Boffins from the University of Oulu in Finland and Huazhong University in China have put together a prototype that uses an Android smartphone's magnetometer as a short-range one-way communications channel. The applications for such a system are limited, of course: magnetic fields decay very quickly, so the system has a range of …
Kim Philby and his gang would have found this an invaluable tool.....
No more dead letter drops, just place the phone on the counter and transfer.
I like the passing keys idea. That could be useful. You could have decently encrypted exchanges between those 2 phones from then on; and it wouldn't require the user to have any real expertise.
Stop press - sensors sense
Presumably it doesn't need boffins to work out that sensors in phones can sense things. You could have plenty of one-way communication channels: as well as using electromagnets to signal via the magnetometer, shake it to pass data via the accelerometer, flash lights to signal via the camera, warble data into the microphone, turn satellites on and off to signal via GPS [er, is this one right?], etc etc
Re: Stop press - sensors sense
Well, one could also consider turning gravity on and off, for the accelerometer...
RE. Re: Stop press - sensors sense
Ask the Russians, I gather they have experience in such areas :-)
Modulated HFGWs for covert comms using a temporal key is pretty ingenious, and any phone can be used as a receiver.
Might explain where Ning Li got to, presumably figuring out how to fit this to Chinese and Russian submarines.
Doesn't sound as useful as the bidirectional ultrasound handshaking incoming for Chromecast. Still, best to do all the sensors before Apple patent them ;)