Electric cars charge themselves while driving through magnetic coils in the road. Such wireless charging units are already available. However, up to now, charging by induction only works when both objects are stationary. In order for the alternating magnetic field to generate current in the receiver coil, the frequency of both coils must be exactly matched. If the distance or angle change, the charging power is immediately reduced or even completely broken.

Shanhui Fan, Stanford University, CA, USA, and colleagues have modified the charge coil to automatically adjust the frequency of the magnetic field to the distance of the receiver coil. A voltage amplifier and a resistor coupled to it ensure effective transfer.

The researchers demonstrated that their system works with a prototype: They transferred power wirelessly to an LED, which was slowly moved past the charge coil. Normally, the brightness of the LED is dependent on the distance to the coil. In the experiment, the brightness always stays the same.

So far, the researchers have transferred only one milliwatt with their wireless charge system and also only to a moving object in a one meter radius. But they are confident that this can be significantly increased. They consider charging of electric cars with this technique quite feasible. Mobile devices such as smartphones could also benefit from their technology.

• Robust wireless power transfer using a nonlinear parity–time-symmetric circuit,
  Sid Assawaworrarit, Xiaofang Yu, Shanhui Fan
  Nature 2017, 546, 387–390.
  DOI: 10.1038/nature22404