Eh, it's basically a new(-ish) way to make a small radio receiver but instead of converting the output power of the antenna into an input signal for an amplifier, they try to feed a small electric device with it. This is super possible and doesn't violate known physics in any way, but the amount of power you can transmit is minuscule. Still, it might be useful for some type of sensor where it is non desirable to have to replace the battery for some reason.

The problem is getting enough power to broadcast the sensor report. It means you have to run your sensors with a thimble of energy and store up remaining energy for a burst of reports (hope someone is listening :))

> a burst of reports

That assumes the device needs to be running constantly collecting data.

What if a circuit passively charges a capacitor that once full powers up the device just long enough to take a reading and transmit it? This wouldn't be viable for things like air monitors where the sensors need to warm up for minutes, but it seems like it would be ideal for temperature sensors which can be influenced by the waste heat of the device.

If you don't need much range (a couple feet, say), you can use ambient backscatter which doesn't need to store up energy for bursts.

Here's a video about it from 2013 [1] showing some test devices. Here's an article about it [2].

A bit later they extended this to make it work with off-the-shelf WiFi devices, so you could have a battery-free device that can communicate to ordinary WiFi devices [3]. Still short range, but now you could make it so the reader is a commodity WiFi device like a smartphone, instead of a specialty device.

Here are some other researchers who demonstrated a backscatter tag that diddled with Bluetooth Low Energy (BLE) instead of WiFi, and was correctly received by an iPad at over 9 m using the existing iOS Bluetooth stack with no modifications [4].

And here is a battery-fee phone that communicates with a powered base station [5]. It works up to 30 ft away. The phone uses energy from ambient RF and small photodiodes.

[1] https://www.youtube.com/watch?v=gX9cbxLSOkE

[2] https://www.washington.edu/news/2013/08/13/wireless-devices-...

[3] https://iotwifi.cs.washington.edu/files/wifiBackscatter.pdf

[4] https://www.washington.edu/news/2017/07/05/first-battery-fre...

> and small photodiodes.

I bet the small photodiodes collect far more energy than the RF harvesting...

There is this company [https://uwinloc.com] , I've been working for. However ,they are using 968MHz band to harvest energy, thus (lower freq = less loss due to distance). It works up to ~20m if the antenna are correctly "aligned".

Products similar to that already exist, albeit not as low power as the device in the paper. For example, this passively powered UHF RFID tag has a moisture sensor built in: https://www.dipolerfid.com/RFID-Tags/Smartrac-Dogbone-Humidi...

How would you handle payment for the energy and how would you make sure the energy ends up at the proper device and not at some man-in-the-middle?

The energy to generate the 2.4GHz signal is already being lost; I don't believe that placing a spintronic device attenuates the signal in any meaningful way. In certain ways it's similar to tidal energy; one small tidal generator is not going to have any measurable effect on an ocean-scale system. The power absorbed is measured in microwatts whereas the power consumption of a consumer router is measured in watts.

It isn't meant for energy delivery, but energy harvesting. Put a metal stick in place of the antenna, the same power is harvested, but dissipated away.

I can see this useful for very low power devices. I don't have specific example in mind, but you can picture something like sensors embedded inside smart concrete high-rise buildings, that alert when crackling is found.