Por quite a few years, scientific and technological advances have made commonplace the possibility of encoding information in the modulation of light beams. Several techniques have been investigated to optimise such methods in the temporal domain or in its dual counterpart, ie in the radiation spectrum, with a procedure similar to encoding information in our voice. On the other side, encoding information in space is an everyday task we accomplish, with a wealth of technical means, when we" write". It all amounts to getting control of a homogeneous support, and breaking its translational symmetry with suitable procedures and tools (eg a pen on a sheet ofpaper, a photographic impression on a photosensitive surface).

Light, and particularly coherent, highly directional and controllable beams are suitable to encode information in space, especially due to the intrinsic parallelism of linear wave-like propagation. In optics, a widely exploited way to encode information in space makes use of arrays of micropixels, each emitting or transmitting a bright spot (see Box la). The price to pay of course is the rigidity of the array wherein the" units" are arranged, similar to what we have when we use a typewriter. Rigidity is a limitation indeed: beyond handwriting, educators make use of an even more flexible support, the blackboard, where writing/erasing, changing the layout of the encoded information etc. is even easier than with ink and paper. One might even dream of a blackboard where you can grasp an alreadyjotted formula and" carry" it across a couple of panels without destroying it or what else you already wrote. To overcome the rigidity constraint, one may think of using as homogeneous support (blackboard) the transverse cross-section of a single broad area laser beam. However, in most laser beams the transverse configuration corresponds to a spatial mode of the laser cavity, whose parts are all correlated in space, and you cannot" act" to break its symmetry in one side, without having some other place in the beam changing its intensity profile in an unwanted manner. What we need is to find a" sign", an elementary structure of light that can be used as a minimal alphabet, can be" written" and" erased" and remains independent ofwhat happens to the rest ofthe radiation profile, no matter what we do somewhere else in the homogeneous support.