I'm not saying it is competitive or practical but optical multi-level storage exists. https://www.nature.com/articles/nphoton.2015.182 https://www.nature.com/a

I'm not saying it is competitive or practical but optical multi-level storage exists. https://www.nature.com/articles/nphoton.2015.182 https://www.nature.com/articles/nphoton.2015.182