PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 765 February14, 2006 by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi
ATTACK OF THE TELECLONES: Should quantum cryptographers begin to worry? In contrast with everyday matter, quantum systems such as photons cannot be copied, at least not perfectly, according to the "no-cloning theorem." Nonetheless, imperfect cloning is permitted, so long as Heisenberg's Uncertainty Principle remains inviolate. According to Heisenberg, measuring the position of a particle disturbs it, and limits the accuracy to which its complementary property (momentum) can be determined, making it impossible to reliably replicate the particle's complete set of properties. Now, quantum cloning has been combined with quantum teleportation in the first full experimental demonstration of "telecloning" by scientists at the University of Tokyo, the Japan Science and Technology Agency, and the University of York (contact Sam Braunstein, [EMAIL PROTECTED] and Akira Furusawa, [EMAIL PROTECTED]). In ideal teleportation, the original is destroyed and its exact properties are transmitted to a second, remote particle (Heisenberg does not apply because no definitive measurements are made on the original particle). In telecloning, the original is destroyed, and its properties are sent to not one but two remote particles, with the original's properties reconstructed to a maximum accuracy (fidelity) of less than 100%. (Heisenberg limits the ability to make clones as otherwise researchers could keep making copies of the original particle and learn everything about its state.) In their experiment, the researchers didn't just teleclone a single particle, but rather an entire beam of laser light. They transmitted the beam's electric field, specifically its amplitude and phase (but not its polarization) to two nearly identical beams at a remote location with 58% accuracy or fidelity (out of a theoretical limit of 66%). This remarkable feature of telecloning stems from the very magic of quantum mechanics: quantum entanglement. Telecloning stands apart from local cloning and from teleportation in requiring "multipartite" entanglement, a form of entanglement in which stricter correlations are required between the quantum particles or systems, in this case three beams of light. (An example of a multipartite entanglement is the GHZ state between three particles that was featured in Update 414.) In addition to representing a new quantum-information tool, telecloning may have an exotic application: tapping quantum cryptographic channels. Quantum cryptographic protocols are so secure that they may discover tapping. Nonetheless, with telecloning, the identity and location of the eavesdropper could be guaranteed uncompromised. (Koike et al., Physical Review Letters, 17 February 2006; for an earlier partial demonstration of telecloning, between an original photon and one clone at a remote location and another clone local to it, see Zhao et al., Phys Rev Lett, 13 July 2005) STOCK MARKET CRITICALITY. In the months before and after a major stock market crash, price fluctuations follow patterns similar to those seen in natural phenomena such as heartbeats and earthquakes, physicists find in a study to appear in Physical Review Letters. A University of Tokyo team studied the Standard & Poor's S&P 500 index, focusing on small deviations from long-term index trends. Such up-and-down blips in stock prices are usually "Gaussian," or "normally" random, at least when measured over sufficiently long time scales---for example, for more than one day. That means that fluctuations are likely to be small, while larger fluctuations are less likely, their probabilities following a bell curve. But when the team looked at 2-month periods surrounding major crashes such as the Black Monday event of October 19, 1987, they saw a different story: Fluctuations of all magnitudes were equally probable. As a consequence, the graph of index fluctuations looked statistically similar if plotted over different time scales, anywhere between time scales of 4 minutes and two weeks. Such behavior is called critical in analogy with a ferromagnetic metal at the "critical temperature," when regions form where the metal's atoms arrange their spins in the same direction, and these regions look similar at different levels of magnification. This self-similarity is also seen in the time intervals between heartbeats, or between earthquakes. Mathematically, however, the stock market case differs in that the probabilities do not change with the size of the event, while in other cases of non-critical self-similarity, the probabilities usually follow a so-called power law. It is unclear what individual trading decisions lead to criticality in the stock market, co-author Zbigniew Struzik ([EMAIL PROTECTED]) says, although he and the team at the University of Tokyo are working on finding explanations. Also unclear is whether the findings could one day lead to an early-warning system to predict crashes, and if such a system would precipitate a crash -- or create one artificially -- by inducing panic. "It could compensate for or neutralize the crashes, or make them worse," Struzik says. (Kiyono et al., Physical Review Letters, 17 February) *********** PHYSICS NEWS UPDATE is a digest of physics news items arising from physics meetings, physics journals, newspapers and magazines, and other news sources. It is provided free of charge as a way of broadly disseminating information about physics and physicists. For that reason, you are free to post it, if you like, where others can read it, providing only that you credit AIP. 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