State preservation by repetitive error detection in a superconducting quantum circuit

A quantum error correction scheme is demonstrated in a system of superconducting qubits, and repeated quantum non-demolition measurements are used to track errors and reduce the failure rate; increasing the system size from five to nine qubits improves the failure rate further. A milestone in quantum error correction Quantum states are fragile and easily destroyed, which presents a major obstacle in quantum computing. Quantum error correction can mitigate this problem by identifying and correcting environmentally induced quantum errors. Here, the authors demonstrate aspects of a quantum error correction scheme in a system of superconducting qubits. Using repeated quantum non-demolition measurements, they track bit-flip errors and manage to reduce the failure rate. Increasing the system size from five to nine qubits improves the failure rate further, and the authors demonstrate that an increase in code complexity enhances the fidelity. Although many more developments are still necessary for quantum error correction schemes to be applicable in a sizeable quantum computer, this work is an important step in this direction. Quantum computing becomes viable when a quantum state can be protected from environment-induced error. If quantum bits (qubits) are sufficiently reliable, errors are sparse and quantum error correction (QEC) 1 , 2 , 3 , 4 , 5 , 6 is capable of identifying and correcting them. Adding more qubits improves the preservation of states by guaranteeing that increasingly larger clusters of errors will not cause logical failure—a key requirement for large-scale systems. Using QEC to extend the qubit lifetime remains one of the outstanding experimental challenges in quantum computing. Here we report the protection of classical states from environmental bit-flip errors and demonstrate the suppression of these errors with increasing system size. We use a linear array of nine qubits, which is a natural step towards the two-dimensional surface code QEC scheme 7 , and track errors as they occur by repeatedly performing projective quantum non-demolition parity measurements. Relative to a single physical qubit, we reduce the failure rate in retrieving an input state by a factor of 2.7 when using five of our nine qubits and by a factor of 8.5 when using all nine qubits after eight cycles. Additionally, we tomographically verify preservation of the non-classical Greenberger–Horne–Zeilinger state. The successful suppression of environment-induced errors will mo