Shannon-Hartley says the theoretical maximum data rate for a channel with AWGN is proportional to bandwidth and the log of signal-to-noise ratio. For an off-the-shelf microphone/speaker pair, I think 16 kHz and 80 dB are probably decent guesses. That would give a theoretical maximum data rate of about 425 kb/s. The practical limit is probably much lower.
It may be possible to increase the bandwidth by increasing the sample rate on both ends, but this quickly leaves the realm of consumer audio equipment (and consumer pricing). At some point you'd exceed the reasonable frequency responses for each device, as well as the medium. I imagine that air attenuates ultrasonic frequencies more than lower ones, but that's just a guess.
I think they only care about preventing data flow in one direction while still allowing it in the other. This isn't strictly an air gap, but it fits their use of the term "data diode". The fact that the unidirectional flow of information is achieved through galvanic isolation is probably just a side effect. In the ideal case, no information can flow from the photosensitive element to the LED. A determined attacker could probably exploit lots of side channels here, though.
> 25 years ago it seemed like physics was mostly complete, and the only remaining work was exploring the corner cases and polishing out all the imperfections. It doesn't feel that way anymore!
Physicists thought the same thing c. 1900, but then one of the "corner cases" turned into the ultraviolet catastrophe[1]. The consequences of the solution to that problem kept the whole field busy for a good part of the 20th century.
I'm highly skeptical of the idea that physics is anywhere near complete. The relative success of our technology gives us the illusory impression that we're almost done, but it's not obvious that physics even has a single, complete description that we can describe. We assume it does for convenience, in the same way that we assume the laws are constant everywhere in spacetime. I view this as both exciting and terrifying, but mostly exciting.
It may be possible to increase the bandwidth by increasing the sample rate on both ends, but this quickly leaves the realm of consumer audio equipment (and consumer pricing). At some point you'd exceed the reasonable frequency responses for each device, as well as the medium. I imagine that air attenuates ultrasonic frequencies more than lower ones, but that's just a guess.