Northwestern Events Calendar


CMP Seminar: Jay Lawrence: Observing a Quantum Measurement

When: Tuesday, June 30, 2020
11:00 AM - 1:00 PM  

Where: Online

Audience: Faculty/Staff - Student - Post Docs/Docs - Graduate Students

Contact: Cristian Pennington  

Group: Physics and Astronomy Condensed Matter Physics Seminars

Category: Academic


To look inside of a Stern-Gerlach measurement, we consider the system of a spin-1/2 atom and two virtual detectors (quantum two-state systems or ancillae) with one placed in each path of a Stern-Gerlach apparatus. Each ancilla registers the passage (or not) of the atom along its path. The two paths are then brought back together to form a single wave packet. We then perform "real" measurements on this three-quit system, by reading out the ancilla states and by (re)measuring the atomic spin state downstream with another Stern-Gerlach apparatus. We show that a superposition of both paths may be observed compatibly with two "collapse-like" properties: (i) the "projection" property - the subsequent atomic spin state conforms to the detector readings, and (ii) the "single-outcome" property - the detectors are perfectly anti-correlated (i.e., one can never "find" the atom on both paths) These measurements are nondestructive if made without acquiring "which path" information. Otherwise, they destroy the initial state, but confirm all of its correlations. The model gives a novel perspective on why ordinary detectors are blind to superpositions.

Jay Lawrence, Physics Department, Dartmouth College; The James Franck Institute, University of Chicago

Host: Jens Koch

Keywords: Physics, Astronomy, Condensed Matter

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