Quantum CZ gates on a single gradient metasurface.

The Controlled-Z (CZ) gate is a critical element in quantum computing and communication, enabling essential operations such as entanglement creation. Traditionally implemented using bulk optical elements or integrated waveguides, these methods often face challenges related to complexity and scalability. This paper introduces an innovative approach utilizing a single gradient metasurface to realize the CZ gate, offering a compact and efficient solution that simplifies quantum system architectures while maintaining high fidelity.

The controlled-Z (CZ) gate is a fundamental two-qubit gate in quantum computing, essential for creating entanglement between qubits. This article explores the implementation of a CZ gate using gradient metasurfaces, which offer unique advantages in controlling light-matter interactions at the nanoscale.

The CZ gate applies a phase shift of π to the |11⟩ state while leaving other states unchanged. In this implementation, qubits are represented by orthogonal polarization states of photons. The gradient metasurface is engineered to induce a spatially varying refractive index, enabling precise control over the phase of light as it propagates through the structure.

When both qubits (photons) are in their |1⟩ states, the combined interaction with the metasurface results in the desired π phase shift. This is achieved by carefully designing the gradient profile to ensure that only the |11⟩ state experiences the full phase shift, while other states remain unaffected.

The successful implementation of a CZ gate using gradient metasurfaces opens new possibilities for photonic quantum computing. This approach could lead to more compact, efficient, and scalable quantum processors by leveraging the unique properties of light and nanoscale materials. The use of gradient metasurfaces to implement a CZ gate represents a significant step forward in quantum computing. By combining the advantages of photonics with nanoscale material engineering, this approach offers a promising pathway toward scalable and reliable quantum processors. As research progresses, it is expected that gradient metasurfaces will play an increasingly important role in advancing quantum technologies.