07. November 2022

Thomas Juffmann (Universität Wien)

Electrons and Light: Ponderomotive Beam Shaping and Optical Near-field
Electron Microscopy

Electron-induced damage is one of the major challenges in imaging biological matter with electron beams. We will discuss two techniques that open possibilities for dose-effective measurements:

First, we will discuss ponderomotive electron beam shaping [1]. An electron beam interacts with a shaped high-intensity laser pulse, which induces a phase shift proportional to the local light intensity. Since the light intensity distribution can be shaped arbitrarily, we can use this to arbitrarily shape the wavefront of an electron beam. We demonstrate concave and convex electron lensing and the ability to program complex electron deflection patterns. This beam-shaping technique is lossless and doesn't induce inelastic scattering. It can be used for the realization of a phase plate, an aberration correction plate, or adaptive measurement schemes.

Second, we will discuss optical near-field electron microscopy (ONEM) which has been proposed as a way to circumvent electron-induced damage [2]: In ONEM, a sample is probed non-invasively using light, and the resulting near-field interference patterns are converted into an electron current using a photocathode. The emitted electrons are then imaged with nanometric resolution using an aberration-corrected Low Energy Electron Microscope (LEEM). For sample-photocathode distances much smaller than the optical wavelength (z≪λ) this allows for label-free superresolution microscopy of interfaces. After introducing the basics of this new imaging concept, we will discuss the first steps toward the implementation of ONEM.

1. Marius Constantin Chirita Mihaila, Philipp Weber, Matthias Schneller, Lucas Grandits, Stefan Nimmrichter, Thomas Juffmann, Transverse Electron Beam Shaping with Light. Phys. Rev. X 12, 031043 (2022)
2. Marchand, R., Šachl, R., Kalbac, M., Hof, M., Tromp, R., Amaro, M., van der Molen, S. J. & Juffmann, T., Optical Near-Field Electron Microscopy. Physical Review Applied 16, 014008 (2021)

21. November 2022

Rob Spekkens (Perimeter Institute, Canada)

Why interference phenomena do not capture the essence of quantum theory

Quantum interference phenomena are widely viewed as posing a challenge to the classical worldview. Feynman
even went so far as to claim that they are the only mystery and the basic peculiarity of quantum mechanics. We
challenge this claim by describing an alternative to quantum theory, a statistical theory of a classical discrete field—a `toy field theory’—that reproduces the relevant phenomenology of quantum interference while respecting the classical worldview in the sense of being realizable by a local and noncontextual ontological model.  This theory also reproduces a number of related interference phenomena, such as the Elitzur-Vaidman bomb tester, Wheeler's delayed-choice experiment, and the quantum eraser experiment.  Finally, we identify an aspect of interference phenomenology that does resist explanation within the classical worldview, namely, the functional form of the quantum wave-particle duality relation. 

5. December 2022

Ron Folman (Ben-Gurion University)

Realization of a complete Stern-Gerlach interferometer: Towards a test of quantum gravity

The Stern-Gerlach effect, found a century ago, has become a paradigm of quantum mechanics. Unexpectedly, until recently, there has been little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Several theoretical studies have explained why a Stern-Gerlach interferometer is a formidable challenge. Here, we provide a detailed account of the realization of a full-loop Stern-Gerlach interferometer for single atoms and use the acquired understanding to show how this setup may be used to realize an interferometer for macroscopic objects doped with a single spin. Such a realization would open the door to a new era of fundamental probes, including the realization of previously inaccessible tests at the interface of quantum mechanics and gravity.

9. January 2023

Chiara Marletto (Wolfson College, University of Oxford)

Beyond quantum computation: the physics of can and can’t

The theory of the universal quantum computer has brought us rapid technological developments, together with remarkable improvements in how we understand quantum theory. There are, however, reasons to believe that quantum theory may ultimately have to be modified into a new theory: for instance, it will have to be merged with general relativity, to incorporate gravity; and some claim that it may be impossible to have quantum effects beyond a certain macroscopic scale. So what lies ahead of quantum theory, and of the universal quantum computer? To shed some light on these questions, we need a conceptual shift. Specifically, we can harness general principles about possible/impossible transformations, rather than dynamical laws and initial conditions. These general principles are ideal to describe the physics of ‘hybrid quantum systems’ - which include a quantum system interacting with another system whose exact dynamics are intractable or not fully known. I will describe my recent work in developing applications of this new approach to a number of interconnected problems within information theory and thermodynamics. I will also describe a few experimental implementations that I co-designed to illustrate these theoretical ideas.

16. January 2023

Abhay Ashtekar (Pennsylvania State University)

On the Quantum Nature of the Coulombic Interaction

The interface between Quantum Information and Quantum Field Theory – especially Quantum Gravity – is emerging as a forefront area of fundamental physics. But there is some tension between the way the basic concept of “locality” is commonly understood by the two communities. This tension descends to the issues of interpretation of precisely what would be tested by experiments that have been proposed to probe the quantum nature of gravity. Recall that Hilbert spaces Hgrav and Hph of gravitons and photons know only about the “radiative modes” of the gravitational and electromagnetic fields. But matter sources also give rise to “Coulombic modes”. Are the “Coulombic” parts of the gravitational and electromagnetic fields produced by quantum matter also quantum mechanical, then? If so, in what sense? The “Coulombic modes” are not even “registered” in the Hilbert spaces Hgrav and Hph! Will the proposed experiments directly test the quantum nature of the “radiative aspects” or “Coulombic aspects”? The talk will examine such elementary issues by drawing on an exactly soluble, non-perturbative quantum gravity model that is especially well-suited for this purpose.