  At the Planck scale the distinction between elementary particles and black
holes becomes fuzzy. The very definition of a "quantum black hole" (QBH) is an
open issue. Starting from the idea that, at the Planck scale, the radius of the
event horizon undergoes quantum oscillations, we introduce a black hole
mass-radius Generalised Uncertainty Principle (GUP) and derive a corresponding
gravitational wavelength. Next we recover a GUP encoding effective geometry.
This semi-classical gravitational description admits black hole configurations
only for masses higher than the Planck mass. Quantum corrections lead to a
vanishing Hawking temperature when the Planck mass is approached from above.
Finally we replace our semi-classical model by a relativistic wave equation for
the "horizon wave function". The solution admits a discrete mass spectrum which
is bounded from below by a stable ground state with energy close to the Planck
mass. Interestingly higher angular momentum states fit onto Regge trajectories
indicating their stringy nature.
