Quantum evolution in fluctuating backgrounds: Nonideal clocks and foam-like spacetimes

I characterize good clocks, which are naturally subject to fluctuations, in statistical terms, obtain the master equation that governs the evolution of quantum systems according to these clocks, and find its general solution. This master equation is diffusive and produces loss of coherence. Moreover, real clocks can be described in terms of effective interactions that are nonlocal in time. Alternatively, they can be modeled by an effective thermal bath coupled to the system. I also study some aspects concerning the evolution of quantum low-energy fields in a foamlike spacetime, with involved topology at the Planck scale but with a smooth metric structure at large length scales. This foamlike structure of spacetime may show up in low-energy physics through loss of quantum coherence and mode-dependent energy shifts, for instance, which might be observable. Spacetime foam introduces nonlocal interactions that can be modeled by a quantum bath, and low-energy fields evolve according to a master equation that displays such effects. These evolution laws are similar to those for quantum mechanical systems evolving according to good nonideal clocks, although the underlying Hamiltonian structure in this case establishes some differences among both scenarios.