  This paper is devoted to an analysis of the experiment by Nakamura {\it et
al.} (Nature {\bf 398}, 786 (1999)) on the quantum state control in Josephson
junctions devices. By considering the relevant processes involved in the
detection of the charge state of the box and a realistic description of the
gate pulse we are able to analyze some aspects of the experiment (like the
amplitude of the measurement current) in a quantitative way.
