  For a quantum system undergoing non-Markovian open quantum dynamics, we
demonstrate a tomography algorithm based on multi-time measurements of the
system, which reconstructs a minimal environment coupled to the system, such
that the system plus environment undergoes unitary evolution and that the
reduced dynamics of the system is identical to the observed dynamics of it. The
reconstructed open quantum evolution model can be used to predict any future
dynamics of the system when it is further assumed to be time-independent. We
define the memory size and memory complexity for the non-Markovian open quantum
dynamics which characterize the complexity of the reconstruction.
