2026-01-23
On January 22nd, a joint scientific research team involving core members of Logical Qubit Technology published a paper titled "Demonstration of low-overhead quantum error correction codes" in the globally top-tier academic journal Nature Physics. For the first time worldwide, they experimentally demonstrated quantum error correction using high-rate Bivariate Bicycle Codes on a superconducting quantum processor featuring long-range couplers.
New Directions in Quantum Error Correction Technology
The core of achieving large-scale fault-tolerant quantum computing lies in significantly reducing the error rate of logical qubits through quantum error correction techniques. In the field of superconducting quantum computing, the Surface Code, due to its hardware-friendly requirement for nearest-neighbor coupling, has long dominated the mainstream position in fault-tolerant quantum computing, with Google being the most representative company in surface code error correction. However, the low encoding rate of the Surface Code leads to high hardware resource overhead when scaling up. How to reduce hardware resource costs while improving error correction performance has become a new line of exploration.
Against this backdrop, high-rate quantum Low-Density Parity-Check (qLDPC) codes are seen as an important route to reducing resource overhead. In 2023, the IBM team led by Bravyi proposed the Bivariate Bicycle Code (BB Code). This novel error-correcting code cleverly introduces two carefully selected "non-local long-range" connections on the basis of the traditional Surface Code, which only involves four neighboring qubits. Theoretical simulations indicate that, under the adopted noise model and performance metrics, BB Code requires only about one-tenth of the physical qubit overhead of the Surface Code to achieve equivalent error correction performance, pointing a new direction for low-cost quantum error correction.
Innovative Chip Design and Tackling Engineering Challenges in Fabrication
Despite the theoretical superiority of BB Code, implementing it from concept to hardware poses immense challenges. Constructing non-local long-range connections on a two-dimensional planar quantum chip, and achieving high-fidelity quantum gates in parallel on these complex long-range couplings, presents engineering difficulties significantly higher than those of the Surface Code, which only requires nearest-neighbor connections.
To address this challenge, the core members of Logical Qubit Technology participated in the design of the "Kunlun" 32-qubit high-connectivity superconducting quantum chip. This chip introduces additional long-range tunable coupling structures on the foundation of two-dimensional nearest-neighbor connectivity to support the high connectivity and non-local stabilizer measurements required by BB Code.
To tackle the engineering problems brought by long-range coupling, such as wiring crossovers and parasitic coupling, the team performed targeted optimizations in chip fabrication processes, introducing air bridge structures over the long-range couplers. This key process effectively solved the complex wiring crossover issues faced by high-connectivity superconducting chips, providing crucial support for achieving high-fidelity parallel control on long-range couplers.
Experimental calibration results show that the average parallel gate fidelity for single-qubit and two-qubit gates on the "Kunlun" quantum processor reached 99.95% and 99.22%, respectively.
Architecture diagram of the Kunlun quantum processor and the non-local stabilizer extraction circuits for BB Code
Based on the "Kunlun" quantum processor, the team experimentally demonstrated two BB Code schemes: the [[18,4,4]] code and the [[18,6,3]] code. The former encodes 4 logical qubits with a distance of 4 using 18 data qubits, while the latter encodes 6 logical qubits with a distance of 3. By executing efficient non-local stabilizer extraction circuits, the team successfully demonstrated multiple rounds of quantum error correction.
Experimental results indicate that the average logical error rates (statistics per round, per logical qubit) for the [[18,4,4]] code and the [[18,6,3]] code were 8.91% and 7.77%, respectively. Furthermore, numerical simulations by the research team predict that, under the adopted noise model and decoding settings, if the physical operation error rate of the current chip can be reduced to half of its current level, it could surpass the error correction threshold of BB Code, providing an important foundation for future breakthroughs past the break-even point (where logical qubit lifetime surpasses physical qubit lifetime).
Partial experimental results graph for the [[18,4,4]] BB Code
Fault-Tolerant Quantum Chips are the Cornerstone of Universal Quantum Computers
Quantum error correction is the essential path to achieving large-scale fault-tolerant quantum computing. Currently, public understanding of quantum computing comes more from intuitive visual perceptions of quantum computers, while a complete superconducting quantum computer includes crucial components such as quantum chips, quantum control systems, quantum frequency systems, and quantum software and algorithms. Among these, the chip is the indispensable cornerstone of a quantum computer, and fault-tolerant quantum chips are the key determinant of victory in the future strategic competition for universal quantum computing.
Fault-tolerant quantum chips represent a systematic and engineering challenge. It not only tests the capabilities in chip design and fabrication but also equally tests the ability to coordinate and adapt quantum control, radio frequency, software, and other systems. Precisely for this reason, the Logical Qubit Technology team has been committed from the outset to the synergistic power of "technology-driven + engineering innovation." Building on the technological accumulation of the Zhejiang University superconducting quantum computing laboratory in chip design and fabrication, they explore and validate more efficient error correction pathways while advancing surface code error correction. Simultaneously, they continuously enhance their own engineering capabilities, independently mastering core technologies and engineering capabilities from chips, radio frequency, and control to the entire quantum computer system. They are accelerating the arrival of the quantum computing era in a pragmatic manner.
