Over recent decades, quantum key distribution (QKD) has arisen as a promising solution for secure communications; however, like all cryptographic protocols, implementations of QKD can open security vulnerabilities. Until now, the study of physical vulnerabilities in QKD setups has focused primarily on the optical channel. In classical cryptoanalysis, power and electromagnetic side-channel analysis are powerful techniques that can be used to access secret information about the encryption key in symmetric-key algorithms. Such attacks have rarely been used in QKD, since they require an eavesdropper to have access to Alice’s or Bob’s setup; however, security proofs of QKD protocols generally assume that these setups are secure, making it crucial to understand the security measures required to ensure this protection. In this work, we propose and implement a power side-channel analysis of a QKD system by exploiting the power consumption of the electronic driver controlling the electro-optical components of the QKD transmitter. QKD modules typically require very precise electronic drivers, such as field-programmable gate arrays (FPGAs). Here, we show that the FPGA’s power consumption can leak information about the QKD operation and consequently the transmitted key. Our results are consistent and show critical information leakage, having reached a maximum accuracy of 73.35% in predicting transmitted qubits at a 100-MHz repetition frequency. We also discuss possible countermeasures to prevent such an attack.

Here, we show that up-converting nanoparticles are able to function as stochastic non-Poissonian emitters suitable for superresolution optical fluctuation imaging. We uncover ranges of parameters where up-converting media with emitters such as rare-earth ions can exhibit nonexponential statistics of photon emission times. We also show that photon bunching can be induced by interactions between emitters. This effect can be used for a stochastic imaging modality in which only the contributions of interacting emitters to the cumulant image are enhanced. We also develop a method to use spatial cross-correlations of the registered field to enhance resolution.

Entangled photon pairs are of crucial importance in quantum networks. For the future demands of large-scale and secure quantum communication, integrated photon sources are highly effective solutions. Here, we report entanglement distribution over 30 km in a metropolitan area using an on-chip broadband silicon nanowire biphoton polarization-entangled source based on a silicon-on-insulator platform. This source generates a continuous spectrum spanning the entire C band (4.5 THz), achieving locally detected coincidence counts of about 154 kHz within a 100-GHz bandwidth, making it suitable for long-distance entanglement distribution among multiple users. By combining this source with quantum entanglement, enhanced by high-precision clock synchronization that achieves a standard deviation of 56.8 ps over 600 s, we observe a violation of the Clauser-Horne-Shimony-Holt inequality by 27.8 standard deviations. Our results showcase the potential of silicon photonic technology as a scalable and practical platform for quantum technologies.

Ultrafast dynamics of magnetic properties in the high gigahertz (GHz) or even terahertz (THz) frequency range may be exploited in next-generation data storage and processing devices, and therefore suitable methods for detecting ultrafast dynamics of magnetization need to be developed. In this work, we investigate the impact of optically induced ultrafast precession of magnetization on the anomalous Hall conductivity in a ferromagnetic $\mathrm{Co}\text{−}\mathrm{Pt}$ multilayer structure using THz-based pump-probe measurements. From the measured Faraday rotation, the time-dependent anomalous Hall conductivity ${\sigma }_{xy}$ is extracted, and our data reveals an initial ultrafast quench due to laser-induced demagnetization and a subsequent oscillation with a frequency that increases with the strength of the applied external magnetic field ${\mathbf{\text{H}}}_{\mathrm{ext}}$. for $H_{\mathrm{ext}}$= 30 kOe, the oscillation frequency reaches approximately 100 GHz. The magnetic-field dependence of the oscillation frequency is in good agreement with the prediction from the ferromagnetic resonance model, confirming that the observed oscillation is caused by coherent magnetization precession. These findings are considered to be useful for studies that aim to integrate spintronics and THz electronics.

Establishing quantum entanglement between pairs of distant quantum memories (QMs) is essential for numerous applications in quantum networks. In this paper, we propose some heralded schemes for entanglement generation between multiple pairs of QMs placed in long-distance separated nodes, where the qubits are encoded on QMs coupled to a one-dimensional waveguide. By distributing a pair of spatial-polarization hyperentangled photons to two distant nodes, where each photon scatters with QMs, we can generate entanglement between multiple pairs of distant QMs. In our schemes, the faulty scattering events can be filtered out and discarded via detecting the polarization states of scattered photon components within each module QM. Moreover, the responses from single-photon detectors placed at the end of each node enable us to obtain direct entangled states between QMs in a heralded manner. Eventually, we theoretically extend the proposed schemes by demonstrating specific regularities that arise when entangling more pairs of QMs. With recent advancements in hyperentanglement generation, our schemes become increasingly feasible for practical implementation.

Quantum emitters embedded in photonic integrated circuit (PIC) cavities offer a powerful and scalable platform for label-free refractive-index sensing at the nanoscale. We propose and theoretically analyze a sensing mechanism based on Purcell-enhanced modulation of the emitter’s spontaneous-emission lifetime, enabling detection of refractive-index changes via time-correlated single-photon counting (TCSPC). In contrast to traditional resonance-shift sensors, our approach exploits the lifetime sensitivity to variations in the local density of optical states, providing a sensing modality that is intensity independent, spectrally unresolvable, and potentially compatible with complementary metal-oxide-semiconductor technology. We derive analytical expressions linking refractive-index perturbations to relative lifetime shifts and identify an optimal off-resonance operation regime where the lifetime response becomes linear and maximally sensitive to small perturbations. As a PIC material, silicon offers seamless integration of single-photon avalanche detectors suitable for TCSPC. We therefore illustrate the applicability of the proposed mechanism for quantum emitters in silicon. Parametric evaluation of the analytical expressions shows that, for moderate-quality-factor photonic cavities ($Q={10}^5$–${10}^7$), this method enables refractive-index detection limits as low as ${10}^{-9}$ refractive index units—competitive with or even outperforming state-of-the-art plasmonic and microresonator sensors yet with significantly simpler instrumentation. Furthermore, long-lived emitters such as T centers in silicon offer a unique advantage, allowing subnanosecond lifetime shifts to be resolved with standard TCSPC systems. Although room-temperature operation of quantum emitters in silicon has yet to be demonstrated, our results lay the theoretical foundation for scalable, room-temperature, quantum-enabled refractive-index sensing—eliminating the need for spectral resolution and cryogenic infrastructure. Given the generic nature of the proposed approach, and with ongoing advances in PIC technologies such as diamond-on-silicon, hybrid silicon-silicon nitride, and silicon-silicon carbide platforms, the method can be readily extended to materials in which room-temperature operation of quantum emitters has already been experimentally demonstrated.

Quantum random-number generators (QRNGs) produce true random numbers, which are guaranteed by the fundamental principles of quantum physics. Miniaturization of QRNGs is crucial for a wide range of communication and cryptography applications. Here, we first report a fully functional QRNG chip based on vacuum-state fluctuations, with dimensions of $16.6\times 7.8{\mathrm{mm}}^2$. The quantum entropy source, which is achieved via hybrid photonic integration with an ${\mathrm{Si}\mathrm{O}}_2$ waveguide, generates raw quantum random numbers. The hybrid photonic and electrical components are assembled into a compact ceramic package using system-in-package technology. A microcontroller unit acquires the raw data and outputs the processed quantum random numbers via a serial peripheral interface. According to the characterization results, the QRNG chip achieves a constant real-time output rate of 5.2 Mbps across the industrial temperature range of $-40$ to $85{}^{\circ }\mathrm{C}$, making it suitable for practical applications.

Type-II fusion is a probabilistic entangling measurement that is essential to measurement-based linear-optical quantum computing and can be used for quantum teleportation more broadly. However, it remains under-explored for high-dimensional qudits. Our main result gives a Type-II fusion protocol with proven success probability approximately $2/d^2$ for qudits of arbitrary dimension $d$. This generalizes a previous method that only applied to even-dimensional qudits. We believe this protocol to be the most efficient known protocol for Type-II fusion, with the $d=5$ case beating the previous record by a factor of approximately $723$. We discuss the construction of the required $(d-2)$-qudit ancillary state using a silicon spin-qudit ancilla coupled to a microwave cavity through time-bin multiplexing. We then introduce a general framework of extra-dimensional corrections, a natural technique in linear optics that can be used to nondeterministically correct nonmaximally entangled projections into Bell measurements. We use this method to analyze and improve several different circuits for high-dimensional Type-II fusion and compare their benefits and drawbacks.

Photonic topological insulators and self-induced nonreciprocity based on nonlinear effects in asymmetric structures have garnered increased attention due to their potential applications in quantum information technologies and advanced photonic integrated circuits. In this study, we combine these two fields and present asymmetric photonic crystal designs based on all-dielectric checkerboard structures forming topological interfaces. The resulting topological photonic structures lead to the emergence of a narrowband leaky mode in their linear transmission response, accompanied by an edge state with topological protection. The incorporation of the nonlinear optical Kerr effect in the system results in spectral tunability and transmission nonreciprocity in the induced topological edge states, as the input beam intensity is increased, due to the introduction of asymmetric defects along the interface. We envision that the findings presented in our work will lead to tunable photonic topological insulators with a nonreciprocal response that promise to provide new avenues for the development of next-generation photonic devices and quantum optical technologies.

Performing simultaneous depth-of-field (DOF) extension and edge enhancement within a single optical element remains a fundamental challenge in advanced imaging. Here, we propose a wavelength-tunable nonlocal Huygens metasurface capable of simultaneously extracting depth and edge features of images in a single-shot exposure. Using the selective polarization response of the Huygens metasurfaces, the circularly polarized converted component undergoes geometric phase modulation for wave-front shaping to extend the DOF, while the nonconverted component acts as a spatial frequency filter to enhance edge contrast. The integration of a phase-change material, ${\mathrm{Sb}}_2{\mathrm{S}}_3$, enables continuous tuning of the resonance wavelength across a range of 100 nm by modulating its refractive index, granting the system excellent broadband spectral adaptability. This work offers a compact solution for real-time depth sensing and feature extraction in applications such as autonomous navigation and biomedical imaging.

Facilitated by low-noise laser-frequency locking, optical microresonators with the Pockels effect have shown unprecedented high resolutions in sensing electrical field. However, the requirement for tunable and low-noise laser sources considerably increases the cost and the size of the system, thereby limiting the industrial applicability of the microresonator-based technology. Here, we explore the possibility of using a low-cost fixed-frequency semiconductor laser as the pump laser to perform radiofrequency electrometry. A resonant mode in a lithium niobate microresonator is frequency locked to the laser using the electrooptic effect. This same effect also underlies the radiofrequency electric-field sensing mechanism. Our experimental results show that the electrometry resolution can be maintained at signal frequencies beyond the optical resonance bandwidth and that the signal-to-noise ratio does not change with varied coupling conditions as long as the laser frequency noise is the dominant noise source of the system. In addition, narrowband electrooptic sensitivity enhancement is observed at frequencies of the microresonator’s piezoelectric resonances, resulting in a resolution enhancement factor of approximately 3 at signal frequencies around 4 MHz. Our work advances the photonic resonant electrometry technology by studying the bandwidth limitation, and opens the road to the employment of low-cost lasers in high-resolution sensing applications.

We theoretically design and experimentally demonstrate a mechanism to protect intercavity polaritons, a mixed light-matter excitation in strongly coupled optical cavities. This arises from the strategic design of the energy-level landscape, which provides a direct connection with the transparency window, typically discussed in the context of slow light. The protection manifests itself as a remarkable robustness of the intercavity polariton dispersion over a wide momentum range, uncompromised photon-exciton mixing, and spatial separation of its photonic and excitonic components. The consequent tunable heavy mass opens avenues for exploring and using this unique optical excitation in advanced photonics.

Quantum physics has brought enhanced capability in various sensing applications. Despite challenges from noise and loss in the radio-frequency (rf) domain, Xia et al. [Phys. Rev. Lett. 124, 150502 (2020)] demonstrated a route for enhanced rf-receiver empowered by quantum squeezing and entanglement. In this work, we further explore the quantum advantage of imaging in the weak coupling scenario of the rf-photonic receiver. The proposed imaging receiver applies a transducer to up-convert the rf signal to optical to enable high-efficiency connection via low-loss fiber networks. The efficient connection therefore increases the synthetic aperture and improves the resolution of the distributed imaging system. To overcome the challenge from low transduction efficiency in existing devices limited by weak photon interaction, we propose the use of squeezed-state optical sources to suppress the noise. We numerically evaluate the quantum advantage in synthetic aperture radar imaging, where the images are generated from the standard resolution test chart via a Gaussian point spread function with added Gaussian noise. We apply the Wiener filter on the images to restore the objects and find that stronger squeezing significantly improves the quality of the restored image. Our findings push quantum squeezing advantage to real-world applications.

Topological disclination (TD) serves as an ideal platform for trapping localized zero-dimensional modes and has been extensively studied in classical wave systems. Due to the unconventional bulk-disclination correspondence in topological materials, TDs are robust against local lattice perturbations. However, most TD states lack chiral symmetry protection, which complicates the confinement of these modes near or at the midgap, particularly in photonic systems. In this work, we propose a confined Mie-resonance photonic lattice with rotational symmetries that supports an enlarged bandgap, and construct a two-dimensional metamaterial with a TD core that preserves chiral symmetry. This configuration ensures the confinement of degenerate TD states between the zero-energy line, preventing them from merging into bulk bands. Experimental results demonstrate that two pairs of degenerate TD modes are symmetrically located near the midgap. Our work provides a alternate perspective for realizing robust TD states in 2D photonic materials, releasing possibilities for the fabrication of topological optical devices.

We compare methods for signal classification applied to voltage traces from transition edge sensors (TES) which are photon-number resolving detectors fundamental for accessing quantum advantages in information processing, communication, and metrology. We quantify the effect of numerical analysis on the distinction of such signals. Furthermore, we explore dimensionality reduction techniques to create interpretable and precise photon-number embeddings. We demonstrate that the preservation of local data structures of some nonlinear methods is an accurate way to achieve unsupervised classification of TES traces. We do so by considering a confidence metric that quantifies the overlap of the photon-number clusters inside a latent space. Furthermore, we demonstrate that for our dataset previous methods such as the signal’s area and principal component analysis can resolve up to 16 photons with confidence above 90% whereas nonlinear techniques can resolve up to 21 photons with the same confidence threshold. In addition, we showcase implementations of neural networks to leverage information within local structures, aiming to increase confidence in assigning photon numbers. Finally, we demonstrate the advantage of some nonlinear methods to detect and remove outlier signals.

Hyperentangled states are fascinating resources in quantum information processing as they can significantly increase the channel capacity and enhance noise resistance. We explore a hyperfusion mechanism to fuse one $n$-photon hyper-$W$ state and one $m$-photon hyper-$W$ state into a large-scale $(n+m-2)$-photon hyper-$W$ state. Another mechanism to fuse one $n$-photon hyper-$W$ state, one $m$-photon hyper-$W$ state, and one $t$-photon hyper-$W$ state into an $(n+m+t-3)$-photon hyper-$W$ state is also proposed. These two hyperfusion mechanisms are constructed employing only polarizing beam splitters, balanced beam splitters, half-wave plates, single-photon detectors, and cross-Kerr nonlinearities. Conditional quantum gates, path couplers, and ancillary photons are not required in our constructions. Moreover, our fused $W$ states are hyperentangled in the polarization and spatial degrees of freedom of single-photon systems. The presence of only one garbage output state demonstrates that high efficiency can be achieved in our schemes.

The introduction of time as a new degree of freedom to control wave-matter interactions has opened alternative avenues to fully control wave propagation in four dimensions (x,y,z,t). Time interfaces (rapid changes of the constitutive relationships of the medium where a wave propagates) have recently become popular, as they are the temporal analogue of spatial interfaces. While recent groundbreaking experimental work has demonstrated time interfaces from water waves, microwaves, and the optical regime, rapidly changing, for instance, the permittivity of the medium requires carefully engineered structures. Here, we study the possibility of implementing smooth temporal transitions of the permittivity and permeability of the medium (while considering time-independent impedance) to mimic the response of the time-refracted waves produced by photonic time interfaces. It is shown that indeed, as long as the signal is inside the structure during the modulation time, there are some values of rising or falling time of the smooth modulation that enable a full approximate emulation of time refraction. These results may open further avenues to explore by relaxing the speed at which the time modulation should be introduced when designing four-dimensional media.