Quantum Theory Demonstrated: Observation Affects Reality

In a study reported in the February 26 issue of Nature (Vol. 391, pp. 871-874), researchers at the Weizmann Institute of Science have now conducted a highly controlled experiment demonstrating how a beam of electrons is affected by the act of being observed. The experiment revealed that the greater the amount of "watching," the greater the observer's influence on what actually takes place.

The research team headed by Prof. Mordehai Heiblum, included Ph.D. student Eyal Buks, Dr. Ralph Schuster, Dr. Diana Mahalu and Dr. Vladimir Umansky. The scientists, members of the Condensed Matter Physics Department, work at the Institute's Joseph H. and Belle R. Braun Center for Submicron Research.

When a quantum "observer" is watching Quantum mechanics states that particles can also behave as waves. This can be true for electrons at the submicron level, i.e., at distances measuring less than one micron, or one thousandth of a millimeter. When behaving as waves, they can simultaneously pass through several openings in a barrier and then meet again at the other side of the barrier. This "meeting" is known as interference.

Strange as it may sound, interference can only occur when no one is watching. Once an observer begins to watch the particles going through the openings, the picture changes dramatically: if a particle can be seen going through one opening, then it's clear it didn't go through another. In other words, when under observation, electrons are being "forced" to behave like particles and not like waves. Thus the mere act of observation affects the experimental findings.

To demonstrate this, Weizmann Institute researchers built a tiny device measuring less than one micron in size, which had a barrier with two openings. They then sent a current of electrons towards the barrier. The "observer" in this experiment wasn't human. Institute scientists used for this purpose a tiny but sophisticated electronic detector that can spot passing electrons. The quantum "observer's" capacity to detect electrons could be altered by changing its electrical conductivity, or the strength of the current passing through it.

Apart from "observing," or detecting, the electrons, the detector had no effect on the current. Yet the scientists found that the very presence of the detector-"observer" near one of the openings caused changes in the interference pattern of the electron waves passing through the openings of the barrier. In fact, this effect was dependent on the "amount" of the observation: when the "observer's" capacity to detect electrons increased, in other words, when the level of the observation went up, the interference weakened; in contrast, when its capacity to detect electrons was reduced, in other words, when the observation slackened, the interference increased.

Thus, by controlling the properties of the quantum observer the scientists managed to control the extent of its influence on the electrons' behavior. The theoretical basis for this phenomenon was developed several years ago by a number of physicists, including Dr. Adi Stern and Prof. Yoseph Imry of the Weizmann Institute of Science, together with Prof. Yakir Aharonov of Tel Aviv University. The new experimental work was initiated following discussions with Weizmann Institute's Prof. Shmuel Gurvitz, and its results have already attracted the interest of theoretical physicists around the world and are being studied, among others, by Prof. Yehoshua Levinson of the Weizmann Institute.

Tomorrow's Technology

The experiment's finding that observation tends to kill interference may be used in tomorrow's technology to ensure the secrecy of information transfer. This can be accomplished if information is encoded in such a way that the interference of multiple electron paths is needed to decipher it. "The presence of an eavesdropper, who is an observer, although an unwanted one, would kill the interference," says Prof. Heiblum. "This would let the recipient know that the message has been intercepted."

On a broader scale, the Weizmann Institute experiment is an important contribution to the scientific community's efforts aimed at developing quantum electronic machines, which may become a reality in the next century. This radically new type of electronic equipment may exploit both the particle and wave nature of electrons at the same time and a greater understanding of the interplay between these two characteristics are needed for the development of this equipment. Such future technology may, for example, open the way to the development of new computers whose capacity will vastly exceed that of today's most advanced machines.

This research was funded in part by the Minerva Foundation, Munich, Germany. Prof. Imry holds the Max Planck Chair of Quantum Physics and heads the Albert Einstein Minerva Center for Theoretical Physics.

The Weizmann Institute of Science, in Rehovot, Israel, is one of the world's foremost centers of scientific research and graduate study. Its 2,400 scientists, students, technicians, and engineers pursue basic research in the quest for knowledge and the enhancement of the human condition. New ways of fighting disease and hunger, protecting the environment, and harnessing alternative sources of energy are high priorities.