Quantum Mirage

One of the biggest obstacles to the continued shrinkage of electronic elements within integrated circuits is the connection between them. As the size
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One of the biggest obstacles to the continued shrinkage of electronic elements within integrated circuits is the connection between them. As the size of these elements decreases, so must the size of the wires that carry electrons from one to another. But beyond a certain point, a wire's ability to conduct electrons is significantly hampered, preventing the message from getting through. Therefore, if nanotechnology and atomic-scale computers are to become a reality, an alternative means of sending information between circuit elements must be developed.

One exciting possibility was recently announced by IBM (www.research.ibm.com). Led by Donald Eigler, a team of scientists at the company's Almaden Research Center in San Jose, California, demonstrated a remarkable phenomenon called a quantum mirage: a clearly defined "reflection" of an atom located at a different point in space. Understanding how this relates to nanocircuit communication requires some background, so bear with me.

First, we need to take a slight detour to explore a geometric shape called an ellipse, which looks like an oval. Two important points are located on either side of an ellipse's center, on the line that divides the ellipse in half along its longer dimension. These two points are called the foci (rhymes with low sigh); each one is called a focus. Imagine a line extending from either focus to any point on the ellipse itself and another line extending from that point to the other focus-the combined length of both lines will be the same no matter where the point lies on the ellipse.

Eigler's team used a scanning tunneling microscope to assemble 36 cobalt atoms into an ellipse measuring a few nanometers (billionths of a meter) across. They constructed the ellipse on the surface of a single copper crystal that was cooled to 4 degrees kelvin (that is, 4 degrees above absolute zero) within an ultrahigh vacuum. This elliptical structure, called a quantum corral, confines a portion of the two-dimensional "sea" of electrons that exists on the crystal's surface.

One more detour: cobalt atoms exhibit a property called a magnetic moment. When a cobalt atom is deposited on a metallic, nonmagnetic surface (such as copper), the electron sea produces what is called the Kondo effect, after Japanese physicist Jun Kondo, who explained the phenomenon in 1964. Basically, the electrons near the atom align themselves to offset its magnetic moment, effectively canceling it out. The Kondo effect is highly localized and easily detected using spectroscopic techniques.

When the IBM scientists placed a single cobalt atom within the quantum corral, they saw the Kondo effect at the atom's location, as expected. But when they moved this atom to one of the ellipse's foci, something amazing happened: the Kondo effect also appeared at the other focus, even though no atom was there (see Fig. 1). The "phantom" atom is called a quantum mirage; information about the real atom is transmitted to the other focus of the ellipse via the wavelike medium of the electron sea without using any wires. Due to the wave nature of electrons, the physics of the quantum corral is analogous to the vibration of a guitar string or a drum head.

The potential applications of this phenomenon are many and varied; for example, the presence or absence of a quantum mirage might be used to represent one bit of data in a region far smaller than any current electronic device can manage. It will be years before this technology becomes practical, but it could eventually yield computers (and the musical tools they provide) that are many orders of magnitude smaller, faster, and less power-hungry than anything we can conceive today.