E-Mail 'Wie man in den Wald hineinruft, so schallt es heraus' ('shout into the forest and a similar echo will return', or 'What goes around, comes around'), is not only a well-known German proverb, but is also literally true. When a sound wave is reflected, the echo is heard. When exactly it comes back, however, depends on the 'forest' - but first and foremost on the distance between the caller and the place of reflection," explains Professor Torsten Meier from Paderborn University. "Just imagine that you could tailor when you wanted the echo to come back to you," continues the physicist. A team of scientists has now achieved just this. Not for acoustic signals, however, but for optical signals: The scientists succeeded in controlling photon echoes emitted by semiconductor quantum dots with sub-second precision.