  In 1927, Heisenberg heuristically disclosed the tradeoff between the error in
the measurement and the caused disturbance on another complementary observable.
In the quantum theory, most of uncertainty relations are proposed to reveal the
amount of unavoidable uncertainty in the measuring process. In this paper, we
study the error-disturbance relation from the information viewpoint. We ask how
much information, rather than how much uncertainty, can be obtained during the
two sequential measurements. To achieve optimal information gain, we argue that
the strategy for the "intelligent" prior apparatus is to clone the unknown
state, and for the posterior one is to perform the swapping operation. We
propose the coarse-grained random access code, and therein information
causality as a physical principle can be exploited for deriving the upper-bound
of information gain. Finally, we conjecture the information gain of measuring
the position and momentum of a quantum object in the coarse-grained way.
