Irreversibility and the breaking of resonance-antiresonance symmetry.

We consider open quantum systems modeled as discrete lattices. Using a simple model of a single-site coupled to two leads as an example, we show that the time evolution of these systems can be analyzed in terms of an explicitly time-reversal symmetric resolution of unity. This resolution of unity includes both resonant states, which decay in the future, and anti-resonant states, which decay in the past. We show that a time-reversal invariant state contains both resonant and anti-resonant components with equal weights. However, this symmetry is automatically broken as the system evolves in time, with the resonant component becoming much larger than the anti-resonant component for t > 0 (and vice versa for t < 0). We argue that irreversibility is a manifestation of this symmetry breaking. We also compare our present approach with the subdynamics approach developed by Prof. Prigogine and collaborators. Finally, we suggest an extension of our present approach from the level of wave functions to the level of density matrices.