  We present a comprehensive analysis of fidelity decay and error accumulation
in faulty quantum circuit models. Our work devises an analytical bound for the
average fidelity between desired and faulty output states, accounting for
errors that may arise during the implementation of two-qubit gates and
multi-qubit permutations. It is shown that fidelity decays exponentially with
both circuit depth and the number of qubits raised to an architecture-dependent
power, and determine the decay rates as a function of the two types of errors.
Furthermore, we establish a robust linear relationship between fidelity and the
heavy output frequency used in Quantum Volume tests to benchmark quantum
processors, under the considered errors protocol. These findings pave the way
for predicting the behavior of fidelity in the presence of specific errors and
offer insights into the best strategies for increasing Quantum Volume.
