Quantum computing and polynomial equations over the finite field Z_2
  What is the computational power of a quantum computer? We show that determining the output of a quantum computation is equivalent to counting the number of solutions to an easily computed set of polynomials defined over the finite field Z_2. This connection allows simple proofs to be given for two known relationships between quantum and classical complexity classes. 
Christopher M. Dawson, Henry L. Haselgrove, Andrew P. Hines, Duncan   Mortimer, Michael A. Nielsen, Tobias J. Osborne