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Physical Review Letters

Multichannel Distribution and Transformation of Entangled Photons with Dielectric Metasurfaces

Yajun Gao, Zheng Wang, Yue Jiang, Ru‐Wen Peng, Ziyu Wang, Dong‐Xiang Qi, Ren‐Hao Fan, Wenjie Tang, Mu Wang

Published Jul 6, 2022• 52 citations
Publisher

Abstract

Photonic quantum information processing relies on operating the quantum state of photons, which usually involves bulky optical components unfavorable for system miniaturization and integration. Here, we report on the transformation and distribution of polarization-entangled photon pairs with multichannel dielectric metasurfaces. The entangled photon pairs interact with metasurface building blocks, where the geometrical-scaling-induced phase gradients are imposed, and are transformed into two-photon entangled states with the desired polarization. Two metasurfaces, each simultaneously distributing polarization-entangled photons to spatially separated multiple channels M (N), may accomplish M×N channels of entanglement distribution and transformation. Experimentally we demonstrate 2×2 and 4×4 distributed entanglement states, including Bell states and superposition of Bell states, with high fidelity and strong polarization correlation. We expect this approach paves the way for future integration of quantum information networks.

DOI
10.1103/physrevlett.129.023601

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Photonic quantum information processing relies on operating the quantum state of photons, which usually involves bulky optical components unfavorable for system miniaturization and integration. Here, we report on the transformation and distribution of polarization-entangled photon pairs with multichannel dielectric metasurfaces. The entangled photon pairs interact with metasurface building blocks, where the geometrical-scaling-induced phase gradients are imposed, and are transformed into two-photon entangled states with the desired polarization. Two metasurfaces, each simultaneously distributing polarization-entangled photons to spatially separated multiple channels M (N), may accomplish M×N channels of entanglement distribution and transformation. Experimentally we demonstrate 2×2 and 4×4 distributed entanglement states, including Bell states and superposition of Bell states, with high fidelity and strong polarization correlation. We expect this approach paves the way for future integration of quantum information networks.
Conventional approaches to generating and distributing entangled quantum states usually rely on bulky traditional optical devices, posing significant challenges to system miniaturization and integration. Here we report an entanglement engineering scheme that utilizes quantum interference on a metasurface, combined with postselection, to generate and distribute multiple polarization-entangled photon pairs. Two unentangled identical photons, differing only in polarization, are incident on a multichannel metasurface where each channel supports a specific polarization transformation. The metasurface facilitates two-photon interference, generating postselected polarization-entangled states and forming a fully connected entanglement distribution among every two output channels. Experiments demonstrate the generation and distribution of four Bell states across 21 channel pairs, underscoring the potential of metasurfaces as compact, scalable entanglement sources for quantum networking.
With recent advances in nanophotonics, metasurfaces based on nano-resonators have facilitated novel types of optical devices. In particular, the interplay between different degrees of freedom, involving polarization and spatial modes, boosted classical polarization measurements and imaging applications. However, the use of metasurfaces for measuring the quantum states of light remains largely unexplored. Conventionally, the task of quantum state tomography is realized with several bulk optical elements, which need to be reconfigured multiple times. Such setups can suffer from decoherence, and there is a fundamental and practical interest in developing integrated solutions for measurement of multi-photon quantum states. We present a new concept and the first experimental realization of all-dielectric metasurfaces with no tuneable elements for imaging-based reconstruction of the full quantum state of entangled photons. Most prominently, we implement multi-photon interferometric measurements on a sub-wavelength thin optical element, which delivers ultimate miniaturization and extremely high robustness. Specifically, we realize a highly transparent all-dielectric metasurface, which spatially splits different components of quantum-polarization states. Then, a simple one-shot measurement of correlations with polarization-insensitive on-off click detectors enables complete reconstruction of multi-photon density matrices with high precision. In our experiment, we prepare sets of polarization states and reconstruct their density matrices with a high fidelity of over 99% for single photon states and above 95% for two-photon states. Our work provides a fundamental advance in the imaging of quantum states, where multi-photon quantum interference takes place at sub-wavelength scale.
High-dimensional entanglement is of great importance in quantum communications and can be realized by encoding information on multiple degrees of freedom (DoFs) of the photons. Conventionally, the realization of such high-dimensional entanglement involves different combinations of bulky optical elements. In this work, we present the use of a single dielectric metasurface to generate high-dimensional entanglement by modulating multi-DoFs of photons. By sending one of the polarization-entangled photons to interact with the metasurface, we encode path, spin angular momentum, and orbital angular momentum information to the original state. We achieve a four-qubit quantum state in the experiment. To verify it, we experimentally demonstrate the nonlocal correlations between the two photons by recording the correlated images, and we also perform a quantum state tomography measurement. This scheme can be applied to on-chip quantum state manipulation, which is promising in quantum communication with integrated components.
Metasurface Entangles and Distributes PhotonsPhotons passing through a specially engineered wafer emerge entangled and spread among multiple output channels. By Charles Day Before photon pairs can serve as conveyors of quantum information, they must be generated, entangled, and distributed.Each of those steps typically entails using at least one bulky, carefully aligned optical component.Mu Wang of Nanjing University in China and his collaborators have now shown that they can realize both entanglement and distribution with a single, passive metasurface [1].The feat, which relies on tailoring quantum interference, boosts prospects that photon-based quantum technologies could be miniaturized.
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Multiprotocol quantum key distribution (mQKD) enables users to flexibly select protocols for secure quantum communication, though achieving mQKD introduces considerable system complexity and resource demands. Here, we report the first realization of mQKD using a metasurface, which generates multiple hybrid states of photonic spin angular momentum (SAM) and orbital angular momentum (OAM) and distributes them to different users. The incident polarization-entangled photon pair interacts with the metasurface, producing four SAM-OAM hybrid states with high fidelity through spin-orbit conversion. Among these hybrid states, two execute the BB84 protocol, while the other two perform the BBM92 protocol, all demonstrating high secret key rates and low quantum bit error rates. This approach provides a robust, compact solution for generating and distributing SAM-OAM hybrid states and stands out for the remarkable capability of a metasurface in secured information processing.