Experimental observation of entropic-singularity-induced nonadditive quantum communication in a qutrit platypus channel

TL;DR

This study experimentally demonstrates nonadditive quantum communication in a qutrit channel by measuring coherent information and revealing entropic singularities that surpass classical limits.

Contribution

First experimental observation of entropic-singularity-induced nonadditivity in a qutrit channel using photonic entanglement.

Findings

Measured coherent information showing violation of additivity.

Identified entropic singularity as the cause of nonadditivity.

Demonstrated enhancement of quantum communication beyond classical limits.

Abstract

The nonadditivity of channel capacity is a defining feature that distinguishes quantum communication from classical communication. In the quantum realm, the channel capacity is determined by coherent information, which is defined through the von Neumann entropies of the output and its environment. Despite its fundamental importance, experimental evidence of such nonadditive quantum communication has been elusive because of the complexity of the required quantum channel. Here, we experimentally observe entropic-singularity-induced coherent-information nonadditivity using the qutrit platypus channel implemented on a photonic platform. By preparing six-dimensional photonic entanglement, we directly measure the coherent information of a platypus channel, a qubit amplitude damping channel, and their joint uses, revealing a clear violation of additivity. Quantum process tomography further reveals the entropic singularity responsible for this effect, demonstrating how singular entropy landscapes in low-dimensional channels can enhance quantum communication beyond additive limits.