Quantum information can be negative

I didn't read this article yet, it's on Jonathan Oppenheim's home page.

Even the most ignorant cannot know less than nothing. After all, negative information makes no sense. But, although this may be true in the everyday world we are accustomed to, negative information does exist in the quantum world. Small objects such as atoms, molecules and electrons behave radically different than larger objects -- they obey the laws of quantum mechanics.

What could negative information possibly mean? In short, after I send you negative information, you will know less.

One more link to the relevant article.