Efficient error characterization in quantum information processing

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_. 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_. 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 ; measurement of this information has critical implications for assessing the scalability of fault-tolerant quantum computation in any physical setup.