  A fundamental challenge for quantum information processing is reducing the
impact of environmentally-induced errors. Quantum error detection (QED)
provides one approach to handling such errors, in which errors are rejected
when they are detected. Here we demonstrate a QED protocol based on the idea of
quantum un-collapsing, using this protocol to suppress energy relaxation due to
the environment in a three-qubit superconducting circuit. We encode quantum
information in a target qubit, and use the other two qubits to detect and
reject errors caused by energy relaxation. This protocol improves the storage
time of a quantum state by a factor of roughly three, at the cost of a reduced
probability of success. This constitutes the first experimental demonstration
of an algorithm-based improvement in the lifetime of a quantum state stored in
a qubit.
