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Nano Letters

A Multichannel Metasurface for Multiprotocol Quantum Key Distributions

Yue Jiang, Rui Zhong, Hulin Zhang, Zheng Wang, Yifei Liu, Ren‐Hao Fan, Dong‐Xiang Qi, Wenjie Tang, Ziyu Wang, Ru‐Wen Peng, Mu Wang

Published Apr 24, 2025• 8 citations
Publisher

Abstract

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.

DOI
10.1021/acs.nanolett.5c00868

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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.
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.
Metasurfaces showcase unique abilities to arbitrarily manipulate the amplitude, phase, and polarization of electromagnetic (EM) waves, facilitating their application in wireless communication. In the propagation of EM waves, traditional methods cause severe security concerns in channel/deciphering under eavesdropping. Fortunately, quantum communication, based on quantum key distribution (QKD) of the BB84 protocol, endows the transmission with high security. However, the conventional QKD technology relies on complicated manipulation of a single photon and is prone to channel collapses once an eavesdropper intervenes. Furthermore, the merit of the BB84 protocol has not been realized in metasurfaces as a transmission channel. Here, we, for the first time as far as we know, used metasurfaces to implement wireless transmission of quantum information via the first QKD protocol, BB84. These metasurfaces provided four conjugated quantum states to concretely demonstrate the BB84 protocol through a conveyor-type switching method. High-fidelity holographic results were obtained at ∼10 GHz and beyond in both experiment and simulation. As compared with traditional QKD technology, our BB84 protocol based on metasurfaces is more stable and secure in information transmission. In addition, the metasurfaces have electromechanical switchable mode, empowering them to have multidisciplinary functions in wireless communication, e.g., program-controlled antennas, intelligent reflectors, and variable metalens.
Quantum Key Distribution (QKD) is a cutting-edge field that leverages the principles of quantum mechanics to enable secure communication between parties involved. Single-photon quantum emitters offer remarkable on-demand photon emission, near-unitary indistinguishability, and low multiphoton generation, thereby enhancing the performance of QKD protocols. Standard approaches in which the polarization degree-of-freedom is exploited are limited by the precise alignment between the communicating parties. To overcome this obstacle, the Orbital Angular Momentum (OAM) of light represents a suitable candidate for encoding the information, as it allows the implementation of rotational-invariant photonic states that remove the need for a fixed physical reference frame between the communicating parties. Here, we demonstrate an alignment-free BB84 implementation using on-demand single photons from a quantum-dot emitter at 925 nm. Time-to-spatial demultiplexing and Q -plates encode rotationally invariant spin–orbit states that we transmit over a 2 m free-space link while rotating the receiver between 0 ∘ and 90 ∘ . The source exhibits g (2) (0)≈0.03; the measured QBER remains (4.04±0.02)% and is essentially independent of the relative angle. A simple security model tailored to sub-Poissonian emission, which explicitly incorporates g (2) (0), yields a positive asymptotic secret fraction and an effective QBER threshold of ∼9.6% under our conditions. These results validate rotationally invariant QKD with an on-demand single-photon source and outline its scalability to long-distance scenarios.
Quantum key distribution (QKD) can deliver information-theoretic security, but its receivers are still typically assembled from cascaded bulk optics. Metasurfaces offer a radical reduction in this complexity. Here, we make the metasurface function as the QKD measurement device itself. We design wavelength-specific dielectric apertures at 780, 1550, 2000, and 10 600 nm that interweave the linear H/V and circular R/L phase libraries at a 0.44--0.49λ cell pitch, i.e., below half the operating wavelength, and map the complete passive-BB84 measurement directly onto four detector channels. This subwavelength basis co-location prevents an ordinary propagating-wave aperture mask from isolating one basis without simultaneously attenuating or perturbing the other. Simulated full-wave focal-plane intensities are converted directly into conditional Born probabilities using fitted, calibration-fixed detector regions. The selected designs achieve probability fidelities of 97.96--99.08%, four-port collected efficiencies of 17.43--40.94%, and mean intrinsic quantum bit error rates of 1.42--3.96%. At 1550 nm, the selected design gives an asymptotic device-level yield of 0.151 secret bits per incident photon under an ideal single-photon model. Six-state detector tomography gives positive both-basis security bounds for all six reconstructed receiver models, including their polarization-dependent loss, basis imbalance and crosstalk. These results establish protocol-matched meta-optics as a route to compact, passive QKD receivers.