We present quantum schemes that can code/decode information by optical adiabatic passage processes. In a quantum decoder, information encoded on N levels (quNit), with population amplitudes of M possible phases, is processed by transferring their population optically to (predominantly) one of L final levels. In this two-photon adiabatic passage, N intermediate levels are resonantly coupled 'one-to-one' to N initial states and 'one-to-all' to L final states. A quantum encoder works in the opposite way. We discuss practical implementation of the suggested schemes in manifolds of vibrational states of the Na-2 dimer. (C) 2004 Elsevier B.V. All rights reserved.