One 9 volt battery supplies the power for the green power indicator LED, as well as the 4 infrared emitter and 4 infrared collector diodes. The output jack at the back is also a combination strap peg.
The genesis of this idea came from MAKE Magazine. Their Infrared String Bass is a rudimentary four string bass with optical pickups. Look at their project for resources and links, if you want to try something like this.
In the end, this is an overly complicated alternative to using a basic piezoelectric element. I think that I will stick with piezos and traditional wound electric pickup for future projects.
See it in action!