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CERN detector to measure supershort pulses

VIENNA – Conventional measurement techniques soon will be too slow to measure phenomena taking place on timescales faster than an attosecond. To measure such light pulses, scientists have proposed a method to create the world’s most precise stopwatch.

Currently, pulse durations on the order of attoseconds (10-18 s) can be created, but computer simulations at Vienna University of Technology suggest that heavy ion collisions at CERN should be able to produce even shorter pulses – so short that they cannot be measured with the technology available today.

“Atomic nuclei in particle colliders like the LHC [Large Hadron Collider] at CERN or the RHIC [Relativistic Heavy Ion Collider] can create light pulses which are still a million times shorter than that,” said Andreas Ipp of TU Vienna.


Two lead atoms collide, creating a quark-gluon plasma, which can emit ultrashort laser pulses.


In the ALICE experiments at CERN, lead nuclei collide almost at the speed of light. The debris of the scattered nuclei together with new particles created by the power of the impact form a quark-gluon plasma, a state of matter that is so hot that even protons and neutrons melt. Their building blocks – quarks and gluons – can move independently without being bound to one another. This plasma, which has an extraordinarily low viscosity, exists only for several yoctoseconds (10-24 s).

Light pulses can be emitted from the quark-gluon plasma created in the particle collider. These pulses can carry valuable information about the plasma and help to better characterize this state of matter, but conventional measurement techniques are much too slow to resolve these flashes on a yoctosecond timescale.

“That’s why we make use of the Hanbury Brown-Twiss effect, an idea which was originally developed for astronomical measurements,” Ipp said. In a Hanbury Brown-Twiss experiment, correlations between two different light detectors are studied. This enables the diameter of a star to be calculated very precisely.


Because conventional measurement techniques soon will be too slow to measure phenomena taking place on a timescale faster than attoseconds, Peter Somkuti, left, and Andreas Ipp, right, propose a method to create the world’s most precise stopwatch by using a detector to go online at CERN in 2018.


“Instead of studying spatial distances, the effect can just as well be used for measuring time intervals,” said Peter Somkuti, a doctoral student at TU Vienna.

The calculations showed that the yoctosecond pulses of the quark-gluon plasma could indeed be resolved by a Hanbury Brown-Twiss experiment. “It would be hard to do, but it would definitely be achievable,” Ipp said.

This experiment would not require any additional expensive detectors; it could be done with the “forward calorimeter,” which is supposed to go online at CERN in 2018. This would enable the ALICE experiment to become the world’s most accurate stopwatch.

In the future, it is possible that yoctosecond light pulses could be applied to an established method of nuclear research experimentation involving the use of two pulses: one to change the state of the object being investigated, and the second to measure the change. Until now, this approach has been completely inaccessible to certain fields of research.


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