Photonic reservoir computing has been successfully utilized in time-series prediction as the need for hardware implementations has increased. Prediction of chaotic time series remains a significant challenge, an area where the conventional reservoir computing framework encounters limitations of prediction accuracy. We introduce an attention mechanism to the reservoir computing model in the output stage. This attention layer is designed to prioritize distinct features and temporal sequences, thereby substantially enhancing the prediction accuracy. Our results show that a photonic reservoir computer enhanced with the attention mechanism exhibits improved prediction capabilities for smaller reservoirs. These advancements highlight the transformative possibilities of reservoir computing for practical applications where accurate prediction of chaotic time series is crucial.

We present a novel method for the suppression of laser intensity noise and pulse-to-pulse energy fluctuation, based on second-harmonic generation. This method is broadband, passive, has low complexity, and suppresses noise uniformly over the noise spectrum. We theoretically model, analytically analyze, and numerically simulate this technique’s performance for cw and nanosecond pulse lasers with different pulse shapes. We demonstrate broadband relative intensity noise suppression of up to 48 dB, and pulse-to-pulse energy fluctuation suppression of up to 50 dB, and highlight the significance of these findings for extending atom trap storage time, e.g., for quantum information processing.

Visible-band polarization-entangled photon sources (VBPEPSs) are preferred in quantum microscopy, imaging, and light-matter interactions with solid-state quantum memories, $\mathrm{N}$-$V$ centers, and trapped ions. They can be generated directly via spontaneous parametric down-conversion. However, that requires a high-power deep-ultraviolet pump laser. Additionally, the required ultraviolet-band nonlinear crystal is inefficient compared to periodically poled potassium titanyl phosphate or periodically poled lithium niobate crystals. The quantum frequency transduction of a near-infrared polarization-entangled photon source (NIPEPS) provides an effective way to overcome the drawbacks of direct generation. We demonstrate 634-nm VBPEPS generation via polarization-independent frequency transduction of 1080-nm NIPEPS. High-quality entanglement in the visible band is verified via two-photon polarization interference, Bell-inequality, and quantum-state tomography. This method could be applied to other visible wavelengths or be extended to the ultraviolet, which is difficult to generate directly. It has promising applications in quantum imaging, networks, and spectroscopy.

Bound states in the continuum (BICs) defy the conventional wisdom that assumes a spectral separation between propagating waves, that carry energy away, and spatially localized waves corresponding to discrete frequencies. They can be described as resonance states with infinite lifetime, i.e., leaky modes with zero leakage. The advent of metamaterials and nanophotonics allowed the creation of BICs in a variety of systems. Mainly, BICs have been realized by destructive interference between outgoing resonant modes or by exploiting engineered global symmetries that enforce the decoupling of a symmetry-incompatible bound mode from the surrounding radiation modes. Here, we study BICs relying on a different mechanism, namely local symmetries that enforce a field concentration on a part of a complex system without implying any global symmetry. We experimentally implement these BICs using microwaves in a compact one-dimensional photonic network. We demonstrate that such BICs form an infinite ladder in $k$-space and emerge from the annihilation of two topological singularities, a zero and a pole, of the measured scattering matrix. This alternative for achieving BICs in complex wave systems may be useful for applications such as sensing, lasing, and enhancement of nonlinear interactions that require high-$Q$ modes.

Optics and photonics have recently captured interest as a platform to accelerate linear matrix processing, otherwise a bottleneck in traditional digital electronics. In this paper we propose an all-photonic computational accelerator wherein information is encoded in the amplitudes of frequency modes stored in a single ring resonator. Interaction among these modes is enabled by nonlinear optical processes. Both the matrix multiplication and elementwise activation functions on these modes (the artificial neurons) are performed through coherent processes, enabling the direct representation of negative and complex numbers without having to pass through digital electronics, a common limitation in today’s photonic architectures. This design also has a drastically lower hardware footprint compared with today’s electronic and optical accelerators, as the entirety of the matrix multiplication happens in a single multimode resonator on chip. Our architecture is unique in providing a completely unitary, reversible mode of computation, enabling on-chip analog Hamiltonian-echo backpropagation for gradient descent and other self-learning tasks. Moreover, the computational speed increases with the power of the pumps to arbitrarily high rates, as long as the circuitry can sustain the higher optical power. Lastly, the design presented here is a less demanding version of a future room-temperature quantum computational device. Therefore, while this architecture is already viable today, direct reinvestments in it would be enabling its evolution into quantum computational hardware.

Measurement-device-independent quantum key distribution (MDIQKD) can resist all attacks on the detection devices, but there are still some security issues related to the source side. One possible solution is to use the passive protocol to eliminate the side channels introduced by active modulators at the source. Recently, a fully passive QKD protocol was proposed that could simultaneously achieve passive encoding and passive decoy-state modulation using linear optics. In this work, we propose a fully passive MDIQKD scheme that can protect the system from both side channels of source modulators and attacks on the measurement devices, which can significantly improve the implementation security of the QKD systems. We provide a specific passive encoding strategy and a method for decoy-state analysis, followed by simulation results for the secure key rate in the asymptotic scenario. Our work offers a feasible way to improve the implementation security of QKD systems and serves as a reference for achieving passive QKD schemes using realistic devices.

Continuous-variable quantum key distribution protocol using coherent states and heterodyne detection, called the no-switching protocol, is widely used in practical systems due to the simple experimental setup without basis switching and easy assessment to phase information. The security of an ideal no-switching protocol has been proved against general attacks in the finite-size regime and composable security framework, whose heterodyne detector consists of a beam splitter with transmittance of $50\mathrm{\%}$ and two ideal homodyne detectors. However, the transmittance of a beam splitter is inaccurate and the two detectors always have different quantum efficiency and electronic noise, which introduce asymmetry into the heterodyne detection, and further lead to the mismatch between the ideal protocol and practical systems, thereby overestimating the secret key rate and resulting in a practical security loophole. In this paper, we close this loophole by proposing a modified no-switching protocol with biased quadrature detection, where the asymmetry of the heterodyne detection is modeled to match the practical systems, and the security of the protocol is analyzed in asymptotic and finite-size regimes. Further, an optimization strategy is proposed to achieve the optimal secret key rate by adjusting the transmittance of the beam splitter. Simulation results show the necessity of considering the asymmetry in heterodyne detection and the effectiveness of the optimization, which provides a promising way to realize a practical secure and high-performance no-switching system.

A recently proposed fully passive quantum key distribution (QKD) removes all source modulator side channels. In this work, we combine fully passive sources with measurement-device-independent (MDI) QKD to simultaneously remove side channels from source modulators and detectors. We show a numerical simulation of passive MDI QKD, and we obtain an acceptable key rate while achieving much better implementation security, as well as ease of implementation, compared with a recently proposed fully passive twin-field QKD, paving the way towards more secure and practical QKD systems. We prove that a fully passive protocol is compatible with MDI QKD and we propose a novel idea that can improve the sifting efficiency.

Multifunctional integrated optical sensors have attracted considerable attention for their fast response and high precision. However, it remains a challenge to decouple the crosstalk on the multisensory in a single sensor to enhance reliability and practicality. Here, we present a generic strategy for introducing multiple optical parameters, a fluorescence intensity ratio, and a lifetime to monitor alterations of temperature and strain synergistically. An ultrasensitive sensing platform with suitable multiple emissions was constructed by designing luminescent metal halides to realize versatile energy transfer channels between intrinsic self-trapped excitons and dopants’ emission centers. Thus, the flexible fiber was fabricated and committed to monitoring temperature and strain in real-time working conditions with precision and repeatability. Moreover, the decoupling of temperature and strain sensing can be performed by the fluorescence intensity ratio and the lifetime of the ${\mathrm{Mn}}^{2+}$ ions. This work paves the way for emerging luminescent metal halides with multiple emissions applied in advanced multifunctional optical sensors and provides the solution for decoupling the crosstalk on the multisensory to enhance its reliability and practicality.

Recent advances in twisted photonic structures (e.g., twisted bilayer graphene and twisted anisotropic metasurfaces) have enabled many exotic phenomena of light-matter interactions, such as topological transition of the isofrequency contour of surface waves at the magic angle, photonic flatbands, and localization-to-delocalization transition of light. In general, the optical features of these twisted photonic structures are sensitive to the interlayer twist angle. Here we investigate the mode hybridization of surface waves supported by twisted bilayer anisotropic heterometasurfaces, whose two constituent metasurfaces are nonidentical and support surface waves with similar spatial confinements but distinct polarizations. Counterintuitively, we find that the shape of the isofrequency contour of these hybrid surface waves—eigenmodes with both transverse magnetic and transverse electric wave components—can be insensitive to the twist angle, due to the weak interlayer electromagnetic coupling. Moreover, the interlayer distance provides a unique route to reshape the dispersion of one specific kind of hybrid surface wave, while it has negligible influence on the other kinds. Our finding enriches the physics of light-matter interactions in twisted photonic structures and may open new possibilities to mold the flow of light at the subwavelength scale.

Extensive research has been conducted on near-field radiative heat transfer (NFRHT) due to its wide range of applications in energy conversion, radiative cooling, and thermal diodes. The main objective of studying NFRHT at the nanoscale gap is to enhance system performance. This research proposes a new approach to designing and optimizing a near-field thermophotovoltaic (NFTPV) system using deep-learning techniques. Our study utilizes a fully connected network (FCN) and an automated-machine-learning (AutoML) model to simulate radiative heat transfer, aiming to improve radiative heat flux, power generation, and overall system efficiency. By comparing two emitter configurations, we find that the hyperbolic emitter outperforms other configurations, as evidenced by its impact on various system-performance parameters. Significant achievements have been made through our investigations. For the $\mathrm{Si}\mathrm{C}$-plate emitter configuration, we have achieved a notable power density of $0.589\mathrm{W}/{\mathrm{cm}}^2$ and an efficiency of 23% after accounting for nonradiative recombination at a temperature difference of 600 K with a 100-nm gap. Moreover, the four-period emitter configuration has yielded even more impressive results, with a power density of $0.9452\mathrm{W}/{\mathrm{cm}}^2$ and an efficiency of 30% after accounting for nonradiative recombination. This study demonstrates the immense potential of utilizing FCN and AutoML for theoretical modeling and optimization of structural parameters in NFTPV systems, as well as providing an accurate model for predicting photocurrent generation. By highlighting the capabilities of these advanced techniques, we have hopefully paved the way for further advancements and innovations in NFRHT.

Quantum state transmission and quantum information transmission (QIT) through fiber channels hold immense promise for advancing the scope of quantum information applications. However, besides unavoidable transmission loss, channel noise accelerates the decoherence of quantum states and severely limits the transmission distance in practical continuous-variable quantum state transmission. To address the issue of channel noise in metropolitan quantum fiber-optic links, we propose a scheme called dual-channel transferring after interference. This scheme exhibits immunity to additional channel noise while maintaining sufficient communication capacity. Based on this scheme, we experimentally achieve the transmission of a $-5.9$-dB squeezed state at 1.3 $\text{μ}\mathrm{m}$ through 3 km of optical fiber. We obtain a $-3.2$-dB output squeezed state and demonstrate that this decrease is solely attributable to channel loss, independent of channel noise. Incidentally, we achieve an improved measurement sensitivity of guided acoustic-wave Brillouin scattering noise in a fiber channel exploiting squeezed light. In addition, beyond its simplicity of installation and ease of operation, this approach also offers the highest transmission capacity compared with the existing QIT technologies. Our scheme is suitable for almost all modulation-free continuous-variable quantum information protocols, such as quantum secret sharing, quantum entanglement swapping, and some quantum key distribution schemes with entangled states, as well as participating in the construction of future quantum networks.

We propose a terahertz quantum illumination (QI) scheme utilizing emissions from free-electron lasers (FELs) based on joint photon number measurement and quantum state discrimination using maximum a posteriori estimation. We analyze the performance of our QI scheme and quantify it with error probabilities. We numerically demonstrate the quantum advantage of our QI scheme over classical illumination. Our work paves the way to using FELs as entangled photon sources for quantum illumination in the terahertz regime.

We introduce a feasible scheme to produce high-rate long-distance entanglement that uses hybrid entanglement between continuous variables (CVs) and discrete variables (DVs). We show that hybrid entanglement can effectively remove the experimental limitations of existing CV and DV systems to produce long-range entanglement. We benchmark the resulting DV entangled states using an entanglement-based-quantum-key-distribution protocol. We show, using hybrid entangled states, that entanglement-based quantum key distribution is possible with standard telecommunication fibers for 300 km. The key idea is the use of the CV part, which can be adjusted to be robust regarding photon losses, for increasing the transmission distance, and the use of the DV part for achieving high secure key rates. Our results show that hybrid entangled states provide a clear advantage for practical generation of long-distance and high-rate entanglement that may lead to further applications in quantum information processing.

We report on the design, fabrication and performance of novel flat-optical spin-orbit modal beam shapers made from silica glass by femtosecond laser nanostructuring technology, choosing the Laguerre-Gauss basis as a case study. The basic design principle involves endowing a transparent dielectric material, initially isotropic and homogeneous, with doubly inhomogeneous anisotropic characteristics (optical axis orientation and birefringence). The quantitative structural characterization of the fabricated devices is achieved by Stokes polarimetric analysis and the modal beam shaping performance is assessed via modal decomposition. The modal beam shaping capabilities obtained outperform those of previously produced spin-orbit beam shapers based on the same fabrication technology. Also, current challenges and perspectives associated with high-order and spectrally broadband modal beam shaping are discussed. More generally, the proposed approach extends the scope of spin-orbit optical elements to any situation where complex field amplitude shaping is required in a single stage.

In the context of electromagnetic absorption, it is obvious that for an infinite planar periodic structure illuminated by a plane wave the maximum attainable absorptance, i.e., perfect absorption, is theoretically limited to 100% of the incident power. Here we show that an intriguing possibility of overcoming this limit arises in finite-sized resonant absorbing arrays. We present a comprehensive analysis of a simple two-dimensional strip array over an infinite perfectly conducting plane, where the strips are loaded with bulk-impedance loads. The absorptance is defined as the ratio of the dissipated power per unit length of the strips to the incident power on a unit length of the array width. The results show that even regular subwavelength arrays of impedance strips can slightly overcome the limit of 100% absorptance, while with use of aperiodic arrays with optimized loads, absorptance can be significantly increased as compared with the scenario where the strips are identical. In principle, by tuning of the bulk loads, high superunity absorptance can be realized for all angles of illumination.

Integrated Kerr quantum frequency combs (QFCs), with the potential to generate multiple and scalable quantum states, have emerged as a compact, stable, and fundamental resource for broadband entanglement. Here, we build a platform for designing bipartite entangled QFCs via an on-chip silicon nitride microring resonator. By establishing the system quantum dynamics of Kerr nonlinear microresonators, our platform can support up to 12 continuous-variable quantum modes, in the form of six simultaneously two-mode squeezed pairs under the influence of hysteresis. The entanglement degree of frequency-mode pairs is dependent on the resonator structure and ambient temperature. By adjusting the cavity temperature, we can optimize the performance of entanglement under certain injected pump power and pump detuning. Our comprehensive QFC designing process and entanglement distribution control may ameliorate the generation and optimization of entanglement.