Once again we have to tip our hats to the IBM Research team as it managed yet another breakthrough that brings phase-change memory (PCM) closer to its true potential. The big news from IBM is the ability to manufacture multi-bit (multi-level) PCM chips.
Up until now phase-change memory was 'restricted' to single bit-per-cell, which meant lower capacities and higher costs, so IBM's work is a breath of fresh air as it will allow for cheaper, high-density PCM chips that will put current NAND to shame with faster write/read speeds and support for over 10 million write cycles (25nm MLC NAND Flash memory is capable of around 3,000 write cycles).

Seen below, IBM's multi-bit PCM test chip was made using 90-nanometer CMOS technology is supposed to deliver a worst-case write latency of about 10 microseconds, 100 times better than today's most advanced Flash memory.
Unfortunately, multi-bit phase-change memory is still some time away so we'll have to settle for NAND for at least 4-5 more years.

Source :tcmagazine.com