- REHOVOT, Israel, February 26, 1998--One of the most bizarre premises
of quantum theory, which has long fascinated philosophers and physicists
alike, states that by the very act of watching, the observer affects the
observed reality.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- Tomorrow's Technology
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- 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."
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- 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.
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- 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.
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- 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.
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