An experimental printer documented by Cambridge University scientists in a paywalled Royal Society paper is capable of laser-ablating the toner off of sheets of previously printed paper, leaving them ready to be reused. The device uses picosecond pulses of a green laser that passes through the cellulose in the paper, but vaporizes the toner.
The primary goal of unprinting is to cut down on the carbon footprint of the paper and printing industries. Manufacturing paper is incredibly messy business that produces millions of tons of CO2 every year. Recycling paper is definitely a step in the right direction, but it's still a very resource-intensive process. If we could simply delete sheets of paper, rather than re-pulping them, we could cut down on electricity usage, CO2 output, and most importantly fresh water, which is growing more scarce by the year. It would also be rather handy if you never had to buy another ream of paper, too.
In a worst-case scenario, The University of Cambridge unprinting method has half the carbon emissions of recycling; best-case, unprinting is almost 20 times as efficient. It's now a matter of building the technology into a commercial device, which the team admits is probably a long way off. High-powered picosecond lasers are the reserve of labs… for now!