Contents
2021
Volume: 9 Issue 12
22 Article(s)

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Research Articles
Fiber Optics and Optical Communications
Underwater photon-inter-correlation optical communication
Zeng-Quan Yan, Cheng-Qiu Hu, Zhan-Ming Li, Zhong-Yuan Li, Hang Zheng, and Xian-Min Jin
High-capacity, long-distance underwater optical communication enables a global scale optical network covering orbit, land, and water. Underwater communication using photons as carriers has a high channel capacity; however, the light scattering and absorption of water lead to an inevitable huge channel loss, setting an insurmountable transmission distance for existing underwater optical communication technologies. Here, we experimentally demonstrate the photon-inter-correlation optical communication (PICOC) in air–water scenarios. We retrieve additional internal correlation resources from the sparse single-photon stream with high fidelity. We successfully realize the 105-m-long underwater optical communication against a total loss up to 120.1 dB using only a microwatt laser. The demonstrated underwater light attenuation is equivalent to the loss of 883-m-long Jerlov type I water, encouraging the practical air–water optical communication to connect deeper underwater worlds.
Photonics Research
  • Publication Date: Nov. 05, 2021
  • Vol.9 Issue, 12 12002360 (2021)
Holography, Gratings, and Diffraction
High tolerance detour-phase graphene-oxide flat lens
Shibiao Wei, Guiyuan Cao, Han Lin, Haoran Mu, Wenbo Liu, Xiaocong Yuan, Michael Somekh, and Baohua Jia
Photonics Research
  • Publication Date: Nov. 15, 2021
  • Vol.9 Issue, 12 12002454 (2021)
Dual-wavelength in-line digital holography with untrained deep neural networks
Chen Bai, Tong Peng, Junwei Min, Runze Li, Yuan Zhou, and Baoli Yao
Dual-wavelength in-line digital holography (DIDH) is one of the popular methods for quantitative phase imaging of objects with non-contact and high-accuracy features. Two technical challenges in the reconstruction of these objects include suppressing the amplified noise and the twin-image that respectively originate from the phase difference and the phase-conjugated wavefronts. In contrast to the conventional methods, the deep learning network has become a powerful tool for estimating phase information in DIDH with the assistance of noise suppressing or twin-image removing ability. However, most of the current deep learning-based methods rely on supervised learning and training instances, thereby resulting in weakness when it comes to applying this training to practical imaging settings. In this paper, a new DIDH network (DIDH-Net) is proposed, which encapsulates the prior image information and the physical imaging process in an untrained deep neural network. The DIDH-Net can effectively suppress the amplified noise and the twin-image of the DIDH simultaneously by automatically adjusting the weights of the network. The obtained results demonstrate that the proposed method with robust phase reconstruction is well suited to improve the imaging performance of DIDH.
Photonics Research
  • Publication Date: Nov. 30, 2021
  • Vol.9 Issue, 12 12002501 (2021)
Imaging Systems, Microscopy, and Displays
Fluorescent nanodiamonds for characterization of nonlinear microscopy systems
Mantas Žurauskas, Aneesh Alex, Jaena Park, Steve R. Hood, and Stephen A. Boppart
Characterizing the performance of fluorescence microscopy and nonlinear imaging systems is an essential step required for imaging system optimization and quality control during longitudinal experiments. Emerging multimodal nonlinear imaging techniques require a new generation of microscopy calibration targets that are not susceptible to bleaching and can provide a contrast across the multiple modalities. Here, we present a nanodiamond-based calibration target for microscopy, designed for facilitating reproducible measurements at the object plane. The target is designed to support day-to-day instrumentation development efforts in microscopy laboratories. The images of a phantom contain information about the imaging performance of a microscopy system across multiple spectral windows and modalities. Since fluorescent nanodiamonds are not prone to bleaching, the proposed imaging target can serve as a standard, shelf-stable sample to provide rapid reference measurements for ensuring consistent performance of microscopy systems in microscopy laboratories and imaging facilities.
Photonics Research
  • Publication Date: Nov. 02, 2021
  • Vol.9 Issue, 12 12002309 (2021)
Single-shot real-time compressed ultrahigh-speed imaging enabled by a snapshot-to-video autoencoder
Xianglei Liu, João Monteiro, Isabela Albuquerque, Yingming Lai, Cheng Jiang, Shian Zhang, Tiago H. Falk, and Jinyang Liang
Single-shot 2D optical imaging of transient scenes is indispensable for numerous areas of study. Among existing techniques, compressed optical-streaking ultrahigh-speed photography (COSUP) uses a cost-efficient design to endow ultrahigh frame rates with off-the-shelf CCD and CMOS cameras. Thus far, COSUP’s application scope is limited by the long processing time and unstable image quality in existing analytical-modeling-based video reconstruction. To overcome these problems, we have developed a snapshot-to-video autoencoder (S2V-AE)—which is a deep neural network that maps a compressively recorded 2D image to a movie. The S2V-AE preserves spatiotemporal coherence in reconstructed videos and presents a flexible structure to tolerate changes in input data. Implemented in compressed ultrahigh-speed imaging, the S2V-AE enables the development of single-shot machine-learning assisted real-time (SMART) COSUP, which features a reconstruction time of 60 ms and a large sequence depth of 100 frames. SMART-COSUP is applied to wide-field multiple-particle tracking at 20,000 frames per second. As a universal computational framework, the S2V-AE is readily adaptable to other modalities in high-dimensional compressed sensing. SMART-COSUP is also expected to find wide applications in applied and fundamental sciences.
Photonics Research
  • Publication Date: Nov. 15, 2021
  • Vol.9 Issue, 12 12002464 (2021)
Integrated Optics
Broadband adiabatic polarization rotator-splitter based on a lithium niobate on insulator platform | Editors' Pick
Zhaoxi Chen, Jingwei Yang, Wing-Han Wong, Edwin Yue-Bun Pun, and Cheng Wang
Polarization rotator-splitters (PRSs) are crucial components for controlling the polarization states of light in classical and quantum communication systems. We design and experimentally demonstrate a broadband PRS based on the lithium niobate on insulator (LNOI) platform. Both the rotator and splitter sections are based on adiabatically tapered waveguide structures, and the whole device only requires a single etching step. We show efficient PRS operation over an experimentally measured bandwidth of 130 nm at telecom wavelengths, potentially as wide as 500 nm according to simulation prediction, with relatively low polarization crosstalks of ∼-10 dB. Our PRS is highly compatible with the design constraints and fabrication processes of common LNOI photonic devices, and it could become an important element in future LNOI photonic integrated circuits.
Photonics Research
  • Publication Date: Nov. 05, 2021
  • Vol.9 Issue, 12 12002319 (2021)
Wide-steering-angle high-resolution optical phased array
Yingzhi Li, Baisong Chen, Quanxin Na, Qijie Xie, Min Tao, Lanxuan Zhang, Zihao Zhi, Yuxuan Li, Xiaobin Liu, Xianshu Luo, Guoqiang Lo, Fengli Gao, Xueyan Li, and Junfeng Song
Optical phased array (OPA) technology is considered a promising solution for solid-state beam steering to supersede the traditional mechanical beam steering. As a key component of the LIDAR system for long-range detection, OPAs featuring a wide steering angle and high resolution without beam aliasing are highly desired. However, a wide steering range requires a waveguide pitch less than half of the wavelength, which is easily subjected to cross talk. Besides, high resolution requires a large aperture, and it is normally achieved by a high count number of waveguides, which complicates the control system. To solve the mentioned issues, we design two high-performance 128-channel OPAs fabricated on a multilayered SiN-on-SOI platform. Attributed to the nonuniform antenna pitch, only 128 waveguides are used to achieve a 4 mm wide aperture. Besides, by virtue of innovative dual-level silicon nitride (Si3N4) waveguide grating antennas, the fishbone antenna OPA achieves a 100°×19.4° field of view (FOV) with divergence of 0.021°×0.029°, and the chain antenna OPA realizes a 140°×19.23° FOV with divergence of 0.021°×0.1°. To our best knowledge, 140° is the widest lateral steering range in two-dimensional OPA, and 0.029° is the smallest longitudinal divergence. Finally, we embed the OPA into a frequency-modulated continuous-wave system to achieve 100 m distance measurement. The reflected signal from 100 m distance is well detected with 26 dBm input transmitter power, which proves that OPA serves as a promising candidate for transceiving optical signal in a LIDAR system.
Photonics Research
  • Publication Date: Nov. 30, 2021
  • Vol.9 Issue, 12 12002511 (2021)
Medical Optics and Biotechnology
Single-cell detection by enhancement of fluorescence in waveguides for cancer diagnosis and therapy
Hailang Dai, Hongrui Shan, Zhangchi Sun, Daopeng Dai, Yuxi Shang, Zhuangqi Cao, and Xianfeng Chen
Cancer is one of the most common diseases to threaten human health. If individuals are diagnosed with malignant tumors via a single cell, medical workers are greatly advantageous to early diagnose and intervene in malignant tumors therapy. In this paper, we propose a fluorescence detection map to rapidly distinguish whether the chromosomes of a cell are normal or abnormal by detecting the fluorescent intensity of a single cell. Herein, we draw a map from a single cell with an abnormal number of chromosomes that is monitored in real time. Moreover, this way offers precise and prompt detection of the surviving of cancer cells at or near the site of the tumor after treatments for cancer, which can achieve personalized cancer diagnosis and therapy. Therefore, cancer recurrences and metastasis can be effectively identified, utilizing this ultrasensitive detection method of an abnormal chromosome number.
Photonics Research
  • Publication Date: Nov. 12, 2021
  • Vol.9 Issue, 12 12002381 (2021)
Nanophotonics and Photonic Crystals
Synthetic aperture metalens | On the Cover
Feng Zhao, Zicheng Shen, Decheng Wang, Bijie Xu, Xiangning Chen, and Yuanmu Yang
Metalenses are ultrathin optical elements that can focus light using densely arranged subwavelength structures. Due to their minimal form factor, they have been considered promising for imaging applications that require extreme system size, weight, and power, such as in consumer electronics and remote sensing. However, as a major impediment prohibiting the wide adoption of the metalens technology, the aperture size, and consequently the imaging resolution, of a metalens are often limited by lithography processes that are not scalable. Here, we propose to adopt a synthetic aperture approach to alleviate the issue, and experimentally demonstrate that, assisted by computational reconstruction, a synthetic aperture metalens composed of multiple metalenses with relatively small aperture size can achieve an imaging resolution comparable to a conventional lens with an equivalent large aperture. We validate the concept via an outdoor imaging experiment performed with a synthetic aperture metalens-integrated near-infrared camera using natural sunlight for target illumination.
Photonics Research
  • Publication Date: Nov. 12, 2021
  • Vol.9 Issue, 12 12002388 (2021)
Hybrid photonic-plasmonic cavities based on the nanoparticle-on-a-mirror configuration
Angela I. Barreda, Mario Zapata-Herrera, Isabelle M. Palstra, Laura Mercadé, Javier Aizpurua, A. Femius Koenderink, and Alejandro Martínez
Hybrid photonic-plasmonic cavities have emerged as a new platform to increase light–matter interaction capable to enhance the Purcell factor in a singular way not attainable with either photonic or plasmonic cavities separately. In the hybrid cavities proposed so far, the plasmonic element is usually a metallic bow-tie antenna, so the plasmonic gap—defined by lithography—is limited to minimum values of several nanometers. Nanoparticle-on-a-mirror (NPoM) cavities are far superior to achieve the smallest possible mode volumes, as plasmonic gaps smaller than 1 nm can be created. Here, we design a hybrid cavity that combines an NPoM plasmonic cavity and a dielectric-nanobeam photonic crystal cavity operating at transverse-magnetic polarization. The metallic nanoparticle can be placed very close (1 nm) to the upper surface of the dielectric cavity, which acts as a low-reflectivity mirror. We demonstrate through numerical calculations of the local density of states that this hybrid plasmonic-photonic cavity exhibits quality factors Q above 103 and normalized mode volumes V down to 10-3, thus resulting in high Purcell factors (FP≈105), while being experimentally feasible with current technology. Our results suggest that hybrid cavities with sub-nanometer gaps should open new avenues for boosting light–matter interaction in nanophotonic systems.
Photonics Research
  • Publication Date: Oct. 27, 2021
  • Vol.9 Issue, 12 12002398 (2021)
Nonlinear Optics
Femtosecond nonlinear losses in multimode optical fibers
Mario Ferraro, Fabio Mangini, Mario Zitelli, Alessandro Tonello, Antonio De Luca, Vincent Couderc, and Stefan Wabnitz
Multimode optical fibers are attracting a growing interest for their capability to transport high-power laser beams, coupled with novel nonlinear optics-based applications. However, optical fiber breakdown occurs when beam intensities exceed a certain critical value. Optical breakdown associated with irreversible modifications of the refractive index, triggered by multiphoton absorption, has been largely exploited for fiber material micro-structuration. Here we show that, for light beam intensities slightly below the breakdown threshold, nonlinear absorption strongly affects the dynamics of a propagating beam as well. We experimentally analyze this subthreshold regime and highlight the key role played by spatial self-imaging in graded-index fibers for enhancing nonlinear optical losses. We characterize the nonlinear power transmission properties of multimode fibers for femtosecond pulses propagating in the near-infrared spectral range. We show that an effective N-photon absorption analytical model is able to describe the experimental data well.
Photonics Research
  • Publication Date: Nov. 15, 2021
  • Vol.9 Issue, 12 12002443 (2021)
Optical and Photonic Materials
Enhanced multimodal luminescence and ultrahigh stability Eu3+-doped CsPbBr3 glasses for X-ray detection and imaging | Spotlight on Optics
Yao Tong, Qin Wang, Heng Yang, Xiaoting Liu, Enrou Mei, Xiaojuan Liang, Zhijun Zhang, and Weidong Xiang
As an emerging scintillation material, metal halide perovskite (CsPbX3) has been deemed the most potentially valuable candidate in X-ray detection and medical imaging. Nevertheless, it is a continuing challenge to implement efficient radioluminescence (RL) with high radiation stability and moisture resistance. Moreover, the optimized luminescence properties and excellent uniformity of CsPbX3 glass are also key points for obtaining perfect X-ray images. Herein, we have successfully precipitated Eu3+-doped CsPbBr3 nanocrystals (NCs) with improved photoluminescence quantum yield (≈58.6%) because partial Eu3+ entered the perovskite lattice in a robust borosilicate glass matrix by in situ crystallization. The small amount of Eu addition made the lattice of NCs shrink and promoted uniform distribution of CsPbBr3 NCs in the glass, which effectively reduced the light scattering of the sample. Subsequently, multimodal RL intensity of the CsPbBr3/CsPbBr3:xEu NCs glasses (CPB-0Eu/CPB-xEu) as a function of X-ray dose rate showed a superlinear relationship to the benefit of obtaining satisfactory X-ray images. Also, the outstanding radiation stability and water resistance of CPB-xEu were confirmed due to the protection of the robust glass matrix. Finally, an X-ray imaging system using a CPB-xEu scintillator was constructed, and the spring in the opaque sample was legibly detected under the motivation of X-rays, indicating that CsPbX3 glasses possess extensive application prospects in terms of X-ray detection and medical imaging.
Photonics Research
  • Publication Date: Nov. 12, 2021
  • Vol.9 Issue, 12 12002369 (2021)
Optical Devices
Bifunctional ultraviolet light-emitting/detecting device based on a SnO2 microwire/p-GaN heterojunction
Tong Xu, Mingming Jiang, Peng Wan, Kai Tang, Daning Shi, and Caixia Kan
SnO2 has attracted considerable attention due to its wide bandgap, large exciton binding energy, and outstanding electrical and optoelectronic features. Owing to the lack of reliable and reproducible p-type SnO2, many challenges on developing SnO2-based optoelectronic devices and their practical applications still remain. Herein, single-crystal SnO2 microwires (MWs) are acquired via the self-catalyzed approach. As a strategic alternative, n-SnO2 MW/p-GaN heterojunction was constructed, which exhibited selectable dual-functionalities of light-emitting and photodetection when operated by applying an appropriate voltage. The device illustrated a distinct near-ultraviolet light-emission peaking at ∼395.0 nm and a linewidth ∼50 nm. Significantly, the device characteristics, in terms of the main peak positions and linewidth, are nearly invariant as functions of various injection current, suggesting that quantum-confined Stark effect is essentially absent. Meanwhile, the identical n-SnO2 MW/p-GaN heterojunction can also achieve photovoltaic-type light detection. The device can steadily feature ultraviolet photodetecting ability, including the ultraviolet/visible rejection ratio (R360 nm/R400 nm) ∼1.5×103, high photodark current ratio of 105, fast response speed of 9.2/51 ms, maximum responsivity of 1.5 A/W, and detectivity of 1.3×1013 Jones under 360 nm light at -3 V bias. Therefore, the bifunctional device not only displays distinct near-ultraviolet light emission, but also has the ability of high-sensitive ultraviolet photodetection. The novel design of n-SnO2 MW/p-GaN heterojunction bifunctional systems is expected to open doors to practical application of SnO2 microstructures/nanostructures for large-scale device miniaturization, integration and multifunction in next-generation high-performance photoelectronic devices.
Photonics Research
  • Publication Date: Nov. 18, 2021
  • Vol.9 Issue, 12 12002475 (2021)
Optoelectronics
CsPbBr3 perovskite quantum-dot paper exhibiting a highest 3 dB bandwidth and realizing a flexible white-light system for visible-light communication | Editors' Pick
Konthoujam James Singh, Xiaotong Fan, Annada Sankar Sadhu, Chun-Ho Lin, Fang-Jyun Liou, Tingzhu Wu, Yu-Jung Lu, Jr-Hau He, Zhong Chen, Tom Wu, and Hao-Chung Kuo
We propose a flexible white-light system for high-speed visible-light communication (VLC) applications, which consists of a semipolar blue InGaN/GaN single-quantum-well micro-light-emitting diode (LED) on a flexible substrate pumping green CsPbBr3 perovskite quantum-dot (PQD) paper in nanostructure form and red CdSe QD paper. The highest bandwidth for CsPbBr3 PQD paper, 229 MHz, is achieved with a blue micro-LED pumping source and a high data transmission rate of 400 Mbps; this is very promising for VLC application. An 817 MHz maximum bandwidth and a 1.5 Gbps transmission speed are attained by the proposed semipolar blue micro-LEDs. The proposed flexible white light system and the high-bandwidth PQD paper could pave the way for VLC wearable devices.
Photonics Research
  • Publication Date: Nov. 05, 2021
  • Vol.9 Issue, 12 12002341 (2021)
Sensitive direct-conversion X-ray detectors formed by ZnO nanowire field emitters and β-Ga2O3 photoconductor targets with an electron bombardment induced photoconductivity mechanism
Zhipeng Zhang, Manni Chen, Xinpeng Bai, Kai Wang, Huanjun Chen, Shaozhi Deng, and Jun Chen
Sensitive X-ray detection is needed in diverse areas motivated by a common desire to reduce radiation dose. Cold cathode X-ray detectors operating with a photoelectron multiplication mechanism called electron bombardment induced photoconductivity (EBIPC) have emerged as promising candidates for low-dose X-ray detection. Herein, the cold cathode detectors formed by ZnO nanowire field emitters and β-Ga2O3 photoconductor targets were proposed for sensitive direct-conversion X-ray detection. The charge carrier transport mechanism of EBIPC effect in X-ray detectors was investigated to achieve a high internal gain (2.9×102) and high detection sensitivity (3.0×103 μCGyair-1 cm-2) for a 6 keV X-ray at the electric field of 22.5 V μm-1. Furthermore, the proposed X-ray detectors showed the features of fast response time (40 ms), long-term stability (0.6% for 1 h), and low detection limit (0.28 mGyair s-1), suggesting that the direct-conversion cold cathode X-ray detectors are ideal candidates for low-energy X-ray detecting and imaging applications.
Photonics Research
  • Publication Date: Nov. 12, 2021
  • Vol.9 Issue, 12 12002420 (2021)
Ultrasmall and ultradense InGaN-based RGB monochromatic micro-light-emitting diode arrays by pixilation of conductive p-GaN
Zhe Zhuang, Daisuke Iida, and Kazuhiro Ohkawa
We describe 5 μm squircle InGaN-based red, green, and blue (RGB) monochromatic micro-light-emitting diodes (μLEDs) with an interpitch of 4 μm by pixilation of conductive p-GaN using a H2-plasma treatment. The p-GaN was passivated by H2 plasma and prevented the current’s injection into the InGaN quantum wells below. We observed that InGaN-based red μLEDs exhibited a broader full width at half-maximum and larger peak wavelength blueshift at 11.5–115 A/cm2 than the green/blue μLEDs. The on-wafer light output power density of the red μLEDs at a wavelength of 632 nm at 115 A/cm2 was approximately 936 mW/cm2, the highest value reported thus far for InGaN-based red μLEDs. This value was comparable with that of the green/blue μLEDs at 11.5 A/cm2, indicating that the red μLEDs can satisfy the requirement of high brightness levels for specific displays. The color gamut based on InGaN RGB μLEDs covered 83.7% to 75.9% of the Rec. 2020 color space in the CIE 1931 diagram at 11.5 to 115 A/cm2.
Photonics Research
  • Publication Date: Nov. 12, 2021
  • Vol.9 Issue, 12 12002429 (2021)
Low-latency full-field temporal magnification based on spectral compression
Xiangzhi Xie, Jilong Li, Feifei Yin, Kun Xu, and Yitang Dai
Temporal magnification is an emerging technology for the observation of single-shot optical signals with irregular and ultrafast dynamics, which exceed the speed, precision, and record length of conventional digitizers. Conventional temporal magnification schemes suffer from transmission delay and large volume of dispersive elements. Because only the signal envelope can be magnified in the dispersion-based schemes, real-time full-field (phase and amplitude) measurement for a complex ultrafast optical signal remains an open challenge. Here, a bandwidth-compressed temporal magnification scheme for low-latency full-field measurements of ultrafast dynamics is proposed. Unlike the dispersion-based schemes, temporal magnification of a complex optical signal is achieved by bandwidth compression. The bandwidth is coherently compressed by the Vernier effect relying on the detuned free spectral range of a periodic optical filter and time lens. Experimentally, a temporal magnification factor of 224 is realized, and full-field measurements for picosecond pulses are demonstrated. The proposal eliminates the dependence on dispersive elements and shows great potential in integration, which may pave a new path toward full-field measurement for nonrepetitive and statistically rare signals.
Photonics Research
  • Publication Date: Nov. 23, 2021
  • Vol.9 Issue, 12 12002494 (2021)
Physical Optics
Unconventional Weyl exceptional contours in non-Hermitian photonic continua
Qinghui Yan, Qiaolu Chen, Li Zhang, Rui Xi, Hongsheng Chen, and Yihao Yang
Unconventional Weyl points with topological charges higher than 1 can transform into various complex unconventional Weyl exceptional contours under non-Hermitian perturbations. However, theoretical studies of these exceptional contours have been limited to tight-binding models. Here, we propose to realize unconventional Weyl exceptional contours in photonic continua—non-Hermitian anisotropic chiral plasma, based on ab initio calculation by Maxwell’s equations. By perturbing in-plane permittivity, an unconventional Weyl point can transform into a quadratic Weyl exceptional ring, a type I Weyl exceptional chain with one chain point, a type II Weyl exceptional chain with two chain points, or other forms. Realistic metamaterials with effective constitutive parameters are proposed to implement these unconventional Weyl exceptional contours. Our work paves a way toward exploration of exotic physics of unconventional Weyl exceptional contours in non-Hermitian topological photonic continua.
Photonics Research
  • Publication Date: Nov. 15, 2021
  • Vol.9 Issue, 12 12002435 (2021)
Theory of light propagation in arbitrary two-dimensional curved space
Chenni Xu, and Li-Gang Wang
As an analog model of general relativity, optics on some two-dimensional (2D) curved surfaces has received increasing attention in the past decade. Here, in light of the Huygens–Fresnel principle, we propose a theoretical frame to study light propagation along arbitrary geodesics on any 2D curved surfaces. This theory not only enables us to solve the enigma of “infinite intensity” that existed previously at artificial singularities on surfaces of revolution but also makes it possible to study light propagation on arbitrary 2D curved surfaces. Based on this theory, we investigate the effects of light propagation on a typical surface of revolution, Flamm’s paraboloid, as an example, from which one can understand the behavior of light in the curved geometry of Schwarzschild black holes. Our theory provides a convenient and powerful tool for investigations of radiation in curved space.
Photonics Research
  • Publication Date: Nov. 18, 2021
  • Vol.9 Issue, 12 12002486 (2021)
Quantum Optics
Visualization of magnetic fields with cylindrical vector beams in a warm atomic vapor
Shuwei Qiu, Jinwen Wang, Francesco Castellucci, Mingtao Cao, Shougang Zhang, Thomas W. Clark, Sonja Franke-Arnold, Hong Gao, and Fuli Li
Photonics Research
  • Publication Date: Nov. 05, 2021
  • Vol.9 Issue, 12 12002325 (2021)
Experimental realization of a quantum image classifier via tensor-network-based machine learning | Editors' Pick
Kunkun Wang, Lei Xiao, Wei Yi, Shi-Ju Ran, and Peng Xue
Quantum machine learning aspires to overcome intractability that currently limits its applicability to practical applications. However, quantum machine learning itself is limited by low effective dimensions achievable in state-of-the-art experiments. Here, we demonstrate highly successful classifications of real-life images using photonic qubits, combining a quantum tensor-network representation of hand-written digits and entanglement-based optimization. Specifically, we focus on binary classification for hand-written zeroes and ones, whose features are cast into the tensor-network representation, further reduced by optimization based on entanglement entropy and encoded into two-qubit photonic states. We then demonstrate image classification with a high success rate exceeding 98%, through successive gate operations and projective measurements. Although we work with photons, our approach is amenable to other physical realizations such as nitrogen-vacancy centers, nuclear spins, and trapped ions, and our scheme can be scaled to efficient multi-qubit encodings of features in the tensor-product representation, thereby setting the stage for quantum-enhanced multi-class classification.
Photonics Research
  • Publication Date: Nov. 05, 2021
  • Vol.9 Issue, 12 12002332 (2021)
Frequency-multiplexed entanglement for continuous-variable quantum key distribution
Olena Kovalenko, Young-Sik Ra, Yin Cai, Vladyslav C. Usenko, Claude Fabre, Nicolas Treps, and Radim Filip
Quantum key distribution with continuous variables already uses advantageous high-speed single-mode homodyne detection with low electronic noise at room temperature. Together with continuous-variable information encoding to nonclassical states, the distance for secure key transmission through lossy channels can approach 300 km in current optical fibers. Such protocols tolerate higher channel noise and also limited data processing efficiency compared to coherent-state protocols. The secret key rate can be further increased by increasing the system clock rates, and, further, by a suitable frequency-mode-multiplexing of optical transmission channels. However, the multiplexed modes couple together in the source or any other part of the protocol. Therefore, multiplexed communication will experience cross talk and the gain can be minuscule. Advantageously, homodyne detectors allow solving this cross-talk problem by proper data processing. It is a potential advantage over protocols with single-photon detectors, which do not enable similar data processing techniques. We demonstrate the positive outcome of this methodology on the experimentally characterized frequency-multiplexed entangled source of femtosecond optical pulses with natural cross talk between eight entangled pairs of modes. As the main result, we predict the almost 15-fold higher secret key rate. This experimental test and analysis of frequency-multiplexed entanglement source open the way for the field implementation of high-capacity quantum key distribution with continuous variables.
Photonics Research
  • Publication Date: Nov. 05, 2021
  • Vol.9 Issue, 12 12002351 (2021)

About the Cover

A synthetic aperture metalens composed of multiple metalenses with relatively small aperture size can achieve an imaging resolution comparable to a conventional lens with an equivalent large aperture. This concept may allow the construction of metalens on the meter or even larger scale.