No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Implement quantum tomography of polarization-entangled states via nondiffractive metasurfaces
Applied Physics Letters
Abstract
Traditional optical elements, such as waveplates and polarization beam splitters, are essential for quantum state tomography (QST). Yet, their bulky size and heavy weight are prejudicial for miniaturizing quantum information systems. Here, we introduce nondiffractive silicon metasurfaces with high transmission efficiency to replace the traditional optical elements for QST of polarization-entangled states. Two identical silicon metasurfaces are employed, and each metasurface comprises four independent districts on a micrometer scale. The unit cell of each district consists of two silicon nanopillars with different geometrical sizes and orientation angles, and the interference of the scattered waves from the nanopillars leads to a single output beam from the district with a specific polarization state with a transmission efficiency above 92%. When the two-photon polarization-entangled state shines on different districts of two metasurfaces, each photon of the photon pair interacts with the local nanopillars within the district, and the two-photon state is projected onto 16 polarization bases for state reconstruction. We experimentally demonstrate the reconstruction of four input Bell states with high fidelities. This approach significantly reduces the number of conventional optical components in the QST process and is inspiring for advancing quantum information technology.
... Typical examples for polarized light source include circular polarization laser, 52 polarization entangled photon-pair sources 57 with efficient and on-demand uniform polarized beam generation. Metasurface polarization manipulation devices include basic uniform polarization optical elements such as meta-waveplates 53 and vectorial holography elements 55 in classic regime and entanglement modulation 62 and tomography devices 58 in quantum regime. In terms of metasurface polarization detection, singleshot full-Stokes parameters detector and imager 63 and multi-photon quantum entanglement state tomography 35 are widely explored. ...
... 24,56 In Sec. V, we will particularly review the metasurface polarization optics for quantum applications, such as polarization-entangled photon sources, 57 quantum tomography, 35,58 quantum interference, 59 quantum sensing, 60 and quantum imaging. 61 Finally, we conclude with the challenges and prospects of the metasurface polarization optics in both classical and quantum regimes. ...
Metasurface polarization optics, manipulating polarization using metasurfaces composed of subwavelength anisotropic nanostructure array, has enabled a lot of innovative integrated strategies for versatile and on-demand polarization generation, modulation, and detection. Compared with conventional bulky optical elements for polarization control, metasurface polarization optics provides a feasible platform in a subwavelength scale to build ultra-compact and multifunctional polarization devices, greatly shrinking the size of the whole polarized optical system and network. Here, we review the recent progresses of metasurface polarization optics in both classical and quantum regimes, including uniform and spatially varying polarization-manipulating devices. Basic polarization optical elements such as meta-waveplate, meta-polarizer, and resonant meta-devices with polarization singularities provide compact means to generate and modulate uniform polarization beams. Spatial-varying polarization manipulation by employing the pixelation feature of metasurfaces, leading to advanced diffraction and imaging functionalities, such as vectorial holography, classic and quantum polarization imaging, quantum polarization entanglement, quantum interference, and modulation. Substituting conventional polarization optics, metasurface approaches pave the way for on-chip classic or quantum information processing, flourishing advanced applications in displaying, communication, imaging, and computing.
... Quantum state tomography (QST) is a pivotal methodology in quantum computing and quantum information theory, enabling the exhaustive characterization of quantum states [1,2]. It is instrumental in validating quantum operations and elucidating quantum phenomena, thereby serving as a cornerstone for advancing quantum technologies [3,4]. ...
... Zuo et al. [1] introduced an optical neural network for quantum state tomography, which showcased an innovative approach to understanding quantum states. Wang et al. [2] worked on the implementation of quantum tomography of polarization-entangled states via nondiffractive metasurfaces, focusing on the interaction between light and matter. Zheng et al. [3] proposed a reconstruction algorithm for compressive quantum tomography using diverse measurement sets, emphasizing the importance of measurement in quantum state reconstruction. ...
Quantum state tomography (QST) forms the foundational framework in quantum computing, enabling precise characterization of quantum states through specialized measurement arrays. This is crucial for assessing the fidelity and coherence of quantum states in various quantum systems. The complexity and high dimensionality of quantum states require advanced statistical methods to meet modern quantum paradigms' precision and computational needs, as traditional methods often struggle with inefficiencies and inaccuracies. Conventional approaches in QST typically use linear inversion and maximum likelihood estimators, which often face computational redundancies and perform sub-optimally in high-dimensional quantum architectures. This exposition introduces pioneering statistical methodologies that combine Bayesian Inference, Variational Quantum Eigensolver, and Quantum Neural Networks to achieve enhanced fidelity approximation. The analytical discussion is supported by synthetic quantum states, demonstrating the efficacy and applicability of these statistical methods across various quantum matrices. Preliminary empirical results show a significant increase in fidelity and a notable reduction in error margins, highlighting the potential of these advanced statistical methodologies in optimizing quantum state reconstructions. Additionally, leveraging the inherent symmetry properties in quantum systems could further improve the efficiency and accuracy of state reconstructions, offering additional pathways for advancing the field.
... Later, Wang et al. proposed and demonstrated a two-MS scheme for the measurement of twophoton polarization state, where each MS has four districts functioning as a polarizer for four different polarization states [ Fig. 4(b)]. 106 The needed 16 projection measurements were done by spatially translating the MS into different districts. As a result, reconstruction of the four Bell states was achieved with fidelity over 93.45%. ...
Quantum metaphotonics has emerged as a cutting-edge subfield of meta-optics employing subwavelength resonators and their planar structures, such as metasurfaces, to generate, manipulate, and detect quantum states of light. It holds a great potential for the miniaturization of current bulky quantum optical elements by developing a design of on-chip quantum systems for various applications of quantum technologies. Over the past few years, this field has witnessed a surge of intriguing theoretical ideas, groundbreaking experiments, and novel application proposals. This Perspective aims to summarize the most recent advancements and also provides a perspective on the further progress in this rapidly developing field of research.
Quantum state tomography (QST) forms the foundational framework in quantum computing, enabling precise characterization of quantum states through specialized measurement arrays. This is crucial for assessing the fidelity and coherence of quantum states in various quantum systems. The complexity and high dimensionality of quantum states require advanced statistical methods to meet modern quantum paradigms’ precision and computational needs, as traditional methods often struggle with inefficiencies and inaccuracies. Conventional approaches in QST typically use linear inversion and maximum likelihood estimators, which often face computational redundancies and perform sub-optimally in high-dimensional quantum architectures. This exposition introduces pioneering statistical methodologies that combine Bayesian Inference, Variational Quantum Eigensolver, and Quantum Neural Networks to achieve enhanced fidelity approximation. The analytical discussion is supported by synthetic quantum states, demonstrating the efficacy and applicability of these statistical methods across various quantum matrices. Preliminary empirical results show a significant increase in fidelity and a notable reduction in error margins, highlighting the potential of these advanced statistical methodologies in optimizing quantum state reconstructions. Additionally, leveraging the inherent symmetry properties in quantum systems could further improve the efficiency and accuracy of state reconstructions, offering additional pathways for advancing the field.
Metasurface enables the generation and manipulation of multiphoton entanglement with flat optics, providing a more efficient platform for large-scale photonic quantum information processing. Here, we show that a single metasurface optical device would allow more efficient characterizations of multiphoton entangled states, such as shadow tomography, which generally requires fast and complicated control of optical setups to perform information-complete measurements, a demanding task using conventional optics. The compact and stable device here allows implementations of general positive operator valued measures with a reduced sample complexity and significantly alleviates the experimental complexity to implement shadow tomography. Integrating self-learning and calibration algorithms, we observe notable advantages in the reconstruction of multiphoton entanglement, including using fewer measurements, having higher accuracy, and being robust against experimental imperfections. Our work unveils the feasibility of metasurface as a favorable integrated optical device for efficient characterization of multiphoton entanglement, and sheds light on scalable photonic quantum technologies with ultra-thin optical devices.
Holographic metasurfaces and their applications have garnered significant attention owing to their role in polarization control. In this study, we demonstrate that the quantum properties of holographic metasurfaces can be obtained by quantum state tomography (QST) and quantum process tomography (QPT). We perform QST to obtain the experimental output states by extracting information from holograms encoded on the holographic metasurface, and develop a QPT-based method to estimate the quantum process of the metasurface. The theoretical output states derived from the estimated quantum process are in good agreement with the experimental output states, proving the effectiveness of our method. Our work not only provides theoretical and experimental analysis for understanding the quantum properties of holographic metasurfaces, but also paves the way for the application of holographic metasurfaces in quantum field.
Composite vortex beams (CVBs) have attracted considerable interest recently due to the unique optical properties and potential applications. However, these beams are mainly generated using spatial light modulators, which suffer from large volume, high cost and limited resolution. Benefiting from the ultrathin nature and unprecedented capability in light manipulation, optical metasurfaces provide a compact platform to perform this task. We propose and experimentally demonstrate a metasurface approach to creating these CVBs. The design is based on the superposition of multiple circularly polarized vortex beams with different topological charges, which is realized based on a geometric metasurface consisting of metallic nanorods with spatially variant orientations. We experimentally analyze the effects of initial phases, amplitude coefficients, incident polarization state and propagation distance on the generated CVBs, which are in good agreement with the theoretical prediction. Our work has opened a new avenue for engineering CVBs with a minimal footprint, which has promising applications ranging from multiple optical traps to quantum science. This article is protected by copyright. All rights reserved
Metasurfaces have proven themselves an exotic ability to harness light at nano-scale, being important not only for classical but also for quantum optics. Dynamic manipulation of the quantum states is at the heart of quantum information processing; however, such function has been rarely realized with metasurfaces so far. Here, we report an all-optical dynamic modulation of the photonic quantum states using the nonlinear metasurface. The metasurface consists of a metallic nanostructure combined with a photoisomerizable azo layer. By tuning the plasmonic resonance through optically switching the azo molecules between their binary isomeric states, we have realized dynamic control of transmission efficiencies of orthogonally polarized photons and also the phase delay between them, thereby an entangled state was efficiently controlled. As an illustration, a quantum state distillation has been demonstrated to recover a Bell state from a non-maximally entangled one to that with fidelities higher than 98%. Our work would enrich the functions of the metasurface in the quantum world, from static to dynamic modulation, making the quantum metasurface going practical.
From metamaterials to metasurfaces, optical nano-structure has been widely investigated for novel and high efficiency functionalities. Apart from the intrisinsic properties of composite material, rich capabilities can be derived from the judicious design of metasurfaces, which enable more excellent and highly integrated optical devices than traditional bulk optical elements. In the meantime, the abundant manipulation abilites of light in the classical domain can be carried over into quantum domain. In this review, we highlight recent development of quantum optics based on metasurfaces, ranging from quantum plasmonics, generation, manipulation and appplication of quantum light to quantum vaccum engineering etc. Finally, some promising avenues for quantum optics with the help of optical metasurface are presented.
Over the past years, broadband achromatic metalenses have been intensively studied due to their great potential for applications in consumer and industry products. Even though significant progress has been made, the efficiency of technologically relevant silicon metalenses is limited by the intrinsic material loss above the bandgap. In turn, the recently proposed achromatic metalens utilizing transparent, high-index materials such as titanium dioxide has been restricted by the small thickness and showed relatively low focusing efficiency at longer wavelengths. Consequently, metalens-based optical imaging in the biological transparency window has so far been severely limited. Herein, we experimentally demonstrate a polarization-insensitive, broadband titanium dioxide achromatic metalens for applications in the near-infrared biological imaging. A large-scale fabrication technology has been developed to produce titanium dioxide nanopillars with record-high aspect ratios featuring pillar heights of 1.5 µm and ~90° vertical sidewalls. The demonstrated metalens exhibits dramatically increased group delay range, and the spectral range of achromatism is substantially extended to the wavelength range of 650–1000 nm with an average efficiency of 77.1%–88.5% and a numerical aperture of 0.24–0.1. This research paves a solid step towards practical applications of flat photonics. Though broadband achromatic metalens are attractive for biological applications, existing metalenses show limited performance in the biological imaging window. Here, the authors report high-efficiency broadband achromatic metalens featuring record-high aspect ratio titanium dioxide metasurfaces.
Metasurfaces have recently entered the realm of quantum photonics, enabling manipulation of quantum light using a compact nanophotonic platform. Realizing the full potential of metasurfaces at the deepest quantum level requires the ability to tune coherent light-matter interactions continuously in space and time. Here, we introduce the concept of space-time quantum metasurfaces for arbitrary control of the spectral, spatial, and spin properties of nonclassical light using a compact photonic platform. We show that space-time quantum metasurfaces allow on-demand tailoring of entanglement among all degrees of freedom of a single photon. We also show that spatiotemporal modulation induces asymmetry at the fundamental level of quantum fluctuations, resulting in the generation of steered and vortex photon pairs out of vacuum. Space-time quantum metasurfaces have the potential to enable novel photonic functionalities, such as encoding quantum information into high-dimensional color qudits using designer modulation protocols, sculpting multispectral and multispatial modes in spontaneous emission, and generating reconfigurable hyperentanglement for high-capacity quantum communications.
Rapid progress in the development of metamaterials and metaphotonics allowed bulky optical assemblies to be replaced with thin nanostructured films, often called metasurfaces, opening a broad range of novel and superior applications of flat optics to the generation, manipulation and detection of classical light. Recently, these developments started making headway in quantum photonics, where novel opportunities arose for the control of non-classical nature of light, including photon statistics, quantum state superposition, quantum entanglement and single-photon detection. In this Perspective, we review recent progress in the emerging field of quantum-photonics applications of metasurfaces, focusing on innovative and promising approaches to create, manipulate and detect non-classical light. Progress in the field of quantum-photonics applications of metasurfaces is reviewed. Cutting-edge research, including the development of optical chips supporting high-dimensional quantum entanglement and advanced quantum tomography, is summarized.
Mimicry is a biological camouflage phenomenon whereby an organism can change its shape and color to resemble another object. Herein, the idea of biological mimicry and rich degrees of freedom in metasurface designs are combined to realize holographic mimicry devices. A general mathematical method, called phase matrix transformation, to accomplish the holographic mimicry process is proposed. Based on this method, a dynamic metasurface hologram is designed, which shows an image of a “bird” in the air, and a distinct image of a “fish” when the environment is changed to oil. Furthermore, to make the mimicry behavior more generic, holographic mimicry operating at dual wavelengths is also designed and experimentally demonstrated. Moreover, the fully independent phase modulation realized by phase matrix transformation makes the working efficiency of the device relatively higher than the conventional multiwavelength holographic devices with off‐axis illumination or interleaved subarrays. The work potentially opens a new research paradigm interfacing bionics with nanophotonics, which may produce novel applications for optical information encryption, virtual/augmented reality (VR/AR), and military camouflage systems.
Photonic quantum information processing, one of the leading platforms for quantum technologies1,2,3,4,5, critically relies on optical quantum interference to produce an indispensable effective photon–photon interaction. However, such an effective interaction is fundamentally limited to bunching⁶ due to the bosonic nature of photons⁷ and the restricted phase response from conventional unitary optical elements8,9. Here we propose and experimentally demonstrate a new degree of freedom in the optical quantum interference enabled by a non-unitary metasurface. Due to the unique anisotropic phase response that creates two extreme eigen-operations, we show dynamical and continuous control over the effective interaction of two single photons such that they show bosonic bunching, fermionic antibunching or arbitrarily intermediate behaviour, beyond their intrinsic bosonic nature. This quantum operation opens the door to both fundamental quantum light–matter interaction and innovative photonic quantum devices for quantum communication, quantum simulation and quantum computing.
The control of polarization, an essential property of light, is of broad scientific and technological interest. Polarizers are indispensable optical elements for direct polarization generation. However, arbitrary polarization generation, except that of common linear and circular polarization, relies heavily on bulky optical components such as cascading linear polarizers and waveplates. Here, we present an effective strategy for designing all-in-one full Poincaré sphere polarizers based on perfect arbitrary polarization conversion dichroism and implement it in a monolayer all-dielectric metasurface. This strategy allows preferential transmission and conversion of one polarization state located at an arbitrary position on the Poincaré sphere to its handedness-flipped state while completely blocking its orthogonal state. In contrast to previous methods that were limited to only linear or circular polarization, our method manifests perfect dichroism of nearly 100% in theory and greater than 90% experimentally for arbitrary polarization states. By leveraging this attractive dichroism, our demonstration of the generation of polarization beams located at an arbitrary position on a Poincaré sphere directly from unpolarized light can substantially extend the scope of meta-optics and dramatically promote state-of-the-art nanophotonic devices.
Exquisite polarization control using optical metasurfaces has attracted considerable attention thanks to their ability to manipulate multichannel independent wavefronts with subwavelength resolution. Here we present a new class of metasurface polarization optics, which enables imposition of two arbitrary and independent amplitude profiles on any pair of orthogonal states of polarization. The implementation method involves a polarization-dependent interference mechanism achieved by constructing a metasurface composed of an array of nanoscale birefringent waveplates. Based on this principle, we experimentally demonstrate chiral grayscale metasurface and chiral shadow rendering of structured light. These results illustrate a general approach interlinking amplitude profiles and orthogonal states of polarization and expands the scope of metasurface polarization shaping optics.
Metasurfaces consisting of engineered dielectric or metallic structures provide unique solutions to realize exotic phenomena including negative refraction, achromatic focusing, electromagnetic cloaking, and so on. The intersection of metasurface and quantum optics may lead to new opportunities but is much less explored. Here, we propose and experimentally demonstrate that a polarization-entangled photon source can be used to switch ON or OFF the optical edge detection mode in an imaging system based on a high-efficiency dielectric metasurface. This experiment enriches both fields of metasurface and quantum optics, representing a promising direction toward quantum edge detection and image processing with remarkable signal-to-noise ratio.
Geometric-phase metasurfaces, recently utilized for controlling wavefronts of circular polarized (CP) electromagnetic waves, are drastically limited to the cross-polarization modality. Combining geometric with propagation phase allows to further control the co-polarized output channel, nevertheless addressing only similar functionality on both co-polarized outputs for the two different CP incident beams. Here we introduce the concept of chirality-assisted phase as a degree of freedom, which could decouple the two co-polarized outputs, and thus be an alternative solution for designing arbitrary modulated-phase metasurfaces with distinct wavefront manipulation in all four CP output channels. Two metasurfaces are demonstrated with four arbitrary refraction wavefronts, and orbital angular momentum modes with four independent topological charge, showcasing complete and independent manipulation of all possible CP channels in transmission. This additional phase addressing mechanism will lead to new components, ranging from broadband achromatic devices to the multiplexing of wavefronts for application in reconfigurable-beam antenna and wireless communication systems. Here the authors propose an approach to construct metasurfaces, which activate all circularly polarized channels and make full utilization of transmitted energy simultaneously. By introducing chirality-assisted phase all the components in the Jones matrix can be decoupled and independently tuned.
Manipulating the polarization of light on the microscale or nanoscale is essential for integrated photonics and quantum optical devices. Nowadays, the metasurface allows one to build on-chip devices that efficiently manipulate the polarization states. However, it remains challenging to generate different types of polarization states simultaneously, which is required to encode information for quantum computing and quantum cryptography applications. By introducing geometrical-scaling-induced (GSI) phase modulations, we demonstrate that an assembly of circularly polarized (CP) and linearly polarized (LP) states can be simultaneously generated by a single metasurface made of L-shaped resonators with different geometrical features. Upon illumination, each resonator diffracts the CP state with a certain GSI phase. The interaction of these diffractions leads to the desired output beams, where the polarization state and the propagation direction can be accurately tuned by selecting the geometrical shape, size, and spatial sequence of each resonator in the unit cell. As an example of potential applications, we show that an image can be encoded with different polarization profiles at different diffraction orders and decoded with a polarization analyzer. This approach resolves a challenging problem in integrated optics and is inspiring for on-chip quantum information processing.
Phase, polarization, amplitude, and frequency represent the basic dimensions of light, playing crucial roles for both fundamental light–material interactions and all major optical applications. Metasurfaces have emerged as a compact platform to manipulate these knobs, but previous metasurfaces have limited flexibility to simultaneous control them. A multi‐freedom metasurface that can simultaneously and independently modulate phase, polarization, and amplitude in an analytical form is introduced, and frequency multiplexing is further realized by a k‐space engineering technique. The multi‐freedom metasurface seamlessly combines geometric Pancharatnam–Berry phase and detour phase, both of which are frequency independent. As a result, it allows complex‐amplitude vectorial hologram at various frequencies based on the same design strategy, without sophisticated nanostructure searching of massive geometric parameters. Based on this principle, full‐color complex‐amplitude vectorial meta‐holograms in the visible are experimentally demonstrated with a metal–insulator–metal architecture, unlocking the long‐sought full potential of advanced light field manipulation through ultrathin metasurfaces. A multi‐freedom metasurface is proposed to manipulate multiple dimensions of light simultaneously. A general metasurface design strategy seamlessly combining geometric phase and detour phase in the 1st diffraction order is developed, which can modulate phase, amplitude, and polarization in an analytical and wavelength/angle‐independent formation. Frequency multiplexing is further applied by k‐space engineering, ultimately achieving the full‐color complex‐amplitude vectorial holography.
We propose a polarization-reprogrammable digital coding metasurface to realize arbitrarily linear polarization editing for reflected electromagnetic (EM) waves and beams. We present a digital coding element integrated with a PIN diode for switching the polarization conversion, which matches the digital 0 and 1 states. Based on the coding element, we combine the digital states and physical states of EM waves successfully. By arranging distinct coding sequences, we show that the polarization of the EM reflected beam can be controlled flexibly in real time. Not only are two orthogonally linear polarizations switched, but also arbitrary polarization directions can be achieved with specific coding sequences in a programmable manner. Such a manner can be accessibly used in communication and imaging applications. As a proof of concept, six different polarization states are presented to validate the proposed method. A metasurface sample with 30 × 30 elements is fabricated and measured in microwave frequency. The experimental results have good agreement with simulations and theoretical analyses. We envision that this work will promote the research of active polarization manipulation and extend its perspective from traditional physical fields to the information world.
Single photons carrying spin angular momentum (SAM), i.e., circularly polarized single photons generated typically by subjecting a quantum emitter (QE) to a strong magnetic field at low temperatures, are at the core of chiral quantum optics enabling nonreciprocal single‐photon configurations and deterministic spin‐photon interfaces. Here, a conceptually new approach to the room‐temperature generation of SAM‐coded single photons (SSPs) is described, which entails QE nonradiative coupling to surface plasmons being transformed, by interacting with an optical metasurface, into a collimated stream of SSPs with the designed handedness. Design, fabrication, and characterization of SSP sources, consisting of dielectric circular nanoridges with azimuthally varying widths deterministically fabricated on a dielectric‐protected silver film around a nanodiamond containing a nitrogen‐vacancy center, are reported. With properly engineered phases of QE‐originated fields scattered by nanoridges, the outcoupled photons are characterized by a well‐defined SAM (with the chirality >0.8) and high directionality (collection efficiency up to 92%). A novel approach to room‐temperature generation of circularly polarized single photons is described, which entails nonradiative coupling of a quantum emitter to surface plasmons being transformed by an optical metasurface into a collimated stream of circularly polarized single photons. Fabrication and characterization of a metasurface consisting of dielectric circular width‐gradient nanoridges fabricated on a silver film around the emitter are reported.
Vertical cavity surface-emitting lasers (VCSELs) have made indispensable contributions to the development of modern optoelectronic technologies. However, arbitrary beam shaping of VCSELs within a compact system has remained inaccessible until now. The emerging ultra-thin flat optical structures, namely metasurfaces, offer a powerful technique to manipulate electromagnetic fields with subwavelength spatial resolution. Here, we show that the monolithic integration of dielectric metasurfaces with VCSELs enables remarkable arbitrary control of the laser beam profiles, including self-collimation, Bessel and Vortex lasers, with high efficiency. Such wafer-level integration of metasurface through VCSEL-compatible technology simplifies the assembling process and preserves the high performance of the VCSELs. We envision that our approach can be implemented in various wide-field applications, such as optical fibre communications, laser printing, smartphones, optical sensing, face recognition, directional displays and ultra-compact light detection and ranging (LiDAR). Non-intrusive integration of metasurfaces with vertical cavity surface-emitting lasers enables fully arbitrary wavefront control for directional laser emission.
Vector vortex beams (VVBs) possess ubiquitous applications from particle trapping to quantum information. Recently, the bulky optical devices for generating VVBs have been miniaturized by using metasurfaces. Nevertheless, it is quite challenging for the metasurface-generated VVBs to possess arbitrary polarization and phase distributions. More critical is that the VVBs' annular intensity profiles demonstrated hitherto are dependent on topological charges and are hence not perfect, posing difficulties in spatially shared co-propagation of multiple vortex beams. Here, a single-layer metasurface to address all those aforementioned challenges in one go is proposed, which consists of two identical crystal-silicon nanoblocks with varying positions and rotation angles (i.e., four geometric parameters throughout). Those four geometric parameters are found to be adequate for independent and arbitrary control of the amplitude, phase, and polarization of light. Perfect VVBs with arbitrary polarization and phase distributions are successfully generated, and the constant intensity profiles independent of their topological charges and polarization orders are demonstrated. The proposed strategy casts a distinct perception that a minimalist design of just one single-layer metasurface can empower such robust and versatile control of VVBs. That provides promising opportunities for generating more complex vortex field for advanced applications in structural light, optical micromanipulation, and data communication.
Electromagnetic metastructures stand for the artificial structures with a characteristic size smaller than the wavelength, which may efficiently manipulate the states of light. However, their applications are often restricted by the bandwidth of the electromagnetic response of the metastructures. It is therefore essential to reassert the principles in constructing broadband electromagnetic metastructures. Herein, after summarizing the conventional approaches for achieving broadband electromagnetic functionality, some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and several trade‐off approaches are reviewed, followed by some perspectives for the future development of broadband metamaterials. It is anticipated that broadband metastructures will have even more substantial applications in optoelectronics, energy harvesting, and information technology. Broadband electromagnetic response is often required in the applications of metastructures. It is therefore important to reassess the principles of constructing broadband electromagnetic metastructures. Some recent developments in realizing broadband electromagnetic response by dispersion compensation, nonresonant effects, and some trade‐off approaches are reviewed. Perspectives are given on the development of broadband metastructured materials.
Quantitative phase imaging (QPI) of transparent samples plays an essential role in multiple biomedical applications, and miniaturizing these systems will enable their adoption into point-of-care and in vivo applications. Here, we propose a compact quantitative phase gradient microscope (QGPM) based on two dielectric metasurface layers, inspired by a classical differential interference contrast (DIC) microscope. Owing to the multifunctionality and compactness of the dielectric metasurfaces, the QPGM simultaneously captures three DIC images to generate a quantitative phase gradient image in a single shot. The volume of the metasurface optical system is on the order of 1 mm³. Imaging experiments with various phase resolution samples verify the capability to capture quantitative phase gradient data, with phase gradient sensitivity better than 92.3 mrad μm⁻¹ and single-cell resolution. The results showcase the potential of metasurfaces for developing miniaturized QPI systems for label-free cellular imaging and point-of-care devices.
Scientists have developed an optical metasurface capable of entangling and disentangling photon-pairs, providing a path for the development of quantum technologies for applications in computing, imaging, and sensing. Optical metasurfaces are sub-wavelength layers of nanostructures capable of precisely controlling the properties of light. They offer the promise of new miniaturized quantum systems where they remain largely unexplored. Now Thomas Zentgraf and colleagues from the University of Paderborn in Germany, working with researchers from the University of Stuttgart and the Southern University of Science and Technology in China, have developed a nanostructured dielectric metasurface capable of entangling and disentangling the spin states of a photon-pair. Quantum interference of the photons on the metasurface produces a circularly polarized entangled photon-pair, which can be disentangled by passing it through the metasurface a second time.
A tomographic measurement is a ubiquitous tool for estimating the properties of quantum states, and its application is known as quantum state tomography (QST). The process involves manipulating single photons in a sequence of projective measurements, often to construct a density matrix from which other information can be inferred, and is as laborious as it is complex. Here we unravel the steps of a QST and outline how it may be demonstrated in a fast and simple manner with intense (classical) light. We use scalar beams in a time reversal approach to simulate the outcome of a QST and exploit non-separability in classical vector beams as a means to treat the latter as a “classically entangled” state for illustrating QSTs directly. We provide a complete do-it-yourself resource for the practical implementation of this approach, complete with tutorial video, which we hope will facilitate the introduction of this core quantum tool into teaching and research laboratories alike. Our work highlights the value of using intense classical light as a means to study quantum systems and in the process provides a tutorial on the fundamentals of QSTs.
Ultrathin metasurfaces that can efficiently manipulate circularly polarized terahertz waves in transmission rather reflection have been demonstrated by scientists in China. The devices were fabricated by Min Jia and colleagues from Fudan University and Capital Normal University. The tri-layer structures operate at frequencies of around 0.6 THz and suit future on-chip applications for terahertz photonics. Various designs were tested resulting in a circularly polarized terahertz Bessel beam generator and a device exhibiting the photonic spin Hall effect. The metasurfaces consist of a periodic array of meta-atoms, each composed of three thin layers of metal in a U-shape that are separated and surrounded by polyimide. Changing the angular orientation of the meta-atoms allows different phase gradients to be programmed into the metasurfaces bringing the desired functionality.
Vectorial holography: Realizing the potential of metasurface holograms Scientists have developed a novel technique that uses metasurfaces for creating holograms, paving the way for a range of new holographic-based optical devices with potential use in optical displays, optical data storage and security. Holography based on metasurfaces can show exceptional spatial resolution, enormous information capacity and wide field of view. However, current techniques suffer from crosstalk between polarization channels that create undesired effects, preventing the full capacity of all polarization channels from being realized. Now, Thomas Zentgraf and colleagues from the University of Paderborn in Germany, together with colleagues from Beijing Institute of Technology and Southern University of Science and Technology in China, have developed a new technique that integrates multiple polarization channels into a single hologram that completely avoids unwanted crosstalk, and significantly increase protection for optical data security.
Metasurfaces enable manipulation of light propagation at an unprecedented level, benefitting from a number of merits unavailable to conventional optical elements, such as ultracompactness, precise phase and polarization control at deep subwavelength scale, and multifunctionalities. Recent progress in this field has witnessed a plethora of functional metasurfaces, ranging from lenses and vortex beam generation to holography. However, research endeavors have been mainly devoted to static devices, exploiting only a glimpse of opportunities that metasurfaces can offer. We demonstrate a dynamic metasurface platform, which allows independent manipulation of addressable subwavelength pixels at visible frequencies through controlled chemical reactions. In particular, we create dynamic metasurface holograms for advanced optical information processing and encryption. Plasmonic nanorods tailored to exhibit hierarchical reaction kinetics upon hydrogenation/dehydrogenation constitute addressable pixels in multiplexed metasurfaces. The helicity of light, hydrogen, oxygen, and reaction duration serve as multiple keys to encrypt the metasurfaces. One single metasurface can be deciphered into manifold messages with customized keys, featuring a compact data storage scheme as well as a high level of information security. Our work suggests a novel route to protect and transmit classified data, where highly restricted access of information is imposed.
Going quantum with metamaterials
Metasurfaces should allow wafer-thin surfaces to replace bulk optical components. Two reports now demonstrate that metasurfaces can be extended into the quantum optical regime. Wang et al. determined the quantum state of multiple photons by simply passing them through a dielectric metasurface, scattering them into single-photon detectors. Stav et al. used a dielectric metasurface to generate entanglement between spin and orbital angular momentum of single photons. The results should aid the development of integrated quantum optic circuits operating on a nanophotonic platform.
Science , this issue p. 1104 , p. 1101
Quantum tomography: Overcoming quantum complexity by measuring multiple outputs A single chip can be used to fully characterize multi-photon quantum states, without incorporating any reconfigurable elements. This is a fundamental advance since finding a complete description of quantum system normally requires a number of different measurement configurations that grows exponentially with the system size. The group of Andrey Sukhorukov at the Australian National University developed a theoretical concept, and experimental device fabrication and two-photon measurements were realized by the group of Alexander Szameit at the Friedrich-Schiller-Universität Jena and University of Rostock in Germany. The light traversing the device undergoes an optical transformation and expands into a larger number of modes at the output. The authors then use a computationally efficient algorithm to deduce the initial quantum state from the correlations of output photons. The number of modes required only grows linearly with the photon number, demonstrating that the design provides a scalable approach to measuring larger quantum states despite their increasing complexity.
Metalenses consist of an array of optical nanoantennas on a surface capable of manipulating the properties of an incoming light wavefront. Various flat optical components, such as polarizers, optical imaging encoders, tunable phase modulators and a retroreflector, have been demonstrated using a metalens design. An open issue, especially problematic for colour imaging and display applications, is the correction of chromatic aberration, an intrinsic effect originating from the specific resonance and limited working bandwidth of each nanoantenna. As a result, no metalens has demonstrated full-colour imaging in the visible wavelength. Here, we show a design and fabrication that consists of GaN-based integrated-resonant unit elements to achieve an achromatic metalens operating in the entire visible region in transmission mode. The focal length of our metalenses remains unchanged as the incident wavelength is varied from 400 to 660 nm, demonstrating complete elimination of chromatic aberration at about 49% bandwidth of the central working wavelength. The average efficiency of a metalens with a numerical aperture of 0.106 is about 40% over the whole visible spectrum. We also show some examples of full-colour imaging based on this design.
Quantum nonlocality, i.e. the presence of strong correlations in spatially separated systems which are forbidden by local realism, lies at the heart of quantum communications and quantum computing. Here, we use polarization-entangled photon pairs to demonstrate a nonlocal interaction of light with a plasmonic structure. Through the detection of one photon with a polarization-sensitive device, we can prevent or allow absorption of a second, remotely located photon. We demonstrate this with pairs of entangled photons in polarization, one of which is coupled into a plasmon of a thin metamaterial absorber in the path of a standing wave of an interferometer. Thus, we realize a quantum eraser experiment using photons and plasmonic resonances from metamaterials which promises opportunities for probabilistic quantum gating and controlling plasmon-photon conversion and entanglement. Moreover, by using the so-called coherent perfect absorption effect, we can expect near perfect interaction.
As nanofabrication technology progresses, the emerging metasurface has offered unique opportunities for holography, such as an increased data capacity and the realization of polarization-sensitive functionality. Multicolor three-dimensional (3D) meta-hologram imaging is one of the most pursued applications for meta-hologram not yet realized. How to reduce the cross-talk among different colors in broad bandwidth designs is a critical question. On the basis of the off-axis illumination method, we develop a novel way to overcome the cross-talk limitation and achieve multicolor meta-holography with a single type of plasmonic pixel. With this method, the usable data capacity can also be improved. It not only leads to a remarkable image quality, with a signal-to-noise ratio (SNR) five times better than that of the previous meta-hologram designs, but also paves the way to new meta-hologram devices, which mark an advance in the field of meta-holography. For example, a seven-color meta-hologram can be fabricated with a color gamut 1.39 times larger than that of the red, green, and blue (RGB) design. For the first time, a full-color meta-holographic image in the 3D space is also experimentally demonstrated. Our approach to expanding the information capacity of the meta-hologram is unique, which extends broad applications in data storage, security, and authentication.
Dielectric metasurfaces require high refractive index contrast materials for optimum performance. This requirement imposes a severe restraint; devices have either been demonstrated at wavelengths of 700nm and above using high-index semiconductors such as silicon, or they use lower index dielectric materials such as TiO or SiN and operate in the visible wavelength regime. Here, we show that the high refractive index of silicon can be exploited at wavelengths as short as 532 nm by demonstrating a silicon metasurface with a transmission efficiency of 47% at this wavelength. The metasurface consists of a graded array of silicon posts arranged in a square lattice on a quartz substrate. We show full 2{\pi} phase control and we experimentally demonstrate polarization-independent beam deflection at 532nm wavelength. The crystalline silicon is placed on a quartz substrate by a bespoke layer transfer technique and we note that an efficiency >70% may be achieved for a further optimized structure in the same material. Our results open a new way for realizing efficient metasurfaces based on silicon in the visible wavelength regime.
Polarization entangled photon pair source is widely used in many quantum information processing applications such as teleportation, quantum communications, quantum computation and high precision quantum metrology. We report on the generation of a continuous-wave pumped 1550 nm polarization entangled photon pair source at telecom wavelength using a type-II periodically poled KTiOPO4 (PPKTP) crystal in a Sagnac interferometer. Hong-Ou-Mandel (HOM) interference measurement yields signal and idler photon bandwidth of 2.4 nm. High quality of entanglement is verified by various kinds of measurements, for example two-photon interference fringes, Bell inequality and quantum states tomography. The source can be tuned over a broad range against temperature or pump power without loss of visibilities. This source will be used in our future experiments such as generation of orbital angular momentum entangled source at telecom wavelength for quantum frequency up-conversion, entanglement based quantum key distributions and many other quantum optics experiments at telecom wavelengths.
Metasurfaces are planar structures that locally modify the polarization, phase and amplitude of light in reflection or transmission, thus enabling lithographically patterned flat optical components with functionalities controlled by design. Transmissive metasurfaces are especially important, as most optical systems used in practice operate in transmission. Several types of transmissive metasurface have been realized, but with either low transmission efficiencies or limited control over polarization and phase. Here, we show a metasurface platform based on high-contrast dielectric elliptical nanoposts that provides complete control of polarization and phase with subwavelength spatial resolution and an experimentally measured efficiency ranging from 72% to 97%, depending on the exact design. Such complete control enables the realization of most free-space transmissive optical elements such as lenses, phase plates, wave plates, polarizers, beamsplitters, as well as polarization-switchable phase holograms and arbitrary vector beam generators using the same metamaterial platform.
An anisotropic quantum vacuum (AQV) opens novel pathways for controlling light-matter interaction
in quantum optics, condensed matter physics, etc. Here, we theoretically demonstrate a strong AQV over macroscopic distances enabled by a judiciously designed array of subwavelength-scale nanoantennas -a metasurface. We harness the phase-control ability and the polarization-dependent response of the metasurface to achieve strong anisotropy in the decay rate of a quantum emitter located over distances of hundreds of wavelengths. Such an AQV induces quantum interference among radiative decay channels in an atom with orthogonal transitions. Quantum vacuum engineering with metasurfaces holds promise for exploring new paradigms of long-range light-matter interaction for atom optics, solid-state quantum optics,
quantum information processing, etc.
Multilayered plasmonic metasurfaces have been previously shown to enable multifunctional control of full‐space electromagnetic waves, which are of great importance to the development of compact optical systems. While this structural configuration is practical for acquiring metasurfaces working in microwave frequency, it will inevitably become lossy and highly challenging to fabricate when entering the visible band. Here, an efficient yet facile approach to address this issue by resorting to a dielectric metasurface doublet (DMD) based on two vertically integrated polarization‐filtering meta‐atoms (PFMs) is presented. The PFMs exhibit polarization‐dependent high transmission and reflection, as well as independent and full 2π phase control characteristics, empowering the DMD to realize three distinct incidence‐direction and polarization‐triggered wavefront‐shaping functionalities, including anomalous beam deflection, light focusing, vortex beam generation, and holographic image projection as it is investigated either numerically or experimentally. The presented DMD undoubtedly holds several salient features compared with the multilayered metallic metasurfaces in aspects of design complexity, efficiency, and fabrication. Furthermore, as dielectric meta‐atoms with distinct polarization responses can be deployed to construct the DMD, it is anticipated that diverse full‐space metasurfaces equipped with versatile functionalities can be demonstrated in the future, which will greatly advance the development of multifunctional meta‐optics.
We propose the concept of one-sided destructive quantum interference based on lossy metasurfaces. It corresponds to one-sided beam splitting with 50:25:0 in percentage ratio among the transmission and the two reflection efficiencies, which can be realized by a metasurface with unidirectional zero reflection (UZR) from a non-Hermitian exceptional point. When two identical photons enter both sides of the lossy metasurface, quantum interference only occurs for the single-photon output state towards one side but not the other due to UZR. Such one-sided quantum interference can be further made totally destructive. This effect along with UZR provides more degrees of freedom in quantum interference control and entangled states construction.
We develop a concept of metasurface-assisted ghost imaging for nonlocal discrimination between a set of polarization objects. The specially designed metasurfaces are incorporated in the imaging system to perform parallel state transformations in general elliptical bases of quantum-entangled or classically correlated photons. Then, only four or fewer correlation measurements between multiple metasurface outputs and a simple polarization-insensitive bucket detector after the object can allow for the identification of fully or partially transparent polarization elements and their arbitrary orientation angles. The approach can find applications for real-time and low-light imaging across diverse spectral regions in dynamic environments.
Manipulation of polarization states with metasurfaces is a compelling approach for on-chip photonics and portable information processing. Yet it remains challenging to generate different types of polarization states with a single piece of a metasurface. This paper demonstrates a metasurface to resolve this issue, which is made of L-shaped resonators with different geometrical sizes. Each resonator diffracts a right- or left-handed circularly polarized state with an additional geometrical-scaling-induced phase. The type of polarization state of each diffracted beam is determined by the enantiomorphism, size, and spatial sequence of the resonators in the unit cell. The number of the beams is modulated by the geometry and separation of the resonators. We provide examples to illustrate how to achieve the specific number of diffracted beams with the desired polarization states in experiments. We suggest that this strategy can be applied for integrated photonics and portable quantum information processing.
Quantum no-cloning, the impossibility of perfectly cloning an arbitrary unknown quantum state, is one of the most fundamental limitations due to the laws of quantum mechanics, which underpin the physical security of quantum key distribution. Quantum physics does allow, however, approximate cloning with either imperfect state fidelity and/or probabilistic success. Whereas approximate quantum cloning of single-particle states has been tested previously, experimental cloning of quantum entanglement—a highly nonclassical correlation—remained unexplored. Based on a multiphoton linear optics platform, we demonstrate quantum cloning of two-photon entangled states for the first time. Remarkably our results show that one maximally entangled photon pair can be broadcast into two entangled pairs, both with state fidelities above 50%. Our results are a key step towards cloning of complex quantum systems, and are likely to provide new insights into quantum entanglement.
Metalens-array–based quantum source
Spontaneous down-conversion is an exotic optical process in a nonlinear crystal in which a high-energy photon splits into two lower-energy photons that are quantum mechanically entangled. These entangled pairs are valuable commodities for quantum information processing and quantum communications. Because the experimental setup is usually performed with bulk optical components, there is a need to decrease the size scale for application. Li et al. combined an array of specialized metalenses with a nonlinear crystal and show that the scale of the process can be shrunk substantially. The approach should prove useful for developing miniaturized integrated quantum optical technologies.
Science , this issue p. 1487
Quantum entanglement is a key resource that can be exploited for a range of applications such as quantum teleportation, quantum computation, and quantum cryptography. However, efforts to exploit entanglement in imaging systems have so far led to solutions such as ghost imaging, that have since found classical implementations. Here, we demonstrate an optical imaging protocol that relies uniquely on entanglement: Two polarizing patterns imprinted and superimposed on a metasurface are separately imaged only when using entangled photons. Unentangled light is not able to distinguish between the two patterns. Entangled single-photon imaging of functional metasurfaces promises advances towards the use of nanostructured subwavelength thin devices in quantum information protocols and a route to efficient quantum state tomography.
Going quantum with metamaterials
Metasurfaces should allow wafer-thin surfaces to replace bulk optical components. Two reports now demonstrate that metasurfaces can be extended into the quantum optical regime. Wang et al. determined the quantum state of multiple photons by simply passing them through a dielectric metasurface, scattering them into single-photon detectors. Stav et al. used a dielectric metasurface to generate entanglement between spin and orbital angular momentum of single photons. The results should aid the development of integrated quantum optic circuits operating on a nanophotonic platform.
Science , this issue p. 1104 , p. 1101
Entanglement-based quantum science exploits subtle properties of quantum mechanics into applications such as quantum computing, sensing and metrology. The emerging route for quantum computing applications, which calls for ultracompact, integrated and scalable architecture, aims at on-chip entangled qubits. In this context, quantum entanglement among atomic qubits was achieved via cold controlled collisions which are only significant at subwavelength separations. However, as other manifolds of quantum state engineering require single site addressability and controlled manipulation of the individual qubit using diffraction-limited optics, entanglement of qubits separated by macroscopic distances at the chip level is still an outstanding challenge. Here, we report a novel platform for on-chip quantum state engineering by harnessing the extraordinary light-molding capabilities of metasurfaces. We theoretically demonstrate quantum entanglement between two qubits trapped on a chip and separated by macroscopic distances, by engineering their coherent and dissipative interactions via the metasurface. Spatially scalable interaction channels offered by the metasurface enable robust generation of entanglement, with large values of concurrence, and remarkable revival from sudden death. The metasurface route to quantum state engineering opens a new paradigm for on-chip quantum science and technologies.
Multi-photon absorption processes have a nonlinear dependence on the amplitude of the incident optical field i.e. the number of photons. However, multi-photon absorption is generally weak and multi-photon events occur with extremely low probability. Consequently, it is extremely challenging to engineer quantum nonlinear devices that operate at the single photon level and the majority of quantum technologies have to rely on single photon interactions. Here, we demonstrate experimentally and theoretically that exploiting coherent absorption of N=2 N00N states makes it possible to enhance the number of two-photon states that are absorbed. An absorbing metasurface placed inside a Sagnac-style interferometer into which we inject an N = 2 N00N state, exhibits two-photon absorption with 40.5% efficiency, close to the theoretical maximum. This high probability of simultaneous absorption of two photons holds the promise for applications in fields that require multi-photon upconversion but are hindered by high peak intensities, for example spectroscopic measurements of delicate biological samples and quantum photolithography.
Chiroptical effects characterized by different optical responses for left- (LCP) and right-handed circularly polarized light (RCP) are powerful and valuable tools in optics with wide applications in polarization-resolved imaging and sensing. Previously observed strong chiroptical effects are limited to metamaterials with complex three-dimensional chiral structures at the subwavelength scale. Although asymmetrical transmission of LCP and RCP have been investigated in planar chiral metasurfaces, the observed weak chiroptical effects result from anisotropic Ohmic dissipation of the metal constituents. Here, we demonstrate by theory and proof-of-concept experiments that a large difference in transmittances of LCP and RCP can be attained in a single-layer planar chiral metamaterial with a subwavelength thickness. Without violating the reciprocity and mirror symmetry, the strong chiroptical effect, independent of dielectric loss, arises from a mechanism of multimode interference. The described effect may lead to a gateway towards chiral manipulations of light and chiral optical devices.
Metasurfaces have shown extraordinary abilities to manipulate the phase and polarization of light and thus hold great promise for applications in photonics and optoelectronics. Here, we propose a unique application of the dielectric metasurface for quantum weak measurements. In our scheme, the dielectric metasurface introduces a tiny phase gradient in the procedure of measurements and keeps the measured system almost undisturbed. The dielectric metasurface may improve and simplify already existing schemes in quantum weak measurements and thereby provide potential applications in precise measurements of phase, polarization, and frequency of light.
We present a method allowing for the imposition of two independent and arbitrary phase profiles on any pair of orthogonal states of polarization—linear, circular, or elliptical—relying only on simple, linearly birefringent wave plate elements arranged into metasurfaces. This stands in contrast to previous designs which could only address orthogonal linear, and to a limited extent, circular polarizations. Using this approach, we demonstrate chiral holograms characterized by fully independent far fields for each circular polarization and elliptical polarization beam splitters, both in the visible. This approach significantly expands the scope of metasurface polarization optics.
Multifunction planar optics
Specially designed two-dimensional (2D) arrays of nanometer-scale metallic antennas, or metasurfaces, may allow bulky optical components to be shrunk down to a planar device structure. Khorasaninejad et al. show that arrays of nanoscale fins of TiO can function as high-end optical lenses. At just a fraction of the size of optical objectives, such planar devices could turn your phone camera or your contact lens into a compound microscope. Maguid et al. interleaved sparse 2D arrays of metal antennas to get multifunctional behavior from the one planar device structure (see the Perspective by Litchinitser). The enhanced functionality of such designed metasurfaces could be used in sensing applications or to increase the communication capacity of nanophotonic networks.
Science , this issue pp. 1190 and 1202 ; see also p. 1177
The technologies of heating, photovoltaics, water photocatalysis and artificial photosynthesis depend on the absorption of light and novel approaches such as coherent absorption from a standing wave promise total dissipation of energy. Extending the control of absorption down to very low light levels and eventually to the single-photon regime is of great interest and yet remains largely unexplored. Here we demonstrate the coherent absorption of single photons in a deeply subwavelength 50% absorber. We show that while the absorption of photons from a travelling wave is probabilistic, standing wave absorption can be observed determi-nistically, with nearly unitary probability of coupling a photon into a mode of the material, for example, a localized plasmon when this is a metamaterial excited at the plasmon resonance. These results bring a better understanding of the coherent absorption process, which is of central importance for light harvesting, detection, sensing and photonic data processing applications.