  A new form of covariant quantum theory based on a quantum version of the
action principle is considered for the case of a free bosonic string. The
central idea of the new approach is to delay conditions of stationarity of the
classical action with respect to Lagrangian multipliers up to the quantum level
where delayed conditions of stationarity are imposed on a quantum action.
Physical states of the ordinary covariant quantum theory are replaced by well
defined stationary states as those which obey quantum action principle. The
stationary states have well defined energies.
