Decoherence by Internal Degrees of Freedom

We consider a composite system consisting of coupled particles, and investigate decoherence due to coupling of the center-of-mass degree of freedom with the internal degrees of freedom. For a simple model of two bound particles, we show that in general such a decoherence effect exists, and leads to suppression of interference between different paths of the center-of-mass. For the special case of two harmonically-bound particles moving in an external potential in one dimension, we show that the coupling between the center-of-mass and internal degrees of freedom can be approximated as parametric driving, and that nontrivial coupling depends on the second derivative of the external potential. We find a partial solution to this parametric driving problem. For a simple interference experiment, consisting of two wave packets scattering off of a square well, we perform numerical simulations and show a close connection between suppression of interference and entanglement between the center-of-mass and internal degrees of freedom. We also propose a measure of compositeness which quantifies the extent to which a composite system cannot be approximated as a single, indivisible particle. We numerically calculate this quantity for our square well example system.