First electric trap for electrons

Principle of the Paul trap first applied to electrons

Conductor electrodes with electron source in the background; the white lines in between are the underlying substrate. The electrons are emitted through a tiny hole in the center of the source, which has a diameter of 20μm and is not visible on the image. They are then passed over the electrodes at a height of half a millimeter and deflected to the left along the curved electrode structure in the foreground. © Hommelhoff
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Until now, electrons could only be guided and captured by a combination of magnetic and electric fields. Now physicists have for the first time developed an electron trap that uses purely electric alternating fields to control the elementary particles. Like light waves in a glass fiber, electrodes on a chip conduct the electrons. The technique, now published in the journal Physical Review Letters, promises a wide application potential, from fundamental quantum experiments to non-invasive electron microscopy.

The study of the properties of electrons plays a key role in understanding the laws of nature. The extremely light and fast particles are difficult to bring under control. Corresponding measurements have so far been carried out above all in so-called Penning traps, in which a combination of electric and magnetic fields is able to direct the electrons into ordered paths. For a series of experiments, however, it would be better to forego the use of magnetic fields and to guide the electrons with purely electrical alternating fields.

Electrical trap sought for electrons

For electrically charged atoms (ions), there are already such purely electrical traps. These so-called Paul traps are based on microstructured electrodes on flat substrates where an alternating electrical voltage is applied, oscillating at radio frequencies. Thereby a restoring force, which holds the particles in the center of the trap. Now, the research group "Ultrafast Quantum Optics" headed by Peter Hommelhoff at the Max Planck Institute of Quantum Optics in Garching near Munich has succeeded in applying this technique to electrons.

It had to overcome the problem that electrons are about 10, 000 times lighter than ions and thus react much faster to electric fields than the comparatively heavy and inert ions. The frequency with which the voltage is reversed at the electrodes must therefore be much larger for trapping electrons than for the inclusion of ions and is in the microwave range at a few gigahertz.

…and found

In their experiment, the Garching physicists use electrons from a thermal electron source, in which a tungsten wire is heated as in a light bulb and the emerging electrons are bundled into a parallel beam with an energy of a few electron volts. From there, the electrons are coupled into the "waveguide". This is a structure made up of five parallel electrodes, fabricated on a flat substrate, to which an alternating voltage with a frequency of approximately one gigahertz is applied. display

At a distance of half a millimeter above the electrodes, this creates an oscillating quadrupole field, which encloses electrons in the center of the field in the radial direction, that is, transversely to the electrodes. In the longitudinal direction, parallel to the electrodes, on the other hand, no force acts on the particles, so that they can move freely along the "conductor". Overall, the electrons are thereby forced to follow the course of the electrodes on the substrate. The inclusion in the radial direction is extremely strong, so that the electrons follow even small-scale directional changes.

(b) The guided electrons provide as a detector signal a bright round spot at the output of the electron conductor (marked with a circle). (c) When the microwave excitation is switched off, the electrons land on the right detector side, where a weaker and more diffuse signal appears due to the divergence of the beam. Hommelhoff

At the end of the structure is a detector for detecting the escaping electrons. When the alternating field is switched on, the detector clearly shows a bright spot in the left half of the image, exactly where the output of the conductor is located. If the field is switched off, the electrons proceed straight from the source, and in the right half of the image a region diffused by the divergence of the electron beam is visible.

Stronger bundling of the jet is necessary

"This fundamental experiment has proved that electrons can be driven by purely electric fields, " says Hommelhoff. "However, the currently used electron source only delivers a poorly bundled beam, which is why electrons are lost." In the future, scientists want to combine the novel waveguide with an electron source based on the field emission of atomically sharp metal tips. Here it is already possible to bundle the electron beam so sharply that its transverse component is limited only by the Heisenberg uncertainty principle.

Under certain circumstances, individual quantum mechanical vibrational states of the electrons could potentially be accommodated in the radial potential of the conductor. StarkeThe strong inclusion of electrons now demonstrated also means that a "quantum leap" of one vibrational state into the other

The next generation would require a great deal of energy change and would thus be relatively unlikely, "explains Johannes Hoffrogge, a doctoral student in the experiment.

Once a quantum state has been prepared, it will remain stable for a long time and can be used well for experiments . Quantum experiments could be carried out under these conditions, such as electron interferometry with guided electrons: here, the wave function of an electron is first split and then brought together again, so that characteristic superpositions of several quantum states of an electron are generated.

But there are also practical applications conceivable, such as a new type of electron microscopy. (Phys. Rev. Lett., Online Edition, May 9, 2011)

(Max Planck Institute for Quantum Optics, 11.05.2011 - NPO)