  Precise characterization of quantum devices is usually achieved with quantum
tomography. However, most methods which are currently widely used in
experiments, such as maximum likelihood estimation, lack a well-justified error
analysis. Promising recent methods based on confidence regions are difficult to
apply in practice or yield error bars which are unnecessarily large. Here, we
propose a practical yet robust method for obtaining error bars. We do so by
introducing a novel representation of the output of the tomography procedure,
the "quantum error bars". This representation is (i) concise, being given in
terms of few parameters, (ii) intuitive, providing a fair idea of the "spread"
of the error, and (iii) useful, containing the necessary information for
constructing confidence regions. The statements resulting from our method are
formulated in terms of a figure of merit, such as the fidelity to a reference
state. We present an algorithm for computing this representation and provide
ready-to-use software. Our procedure is applied to actual experimental data
obtained from two superconducting qubits in an entangled state, demonstrating
the applicability of our method.
