Polarization modulation plays an important role in polarization encoding in quantum key distribution. By using polarization modulation, quantum key distribution systems become more compact and more vulnerable as one laser source is used instead of using multiple laser sources that may cause side-channel attacks. Metasurfaces with their exceptional optical properties have led to the development of versatile ultrathin optical devices. They are made up of planar arrays of resonant or nearly resonant subwavelength pieces and provide complete control over reflected and transmitted electromagnetic waves opening several possibilities for the development of innovative optical components. In this work, the Si nanowire metasurface grating polarizer is designed by COMSOL Multiphysics Software to operate in the visible region and transmit the transverse magnetic polarization of light. The same structure can be rotated by different angles, i.e., 90o, 45o, and -45o to mimic the function of polarization modulation in quantum key distribution systems. The designed structure has an extinction ratio of ~ 60000 and a wide angular tolerance range of (-20o - 20o).
Polarization modulation plays an important role in polarization encoding in quantum key distribution. By using polarization modulation, quantum key distribution systems become more compact and more vulnerable as one laser source is used instead of using multiple laser sources that may cause side-channel attacks. Metasurfaces with their exceptional optical properties have led to the development of versatile ultrathin optical devices. They are made up of planar arrays of resonant or nearly resonant subwavelength pieces and provide complete control over reflected and transmitted electromagnetic waves opening several possibilities for the development of innovative optical components. In this work, the Si nanowire metasurface grating polarizer is designed by COMSOL Multiphysics Software to operate in the visible region and transmit the transverse magnetic polarization of light. The same structure can be rotated by different angles, i.e., 90o, 45o, and -45o to mimic the function of polarization modulation in quantum key distribution systems. The designed structure has an extinction ratio of ~ 60000 and a wide angular tolerance range of (-20o - 20o).
We propose a coherent control network for arbitrary polarization states and phases. The network is based on a rotationally symmetric stacked metasurface structure that enables XOR, AND, and OR logic operations across dual communication bands via all-optical phase modulation. Stack rotational symmetry gives the structure a polarization-independent response at two-beam incidence, while the coherent control network can precisely modulate the resonant absorption effect between the incident polarization and the metasurface through phase manipulation. Here, we apply this specialized metasurface to a quantum key distribution (QKD) system and successfully design a hybrid encoding mode that utilizes three optical dimensions-polarization, phase, and wavelength. This allows Alice and Bob to modulate 20 polarization states, increasing the information capacity by a factor of 5 compared to the traditional BB84 protocol. Our research significantly improves the performance of metasurfaces in arbitrary polarization multiplexing and phase manipulation and broadens the information dimension of the QKD system in polarization optics.
Metasurface holography has great application potential in the fields of optical display, optical storage, and security. Traditional metasurface holography uses the well-designed subwavelength structure to modulate the incident laser beam. Although many researches about laser metasurface holography have been realized, metasurface holography based on quantum light sources is rare. Here, we realized quantum metasurface holography through single-photon and multichannel polarization multiplexing metasurfaces, and we compared the quantum results with laser results. Our work proves that quantum light sources can be well modulated by the subwavelength structure of integrated metasurfaces and extend both fields of metasurfaces and quantum optics. This result shows that metasurfaces have the potential for use in various quantum devices to reduce the size of quantum devices, improve quantum efficiency, and enhance practicability, reliability, and accuracy.
With the development of science and technology, the way to represent information becomes more powerful and diversified. Recent research on digital coding metasurfaces has built an alternative bridge between wave-behaviors and information science. Different from the logic information in traditional circuits, the digital bit in coding metasurfaces is based on wave-structure interaction, which is capable of exploiting multiple degrees of freedom (DoFs). However, to what extent the digital coding metasurface can expand the information representation has not been discussed. In this work, it is shown that classical metasurfaces have the ability to mimic qubit and quantum information. An approach for simulating a two-level spin system with meta-atoms is proposed, from which the superposition for two optical spin states is constructed. It is further proposed that using geometric-phase elements with nonseparable coding states can induce the classical entanglement between polarization and spatial modes, and give the condition to achieve the maximal entanglement. This study expands the information representing range of coding metasurfaces and provides an ultrathin platform to mimic quantum information.
In nuclear magnetic resonance (NMR) co-magnetometers, the non-uniform transverse energy distribution of the pumping Gaussian beam can result in substantial optical pumping inhomogeneity and decoherence of atomic spins, which hinder the improvement of the precision and sensitivity of the sensor. One of the most significant recent technological advances for laser beam homogenization is the utilization of the microlens array system. However, the homogenized characteristics of the microlens array system vary with the propagation distance of the pumping light and are not suitable for chip integration, which will affect the sensitivity and compactness of the NMR system. To solve this issue, a metasurface homogenizer is demonstrated for encoding intensity information into the polarization profile of an incident Gaussian beam by combining the geometric phase and Malus' law with the transverse intensity distribution independent of the propagation distance. Compared to Gaussian beam pumping at identical input power, the metasurface homogenizer enhances the measured optical magnetic sensitivity by 23%. The proposed metasurface homogenizer not only realizes the higher precision and sensitivity in NMR co-magnetometers, but also highlights how metasurface-based technologies can contribute to the integrated quantum sensing regime.
Engineering light-matter interaction using cold atomic arrays is one of the central topics in modern optics. Here we have demonstrated the capability of two-dimensional asymmetric cold atomic arrays as microscopic metasurfaces for controlling polarization states of light. The designed linear polarizer can lead to an extinction ratio over 20dB as well as a high transmittance over 0.8 for the permitted polarization at zero detuning. For detuned driving light, changing lattice constants can also achieve high performance linear polarizers. We have also accomplished a circular polarizer by manipulating the phases of transmitted light. A theoretical analysis based on Bloch theorem shows the underlying mechanism for this performance is actually attributed to cooperative effects in periodic lattices. Finally, we discuss in detail the effects of system size, lattice imperfection and nonzero driving light linewidth in practical implementation. The present study paves a way to design extremely miniaturized metasurfaces using cold atoms and other two-level systems, showing great potential in quantum information and quantum metrology sciences as well as the fundamental physics of light-matter interaction.
Cold atoms produced in magneto-optical traps (MOTs) have underpinned advances in emerging quantum technologies, including quantum computing, precision metrology, and fundamental physics. Grating-based MOT (gMOT), which employs a reflective diffraction grating to enable laser cooling from a single incident beam, offers a compact alternative to conventional six-beam MOTs. However, the inherent asymmetry of gMOT beam geometry can compromise the trapping stability and efficiency, particularly for atomic species with complex internal level structures. Here, we present a geometric-phase dielectric metasurface that simultaneously engineers the transmitted and diffracted wavefronts with desired polarization states, enabling full MOT operation under axial illumination using a single planar optical element. This metasurface-based architecture restores the symmetry in the trapping forces while preserving the compactness and simplicity of gMOTs. Using this approach, we demonstrate the trapping and cooling of cesium─an atomic species widely regarded as challenging for standard gMOT implementations─and analyze the trapping mechanism through numerical calculations. By providing independent control over key trapping parameters within a planar and alignment-simplified platform, this work establishes a route toward compact, robust, and species-flexible cold-atom systems, thereby expanding the applicability of miniaturized MOTs for integrated quantum technologies.