Sunday, September 9, 2012

The Ghost Barrier And Electrons

This summer in London, competitors vied to see who could claim the title of fastest in the world – their speeds often measured down to hundredths of seconds. But for truly record-breaking speed measurements, one needs to look in a modest lab at the end of a basement hallway in a Weizmann Institute physics building. That is where Dr. Nirit Dudovich has her lab. Here, Dudovich recently succeeded in measuring the amount of time it takes for an electron to briefly pop out of its home in the atom. The results appeared in Nature.

(l-r) Michal Dagan, Dr. Barry Brunner, Hadas Soifer, Oren Pedatzur, Dr. Nirit Dudovich and Dr. Dror Shafir
Nirit Dudovich and group
Credit:  Weizmann Institute 

Normally, an electron is like a ping-pong ball sitting inside an upright ice-cream cone. As long as nothing changes, there is no reason for the ball to suddenly leave the cone. And yet, in certain situations, an electron can occasionally escape its snug hole. When it does so, the electron passes, ghost-like, through the barrier – a quantum phenomenon known as tunneling, which arises from the fact that such quantum particles as electrons can also act as waves. This is one of the most basic principles of quantum physics, but because it has no parallel in the everyday world, it still challenges the imagination of laypeople and physicists, alike.

The type of quantum tunneling Dudovich researches takes place when a strong laser field is applied to a material. Such laser fields make tunneling a bit easier for electrons by “bending” the rim of the cone. Tunneling electrons must move swiftly. The bending grants them an incredibly narrow window of opportunity, opening for just 200 attoseconds, give or take – about the amount of time a light wave remains at its peak. An attosecond is a billionth of a billionth of a second. And that tiny window has, until now, been much too small for scientists to measure directly.

Different electron trajectories are separated along the time axis. Only a narrow region in time is selected by the kicking mechanism (shaded red)

Credit:  Weizmann Institute  

It’s not just that electrons prefer to stay put. They are also loyal to their home bases, so that even when one takes a brief “trip,” it may return straight home. Scientists can observe the instant that an electron gets back home because as it flies into place, a photon is emitted. Measurements of these emitted photons enabled Dudovich to trace tunneling dynamics.

Dudovich essentially played a trick on the tunneling electrons, which pop out and back again in a straight line. She devised a way of “kicking” the electron (with another laser) so that it could not be absorbed back into place – and thus no photon emitted. With these measurements, she was able to trace the electron's movements back to the moment when it left the atom.

But just like Olympic runners, electrons can have different energy levels, and thus some take longer than others to pass beyond the rim of the cone. Dudovich wanted to accurately measure this time difference. To find out, she again made use of the phenomenon of wave-particle duality. When electrons return home, they can appear as waves; such waves interfere with one another, and the patterns of that interference can be observed. When the interference is destructive – that is, the waves are opposite and cancel one another out – their absence reveals the time difference between the two events. Dudovich succeeded in measuring this difference – around 50 attoseconds. This is apparently among the shortest periods of time ever measured.

On one the hand, this research has shed new light on some very basic phenomena in atomic physics. On the other, it may give rise to new knowledge that will, in the future, be used to create new technologies.

Dr. Nirit Dudovich’s research is supported by the Jay Smith and Laura Rapp Laboratory for Research in the Physics of Complex Systems; the Jacques and Charlotte Wolf Research Fund; the Enoch Foundation; and the Crown Photonics Center.

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