# Entanglement without indistinguishability: Routing of coupling-induced path-entangled photon pairs

## TL;DR

This paper demonstrates an on-chip method to generate and route path-entangled photon pairs using coupled waveguides, leveraging linear coupling to induce indistinguishability and enable reconfigurable quantum photonic networks.

## Contribution

It introduces a novel on-chip scheme for deterministic creation and routing of path-entangled photon pairs via coupled waveguides, without relying on photon indistinguishability.

## Key findings

- Successful generation of path-entangled photon pairs in integrated waveguides.
- Ability to reconfigure photon routing by tuning classical pump parameters.
- Linear coupling induces indistinguishability, enabling entanglement without photon indistinguishability.

## Abstract

Realizing an on-chip reconfigurable source of path-entangled photons is of critical importance for the advancement of quantum information processing and networking. Achieving this goal has proven challenging to date. We present an on-chip scheme for the deterministic creation of co-propagating or counter-propagating path-entangled photon pairs that can be routed in multiple configurations by tuning a classical parameter. The simplest manifestation of this approach makes use of two \textit{coupled} waveguides: a \textit{nonlinear} waveguide that produces photon pairs via spontaneous parametric downconversion from an externally incident unguided optical pump, and an auxiliary \textit{linear} waveguide. Although the photon pairs are born in only one waveguide, which alone cannot create path-entanglement, linear coupling over an extended length to the passive waveguide introduces unexpected indistinguishability that induces path-entanglement. Tuning of the classical pump spatial profile allows routing the photon pairs over all possible configurations. The proposed device is a building block for future quantum-optical networks.