TU Wien's Invisible QR Code Could Store 2 TB on One A4 Sheet

QR codes suit archiving because of their built-in error correction, the reason a scribbled-on sticker often still scans. The format also supports four encoding modes: numeric, alphanumeric, byte-sized binary and kanji. It is not better than UTF-8 as a way of encoding text, but archives are meant to last decades or centuries, far longer than magnetic or electronic storage, and that robustness is what makes the format attractive.

Limits of the approach

The catch is the reading equipment. Visible light has wavelengths of about 380 to 750 nm, so the individual cells are too small for an optical microscope. An electron microscope is required.

In theory the codes could go smaller still. The smallest common format, version 1, is a 21×21 grid. With most atoms having a radius of roughly 0.1 to 0.2 nm, an atom-scale version would measure about 2.1 to 4.2 nm per side. Paul Mayrhofer, a professor at TU Wien's institute of materials science and technology, says that is possible but impractical: individual atoms do not reliably stay where they are placed, making atomic-scale storage fundamentally unreliable.

The team's code therefore sits at least an order of magnitude above that scale, with 49 nm cells, and works dependably under an electron microscope. For now, it points to a niche but credible route for long-term archival storage rather than a consumer technology.