  Characterizing and mitigating errors in current noisy intermediate-scale
devices is important to improve performance of next generations of quantum
hardware. In order to investigate the importance of the different noise
mechanisms affecting quantum computation, we perform full quantum process
tomography of single qubits in a real quantum processor in which echo
experiments are implemented. Besides error sources already included in standard
models, the obtained results show the dominant role of coherent errors, which
we practically correct by inserting random single-qubit unitaries in the
quantum circuit, significantly increasing the circuit length over which quantum
computations on actual quantum hardware produce reliable results.
