Like looking through a window, a hologram provides a three-dimensional image. While the optical hologram requires complex laser technology, a hologram of the microwave radiation of a WLAN transmitter can be created using a fixed and a movable antenna. "With this technique, we can create a three-dimensional image of the space in which the Wi-Fi transmitter is located, as if we had eyes for microwave radiation," says Friedemann Reinhard, head of the Emmy Noether Research Group for Quantum Sensors at the Walter Schottky Institute of the Technical University of Munich. The researchers see potential applications above all in the context of the Industry 4.0 concept, i.e. in automated industrial plants, where it is often difficult to localize parts or devices.

Simulation of a warehouse: The three-dimensional image (right) can be reconstructed from the 'light' of the WLAN transmitter in the basement.

© TU Munich

"Instead of a movable antenna that measures pixel by pixel, you could also use a larger number of antennas and thus achieve a video-like image frequency," says Philipp Holl from the Technical University of Munich, who carried out the experiments, and adds: "Future WLAN frequencies, such as the planned IEEE 802.11 standard with 60 GHz, will open up a resolution down to the millimeter range."

Methods for image enhancement known from optics can also be used in WLAN holography: One example is the dark-field method known from microscopy, which makes it possible to better recognize weakly scattering structures. Another method is white light holography: here, the researchers used the bandwidth of the WLAN transmitter to eliminate interference from scattered radiation.

The concept of viewing microwave holograms like optical images also makes it possible to combine the microwave image with camera images. Additional information obtained from microwave images could thus be superimposed on the camera image of the cell phone, for example to directly see wireless key fobs on lost objects.

However, the scientists are only at the beginning of technological development with their work. There is still a lack of research into how transparent which materials are. With this knowledge, opaque paints or wallpapers could then be developed to protect the privacy of microwaves, while transparent materials could be used for factory halls where the path of a component through the plant needs to be tracked.

Researchers hope that the technology developed in this way could also be used in the future to search for people buried under an avalanche or in a collapsed house: While previous methods only allow localization, the holographic evaluation of the signals also provided a spatial image of the destroyed structures. Helpers could then avoid heavy pieces of rubble or use remaining cavities for the rescue and thus find the easiest route to the victim.