A resource-efficient and noise-robust entanglement witness based on the swap test

TL;DR

This paper introduces a resource-efficient, noise-robust entanglement witness based on the swap test, applicable to any two-qubit state, validated on photonic hardware, and providing a lower bound on concurrence.

Contribution

It presents a novel SWAP-based entanglement witness that is resource-efficient, noise-robust, and platform-independent, suitable for arbitrary two-qubit states.

Findings

Validates the method on a photonic chip with linear optical components

Demonstrates robustness against photonic hardware noise

Provides a lower bound on concurrence for entanglement quantification

Abstract

Quantum entanglement is an essential resource for quantum technologies, and the controlled swap test provides a versatile tool for its detection and quantification. Here, we propose a SWAP-based entanglement witness that applies to arbitrary two-qubit states - both pure and mixed - and provides a lower bound on the concurrence. The method is resource-efficient, robust to noise, and platform-independent. As an example, we validate the approach on a room-temperature photonic chip, where the swap test is carried out using only linear and well-established integrated optical components. The robustness of the method against photonic-hardware noise is also analysed. Our results establish a simple and reliable tool for entanglement witnessing.