On October 24, a research team led by Professor Kwon Young-hun at the Mathematical Physics Laboratory, Hanyang University ERICA Campus, announced its world-first success in implementing a syndrome extraction circuit using ‘flag qubits’ for quantum error correction on an IBM quantum computer. This represents a significant technical advance in quantum error correction, a core technology for practical quantum computing.

Quantum computers have the potential to rapidly solve complex calculations that classical computers find challenging, but are limited by frequent errors due to qubits' sensitivity to external environments. As circuit complexity increases, accumulated errors severely impact computational accuracy.

To address these limitations, Professor Kwon’s team directly implemented an error correction circuit using flag qubits on IBM quantum hardware with a non-topological structure. Although IBM’s hardware constraints previously made error correction circuit realization difficult, the team introduced flag qubits to successfully build a syndrome extraction circuit which efficiently detects errors.

Experiments applying the repetition code revealed that as code distance(length) increased from 3 to 9, the logical error rate consistently decreased. Similar results were observed even when data qubits and error-detecting qubits were physically separated or had flag qubits between them, confirming the practical effectiveness of flag qubits. This demonstrates that these circuits operate efficiently even under hardware constraints.

Professor Kwon stated, “It is meaningful that we have experimentally demonstrated the effectiveness of error correction circuits on IBM’s commercial quantum processors. By expanding to more complex codes, we expect further improvement in error suppression capabilities.”

This research is regarded as an important milestone for realizing large-scale, fault-tolerant quantum computing, and is expected to contribute to the stability of quantum hardware and the development of practical quantum computers.

The project was supported by the National Research Foundation of Korea’s Quantum Information Science Human Resource Development Program and Individual Basic Research Program, and was published in the leading journal Quantum on October 24.

In the paper Effectiveness of the syndrome extraction circuit with flag qubits on IBM quantum hardware, Kim Young-hoon(Master’s course graduate) was first author and Professor Kwon Young-hun served as corresponding author.