# Experimental verification of entanglement generated in a plasmonic system

## TL;DR

This paper experimentally demonstrates bipartite entanglement in a nanophotonic system using surface plasmon polaritons and verifies it through quantum state tomography, achieving high concurrence values.

## Contribution

It presents the first experimental verification of entanglement generated via HOM interference in a plasmonic system using quantum state tomography.

## Key findings

- Entanglement with concurrence up to 0.77 was achieved.
- Verification techniques are applicable to other nanophotonic quantum devices.
- Entanglement cannot be inferred from visibility alone.

## Abstract

A core process in many quantum tasks is the generation of entanglement. It is being actively studied in a variety of physical settings - from simple bipartite systems to complex multipartite systems. In this work we experimentally study the generation of bipartite entanglement in a nanophotonic system. Entanglement is generated via the quantum interference of two surface plasmon polaritons in a beamsplitter structure, i.e. utilising the Hong-Ou-Mandel (HOM) effect, and its presence is verified using quantum state tomography. The amount of entanglement is quantified by the concurrence and we find values of up to 0.77 +/- 0.04. Verifying entanglement in the output state from HOM interference is a nontrivial task and cannot be inferred from the visibility alone. The techniques we use to verify entanglement could be applied to other types of photonic system and therefore may be useful for the characterisation of a range of different nanophotonic quantum devices.