  Using the tool of concatenated stabilizer coding, we prove that the
complexity class QMA remains unchanged even if every witness qubit is disturbed
by constant noise. This result may not only be relevant for physical
implementations of verifying protocols but also attacking the relationship
between the complexity classes QMA, QCMA and BQP, which can be reformulated in
this unified framework of a verifying protocol receiving a disturbed witness.
While QCMA and BQP are described by fully dephasing and depolarizing channels
on the witness qubits, respectively, our result proves QMA to be robust against
27% dephasing and 18% depolarizing noise.
