Dynamic surface codes open new avenues for quantum error correction

Quantum error correction (QEC) is crucial for reaching the ultra low error rate necessary for useful quantum algorithms. At Google Quantum AI, our quantum processors use physical qubits constructed from small superconducting circuits, which are susceptible to noise. QEC allows us to combine numerous physical qubits into logical qubits, which are robust to noise.

In December 2024, we announced that operation of error correction on our Willow quantum processor was below threshold, signifying that the logical qubit's robustness to errors exponentially increases as more physical qubits are added. This demonstration utilized a surface code for high-performance quantum error-correction. During the operation of this surface code, we employed a static circuit, i.e., a single consistent set of underlying physical operations was executed repeatedly to measure and correct errors. These static circuits, while useful for realizing QEC on a device with full yield, limit the ability to avoid "dropouts" — qubits or couplers that fail.

Today in “Demonstration of dynamic surface codes”, recently published in Nature Physics, we are excited to report the experimental demonstration of a surface code operating with dynamic circuits. Unlike their static counterparts, these dynamic circuits detect errors by alternating between different circuit constructions, which provides greater flexibility in our choice of the types of gates, connectivity, and correlated error suppression. Using dynamic circuits allows us to sidestep some of the big challenges that superconducting qubits face, like leakage out of the computational subspace, hardware layout constraints, and qubit dropouts.

Physical errors are triangulated using detecting regions

Quantum error correction on a hexagonal lattice

Exchanging the roles of data and measure qubits

Make way for the iSWAP gate

What’s next

Acknowledgments

The primary contributors to this work are Alec Eickbusch, Matt McEwen, and Alexis Morvan.