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Number 558, September 26, 2001 by Phil Schewe, James Riordon, and Ben Stein

Entanglement of Macroscopic Objects

Entanglement of macroscopic objects, a pair of gas clouds containing a trillion atoms each, has been achieved by a research team in Denmark (Eugene Polzik, University of Aarhus, 011-45-89423745, polzik@ifa.au.dk), constituting by far the largest material objects entangled on demand and paving the way for quantum teleportation between macroscopic objects. The accomplishment, published in this week's issue of Nature (Julsgaard et al., 27 September 2001), was announced in preliminary form this June at the first International Conference on Quantum Information, sponsored in part by the Optical Society of America and the American Physical Society.

One of the most profound features of quantum mechanics, entanglement is a special interrelationship between objects in which measuring one object instantly influences the other, even if the two are completely isolated from one another. No previous entanglement with atoms has involved more than four particles. Furthermore, atoms have only been entangled at close proximity, either as ions spaced microns apart in a tiny trap (Update 475), or atoms flying over a short range through narrowly spaced cavities (Hagley et al., Phys. Rev. Lett., 7 July 1997).

In the present experiment, researchers sent a light beam through two cesium gas samples, each held in a special paraffin-coated cell. The beam changed each sample's "collective spin," which describes, in a sense, the net direction in which all of the atoms' tiny magnets add up. First, the researchers measured the sum of the two collective spins without knowing the individual collective spin of each sample. A subsequent measurement, nearly a millisecond later, showed that the sum remained the same. This demonstrated that the two gas samples maintained their special interrelationship and were entangled. Although the two samples were just millimeters apart, they could in principle be separated, and thereby entangled, at much longer distances. Entanglement of such large objects enables "bulk" properties, like collective spin, to be "teleported," or transferred, from one gas cloud to another.

The Black Hole of Geneva

Black holes are known as the omnivorous destroyers of stars. In reality black holes not only take but give. Near their event horizons, where space is so drastically warped, black holes spawn particle-antiparticle pairs out of sheer vacuum. In some cases one of the pair escapes beyond the horizon while its counterpart is pulled back into the hole. Thus black holes can shed energy in the form of this "Hawking radiation."

Physicists hope to bring this whole process down to earth by manufacturing tiny black holes amid the stupendous smashups of protons at the Large Hadron Collider (LHC) being built at CERN. Until recently theorists thought gravity was so weak compared to the other forces that it, and gravitationally bound objects like black holes, could be studied on an equal footing with the other forces like the strong nuclear force only at energies of 10¹⁹ GeV.

In the past few years, though, some models featuring extra spatial dimensions hint that the unification of the forces, including gravity, might set in at much more modest energies, even in the TeV realm of the LHC. Thus one can contemplate forming a TeV-mass black hole even as one can imagine creating new particles in that mass range.

But what would a black hole look like? Savas Dimopoulos of Stanford (650-723-4231) and Greg Landsberg of Brown University (landsberg@hep.brown.edu, 401-863-1464) have drawn a picture in which proton-proton collisions could create black holes with a cross section (likelihood of creation) only about a factor of ten less than for producing top quarks and at a rate of up to one per second (see figure). A black hole produced in this way would quickly decay, not in the usual particle way but in a furious burst of Hawking radiation. A particularly striking signature of the black hole would involve an electron, muon, and photon in the final state of debris particles. Properties of Hawking radiation could tell physicists about the shape of extra spatial dimensions. A possibility of recreating the early moments of the universe in the lab would further unite particle physics and cosmology (Dimopoulous and Landsberg, Physical Review Letters, 15 October 2001.)

Turing Model for Ladybug Beetle Patterns

Zebras, leopards, and giraffes are just a few creatures exhibiting intricate patterns that can be duplicated with models pioneered by the late mathematical genius Alan Turing (Update 80). The models are based on diffusion equations, which are often used to describe the spontaneous mixing of materials over time. As a rule, mathematicians and physicists have studied Turing models of biological patterns as though they were formed on flat surfaces. Of course, few animals tend to be flat, unless they've lingered too long on a highway.

Although Turing models can mimic tiger stripes and cheetah spots fairly well despite this simplifying assumption, a group of researchers from the National Chung-Hsing University in Taiwan decided to consider a slightly more complex shape. When S.S. Liaw (liaw@phys.nchu.edu.tw, 011-8864-2284-0427) and colleagues studied Turing models on a portion of a spherical surface, patterns reminiscent of those on lady bug beetles emerged. There are over 4500 species of ladybugs, most bearing unique, recognizable designs in contrasting colors such as black and red.

By adjusting coefficients in the model's equations and varying the initial distributions of hypothetical compounds that mix to create the colors, the researchers could reproduce the stripes, swirls, and spots that decorate many of these predatory insects. The new model shows that an animal's specific geometry is important in determining it's adornment, and adds weight to Turing's proposal that diffusion is potentially a mechanism that helps generate an endless variety of patterns in nature. (S. S. Liaw; C. W. Yang; R. T. Liu; J. T. Hong, Physical Review E, October 2001.)