  Secret sharing is a well-established cryptographic primitive for storing
highly sensitive information like encryption keys for encoded data. It
describes the problem of splitting a secret into different shares, without
revealing any information about the secret to its shareholders. Here, we
demonstrate an all-optical solution for secret sharing based on metasurface
holography. In our concept, metasurface holograms are used as spatially
separable shares that carry an encrypted message in form of a holographic
image. Two of these shares can be recombined by bringing them close together.
Light passing through this stack of metasurfaces accumulates the phase shift of
both holograms and can optically reconstruct the secret with high fidelity. On
the other hand, the holograms generated by the single metasurfaces can be used
for identifying each shareholder. Furthermore, we demonstrate that the inherent
translational alignment sensitivity between the two stacked metasurface
holograms can be used for spatial multiplexing, which can be further extended
to realize optical rulers.
