  The theory of optimal quantum control serves to identify time-dependent
control Hamiltonians that efficiently produce desired target states. As such,
it plays an essential role in the successful design and development of quantum
technologies. However, often the delivered control pulses are exceedingly
sensitive to small perturbations, which can make it hard if not impossible to
reliably deploy these in experiments. Robust quantum control aims at mitigating
this issue by finding control pulses that uphold their capacity to reproduce
the target states even in the presence of pulse perturbations. However, finding
such robust control pulses is generically hard, since the assessment of control
pulses requires the inclusion of all possible distorted versions in the
evaluation. Here we show that robust control pulses can be identified based on
disorder-dressed evolution equations. The latter capture the effect of
disorder, which here stands for the pulse perturbations, in terms of quantum
master equations describing the evolution of the disorder-averaged density
matrix. In this approach to robust control, the purities of the final states
indicate the robustness of the underlying control pulses, and robust control
pulses are singled out if the final states are pure (and coincide with the
target states). We show that this principle can be successfully employed to
find robust control pulses. To this end, we adapt Krotov's method for
disorder-dressed evolution and demonstrate its application with several
single-qubit control tasks.
