Researchers have been looking for a way to make a wireless charger for some time. One idea is to use electromagnetic induction -- passing an electric current through a coil to create a magnetic field that induces a current in a neighbouring coil.

This is the way devices like electric toothbrushes are charged, and has been proposed as the basis of a universal recharger pad before.

The snag as far as mobile devices are concerned is that the charger and device must be in close contact with each other for it to work. Alternative schemes - for instance, transmitting electromagnetic waves in all directions to reach any device in a room - would be hugely wasteful.

Instead, Marin Soljacic at the Massachusetts Institute of Technology wants to use evanescent coupling, which allows electromagnetic energy "trapped" in a charging device to be tapped by a "drain" mobile device if the two have the same resonant frequency.

"The energy is trapped at source, until I bring a device that has the same resonant frequency close to it. Only then can the energy 'tunnel through'," says Soljacic. Crucially, the "charger" only starts powering another device when a compatible gadget comes within range.