Quantum adiabatic brachistochrone for open systems

We propose a variational principle to compute a quantum adiabatic brachistochrone (QAB) for open systems. Using the notion of ``adiabatic speed'' based on the energy gaps, we derive a Lagrangian associated to the functional measuring the time spent to achieve adiabatic behavior, which in turn allows us to perform the optimization. The QAB is illustrated for the nonunitary dynamics of a stimulated Raman adiabatic passage process, the Deutsch-Jozsa quantum computing algorithm, and of a transmon qutrit. A numerical protocol is devised, which allows one to compute the QAB for arbitrary quantum systems for which exact simulations can be afforded. We also establish sufficient conditions for the equivalence between the Lagrangians, and thus the QAB, of open and closed systems.