  Quantum computing becomes viable when a quantum state can be preserved from
environmentally-induced error. If quantum bits (qubits) are sufficiently
reliable, errors are sparse and quantum error correction (QEC) is capable of
identifying and correcting them. Adding more qubits improves the preservation
by guaranteeing 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 precursor of the two-dimensional surface code QEC scheme, and track
errors as they occur by repeatedly performing projective quantum non-demolition
(QND) parity measurements. Relative to a single physical qubit, we reduce the
failure rate in retrieving an input state by a factor of 2.7 for five qubits
and a factor of 8.5 for nine qubits after eight cycles. Additionally, we
tomographically verify preservation of the non-classical
Greenberger-Horne-Zeilinger (GHZ) state. The successful suppression of
environmentally-induced errors strongly motivates further research into the
many exciting challenges associated with building a large-scale superconducting
quantum computer.
