QLC flash is the latest in a long trend of squeezing more bits per cell within a NAND flash device. Here’s a brief overview of how the technology has evolved over time:
Single-level cell (SLC) flash: One bit per cell, two possible voltage states
Multi-level cell (MLC) flash: Two bits per cell, four possible voltage states
Triple-level cell (TLC) flash: Three bits per cell, eight possible voltage states
Quad-level cell (QLC) flash: Four bits per cell, 16 possible voltage states