  Fully quantized mechanical motion of a single-level quantum dot coupled to
two voltage biased electronic leads is studied. It is found that there are two
different regimes depending on the applied voltage. If the bias voltage is
below a certain threshold (which depends on the energy of the vibrational
quanta) the mechanical subsystem is characterized by a low level of excitation.
Above a threshold the energy accumulated in the mechanical degree of freedom
dramatically increases. The distribution function for the energy level
population and the current through the system in this regime is calculated.
