A MODEL OF QUANTUM MEASUREMENT IN JOSEPHSON-JUNCTIONS

A model for the quantum measurement of the electronic current in a Jos ephson junction is presented and analyzed. The model is similar to a S tern-Gerlach apparatus, relying on the deflection of a spin-polarized particle beam by the magnetic field created by the Josephson current. The aim is (1) to explore, with the help of a simple model, some gener al ideas about the nature of the information which call be obtained by measurements upon a quantum system and (2) to find Mew approaches for obtaining information about the nature of the states of a macroscopic quantum system. In the case of sufficiently strong coupling between t he system and the apparatus, we find that the model provides in princi ple a standard ideal measurement of the value of the instantaneous Jos ephson current. In the case of weak coupling, where the measurement is not ideal, we show that the scattering of neutrons neutrons from a ju nction can ill principle be used to measure the average value of the J osephson current, thereby allowing an experimental distinction to be m ade between an eigenstate of relative phase and one of relative Cooper pair number. The possibility of the latter type of measurement sugges ts an experimental approach to answer a question of fundamental intere st, namely whether two isolated superconductors (or superfluids) posse ss a definite relative phase or a definite relative number of supercon ducting (or superflowing) particles.