Yoctosecond light pulses from quark-gluon plasmas

In recent years, photon pulses in the attosecond (10^-18 s) regime have been precisely engineered and are being increasingly put to work—for example, in experimental quantum control and chemical dynamics (see Physics Today, March 2005, page 39). But can much shorter pulses be generated and put to use? Three physicists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have proposed some answers. They modeled the photon emission in the early expansion of a quark–gluon plasma, a hot dense stew of fundamental particles created when heavy nuclei smash into each other at relativistic speeds. Prompt gamma rays in the GeV range, produced primarily by quark–gluon Compton scattering and quark–antiquark annihilation, would exit the expanding QGP in at most a few yoctoseconds (10^-24 s). With certain collision parameters and detectors nearly aligned with the collision axis, the model predicts a double-peaked pulse before the QGP disappears. One peak is blueshifted, arising from the approaching side of the QGP, the other is redshifted from the receding side, and the peaks are separated roughly by the light-travel time across the hot soup. The dip between the peaks occurs during an intermediate time at which the stew acquires an anisotropy and emits nothing in that direction. If the model proves correct, such a double pulse could enable pump–probe experiments at the nuclear scale, though new detection schemes would first need to be invented. (A. Ipp, C. H. Keitel, J. Evers, Phys. Rev. Lett. 103, 152301, 2009.) —Stephen G. Benka