Right now, the platters in Seagate's Barracuda 3TB hard drive pack 620 gigabits into every square inch of surface area. That areal density is high enough to squeeze an entire terabyte onto each platter, but it's nothing compared to what Seagate has come up with in its labs. The company is trumpeting a "technology demonstration" that achieves an areal density of one terabit per square inch. Impressive.
How did Seagate manage such a substantial increase in areal density? Frickin' lasers. Seriously. The tech demo relied on heat-assisted magnetic recording technology (HAMR), which employs ultra-precise lasers to flip bits on the platter. The press release provides few details on the specifics of the implementation, but HAMR is something Seagate has been working on since at least 2006. Although there's no timeline for when the technology will be ready for mass consumption, the perpendicular recording technology currently used by hard drive makers is expected to run out of steam near the 1Tb/in² threshold.
In addition to enabling higher bit densities, HAMR has been associated with faster write performance. Using lasers to flip bits is apparently faster than the traditional magnetic approach, although a magnetic head is still required to read the data once it's been written. As far as we know, lasers can't take part in the read process.