Niobium nitride ($\mathrm{Nb}\mathrm{N}$) is a particularly promising material for quantum technology applications, as it shows the degree of reproducibility necessary for large-scale superconducting circuits. We demonstrate that resonators based on $\mathrm{Nb}\mathrm{N}$ thin films present a one-photon internal quality factor above ${10}^5$ maintaining a high impedance (larger than $2\mathrm{k}\mathrm{\Omega }$), with a footprint of approximately $50\times 100\text{μ}{\mathrm{m}}^2$ and a Kerr nonlinearity of few tenths of a hertz. These quality factors, mostly limited by losses induced by the coupling to two-level systems, have been maintained for kinetic inductances ranging from tenths to hundreds of picoHenry per square. We also demonstrate minimal variations in the performance of the resonators during multiple cooldowns over more than nine months. Our work proves the versatility of niobium nitride high-kinetic-inductance resonators, opening perspectives towards the fabrication of compact, high-impedance, and high-quality multimode circuits, with sizable interactions.

Quantum sensing has performance advantages that far exceed classical sensing, where sensing with photons is one of the most useful branches, and fiberization is a significant development approach to achieve a broader range of applications. However, the birefringence effect in optical fibers can cause polarization drift, which severely reduces the sensitivity of measurement on weak signals and influences the development of quantum sensing. In order to solve this problem, we propose a scheme that builds four time-division-multiplexing channels in the system by switching the polarization states of different optical pulses, and achieves the effect of suppressing polarization drift by analyzing and integrating the effective information of the four polarization channels. Moreover, the sensitivity and linear dynamic range of the system can be adjusted by inserting a reference phase. We demonstrate the feasibility of the system via theoretical analysis, and the numerical simulation results show that the scheme can demodulate the signal well and improve the signal-to-noise ratio by 30 dB, even in the case of the signal of the direct demodulation method is drowned out by noise and completely distorted.

Non-Hermiticity has emerged as a promising tool to induce topological phase transitions, allowing for the design of tunable and functional topological devices such as topological lasers and switches on on-chip integrated optical platforms. In this work, we experimentally demonstrate loss-modulated photonic topological edge states in anti-parity-time-symmetric waveguide arrays. Loss is introduced via carefully designed subwavelength grating (SWG) waveguides, allowing for on-chip control of topological responses. Our results show that loss not only induces topological phase transition but also enhances topological responses, leading to the increased localization of topological edge states and the anomaly reduced intrinsic loss of the topological edge states. Our work provides a platform based on SWG waveguides for exploring non-Hermiticity-induced photonic topological responses and corresponding applications.

We apply inverse-design methods to produce two-dimensional triangular-lattice plasma-metamaterial (PMM) devices, which are then constructed and demonstrated experimentally. Finite-difference frequency-domain simulations are used along with forward-mode automatic differentiation to optimize the plasma densities of each of the plasma elements in the PMM to perform beam steering and demultiplexing under transverse magnetic polarization. The optimal device parameters are then used to assign plasma-density values to elements that make up an experimental version of the device. Device performance is evaluated against both the simulated results and human-designed alternatives, showing the benefits and disadvantages of in silico inverse design and paving the way for future fully in situ optimization.

Remote implementation of quantum operation between two distant nodes is crucial for many quantum applications. Here we consider the implementation of an arbitrary single-photon operation upon a remote quantum system exploiting photon pairs simultaneously entangled in the polarization and temporal degrees of freedom, which is a fascinating resource in long-distance quantum communication with its outstanding merits. To improve the level of security for the above protocol, we also consider a controlled implementation of quantum operation scenario by adding a supervisor, which is necessary for distributed quantum computation in a quantum network. Both of the protocols can be fulfilled by bidirectional state teleportation with the aid of the effective cross-Kerr interaction. Furthermore, the simple construction circuits together with available existing optical elements and common single-photon detectors facilitate the realization of our protocols with current sufficient technologies.

Bound states in the continuum (BICs) in open cavities have attracted considerable attention in wave physics due to their ability to confine light and produce high-quality-factor resonances with promising applications for filtering and sensing. One of the most interesting types of BICs is Friedrich-Wintgen (FW) BICs, which result from destructive interference of two interacting modes belonging to the same radiation channel. Here, we investigate theoretically and experimentally FW BICs in a photonic and plasmonic T-shaped cavity made of two horizontal guides of lengths $d_2$ and $d_3$ coupled to a vertical stub of length $d_1$. We demonstrate that the necessary condition for obtaining BICs consists in taking the lengths of the two horizontal guides $d_2$ and $d_3$ commensurate. This BIC is a common mode of the guides of lengths $d_2$ and $d_3$, such as the electric field vanishes at their connection point with the stub of length $d_1$; this BIC is independent of $d_1$ and the infinite waveguide to which the whole cavity will be attached. We show that, depending on $d_1$, the FW BIC appears as the consequence of the interaction between two eigenmodes of the originally isolated cavity where the width of one mode vanishes giving rise to FW BIC, while the width of the second mode becomes broad. In addition, we show that by slightly deviating from the BIC condition, the latter transforms to either electromagnetically induced transparency (EIT) or reflection or Autler-Townes splitting (ATS) resonances. Both EIT and ATS effects are qualified as a transparency window between two transmission zeros, but with different physical origins. We exploit the Akaike’s information criterion test to discern EIT from ATS and distinguish the regime where the EIT or ATS effect dominates. The theoretical results, obtained by means of the Green’s function method, are validated both by experimental measurements using coaxial cables in the radiofrequency domain and numerical simulations using metal-insulator-metal plasmonic waveguides operating in the infrared domain. The sensitivity of the PIT (the plasmonic analogue of EIT) resonances to the dielectric inside the waveguides can be used to design a highly sensitive sensor, which makes it suitable for an on-chip optical sensing platform.

We show how to generate Bell states and $N$-partite $W$ states of long-distance superconducting (SC) qubits in a SC waveguide QED system, where SC qubits are coupled to an open microwave transmission line. In the two-qubit case, the Bell state of two long-distance qubits can be a dark state of the system by choosing appropriate system parameters. If one proper microwave pulse drives one of two qubits, the two qubits will evolve from their ground states to a Bell state. Further, we extend this scheme to the multiqubit case. We show that $W$ states of $N$ long-distance qubits can also be generated. Because both the Bell and $W$ states are decoupled from the waveguide (i.e., dark states of the system), they are steady and have very long lifetimes in the ideal case without decoherence of qubits. In contrast to the ideal case, the presence of decoherence of qubits limits the lifetimes of the Bell and $W$ states. Our study provides an alternative scheme for generating Bell states and $N$-partite $W$ states in SC waveguide QED, which can be used to entangle long-distance nodes in waveguide quantum networks.

Quantum key distribution (QKD) based on the fundamental laws of quantum physics can allow the distribution of secure keys between distant users. However, imperfections in realistic devices may lead to potential security risks, which must be accurately characterized and considered in practical security analysis. High-speed optical modulators, being one of the core components of practical QKD systems, can be used to prepare the required quantum states. Here, we find that optical modulators based on $\mathrm{Li}\mathrm{Nb}\mathrm{O}$${}_3$, including phase modulators and intensity modulators, are vulnerable to photorefractive effects caused by external light injection. By changing the power of external light, eavesdroppers can control the intensities of the prepared states, posing a potential threat to the security of QKD. We have experimentally demonstrated the influence of light injection on $\mathrm{Li}\mathrm{Nb}\mathrm{O}$${}_3$-based optical modulators and analyzed the security risks caused by the potential light-injection attack, along with the corresponding countermeasures.

We describe the generation of entangling gates on superconductor-semiconductor hybrid qubits by ac voltage modulation of the Josephson energy. Our numerical simulations demonstrate that the unitary error can be below ${10}^{-5}$ in a variety of 75-ns-long two-qubit gates (CZ, $i\mathrm{SWAP}$, and $\sqrt{i\mathrm{SWAP}}$) implemented using parametric resonance. We analyze the conditional $ZZ$ phase and demonstrate that the CZ gate needs no further phase-correction steps, while the $ZZ$ phase error in swap-type gates can be compensated by choosing pulse parameters. With decoherence considered, we estimate that qubit relaxation time needs to exceed $70\text{μ}\mathrm{s}$ to achieve the 99.9% fidelity threshold.

Quantum digital signatures (QDSs), generating correlated bit strings among three remote parties for signatures through quantum law, can guarantee nonrepudiation, authenticity, and integrity of messages. Recently, a one-time universal-hashing QDS framework, exploiting the quantum asymmetric encryption and universal hash functions, has been proposed to significantly improve the signature rate and ensure unconditional security by directly signing the hash value of long messages. However, similar to quantum key distribution, this framework utilizes keys with perfect secrecy by performing privacy amplification that introduces cumbersome matrix operations, thereby consuming large computational resources, causing delays, and increasing failure probability. Here, we prove that, different from private communication, imperfect quantum keys with partial information leakage can be used for digital signatures and authentication without compromising the security while having eight orders of magnitude improvement on signature rate for signing a megabit message compared with conventional single-bit schemes. This study significantly reduces the delay for data postprocessing and is compatible with any quantum key generation protocols. In our simulation, taking two-photon twin-field key generation protocol as an example, QDS can be practically implemented over a fiber distance of 650 km between the signer and receiver. For the first time, this study offers a cryptographic application of quantum keys with imperfect secrecy and paves a way for the practical and agile implementation of digital signatures in a future quantum network.

Recently, three-dimensional (3D) metamaterials have undergone a revolution driven mainly by the popularization of 3D-printing techniques, which has enabled the implementation of modern microwave and photonic devices with advanced functionalities. However, the analysis of 3D metamaterials is complex and computationally costly in comparison with the analysis of their one-dimensional and two-dimensional counterparts due to the intricate geometries involved. In this paper, we present a fully analytical framework based on Floquet-Bloch modal expansions of the electromagnetic fields and integral-equation methods for the analysis of 3D metamaterials and metagratings. Concretely, we focus on 3D configurations formed by periodic arrangements of rectangular waveguides with longitudinal slot insertions. The analytical framework is computationally efficient compared with full-wave solutions and also works under oblique-incidence conditions. Furthermore, it comes associated with an equivalent circuit that allows one to gain physical insight into the scattering and diffraction phenomena. The analytical equivalent circuit is tested against full-wave simulations in cst. Simulation results show that the proposed 3D structures provide independent polarization control of the two orthogonal polarization states. This key property is of potential interest for the production of full-metal polarizers, such as the one illustrated here.

Tuning $\mathrm{Ga}\mathrm{As}$-based quantum emitters to telecom wavelengths makes it possible to use existing mature technology for applications in, e.g., long-haul ultrasecure communication in fiber networks. A promising method redeveloped recently is to use a metamorphic $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ buffer that redshifts the emission by reducing strain. However, the impact of such a buffer also causes the simultaneous modification of other quantum dot (QD) properties. Knowledge of these effects is crucial for actual implementations of QD-based nonclassical light sources for quantum communication schemes. Here, we thoroughly study single $\mathrm{Ga}\mathrm{As}$-based quantum dots grown by molecular-beam epitaxy on specially designed, digital-alloy $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ metamorphic buffers. With a set of structures varying in the buffer indium content and providing quantum dot emission through the telecom spectral range up to 1.6 µm, we analyze the impact of the buffer and its composition on QD structural and optical properties. We identify the mechanisms of quantum dot emission shifts with varying buffer compositions. We also look at charge-trapping processes and compare excitonic properties for different growth conditions with single-dot emission successfully shifted to both the second and third telecom windows.

Topological insulators that exhibit localized states at the edges or interference regions have been extensively investigated. Among these, waterborne acoustic topological interface states have significant potential in applications such as particle manipulation. However, the large impedance of water can no longer be ignored and sets difficulty in the construction of the waveguide in observing the topological states. In this paper, we propose an effective strategy to circumvent this issue and achieve the direct observation of the topological interface state in a waterborne acoustic waveguide as well as the associated nonzero Zak phases. Moreover, the robustness of the interface state against disorder is validated, suggesting a viable method for studying underwater topological acoustic fields.

Transmission and reflection of sound across a dense layer of long rectangular quasi-two-dimensional bubbles in a polymeric matrix are measured over two frequency ranges, 150–260 and 670–1130 kHz. Contrary to what is expected, considering the high acoustic impedance contrast between air and the matrix, there is high transmission across the layer, especially in the lower-frequency range. These results are explained by a monopolar Minnaert-like resonance of bubbles, which radiates sound in both directions of the bubble layer. The Minnaert resonance is computed for two-dimensional bubbles, revealing a resonance frequency within the lowest-frequency range used. The transmission and reflection coefficients are theoretically computed by considering the bubble array as a homogeneous layer with a resonant acoustic impedance.

We present a field-deployed quantum key distribution (QKD) link based on energy-time entanglement. Three nodes are connected, bridging the cities of Nice and Sophia Antipolis, by means of commercial grade optical fibers, spanning over a total distance of 50 km. We have built a packaged and resilient source of energy-time entangled photon pairs and implemented remote interferometer stabilization. Moreover, the clocks of the two end users are synchronized exploiting photon-counting techniques only, therefore requiring no dedicated channel. The system runs over the ITU 100-GHz standard telecom grid, through which a secure key rate of 7.0 kbit/s is extracted for one pair of channels. A homemade software performs all the necessary postprocessing procedures enabling to establish secret keys in real time. We present the first fully operational entanglement-based metropolitan QKD system being totally self-sufficient, i.e., requiring no additional overheads to be operated on a conventional metropolitan fiber network.

Quantum key distribution (QKD) enables private communications with information-theoretic security. To guarantee the practical security of QKD, it is essential that QKD systems are implemented in accordance with theoretical requirements and are robust with regard to side-channel attacks. Here we study a prominent attack on QKD transmitters known as the laser-seeding attack (LSA). It consists in injecting photons into the laser of the transmitter in an attempt to modify the outgoing light in some way that is beneficial to the eavesdropper. In this work we measure the response of a QKD transmitter to the LSA as a function of the optical power injected, allowing us to quantify the level of optical attenuation required to mitigate the attack. Further, we use a laser-rate-equation model to numerically simulate the effects of the LSA on a gain-switched laser. With this model we are able to reproduce previous experimental results, as well as generate new insight into the LSA by examining the effects of the LSA when the QKD transmitter is operated with different laser current driving parameters.

Recent progress on metasurfaces has revealed the nonlocal features of this kind of multiunit artificial structure. It has been shown that there is always intercoupling between the constituent units, which must be considered in their design. Such an intercoupling makes the design process difficult, especially when the number of constituent units becomes large. We show in this paper that a metasurface can be generally described by a multiport structural model, based on which an efficient design method can be established. To show the main idea of the model and the adaptability of the method based on it, nonlocal metastructures with various target fields, including an anomalous reflector, mode convertor in waveguide, wave focuser, and structure for a bound state in a continuum are designed. The successfully designed metastructures show that the suggested method is generally valid for structures with various target fields and is also efficient enough for structures with a large number of constituent units. The model gives not only an efficient method for designing metasurfaces, but also a general understanding of metastructures, based on which nonlocal metastructures for field control in complex environments, such as in organic tissue and architectural structures, can be expected.

Two-dimensional (2D) semiconductors are a promising solution for the miniaturization of electronic devices and for the exploration of novel physics. However, practical applications and demonstrations of physical phenomena are hindered by high Schottky barriers at the contacts to 2D semiconductors. While the process of image-force barrier lowering (IFBL) can considerably decrease the Schottky barrier, IFBL is not fully understood for the majority of prevalent contact geometries. We introduce an alternative technique to determine the IFBL potential energy with application spanning far beyond that of any existing method. We do so by solving Poisson’s equation with the boundary conditions of two metal surfaces separated by an angle $\mathrm{\Omega }$. We then prove that our result can also be obtained with the method of images provided a non-Euclidean, cone manifold space is used. The resulting IFBL is used to calculate the expected contact resistance of the most prevalent contact geometries. Finally, we investigate contact resistance and show how stronger IFBL counteracts the effect of larger depletion width with increasing contact angle. We find that top contacts experience lower contact resistance than edge contacts. Remarkably, our results identify tunable parameters for reducing Schottky barriers and likewise contact resistance to edge-contacted 2D materials, enhancing potential applications.