We recently learned the sad news that Dr. Akira Tonomura – a truly great experimentalist – passed away on May 2, 2012 during the course of treatment on pancreatic cancer.
We have been great admirers of Dr. Tonomura. Our blog’s banner is a cartoon representation of an experimental setup developed by Dr. Tonomura, through which in 1986 he showed single-electron buildups of electron wave interference fringe patterns. This experiment clearly revealed the dual nature of electrons and was described by Physics World magazine as the world’s most beautiful physics experiment, ranking above the historical experiments of Galileo Galilei and Robert Millikan.
This figure from our book “Exploring Quantum Physics Through Hands-On Projects” shows a schematic representation of the modifications that Tonomura made to a transmission electron microscope to develop his experimental setup. Electrons are emitted from a very sharp tungsten tip by applying a potential difference of 3 to 5kV between the tip and a first anode ring through an effect known as “field emission.” These electrons are then accelerated to the second anode potential of 50kV (the de Broglie wavelength for the accelerated electrons is λ=0.0055nm). Assorted “electron optics” within the modified electron microscope attenuate and focus the electron beam so that a current of barely 1,000 electrons per second is beamed towards the double-slit.
The double-slit is actually an extremely fine wire filament (1μm diameter) placed between two conductive plates a centimeter apart. The wire is biased at a positive voltage of 10V relative to the plates. This arrangement is known as an electron biprism.
Obviously, any electrons that make it past the biprism must have gone either through one or the other side of the fine wire. Two electron lenses then magnify the interference pattern 2,000 times and project it onto a fluorescent screen. Each 50keV electron hitting the screen produces about 500 photons which generate photoelectrons inside an intensified position detector. A computer then integrates the hits to produce a final image of the electron interference pattern. Through which slit did each of the electrons go? The answer is that somehow each electron goes through both slits at the same time!
Another one of Dr. Tonomura’s major accomplishments was his experimental verification of the Aharonov–Bohm (AB) effect. For this experiment, Dr. Tonomura used electron holography. Tonomura fabricated a tiny toroidal ferromagnet covered with a layer of superconducting niobium to perfectly shield the magnetic field. His group then measured a phase difference between the electrons that traveled through the central hole of the toroid and those outside it. Although the electrons had only progressed through regions free of electromagnetic fields, there was an observable effect produced by the existence of vector potentials, and thus verifying the AB effect.
We are deeply saddened for the untimely death of such a great scientist.