Physicists have utilized radio waves from a Wi-Fi transmitter in order to encode a three dimensional image of a real object in a hologram

April 28, 2017 – Science Mag

In principle, the technique could enable outsiders to “see” the inside of a room using only the Wi-Fi signals leaking out of it, although some researchers say such spying may be easier said than done.

The idea came about a few years ago, says Friedemann Reinhard, an expert on quantum sensors at the Technical University of Munich (TUM) in Germany. “At lunch we had a discussion about what the world would look through Wi-Fi eyes,” he says, “and it became clear that if you want to see the world through Wi-Fi, you could make a hologram.”

A camera makes an image by collecting light reflected from an object and focusing it onto a screen to create a 2D pattern of greater or lesser intensity: the image. In contrast, a hologram more fully exploits the wave nature of light. Typically, lasers are used. The laser beam is split, and half of it reflects off the object and onto a photographic plate. The other half—the reference beam—shines directly on the plate. Like evenly spaced water waves lapping on a beach, the light waves in the reference beam arrive in flat wave fronts. In contrast, those reflected by the object are modified by it, and so some parts of the wave front arrive at the plate earlier and others later, depending on where they bounced off the object. The interference of the two sets of waves creates a pattern of bright and dark spots—the hologram.

Seemingly random, the 2D hologram looks nothing like the object. But the 3D ghost of the object can be retrieved by shining another beam like the reference beam on the plate, as the light waves scatter or “diffract” in a way that reproduces the wave fronts originally reflected from the object.

Now, Reinhard and Philipp Holl, an undergraduate at TUM, have used radio waves from a Wi-Fi router instead of a laser to produce a hologram of a thin aluminum cross measuring about a meter wide, as they report in a paper in press at Physical Review Letters.

Their experiment relies on none of the billions of digital bits of information encoded in Wi-Fi signals, just the fact that the signals are clean, “coherent” waves. However, instead of recording the key interference pattern on a photographic plate, the researchers record it with a Wi-Fi receiver and reconstruct the object in a computer. They placed a Wi-Fi transmitter in a room, 0.9 meters behind the cross. Then they placed a standard Wi-Fi receiver 1.4 meters in front of the cross and moved it slowly back and forth to map out a “virtual screen” that substituted for the photographic plate. Also, instead of having a separate reference beam coming straight to the screen, they placed a second, stationary receiver a few meters away, where it had a direct view of the emitter.