What are subatomic particles for children

26.02.1998 00:00

Observation affects reality

Tal Eizman Publications and Media Relations Department
Weizmann Institute

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Rehovot, Israel. - February 26, 1998

One of the strangest assumptions in quantum theory, which has long fascinated philosophers and physicists, is that by observing a given condition, the observer influences it.

In a study reported in the journal Nature on February 26, 1998, researchers at the Weizmann Institute carried out an extremely detailed controlled experiment that demonstrated how electrons are affected by the act of observation. The experiment showed that the influence of the observer on what actually happens increases with the intensity of the observation.

The research team headed by Prof. Mordehai Heiblum included the doctoral student Eyal Buks, Dr. Ralph Schuster, Dr. Diana Mahalu and Dr. Vladimir Umansky. The researchers are members of the Department of Solid State Physics at the Weizmann Institute and work at the institute's own Joseph H. and Belle Braun Center for Submicron Research.

When a "quantum observer" looks on

Quantum mechanics says that particles can also behave like waves. This can apply to electrons from the sub-micrometer range - i.e. at distances that measure less than a micron, that is, a thousandth of a millimeter. If electrons behave like waves, they can simultaneously travel through several openings in a barrier and meet again on the other side of the barrier. This "meeting" is called interference.

As strange as it sounds, interference can only occur when no one is watching. As soon as an observer observes the particles on their way through the openings, the physicists get a completely different picture: If a particle can be observed while passing through an opening, it is clear that it has not migrated through another opening. In other words - under observation, electrons are "forced" to behave like particles and not like waves. The mere act of observation influences the results of the experiments.

To demonstrate this, the scientists at the Weizmann Institute built a tiny device, barely a micron in size, that contained a barrier with two openings. Then they directed a stream of electrons at the barrier.

The "spectator" in this experiment was not a human. The scientists hid behind a tiny but highly complex electron detector that detects electrons whizzing by. The ability of the "quantum goggle" to recognize electrons could be improved by changing its electrical conductivity, i. H. the strength of the current flowing through it can be changed.

Apart from "watching" or "tracking" the electrons, the detector had no effect on the current. Nevertheless, the team found that the mere presence of the observing detector in the vicinity of one of the openings caused changes in the interference pattern of the electron waves passing through the openings in the barrier. In fact, this effect was dependent on the "strength" of the observation. As the "observer" 's ability to detect electrons increased - in other words, as the degree of observation increased - the interference became weaker; in contrast, the interference became stronger when the electron tracking capacity was reduced - in other words, when the observation waned. By controlling the properties of the quantum observer, the scientists were able to control its influence on the behavior of the electrons.

The theoretical basis for this phenomenon was developed a few years ago by a number of physicists, among them Dr. Adi Stern and Prof. Yoseph Imry from the Weizmann Institute, together with Prof. Yakir Aharonov from the University of Tel Aviv. The new experimental work was initiated in discussions with Prof. Shmuel Gurvitz from the Weizmann Institute; its results have already aroused the interest of numerous theoretical physicists worldwide and are being studied by Prof. Yehoshua Levinson from the Weizmann Institute, among others.

Tomorrow's technology

The result of the experiment, namely that observation "destroys" interference, could be used in future technologies to secure an exchange of information. This can happen when information is encoded in such a way that the interference of numerous electron paths is required for deciphering.

"Any eavesdropping would prevent the interference," says Prof. Heiblum. "This way the recipient would know that the message has been intercepted."

On a broader scale, the Weizmann Institute experiment is an important contribution to the efforts of the research community to develop quantum electronic machines, a goal that may be achieved in the next century. This radically new type of electronics could take advantage of the particle and wave nature of electrons at the same time. To build such devices, however, a better understanding of the interaction between these two properties is necessary. Such a technology of the future could, for example, open the way to new computers, the capacity of which would by far exceed that of today's most advanced models.

The study was partially funded by the Munich Minerva Foundation. Prof. Imry holds the Max Planck Chair for Quantum Physics and heads the Albert Einstein-Minerva Center for Theoretical Physics.

The Weizmann Institute is a major center for scientific research and university studies in Rehovot, Israel. The 2400 scientists, students and other employees of the institute operate over 850 research projects, which cover the whole spectrum of today's science.

The news from the Weizmann Institute is available on the World Wide Web at http://www.weizmann.ac.il and is also available at http://www.eurekalert.org.

Features of this press release:
Mathematics, physics / astronomy
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