We discuss experimental situations that consist of multiple preparation and measurement stages. This leads us to an alternative approach to quantum mechanics. In particular, we introduce the idea of multitime quantum states which are the appropriate tools for describing these experimental situations. We also describe multitime measurements and discuss their relation to multitime states. A consequence of our formalism is to put states and operators on an equal footing. Finally we discuss the implications of our approach to quantum mechanics for the problem of the flow of time.
Aharonov, Yakir, Sandu Popescu, Jeff Tollaksen, and Lev Vaidman. "Multiple-time States and Multiple-time Measurements in Quantum Mechanics." Physical Review A 79.5 (2009). doi: 10.1103/PhysRevA.79.052110
American Physical Society