  Joint measurements of multiple qubits have been shown to open new
possibilities for quantum information processing. Here, we present an approach
based on homodyne detection to realize such measurements in the dispersive
regime of cavity/circuit QED. By changing details of the measurement, the
readout can be tuned from extracting only single-qubit to only multi-qubit
properties. We obtain a reduced stochastic master equation describing this
measurement and its effect on the qubits. As an example, we present results
showing parity measurements of two qubits. In this situation, measurement of an
initially unentangled state can yield with near unit probability a state of
significant concurrence.
