How did they use a living cell as a laser?
I realized that I didn't exactly how a laser worked so I looked it up on Wiki. According to Wikipedia, "Laser" stands for "Light Amplification by Stimulated Emission or Radiation". To build a laser, you basically need a light source, some device that absorbs the original light source and then emits an amplified signal also called "gain medium". The key to lasers is that the light is uniform and does not scatter all over the place - it's one bundle of light. This is performed be some complicated sets of optical mirrors. Normally, crystals (like ruby) are used for the gain medium, but in this case, a kidney cell expressing GFP was used. So technically, a cell does not replace the entire traditional laser setup. It just replaces one component of the laser - the gain medium.
Interesting but what is this useful for?
The authors imagine possible uses for medical diagnostics or ways to study the state of individual cells. Purely speculating here, but I wonder if the property of these cells could be used somehow to further increase the spectrum and penetration of the usable light spectrum into photobioreactors growing photosynthetic organisms to increase efficiencies and eliminate dead spots. Like with the original invention of the laser, individual uses were hard to predict at the time. Nowadays, so many applications rely on lasers. Will the same thing be true for "living" or "bio-based" lasers?
You can read a commentary here: Nature News Commentary
The original article can be obtained here (subscription required): Original Nature Photonics Journal Article