Publishing their results in Nature Letter under the title "Robust wireless power transfer using a nonlinear parity–time-symmetric circuit", the researchers demonstrated both theoretically and experimentally robust wireless power transfer through the use of a parity–time-symmetric circuit incorporating a nonlinear gain saturation element.
Compared to conventional methods where high transfer efficiency can only be maintained by constantly tuning the frequency or the internal coupling parameters of the coils as the transfer distance or the relative orientation of the source and receiver units varies, here transfer efficiency remained near unity over a distance variation of approximately one metre, without the need for any tuning.
To achieve this, the researchers eliminated the radio-frequency source in the transmitter and replaced it with a voltage amplifier and feedback resistor that would automatically figure out the right frequency for different distances without the need for human intervention and tuning.
That approach was tested with a LED bulb placed on the receiving coil. In a conventional setup without active tuning, LED brightness would diminish with distance but in the new setup, the brightness remained constant as the receiver moved away from the source by a distance of about three feet. As reported on Stanford University's news pages, Fan’s team recently filed a patent application for their solution and hope to improve overall efficiency by using custom-made amplifiers with efficiencies over 90% compared to the low efficiency (under 10%) off-the-shelf general-purpose amplifier they used for their demonstration.
That demonstration only involved a 1-milliwatt charge but the researchers are hoping to be able to increase the system's capabilities so it could be considered as an alternative for cars, providing there would be built-in infrastructure (coils embedded into the pavement).