Fig. 1: A quiet sprinkler distributes the water evenly in a circle and the grass grows in a circular pattern - regardless of whether the sprinkler turns clockwise, counterclockwise or randomly. When the wind blows, the grass gets unevenly wet - this is also reflected in its growth. But when the wind changes direction in sync with the sprinkler's rotation, it causes grass growth to be asymmetric. This makes it possible to reconstruct the spin feature of the sprinkler - whether it is a precise, regularly rotating sprinkler or a cheap specimen that rotates at random.

In the investigation, the sprinkler is the short pulse (blue), which lasts only about 10-16 seconds and whose electric field rotates even faster. The "wind" is a linearly polarized and precisely controlled infrared laser field (red). The grass is the measured photoelectron angular distribution (green). The asymmetry in the latter allows for the first time to reconstruct the properties of the ultrashort pulses. (Source: Felipe Morales and Álvaro Jiménez-Galán)

"You can compare the ejected electrons with a one-arm sprinkler that either rotates as desired or repeatedly stuttered and even reverses its direction of rotation," says Misha Ivanov, head of the theory department at the Max Born Institute. Now when the sprinkler runs for a while, it makes the lawn wet around it circularly - regardless of whether it rotates evenly or not. To find out if the sprinkler turns exactly in the desired direction, it is not enough just to look at the lawn. "But if a gusty wind blows in addition, we can tell whether the sprinkler turns evenly or irregularly," says Ivanov. If, for example, a gust of wind alternately occurs from left or right each time the sprinkler's arm is to the left or right, then the lawn will not be circularly wet but will have a diagonal ellipse. A wholly irregularly rotating sprinkler would cast a wind-directed ellipse on the lawn, while a regular-rotating sprinkler would display a crooked ellipse.

The "wind" is an infrared laser pulse whose vibrations are precisely synchronized with the ultrashort pulses. The infrared radiation accelerates the electron either to the left or to the right - just like the wind drops the water.

"With a measurement on the electrons, we can then prove whether the light pulse has possessed the desired uniform rotation or not," says Álvaro Jiménez-Galán, a scientist at the Max Born Institute and first author of the publication in "Nature Communications". "With our method it is possible to determine the properties of ultrashort light pulses with unprecedented precision," says Jiménez-Galán. And once the light pulses are sharply defined, the information from the electron about its place of origin within exotic materials can be read out all the more precisely.

This is particularly important for investigation on a whole range of novel materials. This could include on the one hand superconductors that can conduct electricity without electrical resistance, and on the other hand topological materials that have exotic properties and for their research was awarded the 2016 Nobel Prize in Physics. These materials could be used in a quantum computer or make particularly fast and energy-efficient processors and memory chips possible in normal computers and smartphones.

Although the new sprinkler process exists for the time being only in theory, it should be ready in the near future. "Our specifications correspond to the current state of the art, so there is no reason for an imminent realization in the laboratory," says Ivanov.