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Cascadable Quantum Controlled-Z Gates on a Single Metasurface

Ying Gu, Qi Liu, Yu Tian, Yali Jia, Zhaohua Tian, Guixin Li, Xi‐Feng Ren, Qihuang Gong

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We theoretically demonstrate several cascadable quantum controlled-Z gates on a single metasurface by utilizing its parallel beam-splitting capability, which is valid for both polarization and path-encoding qubits. The path-encoding case can filter out bit-flip errors.
DOI
10.1364/cleo_at.2025.jps200_79

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We propose a scheme to realize quantum controlled-Z (CZ) gates through single gradient metasurface. Using its unique parallel beam-splitting feature, i.e., a series of connected beam splitters with the same splitting ratio, one metasurface can support a CZ gate, several independent CZ gates, or a cascaded CZ gates. Taking advantage of the input polarization determined output path-locking feature, both polarization-encoded and path-encoded CZ gates can be demonstrated on the same metasurface, which further improves the integration level of quantum devices. Our research paves the way for integrating quantum logical function through the metasurface.
Gradient metasurface, formed by a set of subwavelength unit cells with different phase modulation, is widely used in polarized beam splitting (BS) in the classical and quantum optics. Specifically, its phase gradient allows the path and polarization of multiple output lights to be locked by corresponding inputs.Using this unique path-polarization locked property, we demonstrate that the single metasurface can function as sequentially linked beamsplitters, enabling the parallelization of a series of BS processes. Such a parallel BS metasurface provides a multi-beam interference capability for both classical and quantum light manipulation. Taking this advantage, we first prepare path and polarization hybrid entangled states of two, three, and multi photons from unentangled photon sources. Then, the ability of parallel BS-facilitated entanglement is applied to demonstrate entanglement fusion among entangled photon pairs, which can greatly enlarge the entanglement dimension. The principle of parallel BS through the metasurface opens up a versatile way to manipulate the quantum state at the micro/nano scale, which will have potential applications in on-chip quantum optics and quantum information processing.
Abstract Two-qubit controlled gates such as the controlled-Y (CY) gate are essential building blocks for universal quantum computation. While linear-optical implementations of controlled-NOT and controlled-Z gates have been realized with discrete optics and integrated waveguides, realizing a faithful CY gate on a compact, single-element platform remains challenging because it requires both a controlled bit-flip and precise ± i phase factors. Here we propose and numerically demonstrate an optical quantum CY gate based on a single-layer Pancharatnam–Berry gradient metasurface operating at 1550 nm. Polarization-encoded photons are processed by a 26-pillar amorphous-silicon supercell that realizes parallel beam splitting, diffraction routing and two-photon interference. After fixed local polarization encoding/analysis transformations and coincidence post-selection, the reconstructed two-photon gate matrix matches the ideal CY unitary with a matrix fidelity of 0.9885. Computational ZZ- and complementary XY-basis truth tables yield fidelities of 0.9902 and 0.9924, respectively, while the four phase-type Bell states generated by the gate exhibit an average fidelity of 0.9902. The average post-selection success probability extracted from full-wave simulations is 0.0976. Wavelength-dependent simulations including material dispersion indicate a simulated bandwidth of approximately 26.5 nm ( F CY ⩾ 0.95 ). The results establish a compact, monolithic metasurface platform for phase-sensitive controlled quantum logic and entangled-state manipulation.
Multiphoton entanglement with high information capacity plays an essential role in quantum information processing. The appearance of parallel beam splitting (BS) in a gradient metasurface provides the chance to prepare the multiphoton entanglement in one step. Here, we use a single metasurface to construct multiphoton path-polarization entanglement. Based on the parallel BS property, entanglement among N unentangled photons is created after they pass through a gradient metasurface. Also, with this ability, entanglement fusion among several pairs of entangled photons is set up, which can greatly enlarge the entanglement dimension. These theoretical results pave the way for manipulating metasurface-based multiphoton entanglement, which holds great promise for ultracompact on-chip quantum information processing.