Sandia and Quantinuum fully characterize a logical qubit's gate set with 'logical GST'
A Sandia–Quantinuum preprint extends gate set tomography to error-corrected operations, mapping the noise left on a Steane-code logical qubit on Quantinuum H1.
Researchers from Sandia National Laboratories and Quantinuum have adapted one of the most thorough tools for diagnosing qubit errors, gate set tomography, so that it works on error-corrected logical qubits rather than bare physical ones. In a preprint titled Logical gate set tomography, the 14-author team reports what it calls the first full, self-consistent characterization of a universal single-qubit logical gate set, run on Quantinuum's 20-qubit trapped-ion System Model H1.
What was announced
The method, shortened to "logical GST," is meant to answer a question that simple error rates cannot: once quantum error correction has done its work, what kind of noise is left? According to the paper, logical GST builds detailed models of noisy logical operations. From those models researchers can derive figures such as gate fidelity and diamond distance, and test for coherent errors and for non-Markovian effects, where noise depends on what happened earlier in the computation.
As Quantum Computing Report describes it, the experiment was the final campaign run on the H1-1 machine. The logical qubit was encoded in the seven-qubit Steane code, and the team ran active error correction with real-time feedforward, applying physical correction pulses to the ions instead of tracking corrections in software. The characterized set covered logical state preparation, measurement, the Clifford gates H and S, an idle/error-correction cycle, and a non-Clifford T gate produced by magic-state injection and teleportation.
The technical details
The paper says the hardware data showed mostly Pauli-stochastic logical errors (random bit and phase flips), no evidence of logical non-Markovianity, and only weak hints of coherent logical error. QCR reports estimated entanglement infidelities of roughly 0.2% to 1.7% for the logical gates, with large error bars, because only 50 shots were taken per circuit.
To look more closely, the authors ran the same protocol with far more shots on Quantinuum's detailed system emulator. There, according to the abstract, logical GST found weak but statistically significant non-Markovianity and strong evidence of coherent logical error. By switching individual error sources on and off in the emulator, the team traced how specific physical errors shape the logical ones. QCR reports that dephasing during ion transport and cooling was the main driver of coherent logical drift, and that non-Markovian behaviour can arise from the syndrome-extraction cycles themselves even when the physical noise is Markovian.
Why it matters
Threshold theorems and decoders usually assume that error correction turns messy physical noise into simple, independent Pauli errors. Logical GST gives hardware teams a way to check that assumption directly on encoded qubits rather than inferring it. The H1 results broadly supported the simple-noise picture, while the emulator results show the kinds of subtle, structured errors that could matter as machines grow.
What to watch
The work is a preprint and has not yet been peer reviewed. The open questions are whether the method scales to multi-qubit logical gate sets and larger codes, and whether hardware groups adopt it routinely on newer systems such as Quantinuum's Helios.
Sources
Facts on this page come from the sources below, retrieved by DesignQuantum.
- Logical gate set tomography (arXiv:2610.19034) arxiv.org
- Sandia and Quantinuum Introduce Logical Gate Set Tomography to Map Fault-Tolerant Qubit Noise (Quantum Computing Report) quantumcomputingreport.com
- Quantinuum quantinuum.com
- Sandia National Laboratories sandia.gov