  We consider a system of two solid state charge qubits, coupled to a single
read-out device, consisting of a single-electron transistor (SET). The
conductance of each tunnel junction is influenced by its neighboring qubit, and
thus the current through the transistor is determined by the qubits' state. The
full counting statistics of the electrons passing the transistor is calculated,
and we discuss qubit dephasing, as well as the quantum efficiency of the
readout. The current measurement is then compared to readout using real-time
detection of the SET island's charge state. For the latter method we show that
the quantum efficiency is always unity. Comparing the two methods a simple
geometrical interpretation of the quantum efficiency of the current measurement
appears. Finally, we note that full quantum efficiency in some cases can be
achieved measuring the average charge of the SET island, in addition to the
average current.
