Overview

Sketch of the experimental setup. Electrons emerging from a point like electron source are injected into a linear quadrupole guide (linear Paul trap) by an electron lens system. There, they are tightly confined in the radial direction, whereas they can freely propagate longitudinally. In future experiments, we plan to extend the geometry of the guide to more complicated structures, like e.g. beam splitters.

In this experiment we guide electrons in a two dimensional Paul trap. These generate an alternating electric field that radially confines electrons and allows for the guiding of electrons in the third direction. In the transverse direction, an approximately harmonic potential is generated. The quantum mechanical states of this potential could be used as information carrier in future experiments. One might also realize interference experiments with guided electrons as they have so far only be performed with free particles.

Planar Paul traps

Planar five wire structure with cut through the time averaged potential seen by electrons above the wires. A microwave voltage is applied to the red electrodes (MW), whereas the blue electrodes are held at ground potential (GND). The plot shows regions of high potential energy in red and those of low potential energy in blue. Electrons are guided in the blue minimum in the lower center of the image.

Paul traps allow for the trapping of charged particles (normally ions) in a purely electric alternating field. For field oscillations that are fast compared to the motion of the trapped particle, the trajectory can be described by a ponderomotive force that always directs the particle towards regions of low electric field and therefore enables stable trapping at field minima. Due to their low mass, electrons move fast in electric fields, so that high driving frequencies are necessary in order to provide stable confinement. Typical values are in the microwave region between 1GHz and 10GHz. Ions, for comparison, can be trapped in much slower fields of 10MHz to 100MHz.

Experiment

Top view of the experimental setup with the substrate carrying the electrodes in the center. For no voltage applied, the electrons emerging from the gun on the right pass over the substrate in straight trajectories (blue line). Guided electrons are deflected along the guide and are detected more to the left (orange).

In order to realize these high driving frequencies we use in our experiments a planar electrode layout fabricated on a microwave substrate. The cross section of the wire structure is small compared to the wavelength of the driving field. This on the one hand permits the integration of planar microwave feeding structures (coplanar waveguides) to the electrode design; on the other hand more complicated guiding structures can easily be realized by lithographic patterning. Planar Paul traps are a comparable new technique that has been developed for ion trapping experiments only recently.

First results

Guided electrons. The position of the substrate is indicated by a horizontal line, whereas the exit of the guide is highlighted by an orange circle. Without a microwave field applied, only a diffuse spot of electrons is visible in the right part of image a). With the voltages switched on, a bright spot of guided electrons is visible at the end of the guide (image b)). The squared patterning of the spot is caused by a mesh placed behind the guide, through which the electrons are imaged.

In our experiment we have for the first time successfully confined electrons in a planar guide driven at microwave frequencies. Slow electrons with energies of several electron volts are directed along a curved electrode structure. This deflection can be used to demonstrate the transverse confinement of the particles.
In future experiments, this principle will be extended to more complex geometries like beam splitters, for example. We also plan to combine our guides with laser triggered electron sources that we also develop in our group.