  We develop the quantization of unimodular gravity in the Plebanski and
Ashtekar formulations and show that the quantum effective action defined by a
formal path integral is unimodular. This means that the effective quantum
geometry does not couple to terms in the expectation value of the
energy-momentum tensor proportional to the metric tensor. The path integral
takes the same form as is used to define spin foam models, with the additional
constraint that the determinant of the four metric is constrained to be a
constant by a gauge fixing term. We also review the proposal of Unruh, Wald and
Sorkin- that the hamiltonian quantization yields quantum evolution in a
physical time variable equal to elapsed four volume-and discuss how this may be
carried out in loop quantum gravity. This then extends the results of
arXiv:0904.4841 to the context of loop quantum gravity.
