  In this paper, we study decoherence on Grover's quantum searching algorithm
using a perturbative method. We assume that each two-state system (qubit)
suffers \sigma_{z} error with probability p (0\leq p\leq 1) independently at
every step in the algorithm. Considering an n-qubit density operator to which
Grover's operation is applied M times, we expand it in powers of 2Mnp and
derive its matrix element order by order under the n\to \infty limit. (In this
large n limit, we assume p is small enough, so that 2Mnp(\geq 0) can take any
real positive value or 0.) This approach gives us an interpretation about
creation of new modes caused by \sigma_{z} error and an asymptotic form of an
arbitrary order correction. Calculating the matrix element up to the fifth
order term numerically, we investigate a region of 2Mnp (perturbative
parameter) where the algorithm finds the correct item with a threshold of
probability P_{th} or more. It satisfies 2Mnp<(8/5)(1-P_{th}) around 2Mnp\simeq
0 and P_{th}\simeq 1, and this linear relation is applied to a wide range of
P_{th} approximately. This observation is similar to a result obtained by E.
Bernstein and U. Vazirani concerning accuracy of quantum gates for general
algorithms. We cannot investigate a quantum to classical phase transition of
the algorithm, because it is outside the reliable domain of our perturbation
theory.
