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

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 detail the properties of the POVM matrix elements, and investigate their values for given ranges of the free parameters. We also analyze in detail 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.

Synchronous Bimanual Movements Performed by Homologous and Non-Homologous Muscles

Synchrony of bimanual wrist movements by 12 normal Ss was compared when homologous muscles (e.g., left and right wrist flexors) and when non-homologous muscles (e.g., left flexors and right extensors) were simultaneously active. Movements of the two limbs were less synchronous and more variable in the non-homologous condition. These data imply the existence of an unitary coupling mechanism which facilitates simultaneous action of homologous muscles of the upper limbs.