Surface-code Superconducting Quantum Processors: From Calibration to Logical Performance

Quantum computers hold great potential to solve complex physics and chemistry problems that are beyond the capabilities of classical computers. Unlike classical systems that use bits represented as deterministic 0s and 1s, quantum computers operate with quantum bits or qubits, which can exist in a superposition of states. In addition to superposition, quantum computers harness the power of entanglement and quantum interference that enable quantum computers to encode and process information in ways that classical systems fundamentally cannot. These properties allow quantum computers to explore vast solution spaces in parallel, solving certain computational problems far more efficiently than classical systems. This has numerous potential applications ranging from the factoring of large numbers, optimizing complex systems, computational chemistry, machine learning, cryptography, and artificial intelligence.

Yet, current quantum processors are fragile, noisy and fairly limited in both quantity and quality with tens of qubits and physical error rates of around 10-³. To realize practical quantum applications, however, error rates need to be below 10-¹⁵ across millions of qubits. To bridge this gap and fully harness the potential of quantum computers, quantum error correction (QEC) is essential. QEC codes are designed to protect quantum information by redundantly encoding it onto multiple physical qubits. This encoding allows for the detection and correction of local errors affecting individual qubits, e.g., through stabilizer measurements. Importantly, if the physical error rates are below a specific threshold, QEC codes can exponentially suppress logical error rates by increasing the number of physical qubits involved. This is essential for achieving fault-tolerant computations, which are key to unlocking the full potential of quantum computers…