Two counts answer different questions
Physical qubits are the underlying quantum components. A logical qubit encodes information across multiple physical qubits so errors can be detected and handled. Google Quantum AI’s December 2024 paper describes this distinction through experiments with quantum memory. [1]
- Physical: components and operations
- Logical: information encoded across them
- Check: how long is it protected?
More hardware can buy protection
The paper reports that increasing the size of its error-correcting code reduced the logical error rate under the tested conditions. Its introduction stresses a condition: the physical operations must already be below a critical noise threshold. Adding components alone is not the whole recipe. [1]
Storing information is a milestone
The reported experiment concerns protecting a quantum state through error-correction cycles. The authors also discuss requirements such as sustained stability and fast classical decoding. Those requirements help explain why a memory result is a step toward a larger computing task, rather than a direct measure of every application’s capacity. [1]
Ask what the headline counted
Our reading checklist: does the number describe physical components or logical qubits? What operation was demonstrated, and under what conditions? Only then ask how the result relates to the problem you want solved. Avoid turning one impressive count into a universal measure of usefulness.
Go a little deeper
Optional reading · about 1 more minute
Why we checked the correction
An April 28, 2026 author correction changes the labels in Figure 3a, including identifying its horizontal axis as repetition-code distance. We checked that notice and the corrected article. This explainer uses the physical/logical distinction and memory scope; it does not reproduce or compare that figure’s numerical results. [2]
A useful distinction without a fixed conversion
Our interpretation: think of the physical count as part of the equipment description and the logical count as part of the information-protection description. Do not infer a fixed exchange rate between them from this one experiment. This article explains an earlier study, not a September 2026 hardware announcement.
Original sources
Attributed synthesis, not original reporting. Examples labeled hypothetical or illustrative are explanatory. Reviewing a source does not independently validate its findings.
- Google Quantum AI and Collaborators: Quantum error correction below the surface code threshold ↗
Published online December 9, 2024. Corrected article abstract, main introduction and surface-code memory explanation read in Chrome September 14, 2026. Developer-led study; no independent replication.
- Author Correction: Quantum error correction below the surface code threshold ↗
April 28, 2026 notice read in Chrome September 14. Figure 3a axis and key labels corrected; no figure or numerical results reproduced here.