  In quantum information processing quantum operations are often processed
alongside measurements which result in classical data. Due to the information
gain of classical measurement outputs non-unitary dynamical processes can take
place on the system, for which common quantum channel descriptions fail to
describe the time evolution. Quantum measurements are correctly treated by
means of so-called quantum instruments capturing both classical outputs and
post-measurement quantum states. Here we present a general recipe to
characterize quantum instruments alongside its experimental implementation and
analysis. Thereby, the full dynamics of a quantum instrument can be captured,
exhibiting details of the quantum dynamics that would be overlooked with common
tomography techniques. For illustration, we apply our characterization
technique to a quantum instrument used for the detection of qubit loss and
leakage, which was recently implemented as a building block in a quantum error
correction (QEC) experiment (Nature 585, 207-210 (2020)). Our analysis reveals
unexpected and in-depth information about the failure modes of the
implementation of the quantum instrument. We then numerically study the
implications of these experimental failure modes on QEC performance, when the
instrument is employed as a building block in QEC protocols on a logical qubit.
Our results highlight the importance of careful characterization and modelling
of failure modes in quantum instruments, as compared to simplistic
hardware-agnostic phenomenological noise models, which fail to predict the
undesired behavior of faulty quantum instruments. The presented methods and
results are directly applicable to generic quantum instruments.
