  Masking of quantum information is a way of hiding information in correlations
such that no information is accessible to any local observer. Although the set
of all quantum states as a whole cannot be masked into bipartite correlations
according to the no-masking theorem, the set of real states is maskable and is
a maximal maskable set. In this work, we experimentally realize a masking
protocol of the real ququart by virtue of a photonic quantum walk. Our
experiment clearly demonstrates that quantum information of the real ququart
can be completely hidden in bipartite correlations of two-qubit hybrid
entangled states, which are encoded in two different degrees of freedom of a
single photon. The hidden information is not accessible from each qubit alone,
but can be faithfully retrieved with a fidelity of about 99% from correlation
measurements. By contrast, any superset of the set of real density matrices
cannot be masked.
