Physics News Update, Number 475

Number 475, March 17, 2000 by Phillip F. Schewe and Ben Stein

STRANGE HALO ORBITS EXPECTED AT SATURN. Consider particles in orbit above a planet. If the particles are uncharged or have a very low charge-to-mass ratio, they will follow a conventional ("Keplerian") trajectory centered about the axis of the planet at the equator (Saturn's rings are an example of such particles). If, however, the particles are highly charged, their motions are dominated by an electromagnetic interaction with the planet's magnetic dipole (Earth's van Allen belts are an example). If the charge is somewhere in between these two cases, and gravity and electromagnetic forces are comparable, then strange orbits are possible.

Scientists at the University of Colorado (Mihaly Horanyi and Jim Howard, 303-492-6903) and Loughborough University (Holger Dullin) in the UK estimate that if conditions are just right some particles could race around a planet in orbits (stable for as long as 10 years) that never cross the planet's equatorial plane (see figure at Physics News Graphics). The dust analyzer on the Cassini craft now gliding toward Saturn might be able to detect particles in these novel orbits. (Howard et al., Physical Review Letters 10 April 2000;Select Article.)

THE FIRST ENTANGLEMENT OF FOUR PARTICLES has been experimentally achieved by researchers at NIST (Christopher Monroe, 303-497-7415), demonstrating a technique that significantly advances the difficult prospect of building a useful quantum computer. To perform powerful calculations, such as factoring huge numbers or quickly finding items in large databases, a quantum computer typically must contain many particles "entangled" with each other. Entanglement describes a very special interlinking that can occur between particles (such as photons or ions) even if they are physically separated or otherwise isolated from one another.

While entangled, each particle is in a fuzzy, noncommital state (for example, being in a combination or "superposition" of a low and high energy state) but has a precisely defined relationship with its partners. Specifically, when one particle eventually "collapses" into a definite state, it essentially causes its entangled partner to collapse into a complementary state, even if it is halfway across the galaxy.

Entanglement is difficult enough to achieve in two particles, or even three (Update 414), but last year, theorists in Denmark proposed a practical method for entangling any number of particles. (Molmer and Sorensen, Phys. Rev. Lett., 1 Mar 1999; see article at Physics News Select Articles.) Their proposal, based in turn on a earlier idea (Cirac and Zoller, Phys. Rev. Lett., 15 May 1995), involves trapping a string of ions in electromagnetic fields. To create multiple entanglement, laser pulses can interlink each ion's internal state (known as its spin) to the overall motion of the ions rocking back and forth.

The Molmer-Sorensen technique enables researchers to accomplish this in a single pulse. NIST researchers demonstrated this technique with four ions (electrical noise made it difficult to do more), but they showed that entanglement of many more particles is now possible. (Sackett et al, Nature, 16 March 2000.)

REMOVING A COMMUNICATIONS BOTTLENECK WITH INKJETS. At last week's Optical Fiber Communication Conference, held in Baltimore by IEEE and the Optical Society of America, researchers from Agilent Technologies (a spinoff of Hewlett Packard) unveiled technology that makes possible a faster, all-optical communications network. Currently, fiber optics networks are not completely optical.

Traditional switches for re-routing a fiber-optic signal are devices that convert photons into electrons and then back into photons. However, the new device employs a specially designed "planar-lightwave circuit"--a flat circuit through which multiple light signals can travel. The waves converge at "cross points" filled with a fluid possessing the same optical properties as the rest of the material. Through the fluid, optical signals can pass through undisturbed. Rerouting the signal is accomplished by injecting an inkjet bubble at the cross point. The bubble displaces the fluid and changes the optical properties of the cross point, enabling the signal to switch direction. The bubbles, which can be added and removed hundreds of times a second, thereby allow signals to be rerouted without any moving parts or mirrors, let alone electrons. This "Photonic Switching Platform" should be available commercially by the end of the year. (For more details, see http://www.agilent.com/about/feature/photonic.html)