Experimental Masking of Real Quantum States

TL;DR

This paper experimentally demonstrates that real quantum states, specifically a real ququart, can be fully masked in bipartite correlations using photonic quantum walks, highlighting the maximal maskable set of states.

Contribution

The work experimentally realizes a masking protocol for real ququart states via photonic quantum walks, showing complete hiding of quantum information in bipartite correlations.

Findings

Quantum information of real ququart can be hidden in bipartite correlations.

Correlation measurements can retrieve the hidden information with about 99% fidelity.

The set of real states is maximal maskable, and larger sets cannot be masked.

Abstract

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.