The role of anharmonicity and coupling in quantum computing based on vibrational qubits

We analyse the effects of molecular characteristics on the structure of global quantum gates and the complexity of the resulting mechanisms systematically with the goal of rating a molecule's suitability for molecular quantum computing. One decisive property of a molecular vibration is the anharmonicity and in an extension to multimode systems the mode coupling has to be taken into account additionally. In a parametrized two-dimensional model system, we tune these characteristic properties and explore their effects on quantum gates. We find that the interplay of the anharmonicity and the coupling is of prime importance and leads to two basic control mechanisms for all systems. The features of quantum gate laser fields are explained with characteristic transition frequencies, determined by the molecular parameters, and the limits to obtain simple structures are identified.