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ScienceWeek
QUANTUM PHYSICS: ON ENTANGLEMENT IN FREE SPACE
The following points are made by J.G. Rarity (Science 2003 301:604):
1) Entanglement is a property of quantum theory that allows two particles (such as photons) to be much more strongly correlated than is possible in classical physics. Conventional quantum theory sets no limit on the range or duration of these correlations. Over the past 20 years, many optical experiments have demonstrated these apparently nonlocal effects in the laboratory (1-4) and more recently over ranges of up to 10 km in optical fibers (5).
2) Aspelmeyer et al (2003) report the latest of these demonstrations. In contrast to earlier studies, they study entanglement not in optical fibers but in free space. The authors show that strong correlations can be maintained in free space over a separation of 600 m. The optical losses were comparable to those in a putative space-based experiment, in which a satellite would send correlated photon pairs to two separate observers on Earth about 600 km away.
3) Key to the Aspelmeyer et al (2003) experiment is a source that creates entangled pairs of photons with anticorrelated polarizations. If the source emits a horizontal photon in direction 1, a vertical photon is emitted in direction 2, and vice versa. This anticorrelation holds for any polarization direction. The photons are sent in opposite directions to two well-separated polarizers and single-photon detectors. The polarizers, which can be rotated to any angle, allow those photons to pass that are polarized parallel to this angle, while blocking all photons polarized at 90º to this angle.
4) The detectors fire when photons are detected, producing a series of clicks that measure the arrival times of photons. Separate measurements in arm 1 or arm 2 show no change in the click rate when the polarizer angle is changed. Hence, there is no favored polarization emitted by the source. But when both detectors fire simultaneously, indicating the detection of photon pairs, the click rate increases when the polarizers are set at 90º to each other: For instance, a detector click in channel 1 behind a vertical (V) polarizer immediately implies a click in channel 2 when using a horizontal (H) polarizer.
5) In a classical (or "local realistic") view, we would make the naïve assumption that photons have a real (locally labeled) polarization when they leave the crystal. For instance, an H-polarized photon emitted in direction 1 would imply a V-polarized photon emitted in direction 2. After emission, we could set the angle for the polarizer to 45º in channel 1 (-45º in channel 2), such that only half the intensity of horizontally or vertically polarized light will pass through the polarizer. Each individual H or V photon then has a 50% chance of passing through the polarizer, effectively tossing a coin to decide whether to pass (and be detected) or not. We expect no correlation between the results of random coin tosses at each end of the apparatus, effectively removing the anticorrelation.
6) What distinguishes the quantum state is that the anticorrelation is retained even when we rotate the polarizers: If one photon is measured as +45º, then its partner will always pass through a -45º polarizer. The effect is "nonlocal" because we apparently toss the coin only once to decide whether both photons pass or do not pass, and information on the result of the coin toss is sent (apparently) instantaneously to both ends of the apparatus. The quantum mechanical explanation for this ascribes no local value of polarization until after the photon has been measured (a click has been seen). However, there is a strong correlation of polarizations. This arises because the entangled state cannot be factored into a product of two subsystems but must be described within the same single (quantum mechanical) wave function.
References (abridged):
1. A. Aspect, J. Dalibard, G. Roger, Phys. Rev. Lett. 49, 1804 (1982)
2. Y. H. Shih, C. O. Alley, Phys. Rev. Lett. 61, 2921 (1988)
3. Z. Y. Ou, L. Mandel, Phys. Rev. Lett. 61, 50 (1988) [APS].
4. J. G. Rarity, P. R. Tapster, Phys. Rev. Lett. 64, 2495 (1990)
5. P. R. Tapster, J. G. Rarity, P. C. M. Owens, Phys. Rev. Lett. 73, 1923 (1994)
Science http://www.sciencemag.org
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ON QUANTUM ENTANGLEMENT
The following points are made by B.M. Terhal et al (Physics Today 2003 April):
1) Erwin Schroedinger (1887-1961) coined the word entanglement in 1935 in a three-part paper (Naturwiss. 1935 48:807; 49:823,844; Engl. trans.: Proc. Am. Philos. Soc. 1980 124:323) on the "present situation in quantum mechanics." His article was prompted by Albert Einstein, Boris Podolsky, and Nathan Rosen's now celebrated "EPR paper" that had raised fundamental questions about quantum mechanics earlier that year.
2) Einstein and his coauthors had recognized that quantum theory allows very particular correlations to exist between two physically distant parts of a quantum system; those correlations make it possible to predict the result of a measurement on one part of a system by looking at the distant part. On that basis, the EPR paper argued that the distant predicted quantity should have a definite value even before being measured if the theory were to claim completeness and respect locality. However, because quantum mechanics disallows such definite values prior to measuring, the EPR authors concluded that, from a classical perspective, quantum theory must be incomplete.
3) Schroedinger's 1935 perspective comes closer to the modern view: The wavefunction or state vector gives us all the information that we can have about a quantum system. About entangled quantum states, he wrote, "The whole is in a definite state, the parts taken individually are not," which we now understand as the essence of pure-state entanglement. In that same 1935 article, Schroedinger also introduced his famous cat as an extreme illustration of entanglement: A cat physically isolated in a box with a decaying atom and vial of cyanide represents a quantum state having macroscopic degrees of freedom. If the atom were to decay and trigger the release of cyanide, the cat would die. The quantum-mechanical description of the system is a coherent superposition of one state in which the atom is still excited and the cat alive, and another state in which the atom has decayed and the cat is dead. The isolated cat-trigger-atom-cyanide system as a whole is in a definite entangled state, even though the cat itself exists as a probabilistic mixture of being alive or dead.
4) For the three decades following the 1935 articles, the debate about entanglement and the "EPR dilemma" -- how to make sense of the presumably nonlocal effect one particle's measurement has on another -- was philosophical in nature, and for many physicists it was nothing more than that. The 1964 publication (J.S. Bell: Physics 1964 1:195) by John Bell changed that situation dramatically. Bell derived correlation inequalities that can be violated in quantum mechanics but have to be satisfied within every model that is local and complete -- so-called local hidden-variable models. Bell's work made it possible to test whether local hidden-variable models can account for observed physical phenomena. Early and ongoing recent experiments showing violations of such Bell inequalities have invalidated local hidden-variable models and lend support to the quantum-mechanical view of nature. In particular, an observed violation of a Bell inequality demonstrates the presence of entanglement in a quantum system.
Physics Today http://www.physicstoday.org
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Notes:
A "hidden variables theory" is one of a class of physical theories which deny that the quantum state of a physical system is a complete specification. The hidden variables are those components of the hypothetical complete state that are not contained in the quantum state.
"Bell's inequality", formulated by John Bell (1928-1990) in 1964, is one of a family of inequalities concerning the probabilities of joint occurrence of certain events in two well-separated parts of a composite system, the inequality implied by any hidden variables theory that satisfies an appropriate locality condition. In this context, in general, a locality condition is a condition such that no interaction between two entities can occur in less time than the time required for light to travel from one entity to the other. For example, any apparent instantaneous effect of one entity upon the other entity implies locality is not obeyed.
"Bell's theorem" is the theorem that no hidden variables theory satisfying an appropriate locality condition can make statistical predictions in complete agreement with those of quantum mechanics. In other words, there are situations in which quantum mechanics predicts a violation of Bell's inequality. Another formulation is that any hidden variables theory that forbids instantaneous interactions cannot make predictions in complete agreement with those of quantum mechanics.
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