Dark-state photonic entanglement filters

TL;DR

This paper demonstrates that robust photonic entanglement filtering can be achieved through dark states and post-selection in simple waveguide networks, eliminating the need for complex symmetry constraints and bath engineering.

Contribution

It introduces a simplified, universal approach to entanglement filtering based on dark states and post-selection, bypassing the need for non-Hermitian symmetry constraints.

Findings

Dark states enable entanglement filtering without symmetry constraints

Simple waveguide networks can achieve robust entanglement preservation

The approach extends beyond complex architectures and specific configurations

Abstract

Preserving entanglement in the presence of decoherence remains a major challenge for quantum technologies. Recent proposals [M.A. Selim et al., Science 387, 1424 (2025)] have employed photonic filters based on anti-parity-time symmetry to recover certain entangled states, but these approaches require intricate, symmetry-constrained waveguide architectures and precise bath engineering. In this work, we show that such strict non-Hermitian symmetry constraints are not necessary for entanglement filtering. Instead, we identify post-selection and the emergence of dark states -- arising naturally through destructive interference in simple photonic settings -- as the essential mechanisms. By avoiding the need for special bath engineering or non-Hermitian symmetries, our approach significantly simplifies the design and architecture, enhances universality, and extends applicability beyond previously studied dimer configurations. We demonstrate this concept using minimal waveguide network designs, offering a broadly accessible route to robust entanglement filtering.