Tunneling

One of the first applications of tunneling was an atomic clock based on the tunneling frequency of the nitrogen atom in an ammonia (chemical formula NH₃) molecule. The rate at which the nitrogen atom tunnels back and forth across the energy barrier presented by the hydrogen atoms is so reliable and so easily measured that it was used as the timing mechanism in one of the earliest atomic clocks.

A current and quickly advancing application of tunneling is Scanning Tunneling Microscopy (abbreviated STM). This technique can render high-resolution images, including individual atoms, that accurately map the surface of a material. As with many high-tech tools, its operation is fairly simple in principle, while its actual construction is quite challenging.

The working part of a tunneling microscope is an incredibly sharp metal tip. This tip is electrically charged and held near the surface of an object (known as Figure 2. Rutherford's potential energy map (dark curve) and the theoretical other side of the barrier (dashed curve), which serves to contain the pieces of the nucleus; (a) shows an alpha particle as part of the nucleus and (b) shows its troublesome appearance outside the uranium atom. Illustration by Hans & Cassidy. Courtesy of Gale Group. the sample) that is to be imaged. The energy barrier in this case is the gap between the tip and the sample. When the tip gets sufficiently close to the sample surface, the energy barrier becomes thin enough that a noticeable number of electrons begin to tunnel from the tip to the object. Classically, the technique could never work because the electrons would not pass from the tip to the sample until the two actually touched. The number of tunneling electrons, measured by incredibly sensitive equipment, can eventually yield enough information to create a picture of the sample surface.

Another application of tunneling has resulted in the tunnel diode. The tunnel diode is a small electronic switch and, by incorporating electron tunneling, it can process electronic signals much faster than any ordinary physical switch. At peak performance, it can switch on and then off again ten billion times in a single second.