An invisibility cloak that works in the microwave region of the electromagnetic spectrum has been unveiled by researchers in the US. The device is the first practical version of a theoretical set-up first suggested in a paper published earlier in 2006. The cloak works by steering microwave light around an object, making it appear to an observer as if it were not there at all. Materials that bend light in this way do not exist naturally, so have to be engineered with the necessary optical properties.

To simplify the problem, Smith's cloak works in only two dimensions. It is about the size of a movie reel canister and consists of a series of concentric rings, each housing a set of simple electronic components that distort an electromagnetic field as it passes through. "Rather than the cloak's material properties being the same everywhere, its material properties vary from point to point and vary in a very specific way," says Smith. The combined effect of the rings is to steer microwaves around the central region of the device in which Smith hid a copper ring. To study the effect of his cloak, Smith took images of microwaves flowing through the rings, like water waves moving across a pond. Without the cloak in place, the microwaves were reflected and diffracted by the copper ring. But with the cloak in place, the distortion was dramatically reduced.