  We describe how to use the fidelity decay as a tool to characterize the
errors affecting a quantum information processor through a noise generator
$G_{\tau}$. For weak noise, the initial decay rate of the fidelity proves to be
a simple way to measure the magnitude of the different terms in $G_{\tau}$.
When the generator has only terms associated with few-body couplings, our
proposal is scalable. We present the explicit protocol for estimating the
magnitude of the noise generators when the noise consists of only one and
two-body terms, and describe a method for measuring the parameters of more
general noise models. The protocol focuses on obtaining the magnitude with
which these terms affect the system during a time step of length $\tau$;
measurement of this information has critical implications for assesing the
scalability of fault-tolerant quantum computation in any physical setup.
