Quantum Measurements using Interferometric STEM-EELS

BIO Web of Conferences 129, 04019 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412904019 Quantum Measurements using Interferometric STEM-EELS Professor Benjamin McMorran1, Dr. Cameron Johnson1,2, Dr. Amy Turner1 1Department of Physics, University of Oregon, Eugene, USA, 2Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, USA Background incl. aims Nanoscale amplitude beamsplitters for electrons enable flexible electron interferometry in STEM instruments. Nanofabricated materials phase gratings provide a way to coherently diffract electron wavefunctions into different paths, and can easily be placed in probe-forming apertures of unmodified TEM instruments. These tools provide a way to manipulate both the phase, amplitude, and momentum of electrons [1]. This lends itself to the implementation of new interferometric methods in electron microscopy [2-4], with the ultimate goal of performing quantum measurements in the TEM. Methods We used a pair of nanoscale phase gratings as diffractive amplitude beamsplitters to provide a Mach-Zehnder electron interferometer inside an unmodified TEM [2]. One phase grating beamsplitter coherently divides the electrons into separated probes before the specimen, and the second beamsplitter recombines the paths after the specimen, creating a set of discrete interfering outputs. This enables several new imaging and measurement modalities. For example, phase contrast imaging can be provided if one of the probes transmits through a specimen region while the other passes through vacuum, acquiring a relative phase shift that can be recorded by monitoring a discrete electron beam output. The setup also enables quantum-inspired measurements, such as interfering electron paths that have lost energy to the specimen. If the interferometer is tuned to provide destructive interference at the detector, blocking one of the probe paths with an object eliminates the destructive interference, allowing a non-zero probability current to be incident on the detector. This is a quantum interrogation method sometimes called “interaction-free” measurement, because a detection event in a dark detector indicates the presence of an absorbing sample; the electrons that did not scatter from the sample indicate its presence. Results We demonstrated using the 2-grating electron interferometer for STEM phase imaging of nanostructures [2], even using only inelastically scattered electrons [3]. We used this to determine that fast electrons passing on either side of a metallic nanoparticle that excite a plasmon acquire a relative 𝜋𝜋𝜋𝜋 phase © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/). BIO Web of Conferences 129, 04019 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412904019 difference, corresponding to the symmetry of an optical dipole excitation. We also used the setup to demonstrate a quantum “interaction-free” measurement of the presence of an opaque object [4]. In this case, single electron events recorded at the detector indicate the presence of an object without scattering from or transmitting through it. Conclusion We used nanoscale phase gratings to implement electron interferometry in STEM without modifying the instrument other than an aperture change. Potential advantages of the techniques this enables relative to other methods will be discussed. Graphic: Keywords: Electron interferometry, electron holography Reference: [1] Johnson, C. W., Bauer, D. H. & McMorran, B. J. Improved control of electron computer-generated holographic grating groove profiles using ion beam gas-assisted etching. Appl. Opt. 59, 1594–1601 (2020). [2] Johnson, C. W., Turner, A. E. & McMorran, B. J. Scanning two-grating free electron Mach-Zehnder interferometer. Phys. Rev. Research 3, 043009 (2021). [3] Johnson, C. W., Turner, A. E., García de Abajo, F. J. & McMorran, B. J. Inelastic Mach-Zehnder Interferometry with Free Electrons. Phys. Rev. Lett. 128, 147401 (2022). [4] Turner, A. E., Johnson, C. W., Kruit, P. & McMorran, B. J. Interaction-Free Measurement with Electrons. Phys. Rev. Lett. 127, 110401 (2021). 2 BIO Web of Conferences 129, 04019 (2024) EMC 2024 https://doi.org/10.1051/bioconf/202412904019 [5] The author gratefully acknowledges collaboration with Pieter Kruit (Delf University) and Javier García de Abajo (ICFO). This material is based upon work supported by the National Science Foundation under Grant No. 2012191. 3  (...truncated)