  We examine how amplitude noise in queries to the oracle degrades a
performance of quantum search algorithm. The Grover search and similar
techniques are widely used in various quantum algorithms, including cases where
rival parties are fighting over confidential data. Hence, the oracle-box wires
become the subject of competing activity with an alteration of their normal
functioning. Of course, many kinds of errors could arise in this way. Possible
influence of dephasing on quantum search was already addressed in the
literature. Amplitude damping is another type of errors that should be analyzed
first. To study the problem, we introduce a simple model of collective
distortions with the use of amplitude damping channel. All the quantities of
interest should be considered as functions of the number of Grover's
iterations. In particular, we investigate the success probability with respect
to the parameter that characterizes the level of amplitude errors. The success
probability degrades significantly even if the error amount is not essential.
Namely, this probability soon enough reduces to a value close to one half. We
also study trade-off relations between quantum coherence and the success
probability in the presence of amplitude noise.
