Coherent phonons can give rise to phenomena and physical mechanisms in different systems. Understanding phonon-boundary scattering is critical for the manipulation of thermal properties. In this paper, it is found that the thermal conductivity of rough graphene nanoribbon first monotonically changes, and then exhibits an oscillatory manner with the varying of surface boundary roughness. An obvious increase in thermal conductivity, up to 25.33%, can be observed as surface boundary roughness increases from 0.61 to 0.72. This is in contrast to the conventional understanding that thermal conductivity typically decreases when surface boundary roughness increases. Further, a frequency-resolved picture and lattice dynamics analysis identify that the anomalous boundary effect originates from the coherent nature of phonons, which results in the roughness-selected destructive interference of different modes. Besides, the oscillation will be reduced by introducing rough boundaries with different sinusoidal shapes, which increases the randomness. This abnormal boundary effect can also be extended to other materials, for example, hexagonal boron nitride monolayer, depending mainly on the anharmonicity. The study reveals physical insights into phonon-boundary scattering and may aid the design of heat management and thermoelectric devices based on the boundary effect.

Motivated by an interest in measuring electric fields in low-pressure noble-gas discharge plasmas and in dusty plasmas via noninvasive Stark-effect spectroscopy, we investigate Rydberg electromagnetically induced transparency (EIT) of ${}^{85}\mathrm{Rb}$ atomic vapor in a glass cell that contains a 5-Torr neon buffer gas. We find that at low probe power the $\mathrm{Ne}$ buffer gas induces a positive frequency shift of the Rydberg-EIT lines of about 70 MHz as well as a broadening of about 120 MHz, with minimal dependence on the principal quantum number of the Rydberg states. The EIT line shift predominantly is a sum of a positive term from $s$-wave scattering between the Rydberg electron and the $\mathrm{Ne}$ atoms, and a negative term from the polarization of the $\mathrm{Ne}$ atoms within the Rydberg atom. The polarization shift is lesser in magnitude, resulting in the observed net positive shift. The line broadening is largely due to the $\mathrm{Ne}$ polarization. Our experimental results are in good qualitative agreement with our theoretical model, in which the shift is linear in buffer-gas density. Our results suggest that Rydberg EIT is suitable for noninvasive measurement of electric fields in buffer-gas cells that sustain a plasma discharge, and that it can serve as a direct spectroscopic probe for buffer-gas density at low pressure.

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.

Silicon is a key semiconducting material for electrical devices and hybrid quantum systems where low temperatures and zero-spin isotopic purity can enhance quantum coherence. Electrical conductivity in $\mathrm{Si}$ is characterized by carrier freeze out at around 40 K allowing microwave transmission, which is a key component for addressing spins efficiently in silicon quantum technologies. In this work, we report an additional sharp transition of the electrical conductivity in a $\mathrm{Si}$-28 cylindrical cavity at around 1 K. This is observed by measuring microwave resonator whispering gallery mode frequencies and $Q$ factors with changing temperature and comparing these results with finite-element models. We attribute this change to a transition from a relaxation mechanism-dominated to a resonant phononless absorption-dominated hopping conduction regime. Characterizing this regime change represents a deeper understanding of a physical phenomenon in a material of high interest to the quantum technology and semiconductor device community and the impact of these results is discussed.

Predicting properties of large-scale quantum systems is crucial for the development of quantum science and technology. Shadow estimation is an efficient method for this task based on randomized measurements, where many-qubit random Clifford circuits are used for estimating global properties like quantum fidelity. Here we introduce the minimal Clifford measurement (MCM) to reduce the number of possible random circuits to the minimum, while keeping the effective postprocessing channel in shadow estimation. In particular, we show that MCM requires $2^n+1$ distinct Clifford circuits, and it can be realized by mutually unbiased bases, with $n$ as the total qubit number. By applying the $Z$-tableau formalism, this ensemble of circuits can be synthesized to the $-\mathrm{S}-\mathrm{CZ}-\mathrm{H}-$ structure, which can be decomposed to $2n-1$ fixed circuit modules, and the total circuit depth is at most $n+1$. Compared to the original Clifford measurements, our MCM reduces the circuit complexity and the compilation costs. In addition, we find the sampling advantage of MCM on estimating off-diagonal operators, and extend this observation to the biased-MCM scheme to enhance the sampling improvement further.

Resolving the locations and discriminating the spin states of individual trapped ions with high fidelity is critical for a large class of applications in quantum computing, simulation, and sensing. We report on a method for high-fidelity state discrimination in large two-dimensional (2D) crystals with over 100 trapped ions in a single trapping region, combining a hardware detector and an artificial neural network. A high-data-rate, spatially resolving, single-photon sensitive timestamping detector performs efficient single-shot detection of 2D crystals in a Penning trap, exhibiting rotation at about $25\mathrm{kHz}$. We then train an artificial neural network to process the fluorescence photon data in the rest frame of the rotating crystal in order to identify ion locations with a success rate of $90\mathrm{\%}$, accounting for substantial illumination inhomogeneity across the crystal. Finally, employing a time-binned state detection method, we arrive at an average spin-state detection fidelity of $94(2)\mathrm{\%}$. This technique can be used to analyze spatial and temporal correlations in arrays of hundreds of trapped-ion qubits.

The breakdown mechanism of the anodic-oxide-film–electrolyte system has been studied for a long time, but there are still very few experimental studies that can summarize its physical image. Here, we report a finding on the breakdown mechanism of the tantalum ($\mathrm{Ta}$) oxide-electrolyte system. We observe the evolution process of oxygen bubbles in $\mathrm{Ta}$ oxide films and demonstrate that oxygen generation is a cause of breakdown. The oxygen evolution reaction not only provides the initial electron injection for avalanche breakdown, but the resulting oxygen bubbles also lead to a concentration of stress and electric field. Further studies have shown that oxygen vacancies in oxide films are key to driving oxygen evolution and electron-current formation. Based on these findings, we attenuated oxygen evolution by the addition of a reducing agent, and the crystallization of the $\mathrm{Ta}$ oxide films was consequently drastically reduced, with the leakage current density of the dielectric being only 12.6% of the original. Therefore, this study not only contributes to an in-depth understanding of the breakdown mechanism of anodic oxide films, but also provides a promising prospect for the fabrication of high-quality $\mathrm{Ta}$ capacitors.

Research on optimal eavesdropping models under practical conditions will help to evaluate realistic risk when employing a quantum key distribution system for secure information transmission. Intuitively, fiber loss will lead to the optical energy leaking to the environment, rather than harvested by the eavesdropper, which also limits the eavesdropping ability while improving the quantum key distribution system performance in practical use. However, defining the optimal eavesdropping model in the presence of lossy fiber is difficult because the channel is beyond the control of legitimate partners and the leaked signal is undetectable. Here we investigate how the fiber loss influences the eavesdropping ability based on a teleportation-based collective attack model, which requires two distant stations and a shared entanglement source. We find that if the distributed entanglement is limited due to the practical loss, the optimal attack occurs when the two teleportation stations are merged to one and placed close to the transmitter site, which performs similar to the entangling-cloning attack but with a reduced wiretapping ratio. Assuming Eve uses the best available hollow-core fiber, the secret key rate in the practical environment can be 20–40% higher than that under ideal eavesdropping. While if the entanglement distillation technology is mature enough to provide high quality of distributed entanglement, the two teleportation stations should be distantly separated for better eavesdropping performance, where the eavesdropping can even approach the optimal collective attack. Under the current level of entanglement purification technology, the unavoidable fiber loss can still greatly limit the eavesdropping ability as well as enhance the secret key rate and transmission distance of the realistic system, which promotes the development of quantum key distribution systems in practical application scenarios.

Optical manipulation of the magnetization of thin films opens up exciting possibilities for ever-faster magnetic storage applications. In this context, ${L1}_0$ chemically ordered $\mathrm{Fe}\mathrm{Pt}$ thin films are of particular interest due to their high perpendicular magnetic anisotropy and their use as a storage material for heat-assisted magnetic recording devices. However, these materials are difficult to manipulate with external fields due to their high coercivity field. Thus, we want to explore the possibility of tailoring the properties of these materials to enable switching using all-optical techniques. While stochastic all-optical switching between partially magnetized states has been reported for undoped $\mathrm{Fe}\mathrm{Pt}$ thin films, we have investigated to what extent doping with third elements can influence the switching behavior. Reducing the saturation magnetization may be one way to facilitate all-optical switching. While this is expected with the introduction of additional elements, we also want to highlight the role of the inverse Faraday effect and the magnetic circular dichroism in stochastic all-optical switching. In this study, $\mathrm{Cr}$ was found to be a promising dopant, which can almost double the relative magnetization change of the partially magnetized states compared to pure $\mathrm{Fe}\mathrm{Pt}$.

We use a floating tunable coupler to mediate interactions between qubits on separate chips to build a modular architecture. We demonstrate three different designs of multichip tunable couplers using vacuum-gap capacitors or superconducting indium-bump bonds to connect the coupler to a microwave line on a common substrate and then connect to the qubit on the next chip. We show that the zero-coupling condition between qubits on separate chips can be achieved in each design and that the relaxation rates for the coupler and qubits are not noticeably affected by the extra circuit elements. Finally, we demonstrate two-qubit gate operations with fidelity at the same level as qubits with a tunable coupler on a single chip. Using one or more indium bonds does not degrade qubit coherence or impact the performance of two-qubit gates.

The semiconductor industry has been facing obstacles in designing low-power (LP) nanoelectronic devices due to the lack of accurate theoretical simulations and low-resistance contacts of heterojunctions. Here, based on experimental data, we discover that the generalized gradient approximation using the Fritz Haber Institute pseudopotential accurately describes the lattice constants and electronic properties of monolayer ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$. It is further confirmed that this method is also applicable to characterize the electrical contact properties and device transport characteristics of metal$/{\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ heterojunctions (where metal includes $\mathrm{Sc},\mathrm{Bi},\mathrm{Ag},\mathrm{Al},\mathrm{Ti}$, graphene, $\mathrm{Cr}$, $\mathrm{Fe},\mathrm{Cu},\mathrm{Co},\mathrm{Au},\mathrm{Pd},\mathrm{Ni},$ and $\mathrm{Pt}$). Calculations show that various bulk metal/${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ heterojunctions exhibit n-type Schottky barrier contact features, and it is possible to achieve 100% carrier injection for ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ with $\mathrm{Sc},\mathrm{Ag},\mathrm{Ti},\mathrm{Pd},$ and $\mathrm{Pt}$ contacts. Using the Schottky-Mott rule and quantum transport simulation, a strong Fermi-level pinning effect at the metal$/{\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ interfaces is observed, and $\mathrm{Sc}$ is identified as the best metal electrode. The 5.1-nm p-i-n field-effect transistor (FET) with $\mathrm{Sc}$ electrodes can meet the off-state current requirement of the 2013 International Technology Roadmap for Semiconductors standard for high-performance and LP FET devices. By employing a high-k gate, the on-state current for the undoped LP device can be improved by 2 orders of magnitude, and its high on:off ratio is up to 3 × ${10}^6$. These findings provide insights into the metal/${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$ interface behaviors and provide a theoretical reference for designing ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{N}}_4$-based nanoelectronic devices by selecting suitable electrodes with low contact resistance.

We present a unified approach for designing isolators based on temporally modulated coupled resonator networks. Our method leverages standard superconducting quantum interference device–based resonators as building blocks, arranged in series-coupled resonators to realize a wide range of on-chip nonreciprocal devices. The devices operate by translating microwave power into out-of-band intermodulation products in a direction-dependent way. Our theoretical studies demonstrate the effectiveness of the proposed approach, achieving isolators with near-zero insertion losses and isolation greater than 20 dB. To validate our findings, we implemented and measured a series-coupled three-resonator superconducting isolator using a single-layer superconducting process. At a base temperature of 20 mK, our device exhibited an insertion loss of 1.3 dB in the forward direction and isolation of up to 25 dB at the center frequency and greater than 15 dB across a bandwidth of 250 MHz in the reverse direction. Our approach promises to enable the design of a broad range of high-performance nonreciprocal devices for superconducting circuits.

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.

Due to its low symmetry, $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$ exhibits a strongly anisotropic optical response. As a result, the absorption spectra change with the polarization state of the incoming photons. To understand this phenomenon, here we calculate the complete electromagnetic wave equation solutions as a function of linear polarization angle and photon energy for $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$ using its previously measured complex dielectric function tensor. The significant off-diagonal terms in this tensor can result in a nonexponential decay in the photon flux, indicating that the Beer-Lambert law is not generally valid in this anisotropic material. However, for above-band-gap spectral regions that depend on crystallographic orientations [>5.8 eV (001-plane) and >5.2 eV (010-plane)], an effective absorption coefficient closely approximates the photon flux decay with depth. On the other hand, near the optical absorption edge [4.9–5.8 eV (001-plane) and 4.65–5.2 eV (010-plane)], the photon flux decay exhibits a sum of two exponential decays, such that two effective absorption coefficients are necessary to model the loss behavior versus the absorption depth. This behavior manifests from the presence of dichroism in $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$. A single effective absorption coefficient can only be recovered for this energy range by augmenting the isotropic Beer-Lambert law with a critical penetration depth and polarization dependence. Using these results, we calculate the polarization-dependent photoresponsivity spectra for light polarized along different crystallographic directions.

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.

Recent breakthroughs in Josephson diodes dangle the possibility of extending conventional nonreciprocal electronics into the realm of superconductivity. While a strong magnetic field is recognized for enhancing diode efficiency, it concurrently poses a risk of undermining the essential superconductivity required for nondissipative devices. To circumvent the need for magnetic based tuning, we propose a field-effect Josephson diode based on the electrostatic gate control of finite momentum Cooper pairs in asymmetric spin-momentum-locking states. We propose two possible implementations of our gate-controlled mechanism: (i) a topological field-effect Josephson diode in time-reversal-broken quantum spin Hall insulators; and (ii) semiconductor-based field-effect Josephson diodes attainable in current experimental setups involving a Zeeman field and spin-orbit coupling. Notably, the diode efficiency is highly enhanced in the topological field-effect Josephson diode because the current carried by the asymmetric helical edge states is topologically protected and can be tuned by local gates. In the proposed Josephson diode, the combination of gates and asymmetric spin-momentum-locking nature is equivalent to that of a magnetic field, thus providing an alternative electrical operation in designing nonreciprocal superconducting devices.

Kernel function plays a crucial role in machine learning algorithms such as classifiers. In this paper, we aim to improve the classification performance and reduce the reading out burden of quantum classifiers. We devise a universally trainable quantum feature mapping layout to broaden the scope of feature states and avoid the inefficiently straight preparation of quantum superposition states. We also propose an improved quantum support vector machine that employs partially evenly weighted trial states. In addition, we analyze its error sources and superiority. As a promotion, we propose a quantum iterative multiclassifier framework for one-versus-one and one-versus-rest approaches. Finally, we conduct corresponding numerical demonstrations in the qiskit package. The simulation result of trainable quantum feature mapping shows considerable clustering performance, and the subsequent classification performance is superior to the existing quantum classifiers in terms of accuracy and distinguishability.

In practical applications, achieving perfect transmission of electromagnetic waves despite reflections from a complex medium segment in waveguide systems is of great significance. Using a preinstalled antireflective artificial metastructure (ARAMS) to mitigate reflections caused by the presence of a complex medium is an effective solution. The inverse design of ARAMS in waveguide systems is an optimization process. Recent applications of the topology optimization method in designing ARAMS have demonstrated attractive performances [Nature 607, 281 (2022)]. Topology optimization necessitates frequent utilization of full-wave electromagnetic simulations, while designing from scratch for different functional devices incurs exceedingly high computational costs. To address this problem, we introduce a fast inverse design method for ARAMS based on the internal multiport model (IMPM). Based on a single full-wave electromagnetic simulation dataset, IMPM abstracts the ARAMS electromagnetic field model into a microwave network, analytically establishing connections between actual excitation ports and fictitious internal ports while greatly reducing computational costs in optimization design. Furthermore, once IMPM is established, it can be reused for the rapid optimization design of various functional ARAMS devices in the same complex medium-filled waveguide environment. The cases presented in the paper validate the efficiency of this method in providing effective solutions for the perfect transmission of electromagnetic waves in waveguide systems filled with complex medium.