© P. M. Holl and F. Reinhard, Physical Review Letters

© P.M .Holl and F. Reinhard, Physical Review Letters

Your wireless router may be giving you away in manner you never dreamed of. For the first time,similar to the image of Princess Leia projected by R2D2 in the movie Star Wars. In principle, thealthough 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 abouthe says, "and it became clear that if you want to see the world through Wi-Fi, you couldmakes an image by collecting light reflected from an object and focusing it onto a screen toIn contrast,Typically, lasers are used.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,In contrast,and so some parts of the wave front arrive at the plate earlier and others later, depending on where they bounced off the object.Seemingly random, the 2D hologram looks nothing like the object. But thecan 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 usedto 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,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 emiter.For each point on the virtual screen, the researchers compared the signals arriving simultaneously at both receivers, andThe virtual hologram isn't exactly like a traditional one, as researchers can't recover the image of the object by shining more radio waves on it. Instead, the scientists"It's obvious from the image that there's a cross there," says Neal Patwari, an electrical engineer at the University of Utah in Salt Lake City. "That's very impressive." But he cautions that the method might not work well in a cluttered environment.Moreover, the receivers and object were in the same room, where the cross could easily be seen.Reinhard says. However, Mark Coates, a computer engineer at McGill University in Montreal, Canada, who was not involved in the project, cautions that it may be tricky, especially if the walls have metal studs, which would also reflect radio waves. "Basically, there are waves coming from so many directions that this technique is going to much more challenging," Coates says.Primarily Reinhard imagines putting an array of Wi-Fi sensors in the ceiling of a factory to make holograms that can better track products labeled with radio-frequency identification tags. Patwari notes that other researchers have already developed simpler ways toSuch radio tracking techniques will soon take off, Patwari predicts: