Latest development status
A researcher at University of Texas at Austin, USA, has devised an invisibility cloak that could work over a broad range of frequencies, including visible light and microwaves. This is a significant upgrade from current invisibility cloaks that only cloak a very specific frequency, say, a few hertz in the microwave band, and, more importantly, actually make cloaked objects more visible to other frequencies. This cloak could achieve the goal by being active and electrically-powered, rather than dumb and passive like existing invisibility cloaks.
Another proposed design consists of a conventional meta-material base, but with CMOS negative impedance converters (NICs) placed at the corner of each meta-material square. An NIC is an interesting electronic component that adds negative resistance to a circuit, injecting energy rather than consuming it. NICs are not widely used as researchers are not entirely sure how to use these. This seems to propose that, by interspersing NICs (which must be powered) with the meta-material, multiple frequencies can be cloaked. The proposed cloak is invisible over a large range of frequencies, while a standard passive cloak is only invisible for a small range, and more visible than non-cloaked devices in other ranges.