  Necessary conditions for generating phase controllable asymmetry in spatially
symmetric systems using lasers are identified and are shown to be identical in
quantum and classical mechanics. First, by studying the exact dynamics of
harmonic systems in the presence of an arbitrary radiation field, it is
demonstrated that anharmonicities in the system's potential are a necessary
requirement for phase controllability. Then, by analyzing the space-time
symmetries of the laser-driven Liouville dynamics for classical and quantum
systems, a common set of temporal symmetries for the driving field that need to
be violated to induce transport are identified. The conditions apply to
continuous wave lasers and to symmetry breaking effects that do not rely on the
control of the absolute phase of the field. Known examples of laser fields that
can induce transport in symmetric systems are seen to be particular cases of
these symmetry constraints.
