The things are much easier: two parts, one has blinking LED, another is photosensor. This is called "data diode" and there is a lot of them.
Here is a random vendor with nice pictures: https://owlcyberdefense.com/learn-about-data-diodes/
Here is a random vendor with nice pictures: https://owlcyberdefense.com/learn-about-data-diodes/
…but…what? Why are we doing the blinking-light song and dance at all then?
If data diode points to outside, like a power plant exporting its status to web, then photosensor can be completely taken over. Sure, the web page might be completely bogus, but there will be no disruption in power plant's system. The hardware design guarantees it. That is the strongest case for data diodes.
If data diode points to inside, like a power plant getting new data from the outside, then sure, photosensor software is a concern, but since it's relatively simple, this would not be my biggest worry. I'd worry about app that runs on target PC and receives files; if file is an archive, about un-archiver exploits; an finally about the files themselves. If there a doc, are you sure it's not exploiting Word? If there is an update, are you sure it's not trojaned? Are you sure users are not click on the executable thinking it's a directory?
I don't think this property can be guaranteed for the alternatives you proposed.
So a data diode wouldn't stop a "stuxnet" scenairo where the malware is trying to sabotage the air-gapped. But it would prevent secret information being leaked out.
(Btw. I'm just explaining what a data diode is, and what guarantees it provides. I don't actually think that it would be useful in practice, because it feels to be too cumbersome to use it and therefore the users/IT would poke holes into the security it would provide otherwise.)
Love to read your findings!
Why not USB or internet:
Transmitter is totally safe from compromised receiver. If you insert USB stick to upload file, it could maliciously pretend to be a keyboard. If you connect to Internet to upload a file, your network stack can be exploited (and if you have firewall, then firewall must be exploited first, not impossible). Only data diode lets you push the data to unsecure zone and not worry about getting infected in the process.
If receiver has to be secure, things are not as clear-cut, but there is still advantages from great reduction in complexity. None of existing protocols work, so vendor usually implement something minimally simple to allow file transfer and maybe mailbox-like messages. This system will always have some risks present - even if you securely sent PDF to airgapped site, it might still exploit the PDF viewer. But at least the malware won't be able to report status to C&C and exfiltrate the data.
(1) protected computer has a built-in PC speaker (for example, the computer I am typing this message on does not)
(2) There is an insecure PC with sound card and a microphone (or at least headphones which can be used as microphone)
(3) Secure and insecure PCs are close to each other, as opposed to being in different rooms
(4) It's quiet enough, and no one will notice the sounds (because PC speakers are crappy and can't do infra/ultra sound)
Likelihood of this succeeding depends on a lot of factors, the biggest of them being "how good is the security team". Presumably if they are buying data diodes, they at least have some knowledge?
Other exfil ideas I've read were to emit sounds using HDD, emit sounds by changing fan speed, blink code messages on lights ("sleep mode" or caps/num lock), show special patterns on monitors to transmit RF, add hidden dots to printed pages, abuse wireless keyboard or mice.. There are many idea and most of them are pretty impractical outside of very limited circumstances.