October 15, 2009--The creation of attosecond-scale light pulses (10^-18 s, or a quintillionth of a second, in duration) is an astounding feat, and opens up whole new areas in advanced spectroscopy. But what if we could jump from atto, right past zepto, all the way to yocto?

Yoctosecond (10^-24 s, or a septillionth of a second) scale pulses would enable the light-based investigation of structures the size of atomic nuclei. But, spatially, a light pulse a few yoctoseconds in duration is only as long as an atomic nucleus is wide, so how could such a pulse be produced?

Perhaps a quark-gluon plasma could help.

Precise pulses from plasma
Quarks and gluons are the building blocks that make up neutrons and protons. A quark-gluon plasma can be created in high-energy heavy-ion collisions, which are studied at the Relativistic Heavy Ion Collider (Brookhaven, NY), and soon at the Large Hadron Collider (Geneva, Switzerland). Calculations show that these collisions could produce light flashes of a few yoctoseconds duration.¹ Under certain conditions, the quark-gluon plasma would create a double flash of light that could be used to visualize the dynamics of atomic nuclei (in pump-probe experiments, two light pulses of precisely controllable spacing are used to observe rapid system changes in "slow motion").

The quark-gluon plasma is a state of matter that the universe was made of right after the big bang. In such a state, the temperatures are so high that even neutrons and protons are split into their quark and gluon constituents. Such a state of matter can nowadays be created in modern colliders.

In the collision of heavy ions (atoms of heavy elements from which all electrons have been removed) at relativistic velocities, such a quark-gluon plasma is created for a few yoctoseconds at a size on the order of a nucleus (Figure 1). Among many other particles, it also creates gamma-ray photons with energies of a few GeV. These high-energy flashes of light are as short as the lifetime of the quark-gluon plasma and consist of only a few photons.

The researchers have simulated the time-dependent expansion and internal dynamics of the quark-gluon plasma. It was found that at some intermediate time the photons are not emitted in all directions, but preferably perpendicular to the collision axis. A detector that is placed close to the collision axis will measure practically nothing during this period. Therefore, overall it detects a double pulse. By suitable choice of the setup geometry and observing direction, the double pulses can in principle be selectively varied in spacing. Thus, they open up the possibility of future pump-probe experiments in the yoctosecond range at high energies. This could lead to time-resolved observation of processes in atomic nuclei; conversely, a detailed analysis of the gamma-ray flashes would allow researchers to draw conclusions about the quark-gluon plasma.

REFERENCE

1. Andreas Ipp, et al., Physical Review Letters, 9 October 2009.
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John Wallace, johnw@pennwell.com

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