  We review single-qubit quantum process tomography for trace-preserving and
nontrace-preserving processes, and derive explicit forms of the general
constraints for fitting experimental data. These forms provide additional
insight into the structure of the process matrix as well as reveal a tighter
bound on the trace of a nontrace-preserving process than has been previously
stated. We also describe, for completeness, how to incorporate measured
imperfect input states.
