Probing qubit by qubit: properties of the POVM and the information/disturbance tradeoff

Abstract: We address the class of positive operator-valued measures (POVMs) for qubit systems that are obtained by coupling the signal qubit with a probe qubit and then performing a projective measurement on the sole probe system. These POVMs, which represent the simplest class of qubit POVMs, depends on 3+3+2=8 free parameters describing the initial preparation of the probe qubit, the Cartan representative of the unitary coupling, and the projective measurement at the output, respectively. We analyze in some details the properties of the POVM matrix elements, and investigate their values for given ranges of the free parameters. We also analyze in details the tradeoff between information and disturbance for different ranges of the free parameters, showing, among other things, that i) typical values of the tradeoff are close to optimality and ii) even using a maximally mixed probe one may achieve optimal tradeoff.