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Measuring the past of quantum systems: from counting quantum pigeons to watching atoms as they tunnel

APS March Meeting 2018', Harvard University
Steinberg, A. Department of Physics at the University of Toronto,
2018 Physics

In quantum mechanics, as in the classical world, one can draw some conclusions from present observations about the past behaviour of a system. The question of just what one can say about a system given knowledge of its preparation and its final state remains a topic of discussion. I will present a number of recent and ongoing experiments which address these issues by applying weak measurements (in the sense of Aharonov, Albert, and Vaidman) and post-selected strong measurements (à la Aharonov, Bergmann, and Lebowitz) to systems ranging from entangled photons to tunneling atoms to quantum-level laser beams interacting through an optical nonlinearity.

Such measurements offer insight into processes such as tunneling, but are also well known to lead to some results which violate our intuitive expectations. Applying variable-strength measurements to the "pigeonhole paradox" will allow us to probe just how certain aspects of these "conditional measurements" remain robust, independent of measurement strength, while other axioms and sum rules behave quite differently in different regimes. I will also discuss how a weak measurement of photon number may exceed the number of prepared photons, and what weak measurements tell us about where particles "spend their time" while tunneling through a barrier.

The paper was presented at: 'APS March Meeting 2018', Harvard University.