In a recent paper the experimental physicist Ursula Keller described how circularly polarized light can be used to measure events in the attosecond range. A measurement of this kind could perhaps soon give an experimental answer to the question of whether a measurable tunnelling time of electrons as a result of the tunnelling effect really exists – one of the big unsolved riddles of physics.
Ursula Keller, Professor at the Institute of Quantum Electronics at ETH Zurich, says “What we are doing here is rather ‘exciting’ right now.” You can take her word for it. With all the vividness and excitement in Keller’s voice, one really feels the experimental physicist must be on the trail of something big. Keller and her team are no longer an unknown quantity when it comes to generating ultra-short laser pulses: “Essential Science Indicators” named the Professor as the third most frequently cited scientist in opto-electronics anywhere in the world between 1991 and 1999.
For a long time her group also held the world record for the shortest infra-red pulse in the femtosecond region (10-15 s). Such laser pulses are used in chemistry or medicine, for example, to enable ultra-fast phenomena such as biological processes in the human body or chemical reactions to be measured and followed. They are also used to observe charge displacements in semiconductors in the development of microchips. Finally, femto-laser pulses are also used in a kind of clockwork for extremely precise optical atomic clocks.
A relatively new but all the more spectacular area of application of high-energy lasers is the generation of attosecond pulses (10-18 s) in the soft X-ray region. Until recently the record was held by 80 attosecond (as) pulses. These can be used to measure electron movements in real time. However, the generation and detection of laser radiation of this kind needs enormous experimental effort, and time-resolved measurements are severely limited by the small pulse energies and low pulse repetition rates.
The energy field as the dial and the electron as the pointer
Ursula Keller and her team published on the Internet on 30 May a paper describing a new measurement technique in the as region. The associated printed publication is expected to appear in the August issue of “Nature Physics”. In the system that is described, an even better time resolution in the range of 25 as is achieved with less experimental effort and fewer interfering side-effects, and without using attosecond pulses. Keller’s team did this by almost circularly polarising the infra-red laser pulse that was used.
As a result the pulses are no longer linear, wave-shaped vibrations as is usually the case for a light beam, and instead they are circular movements in space. The energy field of the infra-red light rotates through 360° in three dimensions once during a period of 2.4 femtoseconds (fs). This creates a kind of clock that has, instead of a second hand, an attosecond hand that further sub-divides the 2.4 fs of an entire revolution into as steps. Keller explains that “As a result it is possible to make measurements in the as region with an initial pulse of about five femtoseconds and without needing the elaborate apparatus to generate and measure attosecond pulses.”
The tunnel ionisation of a helium atom is described in “Nature Physics” as a “proof of principle”. The angle of the flight path of the electron split from the atom can be measured on the “dial” of the attosecond clock, i.e. the circularly polarised laser beam, and the time of the event calculated from it – and this can be done with a precision of 25 as. The results aroused great interest in the scientific community, since ultimately Keller is claiming no less than to have developed the world’s “most exact” clock.
Possible key to a great mystery in physics
Ursula Keller believes the experimental apparatus that has been described allows the tunnelling times (see box) of electrons to be measured directly – if they really do exist at all. Because of the required time resolution of less than 100 as, this quantum mechanical process could neither be proved nor disproved experimentally up to the present. However, steadily increasing doubts are being expressed in the scientific community as to the existence of a measurable tunnelling time.
The physicist hopes that “With our technology we might soon be the first to resolve the mystery of such electron tunnelling times by an experiment.” The theories assume that the electron needs a certain time to “tunnel through” the energy barrier. This is exactly what Keller believes she can measure with the attosecond laser clock: “In acknowledged theoretical calculations the “tunnelling” time amounts to about 400 as. In our initial helium experiments we successfully made measurements that are 16 times more precise. Thus a delay of this magnitude ought to be measurable by using our system.”
The on-line publication and the forthcoming “Nature Physics” article merely describe the mode of functioning and the technology underlying the attosecond laser clock. However, the following publication should already contain initial experiments relating to tunnelling times. Keller still remains tight-lipped. Nonetheless, her sparkling eyes and the hopeful allusions make one prick up one’s ears. Keller says “We have learnt from the past. Once the technology to measure a theoretical phenomenon becomes available, one is often surprised by the results. This has led to a breakthrough in physics on more than one occasion. A breakthrough of this kind is the dream of every experimental physicist, ‘that’s what makes us tick’!”
The Tunnelling Effect
“Tunnelling” is a fundamental phenomenon in quantum physics. It means that under certain conditions elementary particles violate the principles of classical mechanics by passing through a sort of energy barrier whose energy is larger than the particle’s kinetic energy. Overcoming the energy barrier is possible quantum mechanically only through a kind of “tunnelling through”, whereby the barrier must be neither too thick nor too high. Well-known physicists have dealt with the tunnelling effect and the tunnelling time in hundreds of scientific publications during the past 60 years, and have derived, described and discussed various theoretical models to explain them.
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