ultra low power mini wireless transmitter / receiver?
A 433MHz ASK transmitter/receiver pair should do very well at the ranges you are talking about. I have used them quite successfully in my Wicked Node and Wicked Receiver products, at hundreds of feet, so you should easily be able to use them to make it work over such a small distance. The transmitter can be quite low power because you can basically draws no current when not transmitting. The receiver on the other hand, needs to always be on and continuously outputs "data" that you have to sift through to extract actual transmissions from the noise. You could use the same exact product set to do what you want here but it will be a little bit longer than 1 inch, but you might be able to hack it into a 1 cubic inch with some creativity.