Discreteness of the volume of space from Bohr-Sommerfeld quantization
  A major challenge for any theory of quantum gravity is to quantize general relativity while retaining some part of its geometrical character. We present new evidence for the idea that this can be achieved by directly quantizing space itself. We compute the Bohr-Sommerfeld volume spectrum of a tetrahedron and show that it reproduces the quantization of a grain of space found in loop gravity. 
Eugenio Bianchi and Hal M. Haggard
gr-qc hep-th math-ph math.MP