  Quantum optimal control can be used to shape the control pulses for realizing
unitary and non-unitary transformations of quantum states. These control pulses
provide the fundamental interface between the quantum compiler and the quantum
hardware. Most current software for quantum optimal control (e.g. Qutip or
Krotov) is restricted to run on shared memory platforms, limiting their
applicability to smaller quantum systems, in particular if interactions with
the environment are taken into account. This paper gives an overview of the
open-source code Quandary, which is designed to solve quantum control problems
in larger open quantum systems modelled by Lindblad's master equation.
Implemented in C++, Quandary uses the message passing paradigm for distributed
memory computers that enables scalability to large numbers of compute cores.
Accompanied by numerical examples, this paper presents an overview on existing
theoretical developments for open optimal quantum control realizing
state-to-state transfer, unitary gate optimization as well as
state-preparation, and presents the numerical tools and implementation aspect
as realized in Quandary, for deployment on modern high-performance computing
platforms.
