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arXiv:2504.05581 · Science

Selective filtering of photonic quantum entanglement via anti–parity-time symmetry

Mahmoud A. Selim, Max Ehrhardt, Yuqiang Ding, Hediyeh M. Dinani, Qi Zhong, Armando Pérez-Leija, Şahin Kaya Özdemir, Matthias Heinrich, Alexander Szameit, Demetrios N. Christodoulides, Mercedeh Khajavikhan

Published Mar 27, 2025• 26 citations
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Abstract

Entanglement is a key resource for quantum computing, sensing, and communication, but it is susceptible to decoherence. To address this, research in quantum optics has explored filtering techniques such as photon ancillas and Rydberg atom blockade to restore entangled states. We introduce an approach to entanglement retrieval that exploits the features of non-Hermitian systems. By designing an anti-parity-time two-state guiding configuration, we demonstrate efficient extraction of entanglement from any input state. This filter is implemented on a lossless waveguide network and achieves near-unity fidelity under single- and two-photon excitation and is scalable to higher photon levels, remaining robust against decoherence during propagation. Our results offer an approach to using non-Hermitian symmetries to address central challenges in quantum technologies.

DOI
10.1126/science.adu3777

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Preserving entanglement in the presence of decoherence remains a major challenge for quantum technologies. Recent proposals 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.
Quantum entanglement is essential for various quantum information technologies that are beyond the possibilities of classical physics. However, it is susceptible to decoherence that may arise from noisy channels and environmental noise. While this deleterious problem has been explored by using filtering techniques, such as photon ancillas and Rydberg atom blockade, a concise yet efficient approach to entanglement retrieval is still a formidable challenge. Here, we present that nonunitary material provides an alternative tool to flexibly control the anomalous Hong–Ou–Mandel (HOM) interference and enables the selective entanglement filter with coherent perfect absorption of anti-symmetric quantum states. An elaborate Cr–SiN–Cr film is prepared as a nonunitary beam splitter such that its tunability over the effective photon–photon interaction in frequency modes can be readily explored in the experiment, which may introduce sophisticated nonunitary realization into integrated optical platforms. In addition, we confirm its ability for selective filtering of high-dimensional frequency entanglement by observing the output quantum state of the conventional HOM interference with a unitary beam splitter and the anomalous HOM interference with a nonunitary beam splitter. These results may offer new insights into investigating the quantum light–matter interaction and facilitating the use of two-photon interference for addressing central challenges in quantum technologies.
Quantum nonlinear optics seeks to harness strong photon-photon interactions for scalable quantum technologies, although dissipative losses still pose a major barrier to near-unity conversion efficiency. Here, we bridge non-Hermitian physics with the quantum nonlinear domain by exploiting perfect absorption to identify and optimize few-photon nonlinear processes. We theoretically investigate two circuit QED systems, operating in the light-matter ultrastrong coupling regime. The first (i) enables simultaneous two-atom excitations by single photons, while the second (ii) realizes the strong coupling between a single-photon and a two-photon Fock states. We demonstrate that, since the strong optical nonlinearities cause quantum spectral features to emerge already at the level of linear response theory, the perfect absorption condition in ∣S11∣|S_{11}|∣S11​∣ enables near-deterministic single-photon down-conversion into (i) a qubit-qubit-correlated pair and (ii) a two-photon pair. We show that the conversion efficiency can be systematically optimized through experimentally accessible parameters, both linked to the emergence of Hermitian subspaces within the effective non-Hermitian Hamiltonians. These findings position non-Hermitian engineering as a broadly applicable route to optimizing quantum devices at the single-photon level, even beyond circuit-QED platforms.
Engineering quantum bath networks through non-Hermitian subsystem Hamiltonians has recently emerged as a promising strategy for qubit cooling, state stabilization, and fault-tolerant quantum computation. However, scaling these systems while maintaining precise control over their complex interconnections, especially in the optical domain, poses significant challenges in both theoretical modeling and physical implementation. In this work, drawing on principles from quantum and mathematical physics, we introduce a systematic framework for constructing non-Hermitian subsystems within entirely Hermitian photonic platforms. In particular, controlled exponential decay without actual absorption loss is realized in finite 1-D waveguide chains through discrete-to-continuum coupling and Lanczos transformations. Using this new methodology, we implement parity-time symmetric quantum systems and experimentally demonstrate that these artificial bath environments accurately replicate the dynamics of non-Hermitian arrangements in both single- and multi-photon excitation regimes. Since the non-Hermitian subsystem response deterministically arises from an artificially built Hermitian bath, the quantum evolution can be monitored via post-selection in this fully conservative configuration. This approach bridges the gap between theoretical models and experimental realizations, thus paving the way for exploiting quantum bath engineering in advanced information processing and emerging quantum technologies.