ScienceWeek

THEORETICAL PHYSICS: ON THE REALITIES OF QUANTUM PHYSICS

The following points are made by Anton Zeilinger (Nature 2005 438:743):

1) In the first of his papers from 1905, his Annus Mirabilis, Einstein proposed the idea of particles of light, later called photons. From this paper, a very realistic picture of light particles emerged, as being much like the particles in an ideal gas. But the paper also contained the seeds of Einstein's later criticisms of quantum mechanics. As he described in his Autobiographical Notes, Einstein challenged physics, including the concepts of quantum mechanics, to describe "the real factual situation", or, in other words, what is out there.

2) The concepts that Einstein criticized were randomness, entanglement and complementarity. These have become the core principles of newly emerging quantum information technologies: quantum computation, quantum teleportation and quantum cryptography. But although we may have realized that Einstein was wrong about these concepts, have we today understood the message of the quantum?

3) The discovery that individual events are irreducibly random is probably one of the most significant findings of the 20th century. Before this, one could find comfort in the assumption that random events only seem random because of our ignorance. For example, although the brownian motion of a particle appears random, it can still be causally described if we know enough about the motions of the particles surrounding it. Thus, as Werner Heisenberg put it, this kind of randomness, of a classical event, is subjective.

4) But for the individual event in quantum physics, not only do we not know the cause, there is no cause. The instant when a radioactive atom decays, or the path taken by a photon behind a half-silvered beam-splitter are objectively random. There is nothing in the Universe that determines the way an individual event will happen. Since individual events may very well have macroscopic consequences, including a specific mutation in our genetic code, the Universe is fundamentally unpredictable and open, not causally closed.

5) Most striking is the case of entanglement, which Einstein called "spooky", as it implies that the act of measuring a property of one particle can instantaneously change the state of another particle no matter how far apart the two are. Distances over which this phenomenon have been verified experimentally are in the order of 100 kilometers. How is it possible that two events, each one objectively random, are always perfectly correlated? John Bell showed that the quantum predictions for entanglement are in conflict with local realism. From that "natural" point of view any property we observe is (a) evidence of elements of reality out there and (b) independent of any actions taken at distant locations simultaneously with the measurement. Most physicists view the experimental confirmation of the quantum predictions as evidence for nonlocality. But the concept of reality itself may be at stake, a view that is supported by the Kochen Specker paradox. This observes that even for single particles it is not always possible to assign definite measurement outcomes independently of and prior to the selection of specific measurement apparatus in the specific experiment.

References:

1. Zeilinger, A., Weihs, G., Jennewein, T. & Aspelmeyer, M. Nature 433, 230 238 (2005)

2. Schilpp, P. A. Albert Einstein: Philosopher-Scientist (Open Court Publishing, Library of Living Philosophers, Peru, Illinois, 1949)

Nature http://www.nature.com/nature

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ENTANGLEMENT, DECOHERENCE, AND THE QUANTUM-CLASSICAL BOUNDARY

Notes by ScienceWeek:

Quantum mechanical entanglement is a phenomenon that has caught the imagination of the public as one of the more bizarre consequences of fundamental physical theory. Entanglement is unique to quantum mechanics, and involves a relationship (a "superposition of states") between the possible quantum states of two entities such that when the possible states of one entity collapse to a single state as a result of suddenly imposed boundary conditions, a similar and related collapse occurs in the possible states of the entangled entity no matter where or how far away the entangled entity is located.

Entanglement arises from the wave function equation of quantum mechanics, which has an array of possible function solutions rather than a single function solution, with each possible solution describing a set of possible probabilistic quantum states of the physical system under consideration. Upon fixation of the appropriate boundary conditions, the array of possible solutions collapses into a single solution. For many quantum mechanical physical systems, the fixation of boundary conditions is a theoretical and fundamental consequence of some interaction of the physical system with something outside that system, e.g., an interaction with the measuring device of an observer.

In this context, two entities that are described by the same array of possible solutions to the wave function equation are said to be "coherent", and when events decouple these entities, the consequence is said to be "decoherence". As a physical phenomenon, entanglement was discussed many years ago, most particularly following the publication in 1935 of the often quoted Einstein-Podolsky-Rosen paper (Phys Rev 1935 47:777). These discussions have been in the form of "gedanken" (thought) experiments involving two quantum-mechanical entangled entities. More recently, however, there have been laboratory constructions of actual quantum mechanical systems exhibiting such entanglement phenomena, and the reportage of these laboratory arrangements by the media have engaged the public fancy. Essential here is that any purely verbal account of quantum mechanical phenomena is severely limited by the constraint that the properties of quantum mechanical systems can be precisely described only by the equations relevant for those systems, and all other descriptions usually introduce serious ambiguities.

The following points are made by Serge Haroche (Physics Today 1999 July):

1) In quantum mechanics, a particle can be delocalized (simultaneously occupy various probable positions in space), can be simultaneously in several energy states, and can even have several different identities at once. This apparent "weirdness" behavior is encoded in the wave function of the particle.

2) Recent decades have witnessed a rash of experiments designed to test whether nature exhibits implausible nonlocality. In such experiments, the wave function of a pair of particles flying apart from each other is entangled into a non-separable superposition of states. The quantum formalism asserts that detecting one of the particles has an immediate effect on the other, even if they are very far apart, even far enough apart to be out of interaction range. The experiments clearly demonstrate that the state of one particle is always correlated to the result of the measurement performed on the other particle, and in just the strange way predicted by quantum mechanics.

3) An important question is: Why and how does quantum weirdness disappear (decoherence) in large systems? In the last 15 years, entirely solvable models of decoherence have been presented by various authors (e.g., Leggett, Joos, Omnes, Zeh, Zurek), these models based on the distinction in large objects between a few relevant macroscopic observables (e.g., position or momentum) and an "environment" described by a huge number of variables, such as positions and velocities of air molecules, number of black-body radiation photons, etc. The idea of these models, essentially, is that the environment is "watching" the path followed by the system (i.e., interacting with the system), and thus effectively suppressing interference effects and quantum weirdness, and the result of this process is that for macroscopic systems only classical physics obtains.

4) In mesoscopic systems, which are systems between macroscopic and microscopic dimensions, decoherence may occur slowly enough to be observed. Until recently, this could only be imagined in a gedanken experiment, but technological advances have now made such experiments real, and these experiments have opened this field to practical investigation.

Physics Today http://www.physicstoday.org

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QUANTUM PHYSICS: ION ENTANGLEMENT IN INFORMATION PROCESSING

The following points are made by D.G. Cory and T.F. Havel (Science 2004 304:1456):

1) Over the past decade, quantum entanglement has been recognized as an increasingly useful property of multiparticle systems (1). Erwin Schroedinger (1887-1961) coined the term "entanglement" to refer to a peculiar mutual quantum interaction in which the properties of two or more physical objects can be correlated, even when separated. Quantum teleportation, error correction, computation, and communication all benefit from (or require) entanglement (2).

2) One current challenge for the field of quantum information processing has been to engineer a sufficiently large and complex controllable system in which questions related to entanglement can be precisely explored. Roos et al (3) and Leibfried et al (4) have reported the creation, control, and potential applications of three-particle entangled states made with trapped ions.

3) Until recently, most of the laboratory examples of entangling and coherently controlling more than two particles have come from optics and ensemble measurements via liquid-state nuclear magnetic resonance. Although useful for furthering the development of coherent control methods, these implementations do not scale easily to larger numbers of particles. Whereas limited manipulations of even seven-qubit systems have been possible in liquid-state NMR (5), it is particularly important to see the continued development of microscopic systems based on pure-state dynamics, which may lead to scalable quantum computation.

4) The complementary studies of Roos et al (3) and Leibfried et al (4) demonstrate the continued progress in engineering quantum systems to perform tasks that are beyond their classical counterparts. These studies open a new path for coherent control by enabling the control operation to be conditional on measurement, and they point to practical applications for entangled states.

References (abridged):

1. B. M. Terhal, M. W. Wolf, A. C. Doherty, Phys. Today 56, 46 (April 2003) [Physics Today]

2. M. Nielson, I. Chuang, Quantum Computation and Quantum Information (Cambridge Univ. Press, Cambridge, 2000)

3. C. F. Roos et al., Science 304, 1478 (2004)

4. D. Leibfried et al., Science 304, 1476 (2004)

5. E. Knill, R. Laflamme, R. Martinez, C. H. Tseng, Nature 404, 368 (2000)

Science http://www.sciencemag.org

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