Hiding Ignorance Using High Dimensions

The absence of information-entirely or partly-is called ignorance. Naturally, one might ask if some ignorance of a whole system will imply some ignorance of its parts. Our classical intuition tells us yes, however quantum theory tells us no: it is possible to encode information in a quantum system so that despite some ignorance of the whole, it is impossible to identify the unknown part [T. Vidick and S. Wehner, Phys. Rev. Lett. 107, 030402 (2011)] Experimentally verifying this counterintuitive fact requires controlling and measuring quantum systems of high dimension (d > 9). We provide this experimental evidence using the transverse spatial modes of light, a powerful resource for testing high-dimensional quantum phenomena.