We study pressure acoustic propagation in asymmetric spoof-fluid-spoof acoustic waveguides and its potential application in acoustic gas sensors. First, a stable and efficient analytical method is established for rapid calculation of the dispersion curves based on spectral expansion and the enforcement of continuity between segments at suitable collocation points. The analysis is validated by using commercial finite-element software. The geometric design of the waveguide is then optimized for the emergence of a nearly flat dispersion curve associated with vertical geometric asymmetry. The waveguide is fabricated using three-dimensional-printing technology, and the measurement results corroborate the numerical simulations. Based on the nearly flat dispersion curve supported by this waveguide, a ${\mathrm{CO}}_2$ sensor is proposed, allowing the phase difference measured between two points in the waveguide to be related to the composition of the gas in the waveguide. The proposed sensor is experimentally validated in a controlled environment and the measurement results match the computational predictions well. The sensor is robust with respect to noise and signal-recording duration due to fast phase measurements and shows high sensitivity to gas concentration due to reliance on the second, nearly flat, dispersion curve. In addition, the sensor is label free and low cost, while exhibiting a rapid response, low-maintenance requirements, and potential for measurements over a wide range of ${\mathrm{CO}}_2$ concentrations without saturation issues.

The outcome of excited-state processes is strongly dependent on initial conditions, i.e., the nature of the state arising from excitation. The physics of photoexcitation and the nature of the excited states generated are well established and applied routinely in the exploration of photochemical mechanisms and the interpretation of laser experiments across a broad range of molecules and materials. However, despite its importance in emerging technologies such as organic light-emitting diodes (OLEDs), the nature of excited states formed by electrical excitation is much less understood. In this paper, we use kinetic Monte Carlo (KMC) simulations to reveal that during charge recombination there is a substantial probability of forming higher-lying excited states with $>0.5$ eV excess energy compared to the lowest excited state. The probability of these hot-exciton pathways depends on the energy gap between the states and the intrinsic energetic disorder in the system. Our simulations also reveal the concept of electrical selection, in that only higher-lying excited states exhibiting $\mathrm{HOMO}\to x$ or $x\to \mathrm{LUMO}$ character (where “HOMO” refers to the highest occupied molecular orbital and “LUMO” refers to the lowest unoccupied molecular orbital) can be formed. Importantly, the potential to form these higher-lying excited states is likely to have significant implications for the lifetime of OLEDs, especially those operating in the blue, as the excess energy will push the core components closer to the damage thresholds, increasing the probability of degradation.

The negatively charged nitrogen-vacancy center (NVC) in diamond has been utilized in a wide variety of sensing applications. The long spin coherence and relaxation times ($T_2^{\ast }$, $T_2$ and $T_1$) of the center at room temperature are crucial to this, as they often limit sensitivity. Using NVCs in nanodiamonds allows for operations in environments inaccessible to bulk diamond, such as intracellular sensing. We report long spin coherence and relaxation times at room temperature for single NVCs in isotopically purified polycrystalline ball-milled nanodiamonds. Using a spin-locking pulse sequence, we observe spin coherence times, $T_2$, up to $786\pm 200\text{μ}\mathrm{s}$. We also measure $T_2^{\ast }$ times up to $2.06\pm 0.24\text{μ}\mathrm{s}$ and $T_1$ times up to $4.32\pm 0.60\mathrm{ms}$. These results represent improvements on the leading literature values for $T_2$ and $T_1$ times in nano- and microdiamond and for $T_2^{\ast }$ times in nanodiamond. Scanning-electron-microscopy and atomic force microscopy measurements show that the diamond containing the NVC with the longest $T_1$ time is smaller than 100 nm. EPR measurements give an $\mathrm{N}$${}_s^0$ concentration of $0.15\pm 0.02$ parts per million for the nanodiamond sample.

Precise understanding of the Coulomb interaction between single electrons is vital to achieve accurate single-electron control toward quantum current standards and quantum information processing. Since the strength of the Coulomb interaction increases with decreasing distance, a collision experiment of single electrons would be an ideal way to investigate it. It would be useful to study such a Coulomb collision in silicon single-electron pumps, which can accurately transfer single electrons one by one, while silicon systems have not been used for making Coulomb collisions at the single-electron level. Here, we made two series-coupled tunable-barrier single-electron pumps in silicon and used one to inject a hot single electron into the other pump in each pumping cycle. The hot single electron collides with a cold single electron confined in the other single-electron pump. We observed a current flow due to ejection not only of the hot single electron but also of the confined cold single electron. The latter leads to an excess current at a current plateau at a certain voltage range. We also found that increasing the number of cold electrons from one to two increased the cold-electron current by at least twofold. These results can be explained by a charging effect due to the Coulomb interaction. This observation is valuable to understand single-electron dynamics in the silicon single-electron devices toward accurate current generation and quantum manipulation of flying single electrons.

Topological quantum catalysts are developing rapidly due to the emergence of exotic quantum materials and their corresponding catalytic performance. Although tens of thousands of topological semimetals have been developed, their low topological surface density of states (DOSs) remains a hindrance to the development of high-performance catalysts for electrochemical hydrogen evolution reactions (HERs). In this work, we investigate the potential of double dual-nodal line (DDNL) semimetals, which exhibit large surface DOSs and fair-low Gibbs free energies, as ideal topological quantum catalysts (TQCs) for HER. Using the $\mathrm{Na}\mathrm{Al}\mathrm{Ge}$ compound as a representative example, we demonstrate that its DDNLs and high surface DOSs around the Fermi level provide advantages for achieving exceptionally high catalytic activity in the electrochemical HER process. Through a comparison of catalytic performance under different (electron and hole) doping and uniaxial strain conditions, we establish a linear correlation between the Gibbs free energy ($\mathrm{\Delta }{G}_{{\mathrm{H}}^{\ast }}$) and the projected surface DOSs on the (001) semi-infinite surface of the DDNL semimetal $\mathrm{Na}\mathrm{Al}\mathrm{Ge}$, specifically for the hydrogen evolution process. Our work introduces an alternative category of high-performance TQCs free of noble metals and contributes to a better understanding of the relationship between catalytic performance for HER and surface DOSs in DDNL semimetals.

The ability to create phase-controlled pulses of light with wavelength tunability has applications spanning quantum and classical communications networks. Traditionally, optical transmitters are able to either produce phase-controlled pulses at a fixed wavelength or require a chain of bulky and expensive external modulators to convert wavelength tunable continuous-wave light into optical pulses. One technology of great interest is quantum key distribution (QKD), a technology for generating perfectly random keys at remote nodes to ensure secure communications. Environments such as data centers, where the user needs change regularly, will require adaptability in the deployment of QKD to integrate into classical optical networks. Here we propose and demonstrate an alternative quantum transmitter design consisting of a multimodal Fabry-Perot laser optically injection locked by a wavelength tunable laser. The transmitter is able to produce phase-controlled optical pulses at GHz speeds with a tunable wavelength range of $>65\mathrm{nm}$ centered at 1550 nm. With this transmitter, we perform proof-of-principle QKD with secure bit rates of order Mb/s.

Rare-earth (RE) atoms in solid-state materials are attractive components for photonic quantum information systems because of their coherence properties even in high-temperature environments. We have experimentally performed the single-site optical spectroscopy and optical addressing of a single RE atom in an amorphous silica optical fiber at room temperature. The single-site optical spectroscopy of the tapered RE-doped fiber shows nonresonant emission lines similar to those seen in the case of an unstructured fiber and the autocorrelation function of photons emitted from the fiber shows the antibunching effect due to the spatial isolation given by the tapered fiber structure. The ability to address single RE atoms at room temperature provides a very stable and cost-effective technical platform for the realization of a solid-state system for a large-scale quantum optical network and other quantum technologies based on a large number of spectral channels from visible to midinfrared wavelengths.

Quantum transduction between microwave and optical photons plays a key role in quantum communication among remote qubits. Although the quantum transduction schemes generating communication photons have been successfully demonstrated by using optomechanical interfaces, the low conversion efficiency remains an obstacle to the implementation of a quantum network consisting of multiple qubits. Here, we present an efficient quantum transduction scheme using a one-dimensional diamond optomechanical crystal cavity tuned at a color-center emission without optomechanical coupling. The optomechanical crystal cavity incorporates a thin aluminum nitride ($\mathrm{Al}\mathrm{N}$) pad piezoelectric coupler near the concentrator cavity region, while retaining ultrasmall mechanical and optical mode volumes of about $1.5\times {10}^{-4}({\mathrm{\Lambda }}_p{)}^3$ and $0.2(\lambda /n{)}^3$, respectively. The energy level of a coherent color-center electron is manipulated by a strong mechanical-mode–color-center electron-coupling rate up to 16.4 MHz. In our system, we theoretically predict that the population-conversion efficiency from a single microwave photon into an optical photon can reach 15% combined with current technologies. The coherent conversion efficiency is over 10% with a reasonably pure decay time of $T_2^{\ast }>10\mathrm{ns}$. Our results imply that an atomic color center strongly coupled to the optomechanical crystal cavity will offer a highly efficient quantum transduction platform.

We report the realization of an acoustic quadratic Weyl semimetal, which hosts a pair of quadratic Weyl points with opposite charges ±2 in the two lowest bands. The quadratic Weyl points, protected by $C_4$ and time-reversal symmetries, have quadratic dispersions along the $k_x$ and $k_y$ directions and linear dispersion along the $k_z$ direction. Nonzero charges of the quadrative Weyl points give rise to the Fermi-arc boundary states. By constructing a domain wall, we further achieve the double negative refraction of the boundary acoustic waves without reflection in experiment. These results evidence the quadratic Weyl semimetal, and have potential applications in innovative acoustic devices.

We present a comprehensive study on the high-frequency magnetization dynamics of homogeneously patterned ultrathin magnetic nanowire arrays. The backward volume magnetostatic spin-wave (BVMSW) and Damon-Eshbach (DE) configurations are studied along with the intermediate transition states to understand the edge mode’s evolution in depth. We find at the sub-$10$-$\text{nm}$ ultrathin regime the dynamics are heavily influenced by geometrical parameters such as magnetic layer thickness ($t_{\mathrm{FM}}$), demagnetization factors, and interfaces. Critical entities such as field separation $\delta H$ between uniform to edge mode increase linearly with $1/t_{\mathrm{FM}}$, while the edge saturation field $H_{\text{sat}}^{\mathrm{edge}}$ increase monotonically with increasing $t_{\mathrm{FM}}$, revealing excellent agreement between findings from experimental and micromagnetic simulations. The dynamics are less sensitive to the width of the nanowire but very sensitive to the adjacent material or the interface of the ferromagnet, especially at the ultrathin limits.

We report on multiphoton statistics of correlated twin beams produced in a periodic poled microring resonator on thin-film lithium niobate. Owing to high-cavity confinement and near-perfect quasi-phase-matching, the photon pairs are produced efficiently in single modes at rates reaching 27 MHz per $\text{μ}\mathrm{W}$ pump power. By using a pump laser whose pulse-width impedance matches with the cavity, those photons are further created in single longitudinal modes with purity reaching 99%, without relying on later-on filtering. With a dual-channel photon-number-resolving detection system, we obtain directly the joint detection probabilities of multiphoton states up to three photons, with high coincidence to accidental contrast for each. Used as a single-photon source, it gives heralded $g_H^{(2)}(0)$ around 0.04 at a single-photon rate of 650 kHz on chip. The findings of our research highlight the potential of this nanophotonic platform as a promising platform for generating nonclassical, few-photon states with ideal indistinguishability, for fundamental quantum optics studies and information applications.

Sophisticated positive-operator-valued measures (POVMs) are fundamental to obtain a quantum advantage in many informational problems but are difficult to implement. We design a universal measurement device composed of cascaded quantum-walk modules able to realize arbitrary entangled POVMs on two qubits. Only parameters in the modules need reprogramming for different POVMs. We apply our device to the task of direction guessing, realizing nine different five-outcome entangled measurements (fidelities above 0.9850) via nine-step photonic quantum walks, recovering optimal direction guessing from nonideal input states. Our work marks an important step toward building a universal versatile quantum measurement device able to implement optimal measurements for various quantum information tasks.

We propose an on-chip fiber-based quantum transducer to convert microwave photons into optical-telecommunication-band photons. The device consists of a coplanar-waveguide microwave resonator and a pair of coupled optical cavities integrated into a rare-earth-doped fiber. The microwave resonator is formed by two aluminum strips deposited onto the cladding of the fiber, whose diameter has been etched down to approximately $8\text{μ}\mathrm{m}$. We study the conversion process using the cavity electro-optic formalism for a system operating at liquid-helium temperature (around 4 K), as well as using a three-wave mixing approach for operation at millikelvin temperatures achievable in a dilution refrigerator. Both methods predict that unity conversion efficiency could be achievable with realistically achievable experimental parameters. The proposed device acts as a quantum transducer for coherent microwave-to-optical conversion, which has great promise for application in all-fiber integrated quantum devices for quantum information processing.

We present a scheme of quantum repeater that uses entangled multimode coherent states that are obtained by electro-optic modulation of symmetric and antisymmetric Schrödinger cat states. Part of the generated entangled frequency modes is sent to a symmetric beam splitter at the central node, while the remaining modes are stored locally in quantum memories. The entangled coherent states between remote quantum memories are conditionally prepared by photon-counting measurements at the output channels of the beam splitter. We study how the effects of decoherence in the quantum channel affect statistics of photocounts and, for the heralding outcomes determined by the parity of photocounts, evaluate the probability of success and the fidelity of the prepared entanglement depending on the symmetry of input cat states. It is demonstrated that the generated entanglement can be employed for teleportation of the phase information from the modulated states that are utilized in quantum key distributions with subcarrier wave encoding.

We develop a computational method, based on Gauss’s law, for calculating the effective permeability tensor of materials with magnetic inclusions. Electromagnetic composites find application in a variety of modern technologies since they have properties that are different from their constituent materials. However, many effective medium calculations for the permittivity or permeability tensor detailed over the past 120 years have issues with a limited range of applicability or with ignoring the coupling between different components of the tensor. Here, a computational approach is developed for calculating the effective tensor permeability of a composite material, which eliminates these issues. We show its validity by comparing to analytic results in the relevant limits, namely composites comprised of thin films and regular inclusions of low filling fraction. Examples of irregularly shaped magnetic inclusions and materials with high magnetic filling fractions are then presented for which analytic effective medium methods cannot make predictions.

We have proposed and experimentally verified a tunable interqubit coupling scheme for large-scale integration of superconducting qubits. The key feature of the scheme is the insertion of connecting pads between the qubit and tunable coupling element. In such a way, the distance between two qubits can be increased considerably to a few millimeters, leaving enough space for arranging control lines, readout resonators, and other necessary structures. The increased interqubit distance provides more wiring space for the flip-chip process and reduces crosstalk between qubits and from control lines to qubits. We use the term tunable coupler with capacitively connecting pad (TCCP) to name the tunable coupling part that consists of a transmon coupler and capacitively connecting pads. With the different placement of connecting pads, different TCCP architectures can be realized. We have designed and fabricated a few multiqubit devices in which TCCP is used for coupling. The measured results show that the performance of the qubits coupled by TCCP, such as $T_1$ and $T_2$, was similar to that of traditional transmon qubits without TCCP. Meanwhile, our TCCP also exhibited a wide tunable range of the effective coupling strength and a low residual $ZZ$ interaction between the qubits by properly tuning parameters on the design. Finally, we successfully implemented an adiabatic controlled-$Z$ gate with TCCP. Furthermore, by introducing TCCP, we also discuss the realization of the flip-chip process and tunable coupling qubits between different chips.