  We study the complexity of computational problems from quantum physics.
Typically, they are studied using the complexity class QMA (quantum counterpart
of NP) but some natural computational problems appear to be slightly harder
than QMA. We introduce new complexity classes consisting of problems that are
solvable with a small number of queries to a QMA oracle and use these
complexity classes to quantify the complexity of several natural computational
problems (for example, the complexity of estimating the spectral gap of a
Hamiltonian).
