  We analyse recently proposed physical implementations of a quantum computer
based on polar molecules. A set of general requirements for a molecular system
is presented, which would provide an optimal combination of quantum gate times,
coherence times, number of operations, high gate accuracy and experimental
feasibility. We proceed with a detailed analysis of a scheme utilizing
switchable dipole-dipole interactions between polar molecules. Switchable
dipole-dipole interaction is an efficient tool for realization of two-qubit
quantum gates, necessary to construct a universal set of gates. We consider
three possible realizations of a phase gate using specific molecules, such as
CO, NF, alkali dimers and alkaline-earth monohalides. We suggest suitable
electronic states and ransitions and investigate requirements for the pulses
driving them. Finally, we analyse possible sources of decoherence.
