computer produced a reliable result.
In a new paper, “Sampling hard circuits with verifiably high fidelity,” the researchers describe how they achieved both goals using a newly designed form of encoded quantum circuit. The work represents one of the largest demonstrations of logical quantum computing reported so far.
The circuits and experimental results have also been made publicly available through the Quantum Advantage Tracker.
Why Quantum Results Are So Difficult to Verify
Researchers have long used a benchmark called random circuit sampling (RCS) to explore whether quantum computers can outperform conventional machines.
In simplified terms, RCS challenges a quantum computer to generate patterns that become so complicated that a classical computer cannot efficiently reproduce them. That makes the benchmark useful for testing the limits of classical simulation.
But it also creates a major problem. Once the quantum calculation becomes too difficult for a classical computer to reproduce, verifying that the quantum machine actually produced the correct result becomes increasingly difficult as well. Eventually, checking the answer can itself become infeasible unless researchers make strong assumptions about how the quantum computer behaves internally.
The IBM and University of Chicago team approached this verification problem by developing a more structured alternative to RCS.
The researchers showed that their method preserves the same computational hardness criteria associated with RCS, meaning the problem remains extremely difficult for classical computers. At the same time, the added structure allows errors to be detected during the quantum computation.
“Verification remains one of the biggest challenges in firmly establishing experimental quantum advantage,” said Bill Fefferman, Associate Professor at the University of Chicago. “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.”
Soumik Ghosh, PhD student in Fefferman’s group at the University of Chicago, added, “Beyond strengthening experimental validation, advances in verification have the potential to unlock practical applications for the next generation of quantum computers.”
70 Logical Qubits With Lower Error Rates
The experiment also included one of the world’s largest-known demonstrations of quantum error correction.
The researchers operated 70 logical qubits. Unlike individual physical qubits, logical qubits are encoded in a way that helps protect quantum information from errors and noise.
Using those logical qubits, the team carried out 2,415 logical two-qubit operations and 468 logical “T gates,” two measures that reflect the complexity of the quantum circuit.
The encoded design significantly improved reliability. Effective logical error rates were 10 times lower than the underlying physical error rates, allowing the circuit to maintain unusually high fidelity even while performing a large number of quantum operations.
IBM Says Quantum Advantage Has Entered a New Stage
“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 researchers found that many leading classical simulation methods would face prohibitive runtimes when attempting the same task.
The IBM quantum computer, by comparison, completed the computation in approximately 15 minutes.
A Step Toward Larger, More Trustworthy Quantum Computers
Speed alone is not enough for quantum computing to become useful at larger scales. Researchers also need ways to suppress errors and establish confidence that a quantum system produced a valid result.
This experiment advances both goals at once by combining large-scale logical quantum computing with a method for evaluating the reliability of a calculation that is already beyond practical classical simulation.
Error correction and trustworthy verification are considered essential for scaling quantum computers toward more difficult problems, making the new demonstration an important step toward that goal.
