Jan Gulla - "Photon localization: Local measurements, localized states, and on-demand sources in quantum field theory" (WINQ Seminar)

Tuesday 12 Nov 2024, 10:30 → 11:30 Europe/Stockholm

AlbaNova A3:1003 - Styrelserum/KOMKO (AlbaNova Main Building)

In this talk I will present ideas surrounding particle localization in quantum field theory, based on my PhD work (thesis available at http://hdl.handle.net/10852/113838). I will start the talk by a self-contained proof of why single-particle bosons cannot be localized in space and time, due to the absence of negative frequencies.

This in turn motivates the study of localized states in quantum field theory. I will present the main theory of such states based on works by Knight and Licht. The central idea is that (strictly) localized states are exactly indistinguishable from vacuum by all local measurements outside some spacetime region. Core to this definition is a characterization of what measurements can be done locally in quantum field theory. I will present one answer to this question in the form of the local operator algebra, borrowed from algebraic quantum field theory. I will also briefly discuss some criticisms of this model, such as by Sorkin.

Finally, I will place this theory in the larger picture of establishing aspects of information in quantum field theory. By causality, any signal generated on demand must be undetectable outside the source's light cone, meaning that it must be represented by some localized quantum state. This means that (exact) single photons cannot be generated on demand. We present a new type of optical state which can be localized while being close to a single photon in many experimentally attainable situations. This state can describe non-classical, causally propagating signals in quantum field theory, potentially opening up new methods for analyzing such processes. 

Zoom Meeting URL: https://stockholmuniversity.zoom.us/j/69740734375