Quantum physics to the rescue

Professor Shanhui Fan (left) and graduate student Sid Assawaworrarit with the device they used to show wireless charging of a moving object. (Photo: Mark Shwartz/Stanford University)

Researchers have dreamed of charging cars on the go. And yet most people still lay anchor at gas station islands or spend quality time by a Tesla Supercharger.

Some recent work by Shanhui Fan and his team at Stanford may have brought that day of charging on the run just a wee bit closer.

It was not that wireless charging over a distance is not possible. It has been tested in a variety of conditions and works admirably as long as the sending and receiving units stay put. That system, based on magnetic resonance works best at a fixed distance and not so well on either side of the sweet spot. It relies on an oscillating magnetic field, which causes sympathetic oscillations in electrons across a gap. The result is a more energy at the receiving end.

“If you don’t do any tuning, there’s a particular distance at which the transfer is most efficient. If you move further, transfer efficiency goes down,” Fan said.

Clearly, the old model is not aimed at people pacing around with dying cellphones – or for moving cars.

Fan, a professor of electrical engineering, might have been stumped by the problem but for the fact that his lab also works on photonics, focusing on parity time symmetry systems.

In quantum physics, parity symmetry is about the values in space (the x, y and z axis) being reversed; time symmetry deals with a reversal in, well, time.

Carl Bender, the co-discoverer of PT symmetry, while describing in a paper that parity and time are not symmetries seen in nature, pointed out that “a left-handed laboratory can obtain different experimental results from a right-handed laboratory [parity], and a laboratory traveling backward in time can obtain different results from a laboratory traveling forward in time.”

Speaking to TC, Fan said that “We are certainly inspired by previous work on parity-time symmetry, including the demonstration of novel effects related to parity-time symmetry in optical systems.”

In optics, PT systems have been shown to endure for some distance if complementary waves are allowed to interact. That is what Fan and his team did – but they did not use magnets.

“We improved the robustness. In the conventional scheme, there is only one point where efficiency is highest. We replaced the radio frequency source with an [off-the-shelf] voltage amplifier,” Fan said.

The essential point is that, with an amplifier, the frequency oscillates. The emitter and receiver thus work out a balance that endures for a short distance.

“We are not working on range extension… In terms of range itself, our work does not improve upon it,” Fan said. “That largely comes from the coil design. We are more interested in more complex configurations.” Such as putting together something about nine times more efficient than the off-the-shelf amplifier his team used here.

Fan and his team provided proof of concept using a model reminiscent of a pair of a barbell with weights at both ends. An LED light beneath one “weights” – actually an electrical coil – remained steady when slid the three feet towards the “weight” at the other end of the bar. That is, the light was getting power from the receiver all through a three-foot range.

That may not seem much, but it is more than enough for an emitter embedded in the road to charge a car bouncing over it, or one in a wall to charge a phone with a suitable receiver while someone is wandering around using it.

Their research was described in a letter in the June 15 issue of Nature.

Graduate student Sid Assawaworrarit explains the experiment in this video:

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