Table Of ContentQuantum [Un]speakables
Springer-Verlag
Berlin Heidelberg GmbH
ONLINE LIBRARY
Physics and Astronomy
http://www.springer.de/phys/
R.A. Bertlmann A. Zeilinger
Quantum
[Un]speakables
From Bell
to Quantum Information
With 141 Figures, 4 in Color
t
Springer
Professor Dr. Reinhold A. Bertlmann
University of Vienna, Institute for Theoretical Physics
Boltzmanngasse 5, 1090 Vienna, Austria
e-mail: [email protected]
Professor Dr. Anton Zeilinger
University of Vienna, Institute for Experimental Physics
Boltzmanngasse 5, 1090 Vienna, Austria
e-mail: [email protected]
Library of Congress Cataloging-in-Publication Data
Quantum [un]speakables : from Bell to quantum information I [edited by] R.A. Bertimann,
A. Zeilinger. p. cm. Includes biblographical references and index. ISBN 3540427562 (acid-free pa
per) 1. Bell's theorem--Congresses. 3. Bell, J.S.--Congress. I. Bell, J.S. II. Bertlmann, Reinhold A. III.
Zeilinger, Anton. QC174.17.B45 Q36 2002 530.12--dc21 2002021641
ISBN 978-3-642-07664-0 ISBN 978-3-662-05032-3 (eBook)
DOI 10.1007/978-3-662-05032-2
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John Bell @Renate Bertlmann, 1980
Preface - From Bell to Quantum Information
John Stewart Bell was certainly one of the really outstanding scientists of the
Twentieth Century. The theorem named after him was most aptly called by
Henry Stapp the most profound discovery since Copernicus. Born in Belfast in
1928, his scientific career up to his untimely death in 1990 covered not only the
foundations of quantum mechanics, but also the areas of quantum field theory
and the physics of accelerators, where he made outstanding contributions.
So, the conference organized in his honor at the University of Vienna, 10-14
November 2000, to commemorate the Tenth Anniversary of his death brought
together friends and colleagues working in all three areas.
John Bell's scientific career began in Harwell and Malvern, where he made
calculations of particle trajectories in accelerators and became an expert in
high demand for the focusing of particle beams. During this time, he met his
wife, Mary, who was also an accelerator physicist. We were very happy and
grateful to have Mary Bell as the Guest of Honor at the Vienna Conference.
Later, while working on his thesis with Rudolf Peierls at Birmingham, he
discovered independently from G. Liiders and W. Pauli the CPT theorem, one
of the most basic theorems in physics. This is probably the most fundamental
symmetry principle which states that the joint action of charge conjugation,
inversion of parity and time reversal leaves any physical system unchanged.
Of his contributions to quantum field theory, two others could be men
tioned. One is the discovery, together with Roman Jackiw, of what is to
day called the Bell-Jackiw-Adler anomaly. It explains some hitherto-not
understood pion decays by adding divergences of the quantum current to
the description of the system. The other most important contribution of
John Bell is his suggestion in 1967 that there should exist a gauge theory
describing weak interactions. This idea had an immense influence on Martinus
Veltman and Gerard 't Hooft and resulted in a full understanding of the
weak force. Ultimately, gauge theories became the standard tool of modern
particle physics.
While it is difficult to underestimate the importance of John Bell's contri
butions to accelerator physics and to field theory, his most important contri
butions were certainly in the foundations of quantum mechanics. Evidently,
John Bell had been interested from a very early stage in the philosophical
VIII Preface
issues raised by quantum theory, a topic not very popular during his student
days at Queen's University, Belfast. Apparently, John Bell, who had been
interested in the Bohr-Einstein dialogue, always took the position of Albert
Einstein on philosophical issues. He also felt that a completion of quantum
mechanics using so-called "hidden variables" would be highly desired, as it
would help to regain a realistic and objective picture of the world. That way,
Bell hoped one would be able to arrive at a physics where "measurement"
would not play such a central role as in the Copenhagen interpretation of
quantum mechanics.
Then, a most interesting sequence of events set in. In 1952, David Bohm
had achieved something which had earlier been proclaimed impossible. It
had been proved by John von Neumann that no hidden variable theory could
agree with quantum mechanics. Bohm actually formulated such a theory,
where each particle at any time has both a well-defined position and a well
defined momentum. The conflict raised between von Neumann and Bohm was
elegantly resolved by Bell, who showed that von Neumann's proof contained
a physically unjustifiable assumption. So while John Bell had flung open the
door widely for hidden variable theories, he immediately dealt them a major
blow. In 1964, in his celebrated paper "On the Einstein-Podolsky-Rosen
Paradox", he showed that any hidden variable theory, which obeys Einstein's
requirement of locality, i.e. no influence travelling faster than the speed of
light, would automatically be in conflict with quantum mechanics. This is
the celebrated Bell's theorem, which even found entrance into comics.
Surprisingly, while quantum mechanics was already well established at
the time of the publication of Bell's theorem, no experiments existed which
definitely allowed one to rule out a local realistic interpretation. Which means
that no experiment had measured for those specific two-particle correlations
which are so necessary to demonstrate the violation of Bell's inequality, the
quantitative measure of the border of the validity of local realism. Since then,
experimentation has become better and better, and by now an impressive
body of evidence has been collected, supporting quantum mechanics and
being in conflict with local realism. While a very tiny loophole in principle
remains for local realism, it is a very safe position to assume that quantum
mechanics has definitely been shown to be the right theory. Thus, a very
deep philosophical question, namely, whether or not events observed in the
quantum world can be described by an underlying deterministic theory, has
been answered by experiment, thanks to the momentous achievement of John
Bell. This achievement is even more remarkable as he was able to rule out
a gigantic class of theories, without having to know any details of the theories.
Today, experimental development has gone far beyond this. Bell's in
equality and the underlying physics of entangled states have become cor
nerstones of the newly evolving technology of quantum information. There,
information is encoded, transmitted and processed in completely novel ways
Preface IX
based on quantum laws. A bit of information can be encoded in a quantum
superposition. Two quantum bits or q-bits can be entangled over long dis
tances, and entanglement can be used to encode information in a novel way
unprecedented in classical physics. Experimentally, it has been shown that,
using entanglement, one can encode more information than one bit in a two
state system, one can teleport a quantum state over large distances and one
can use quantum entanglement to provide a cryptographic method which is
secure against eavesdropping by the laws of physics and not by a trick of the
experimentalist. Finally, quantum computation promises exponential speed
up for certain problems, and first steps have been taken in the direction of
quantum computation in various laboratories all over the world.
It is most interesting to note that, despite his own theorem, John Bell
continued to be an advocate of realistic hidden variable theories, which now,
according to his own findings, have to be non-local. Therefore, he became
an advocate of theories proposed by Ghirardi, Rimini, Weber, Peierls and
Gisin, theories that, no matter how little, deviate from quantum mechanics
and thus might be ruled out by experiment someday. Thus, even if John Bell
in the end might not have turned out to be on the right side when it comes
to the fundamentals of the interpretation of quantum mechanics, this should
never be held against him. In contrast, he was one of the few, like Albert
Einstein, who realized how extremely strange the consequences of quantum
mechanics, if it finally should turn out to be the ultimate correct theory, are
for our view of the world, and it is a sign of his high moral distinction that
he was not at all willing to give in easily.
In conclusion, it might very well turn out in the future that Bell's theorem
paved the way to a momentous change of our conception of the world.
Nobody has expressed his appreciation of John Bell, which we fully share,
better than Jack Steinberger:
"One of the greatest privileges of my rewarding life in physics has been the
contact with ( ... ) John Bell. ( ... ) Trying to learn ( ... ) the behavior of neutral
kaons in the light of CP violation, I had the pleasure of benefiting from
John's penetrating understanding and insight, and of his readiness to share
this. Bell was among the most brilliant physicists I have known; in addition
he had the very important human qualities of being accessible, unpretentious
and kind to his less gifted colleagues."
The organization of the conference in Vienna would have been impossible
without the financial support of the Austrian Bundesministerium fiir Bildung,
Wissenschaft und Kultur, the Stadt Wien, Wissenschafts- und Forschungs
forderung, the International Erwin Schrodinger Institut (ESI), the TMR
Network "The Physics of Quantum Information" and the University of Vi
enna. Many thanks also to Larissa Cox and Andrea Aglibut for handling
the correspondence with the speakers and the editing of the present book,
to Gabriele Zobl-Kratschmann and numerous other helpers for organizing
X Preface
the conference, to the Austrian Central Library for Physics for the video
documentation and to Renate Bertlmann for taking a complete photographic
record of the event.
Vienna, Reinhold Bertlmann
January 2002 Anton Zeilinger
