Anyone who has ever left the house without remembering to charge their cellphone can appreciate the concept of wireless power transfer. All you would have to do is remember to drop your phone on your desk, and a wireless charging mat would ensure that it has a full battery by the time you pick it up again.
But current wireless charger systems require specialized hardware on both the sending and receiving ends, and power only flows efficiently when the two are a specific (and short) distance apart. It's possible to expand that distance a bit by carefully adjusting the frequency used to induce current at a distance, but this adds to the complexity and energy overhead of the system. And even the best current systems have losses that mean wasted electricity at a time when energy efficiency is critical.
Now, researchers at Stanford have found a different way to handle wireless charging. Taking advantage of a quantum principle that also applies to the everyday world, they've created a system in which power is transferred over a wider distance with roughly 100-percent efficiency. Better still, the system adjusts itself to the distance, so careful frequency tuning becomes unnecessary. The big downside, however, is that the supporting electronics aren't especially efficient.
Time reversal what?
Current wireless charging systems work on the principle of magnetic induction, in which a resonator and a receiver are coupled through magnetic fields. Transferring power efficiently involves sensing the distance and orientation between the two before adjusting the frequency at which you activate the resonator. This can get you efficient energy transfer out to distances of over 50 centimeters, though the efficiency isn't usually more than 95 percent. Plus it requires the overhead of the circuitry involved in sensing distance and adjusting the resonator.
While very similar looking in structure, the new system works through completely different principles. These principles were originally developed to describe the quantum world: Charge, Parity, and Time Reversal symmetry, or CPT symmetry. In this case, the focus is on parity-time symmetry. As Ars' Chris Lee put it, this symmetry means that quantum systems are indistinguishable if they're moving up and forward in time or moving down and backward in time.
This has actually turned out to work in non-quantum systems as well. If you set up two systems that are mirror images of each other, they start to behave as if they were a single system. While this has mostly been done with optical systems, the Stanford team decided to try it with wireless power.
To make a wireless system that's a mirror image, it's easiest to think about what's on the receiving end: a receiving coil linked to a device that drains off current to use for charging. To make a mirror image, the researchers used an emitting coil and linked it to an amplifier that adds current to the system. Simulations suggested that the combination would be parity-time symmetric. As a result, the two coils would act as a single system, with the energy pumped into the transmitter being evenly shared between the two.
Better yet, the calculations suggested that there would be no need to tune the frequency at which the energy is transferred. As the system approaches a steady state, the frequency that requires the least amplification would naturally grab most of the power and prevent any of the other frequencies from grabbing enough to operate. Even as the orientation and distance change, the system would continue to find the lowest amplification that worked.
The amount of power transferred can be described by an equation that's safe to describe as mind-numbingly complex. Within that equation, however, distance and orientation both feed into the equation in a way that indicates they have a minimal impact on the result. This should mean that the charging system isn't that picky about distance and orientation.
From theory to practice
To see whether any of these calculations matter in the real world, the team built a test system that can only be described as delightfully low tech. The coils for the transmitter and receiver were made from strips of copper wrapped around a short cylinder with a diameter similar to that of a large pizza. The two were mounted on a stick that kept them facing each other, but it allowed the receiver to slide different distances away from the transmitter.
The researchers were able to show that the behavior of the system matched that of the theoretical calculations with a precision that can only be described as eerie. This included a distance (roughly 75cm) for which the system transferred power with full efficiency, followed by a gradual decay before power stopped at about a meter away. They also demonstrated this visually using an LED, as shown in the video below.
Critically, their experiments showed the transition between frequencies happening in the middle of the slide, without any interruption in power transfer. So, the device really is showing off some of the advantages of taking this approach to wireless power.
That's the good stuff. There is some notable bad stuff, and it goes well beyond the issues of needing a pizza-sized device just to charge a phone (presumably, this can be miniaturized). One drawback is that the authors only show this device powering a rather feeble LED, but they are promoting it as a way to charge electric vehicles—as they are moving. Demonstrating that their method can scale would seem to be essential. In addition, although the orientation and distance are supposed to have a minor effect on efficiency, the researchers only tested distance. Being able to charge without precisely aligning everything is a large usability feature and definitely would be required for sending juice to moving vehicles. So that's something else that has to be tested.
Then there's the issue of system efficiency. While the efficiency of the transfer of power may "remain near unity," as the authors put it, the whole system was decidedly less efficient. That primarily comes down to the amplifier, which turned out to have a power efficiency of only 10 percent. Amplifiers exist that could work in this system with efficiencies above 90 percent, but that still means a significant loss of electricity. And, given our energy and climate needs, it's not clear whether these sorts of losses are things we should feel comfortable with.