STOP that light

Credit: Adrian

The fastest thing in the universe was completely stopped and stored for a minute (new record). In vacuum, light travels about 18 million kilometers, in those 60 seconds - which represents more than 20 trips to the moon and back.

"A minute is a very, very long," says Thomas Krauss, St. Andrews University, UK. "This is really an important milestone."

The exploit could allow secure quantum communications to work at long distances.

As the light typically travels just under 300,000 kilometers per second in a vacuum, physicists  were able to slow it down just to only 17 meters per second in 1999, and two years later, stop it completely, though only by a fraction of a second. Earlier this year, researchers increased this time stopping the light for 16 seconds using cold atoms.

Light Trap

To break the minute barrier, George Heinze and colleagues at the University of Darmstadt, Germany, fired a laser control over an opaque crystal, the atoms leading to a quantum superposition of two states. This made it clear to a small range of frequencies. The Heinze's team then stopped a second laser beam which entered the crystal, turning the first laser off and, consequently, the transparency of that crystal disappeared.

The storage time depends upon the crystal's superposition. A magnetic field extends it, but it complicates the configuration of the control laser. Heinze's team used an algorithm to "generate" the magnets and laser combinations, leading them to trap the light for a minute.

Should still be possible to achieve longer storage of light with other crystals, Heinze says, as they did with the physical limits of this material.

Fonte: newscientist

Journal reference: Physical Review Letters, doi.org/m86

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Trapping antihydrogen

By Physics Today on December 6, 2010 11:51 AM

Are there any unexpected differences between matter and antimatter? The international ALPHA collaboration has taken an important step toward answering that question by constructing an apparatus at CERN that can confine freshly made atoms of antihydrogen, the bound state of an antiproton and a positron, for nearly 0.2 seconds—long enough for the antimatter to be examined spectroscopically. A hot plasma of roughly 104 antiprotons—produced by slamming 26-GeV protons into a metal target—is cooled and introduced into one end of the apparatus, while about 106 low-energy positrons from the decay of radioactive sodium are introduced into the other. Electric fields gently nudge the charged species together in the heart of the device, pictured here, where they mix at cryogenic temperatures and form antihydrogen. If their kinetic energies are low enough—in temperature units, less than 0.5 K—the antihydrogen atoms are held in the grip of a superconducting octupole magnet and solenoidal “mirror” coils that together interact with the atoms’ magnetic moments. When the magnetic fields are abruptly turned off, the atoms are released and their spatial distribution captured by a three-layer silicon detector, which locates the atoms’ annihilations and distinguishes them from events triggered by lone antiprotons and stray cosmic rays. In 335 trial runs, the researchers confirmed that 38 antihydrogen atoms had survived in the trap for at least 172 ms. Although the trapping rate per atom produced is low—about 10−5—the achievement sets the stage for precision spectroscopy and antihydrogen tests of fundamental symmetries and gravitation. (G. B. Andresen et al., Nature 468, 673, 2010.)—R. Mark Wilson Read more at blogs.physicstoday.org

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