A way to securely erase paper:
“The key idea was to find a laser energy level that is high enough to ablate – or vaporise – the toner that at the same time is lower than the destruction threshold of the paper substrate. It turns out the best wavelength is 532 nanometres – that’s green visible light – with a pulse length of 4 nanoseconds, which is quite long,” Leal-Ayala told New Scientist.
“We have repeated the printing/unprinting process three times on the same piece of paper with good results. The more you do it, though, the more likely it is for the laser to damage the paper, perhaps yellowing it,” he says. The team have found toner-paper combinations in which almost no appreciable traces of toner can be seen after lasing and in which the paper suffers “no significant mechanical damage.”
EDITED TO ADD (3/21): More than one reader has pointed out that this system is not secure, nor do its inventors make any claims of security.