Experimental Lower Bounds to the Classical Capacity of Quantum Channels

TL;DR

This paper presents an experimental method to certify lower bounds on the classical capacity of noisy qubit channels using entanglement and local measurements, avoiding full process tomography.

Contribution

It introduces a practical, measurement-based approach to estimate classical capacity bounds without requiring complete quantum process characterization.

Findings

High agreement between measured and simulated capacity bounds.

Method effective for Pauli and amplitude damping channels.

Utilizes entanglement and local measurements to estimate capacity.

Abstract

We show an experimental procedure to certify the classical capacity for noisy qubit channels. The method makes use of a fixed bipartite entangled state, where the system qubit is sent to the channel input and the set of local measurements $\sigma_{x}\otimes\sigma_{x}$, $\sigma_{y}\otimes\sigma_{y}$ and $\sigma_{z}\otimes\sigma_{z}$ is performed at the channel output and the ancilla qubit, thus without resorting to full quantum process tomography. The witness to the classical capacity is then achieved by reconstructing sets of conditional probabilities, noise deconvolution, and classical optimization of the pertaining mutual information. The performance of the method to provide lower bounds to the classical capacity is tested by a two-photon polarization entangled state in Pauli channels and amplitude damping channels. The measured lower bounds to the channels are in high agreement with the simulated data, which take into account both the experimental entanglement fidelity $F=0.979\pm 0.011$ of the input state and the systematic experimental imperfections.