Quantum Error Correction with Superconducting Circuits
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Date
2025
Publication Type
Doctoral Thesis
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Abstract
Quantum computing has the potential to solve problems that are intractable for the most powerful supercomputers, which has far-reaching implications for science and technology. However, to realize this potential, quantum computers must overcome errors arising from decoherence and imperfect control. Quantum error correction provides a path to bridge the gap between physical error rates and the extremely low logical error rates required for quantum computing applications, at the expense of additional qubit resources.
A scalable approach to fault-tolerant quantum computation must address two overarching challenges: preserving quantum information in idling logical qubits with a manageable qubit overhead, and performing logical operations on these qubits efficiently.
Superconducting circuits provide a promising platform for addressing these challenges, but until recently, experimental demonstrations were limited to error detection protocols or repetition codes, which do not protect logical qubits from all possible types of errors.
In this thesis, we advance the state-of-the-art in quantum error correction with superconducting circuits through three main contributions. First, we implement repeated quantum error correction using the surface code, a leading approach for error correction due to its high tolerance to errors. Leveraging 17 physical qubits in a superconducting device, we realize a distance-three logical qubit, the smallest instance of the code capable of correcting an error occurring on any of the physical qubits. We preserve logical states by measuring both bit- and phase-flip error syndromes and applying corrections via a minimum-weight perfect-matching decoder. We achieve a low logical error probability of 3% per cycle when using a leakage-rejection method that discards experimental runs in which leakage is detected.
While effective for small-scale experiments, the leakage-rejection approach used in the surface code cannot be extended to larger system sizes, because the fraction of retained data decreases exponentially with the number of qubits. To address this limitation, we present a universal leakage reduction unit (LRU) based on parametric flux modulation capable of suppressing leakage with high fidelity and minimal impact on computational states. The LRU is applicable to both auxiliary and data qubits and operates in just 50 ns with fidelities comparable to single-qubit gates. We integrate the LRU in a distance-three surface code and demonstrate that it reduces errors with the key benefit of retaining all the data.
Finally, we implement logical qubits using the color code, which requires fewer qubits and enables more efficient logical operations than the surface code but involves more complex stabilizer measurements. We scale the code size and provide evidence that larger logical qubits have lower logical error rates, an essential principle of quantum error correction that relies on meeting stringent experimental requirements. Additionally, we realize transversal single-qubit logical gates and multi-qubit logical operations via lattice surgery, thereby demonstrating key techniques for error-corrected logical operations.
Together, these contributions advance the experimental realization of error-corrected quantum computation in superconducting circuits and strengthen the evidence that large-scale quantum computation can be realized in practice.
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Examiner : Wallraff, Andreas
Examiner : Andersen, Christian Kraglund
Examiner : Mueller, Markus
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ETH Zurich
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Subject
Quantum Computing; Quantum error correction; superconducting circuits
Organisational unit
03720 - Wallraff, Andreas / Wallraff, Andreas