before it's in the papers"
August 03, 2010
TO THE WORLD SCIENCE HOME PAGE
Incredibly short light pulses capture our microscopic world
and World Science staff
Researchers say they have used some of the shortest pulses of laser light ever produced to gain new insights into quantum mechanics, the physics of subatomic particles.
The pulses were just over one femtosecond, or a millionth of a billionth of a second, apart, the scientists said. Each pulse itself was measured to last a mere one-fifth of a femtosecond. This time period is to one second, as one second is to 150 million years.
These zaps allows the physicists to extract new information about the state of an electron, the particle that carries electric charge, said researchers at Lund University in Sweden, who led the new research and claim the world record for producing short laser pulses.
Scientists consider subatomic particles as things with two seemingly contradictory natures: they are both particles and waves. This is because they act like one or the other depending on the experiment one does.
One can shoot them into a target like tiny bullets, in which case they act like particles.
But they also move like waves: for instance, they create interference patterns. These are patterns similar to those that appear when one drops two pebbles in a pond. Complex ripple patterns will appear where the two sets of circles, each expanding outward, overlap.
A wave is an oscillation. Thus when there is a wave, something always oscillates. In the case of water, what oscillates is the height of the fluid.
But with particle-waves, bizarrely, it seems there is no physical thing that undulates. Rather, what oscillates is a sort of concept: the probability that the particle will turn up in a given place at a given time if measured. No one actually understands what this means. But physicists just accept that it works this way, because it does.
To try to get a complete picture of the behavior of a particle such as an electron, physicists attempt to understand the properties of its associated wave. But there are notorious difficulties in doing this, both technical and theoretical.
In practice, the Lund researchers said, physicists typically measure only a part of the behavior of a particle-wave: its so-called amplitude. The amplitude is the characteristic of a wave that corresponds to ripple’s height in the water.
But another property of the particle’s behavior, which is usually prohibitively difficult to measure, is its phase, the researchers added.
The phase is a measure of when a given wave reaches a particular place. For instance, imagine dropping a pebble in the water at exactly midnight. Now envision choosing, instead, to do the same thing, but to wait one additional instant before dropping. The two acts would be expected to produce waves with the same amplitude, but a tiny phase difference.
The scientists in the new research said they have developed a technique for measuring the phase of an electron-wave function using the brief light pulses.
Each light pulse was aimed at an atom of the element argon, so as to smack an electron out of the atom. The electrons emerge with different phases. They also start to interfere with each other, not unlike the waves from two dropped pebbles interfere and create patterns.
A close study of the interference patterns produced by the two pebbles would tell you the phase of each of the series of waves that went into producing that pattern. In the same way, scrutinizing the complex interference patterns of the electron-waves in the experiment reveals the phase of the electron-waves, the researchers explained.
This information “is important for predicting the behaviour of atoms, molecules or larger systems,” they wrote in a paper describing their findings, published in this month’s issue of the research journal Nature Physics. The technique combined pulses of intense, laser-generated infrared light with an electron imaging detector built in Amsterdam and moved to Lund for the experiment.
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