  The generation of High-Frequency Gravitational Waves (HFGW) has been
identified as the required breakthrough that will lead to new forms of space
propulsion. Many techniques have been devised to generate HFGW, but most of
them exhibit marginal efficiency, therefore the power emitted in form of
gravitational waves (GW) is orders of magnitude lower than the input power. The
gravitational wave counterpart of the LASER, termed Gravitational-wave LASER or
"GASER" is the quantum approach to the efficient generation of gravitational
waves. Electrons, protons, muons, etc, all have charge and mass, if accelerated
they usually lose energy through the very fast electric and magnetic channels,
this causes a negligible emission through the gravitational channel. Quantum
systems can be engineered to forbid electric and magnetic transitions,
therefore the gravitational spin-2 transitions can take place. A class of
active materials, suitable for making a GASER based on electronic transitions
in the solid state, is identified along with their relevant physical
properties. Means for creating coherence and population inversion and means to
increase the emission probability are described. The expected performances of
the device are derived from quantum gravitational theories. Additional
properties of the active materials are considered to enforce the theoretical
foundation of the device. A proof-of-concept device, operating at about 1 THz,
is described. Experiments are proposed as a natural starting point of the
research.
