Transmission electron microscopy (TEM) provides a versatile platform for investigating physical phenomena with nanoscale resolution, based on the interaction of free electrons with the matter. By exploiting these interactions, scientists can extract classical or semi-classical information, such as atomic positions, crystal lattice orientation, sample morphology, and composition. In recent years, new methods utilizing the quantum nature of electrons have emerged, promising to revolutionize electron microscopy and offer some unexpected capabilities.
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TEM is a well-established technique enabling atomic-scale resolution imaging with a wide range of applications in material science, optoelectronics, condensed matter, and biophysics.
TEM mostly relies on the classical treatment of the electron-matter interaction, where the electrons are treated as classical point particles, scattering through the volume of the specimen in discrete events. However, exploiting the electrons' quantum nature can reveal information previously not accessible with conventional methods.
A notorious problem in high-resolution cryo-TEM of biological molecules is the radiation damage caused by the probing electrons.
The development of cryo-EM has allowed scientists to study individual protein molecules and enhance our understanding of how structure affects biological functions and how biological systems function on the molecular level. As a result, radiation damage is one of the main obstacles to biological high-resolution imaging.
As biomolecules predominantly consist of light atoms that are not arranged in a rigid crystalline lattice, they only can sustain relatively small irradiation doses. The ability to acquire data using a minimal number of electrons is of critical importance in this case.
Quantum Methods Shape Electron Beams for Low-Dose Imaging
A particularly attractive concept in this context is the so-called interaction-free measurement, often referred to as the quantum electron microscope, which takes advantage of the quantum nature of electrons.
Researchers from the University of Oregon in the US and Delft University of Technology in the Netherlands, experimentally demonstrated interaction-free measurements with electrons using a novel electronic analog of the Mach-Zehnder optical interferometer (a device detecting the relative phase shift between two collimated light beams).
The innovative two-grating electron interferometer is used in a conventional transmission electron microscope with discrete output detectors, tunable alignment with independently movable beam splitters, and scanning capabilities for imaging.
By using this path-separating electron interferometer, the researchers demonstrated high-contrast interaction-free measurements using free electrons and achieved a detection efficiency of 14%.
Implementing this quantum-based method in conventional TEM instruments opens a route toward interaction-free electron microscopy The described approach for low-dose imaging, based on quantum interference effects, requires no modification of free electron beam optics.
Exotic Quantum Objects Enhance Electron Microscopy
Another stepping stone toward developing the next-generation quantum-enhanced TEM technology is the emergence of techniques to manipulate the quantum light–electron interactions. While photons can maintain coherence (or quantum entanglement) over relatively large distances and timescales, electrons decohere rapidly due to their stronger interaction with the environment.
Recent advances in the research of interactions between light and relativistic free electrons (traveling at speed close to the speed of light) resulted in the discovery of previously unknown type of quantum objects named quantum free electron wavepackets. These objects are the result of the interaction of ultrashort laser pulses confined in a photonic cavity with a beam of free electrons.
Extreme Light-Electron Interactions Enable Quantum Science Breakthrough
Based on the breakthrough discovery, a research team led by Prof. Ido Kaminer from the Israel Institute of Technology (Technion) in Israel created an ultrafast TEM setup with femtosecond laser pump-probe capabilities. The instrument utilizes sub-100 femtosecond high-power laser pulses to excite the sample and a free electron beam for imaging the sample and probing its transient states.
The researchers installed an optical cavity (or a photonic crystal) into the sample holder of the TEM and, by controlling the delay between the pump laser and probe electron beam, they investigated the dynamics of light confined within the optical cavity at ultrashort timescales.
The direct measurement of the cavity-photon lifetime via the coherent, free-electron beam revealed an enhancement of more than an order of magnitude in the interaction strength compared to previous experiments on electron-photon interactions. More importantly, during the experiment, the researchers directly observed Rabi oscillations (a cyclic quantum behavior of a two-level system in the presence of external oscillatory driving force) of the free electrons in the probe beam, confirming the quantum nature of the electrons.
Quantum Effects Expand the Analytical Capabilities of Electron Microscopy
Prof. Kaminer's team believes that these experiments could lead to advanced methods for ultrafast low-dose electron microscopy of biological specimens and other irradiation-sensitive materials, such as polymers and nanomaterials.
The novel approach, based on measuring free-electron quantum coherence, could also be used to answer fundamental questions regarding the quantum nature of light-matter interactions. Finally, it can also pave the way toward utilizing free electrons for quantum information processing and quantum sensing.
References and Further Reading
Turner, A. E., et al. (2021) Interaction-Free Measurement with Electrons. Phys. Rev. Lett. 127, 110401. https://doi.org/10.1103/PhysRevLett.127.110401
Zhao, Z., et al. (2021) Quantum Entanglement and Modulation Enhancement of Free-Electron–Bound-Electron Interaction. Phys. Rev. Lett. 126, 233402. https://doi.org/10.1103/PhysRevLett.126.233402
Asban, S., et al. (2021) Generation, characterization, and manipulation of quantum correlations in electron beams. npj Quantum Inf 7, 42. https://doi.org/10.1038/s41534-021-00376-4
Mechel, C., et al. (2021) Quantum correlations in electron microscopy. Optica 8, 70-78. https://doi.org/10.1364/OPTICA.402693
Wang, K., et al. (2020) Coherent interaction between free electrons and a photonic cavity. Nature 582, 50–54. https://doi.org/10.1038/s41586-020-2321-x
Madan, I., et al. (2020) The quantum future of microscopy: Wave function engineering of electrons, ions, and nuclei", Appl. Phys. Lett. 116, 230502. https://doi.org/10.1063/1.5143008
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