Particle Physics Seminar: The past of quantum particles in pre- and postselected systems

Abstract:

I examine the question of a quantum particle’s location between two strong measurements, an issue not explicitly addressed by standard quantum mechanics. By incorporating postselection, the particle’s presence can be characterized through the physical traces it leaves on its environment, even via arbitrarily weak interactions. This approach is naturally formulated within the Two-State Vector Formalism, a time-symmetric framework that combines forward and backward evolving quantum states and provides an intuitive quantification of these traces. When applied to a nested Mach-Zehnder interferometer, this analysis leads to the counterintuitive prediction of discontinuous trajectories, with presence inside the inner interferometer but not in the outer arm. Remarkably, a modified version of the setup allows the detection of signals originating from regions where the particle is predicted to have no presence, an effect explained by strong signal amplification arising from a special interference pattern. These insights have motivated the development of high-precision interferometric techniques that exploit dark-port amplification to detect signals originating outside the interferometer.

Seminar Organizer: Dr. Michael Geller