We propose a design for tuning the resonant spectra of Fabry-Pérot nanocavities mediated by the Casimir force. The system involves a suspended gold nanoplate approaching a dielectric-coated gold substrate in a univalent electrolyte solution. The gold nanoplate can be stably suspended due to the delicate balance between repulsive and attractive components of the Casimir forces. In an electrolyte solution, the presence of ionic-charge fluctuations can partially or totally screen the thermal $n=0$ Matsubara term, resulting in strongly modified interactions. As a result, the separation between the gold nanoplate and the substrate experiences a significant modulation in response to variations in salt concentration. Under proper conditions, we find that the modulation of the Casimir force would strongly shift the resonances of Fabry-Pérot nanocavities at the optical frequencies, when the Debye length of the electrolyte decreases from 1000 nm to 10 nm. Finally, the temperature dependence of the thermal Casimir force would provide an additional modulation of Fabry-Pérot nanocavity resonances for their eventual fine tuning. These results open up a promising venue for general tuning of the optical resonances with potential applications in reconfigurable microfluidic nanophotonics.

Topological photonics has established itself as a promising area for the development of photonic integrated circuits due to the robust properties of topological states. In recent years, different logical devices have been proposed to form the fundamental elements of an integrated topological photonic circuit. Despite the rapid theoretical growth of topological photonics, it still lacks experimental foundations concerning logical devices for on-chip integration. For this reason, in this work, we report the design and build of an all-optical photonic switch in a silicon slab working under the higher-order topological spin Hall effect. We have taken advantage of the gapped edge states due to the higher-order topology and combined this feature with changes in the dielectric permittivity of silicon by inducing transitions of free charge carriers. The transitions of charge carriers were realized through the incidence of electromagnetic radiation with a frequency close to the visible-ultraviolet region boundary, making the device operation entirely optical. The behavior of the constructed device has also been numerically simulated and compared with the experimental results obtained. This work contributes to the idea that photonic crystals are a powerful platform for studying topological states and paves the way for the experimental realization of integrated topological optical circuits.

The efficiency of electroluminescent devices based on thermally activated delayed fluorescence (TADF) is a complex interplay of spin-allowed radiative and nonradiative singlet and triplet recombination as well as spin-flip (reverse) intersystem crossing processes. An analytical description of exciton dynamics based on the various singlet and triplet transition rates would strongly facilitate the evaluation of the effects of different processes on device performance. We present unified and analytical expressions for the exciton densities, photoluminescence quantum yield (PLQY), and charge-to-photon (CTP) conversion efficiency in TADF-based electroluminescent devices. The derivation is based on a three-level model that can also be applied to conventional fluorescence- or phosphorescence-based devices. The model allows us to analytically calculate the fundamental kinetic rates in TADF systems as well as the CTP efficiency in devices with only PLQY and transient photoluminescence decay as experimental inputs. Evaluation of the individual kinetic processes reveals that TADF emitters with PLQY as high as around 90% that exhibit pronounced delayed fluorescence, intuitively treated as potential candidates for high-performance electroluminescent devices, can still result in a CTP efficiency of only 50%–60% due to the direct competition between triplet recombination and reverse intersystem crossing.

Parallel-plate capacitors (PPC) significantly reduce the size of superconducting microwave resonators, reducing the pixel pitch for arrays of single-photon energy-resolving kinetic inductance detectors (KIDs). The frequency noise of KIDs is typically limited by tunneling two-level systems (TLS), which originate from lattice defects in the dielectric materials required for PPCs. How the frequency noise level depends on the PPC’s dimensions has not been experimentally addressed. We measure the frequency noise of 56 resonators with $a$-$\mathrm{Si}\mathrm{C}$:$\mathrm{H}$ PPCs, which cover a factor of 44 in PPC area and a factor of 4 in dielectric thickness. To support the noise analysis, we measure the resonators’ TLS-induced power-dependent intrinsic loss and temperature-dependent resonance frequency shift. From the TLS models, we expect a geometry-independent microwave loss and resonance frequency shift, which is set by the TLS properties of the dielectric. However, we observe a thickness-dependent microwave loss and resonance frequency shift; this is explained by surface layers that limit the performance of PPC-based resonators. For a uniform dielectric, the frequency noise level should scale directly inversely with the PPC area and thickness. We observe that an increase in PPC size reduces the frequency noise, but the exact scaling is, in some cases, weaker than expected. Finally, we derive engineering guidelines for the design of KIDs based on PPC-based resonators.

The Toffoli gate takes a special place in the quantum information theory. It opens up a path for efficient implementation of complex quantum algorithms. Despite tremendous progress of the quantum processors based on the superconducting qubits, realization of a high-fidelity three-qubit operation is still a challenging problem. Here, we propose a way to perform a high-fidelity $CCZ$ gate on fluxoniums capacitively connected via a transmon qubit, activated by a microwave pulse on the coupler. The main advantages of the approach are relative quickness, simplicity of calibration and significant suppression of the unwanted longitudinal $ZZ$ interaction. We provide numerical simulation of a 95-ns-long gate of higher than 99.99% fidelity with realistic circuit parameters in the noiseless model and estimate an error of about 0.25% under the conventional decoherence rates.

The concept of perpendicular-shape-anisotropy spin-transfer-torque magnetic random-access memory tackles the downsize scalability limit of conventional ultrathin magnetic tunnel junctions below sub-20-nm technological nodes. This concept uses a thicker storage layer with a vertical aspect ratio, enhancing the thermal stability factor $\mathrm{\Delta }$ thanks to the favorable contribution of the shape anisotropy. However, the increased aspect ratio comes with an increase in switching time under applied voltage and the crossover to nonuniform reversal mechanism at higher aspect ratio, limiting the gain in scalability. Additionally, the larger volume of the magnetic cell significantly increases the stray field acting on the neighboring devices compared to thin magnetic tunnel junctions. In this work, we propose the use of a dipole-coupled core-shell system as a storage layer. This improves both bottlenecks, as predicted by micromagnetic simulations for magnetization reversal, and a macrospin model to estimate the stray field in a dense array.

The integration of two-dimensional (2D) ferroelectrics with other materials holds immense significance for exploring physics at the nanoscale. In this work, we systematically investigate the electronic and transport properties of 2D ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{P}}_4$/${\mathrm{Sc}}_2{\mathrm{CO}}_2$ and ${\mathrm{WSi}}_2{\mathrm{P}}_4$/${\mathrm{Sc}}_2{\mathrm{CO}}_2$ ferroelectric van der Waals heterostructures using density-functional theory and the nonequilibrium Green function method. The results reveal that the semiconductor-metal transition of ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{P}}_4$ and ${\mathrm{WSi}}_2{\mathrm{P}}_4$ monolayers can be flexibly realized by switching the ferroelectric polarization of the ${\mathrm{Sc}}_2{\mathrm{CO}}_2$ monolayer. Moreover, the metallicity of ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{P}}_4$ and ${\mathrm{WSi}}_2{\mathrm{P}}_4$ monolayers is further enhanced as the thickness of the ferroelectric layer increases, and the clamped sandwich structure also allows the nonvolatile electrical control of the metallicity of these two materials. Accordingly, proof-of-concept diodes based on ${\mathrm{Mo}\mathrm{Si}}_2{\mathrm{P}}_4$/${\mathrm{Sc}}_2{\mathrm{CO}}_2$ and ${\mathrm{WSi}}_2{\mathrm{P}}_4$/${\mathrm{Sc}}_2{\mathrm{CO}}_2$ heterostructures exhibit giant current on:off ratios of up to ${10}^6$ and ${10}^5$, respectively. These findings not only provide viable strategies to realize and control metallicity in 2D semiconductors, but also offer promising candidates for the design of advanced memories.

As the quantum computing community gravitates towards understanding the practical benefits of quantum computers, having a clear definition and evaluation scheme for assessing practical quantum advantage in the context of specific applications is paramount. Generative modeling, for example, is a widely accepted natural use case for quantum computers, and yet has lacked a concrete approach for quantifying success of quantum models over classical ones. In this work, we construct a simple and unambiguous approach to probe practical quantum advantage for generative modeling by measuring the algorithm’s generalization performance. Using the sample-based approach proposed here, any generative model, from state-of-the-art classical generative models such as generative adversarial networks (GANs) to quantum models such as quantum circuit born machines, can be evaluated under the same conditions on a concrete well-defined framework. In contrast to other sample-based metrics for probing practical generalization, we leverage constrained optimization problems (e.g., cardinality-constrained problems) and use these discrete datasets to define specific metrics capable of unambiguously measuring the quality of the samples and the model’s generalization capabilities for generating data beyond the training set, but still within the valid solution space. Additionally, our metrics can diagnose trainability issues such as mode collapse and overfitting, as we illustrate when comparing GANs to quantum-inspired models built out of tensor networks. Our simulation results show that our quantum-inspired models have up to a 68 times enhancement in generating unseen unique and valid samples compared to the GANs explored here, and a ratio of 61:2 for generating samples with better quality than those observed in the training set. We foresee these metrics as valuable tools for rigorously defining practical quantum advantage in the domain of generative modeling, where finding a winning model remains an ongoing challenge as new architectures are developed.

Here, we report detailed first-principles calculations of the structural stability, optoelectronic properties, and interaction with water for a wide range of mixed-halide compositions of vacancy-ordered double perovskites ${\mathrm{Cs}}_2{\mathrm{Pt}(\mathrm{Cl}}_x{\mathrm{I}}_{1-x}{)}_6$. Our calculations reveal that lower halide dopant levels subdue phase segregation and enhance the stability. ${\mathrm{Cs}}_2{\mathrm{Pt}(\mathrm{Cl}}_x{\mathrm{I}}_{1-x}{)}_6$ demonstrate improved defect tolerance as compared to ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$ due to the covalent nature of the Pt—X bond. The chloride-rich ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_x{\mathrm{I}}_{1-x}$)${}_6$ exhibit notably improved stability against reaction with water, far surpassing ${\mathrm{Cs}}_2{\mathrm{Pt}\mathrm{I}}_6$ due to the enhanced Cs—Cl bond strength and lower charge transfer between adsorbed ${\mathrm{H}}_2\mathrm{O}$ and surface Cs atoms. The spectroscopic limited maximum photovoltaic efficiency for the optimal composition of ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$ under 1 sun AM1.5G is determined to be $24$% for a 5-$\text{μ}\mathrm{m}$-thick film. Our calculations also suggest that the valence-band edge of this material might be positioned more positive than the standard potential of the oxygen-evolution reaction. These two factors combined with the high stability against reaction with water indicate that ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.04}{\mathrm{I}}_{0.96}$)${}_6$ might be of considerable interest as a photovoltaic absorber, and possibly as a component of anodes for the photoelectrocatalytic water oxidation. Meanwhile, ${\mathrm{Cs}}_2\mathrm{Pt}$(${\mathrm{Cl}}_{0.96}{\mathrm{I}}_{0.04}$)${}_6$ traverses relevant reduction and oxidation redox potentials, affirming it as a promising candidate for the overall photo(electro)catalyst water-splitting reaction.

A classical switch or router determines which of several inputs to the device passes through the output. In analogy, a quantum switch that coherently links the input to a superposition of several outputs plays an important role in quantum information processing, especially when operating at the fundamental limit where a single photon can be controlled. The controllable path superposition of propagating photonic qubits enabled by the quantum switch provides the basis for numerous quantum applications, including entanglement generation and distribution, enhanced quantum communication, and quantum random access memory. In this work, we realize such a quantum switch for microwave single photons with superconducting quantum circuits. We use a superconducting qubit to coherently switch the output path of single microwave photons and confirm quantum entanglement between the control qubit and the photons’ propagation path. We further showcase the generation of quantum entanglement between a time-bin-encoded propagating photonic qubit and a superconducting qubit with the quantum switch, thus providing a versatile approach to generate reconfigurable multinode entanglement in one step. As such, our work provides a prime building block for microwave quantum networks and modular superconducting quantum computing.

Acoustic vortices (AVs) play a significant role in the fields of acoustic communication and microparticle manipulation. Traditional AVs exhibit doughnut-shaped intensity profiles, which may restrict their application scenarios. Here, we propose an acoustic polygonal Bessel vortex (APBV), which can be generated by the coaxial coupling of multiple acoustic asymmetric Bessel vortices (AABVs). The number of sides of the APBV is dependent on the number of superimposed AABVs. Both topological charge and asymmetry coefficient of the AABV impact on the performance of the APBV, and the APBV also exhibits excellent nondiffracting characteristics. Moreover, an acoustic reflection metasurface (ARM) consisting of grooved units is designed to realize the APBV in air. Through simulations and experiments, we verify the generation of the APBV by the proposed ARM. The APBV may find potential applications in the polygonal manipulation of microparticles and acoustic communication.

We investigate atomic chains with different spatial configurations coupled to a ring cavity comprising two counterpropagating traveling modes. We describe the collective atom-light scattering effect with a structure factor of the atomic chain and demonstrate that the interactions between the atoms and the cavity are controlled by the structure factor, resulting in distinctly different collective excitation modes and energy spectrum than for Fabry-Pérot cavities. Remarkably, we observe that a cavity dark mode emerges when the atomic spacings are integer multiples of the half-wavelength. The nodes of this standing-wave dark mode align precisely with the atomic positions, enabling intracavity field conversion without free-space scattering. By adjusting the configuration of the atomic chain, we realize tunable photon routing and a large optical phase shift with almost no photon loss, which can be used to implement versatile building blocks for optical quantum engineering.

The success of a future quantum internet will rest in part on the ability of quantum and classical signals to coexist in the same optical fiber infrastructure, a challenging endeavor given the orders of magnitude differences in flux of single-photon-level quantum fields and bright classical traffic. We theoretically describe and experimentally implement Procrustean entanglement concentration for polarization-entangled states contaminated with classical light, showing significant mitigation of crosstalk noise in dense wavelength-division multiplexing. Our approach leverages a pair of polarization-dependent loss emulators to attenuate highly polarized crosstalk that results from imperfect isolation of conventional signals copropagating on shared fiber links. We demonstrate our technique both on the tabletop and over a deployed quantum local area network, finding a substantial improvement of two-qubit entangled state fidelity from approximately 75% to over 92%. This local filtering technique could be used as a preliminary step to reduce asymmetric errors, potentially improving the overall efficiency when combined with more complex error-mitigation techniques in future quantum repeater networks.

We investigate the Autler-Townes splitting for Rydberg atoms dressed with linearly polarized microwave radiation, resonant with generic $S_{1/2}\leftrightarrow P_{1/2}$ and $S_{1/2}\leftrightarrow P_{3/2}$ transitions. The splitting is probed using laser light via electromagnetically induced transparency measurements, where the transmission of probe laser light reveals a two-peak pattern. In particular, this pattern is invariant under rotation of the microwave field polarization. In consequence, we establish $S\leftrightarrow P$ Rydberg transitions as ideally suited for polarization-insensitive electrometry, contrary to recent findings [A. Chopinaud and J. D. Pritchard, Phys. Rev. Appl. 16, 024008 (2021)].

Reconfigurable intelligent metasurfaces have been proposed as an efficient solution for improving wireless telecommunication systems in multiple-scattering or reverberating media. Concurrently, topology optimization has been successfully used as an inverse-design technique in many fields, and particularly in electromagnetics. In this work, we apply a gradient-based topology optimization for tuning the binary elements of a metasurface for a focusing goal in a complex environment. First, the metasurface unit cells are approximated as point sources and, then, the optimization problem is formulated. Afterwards, the proposed method is applied to find the optimal parameter sets for three distinct environments of increasing complexity, and the resulting focus for each case is demonstrated via numerical simulations. The combination of a reverberating cavity and a metasurface inside the latter is very powerful since everything can be solved analytically for focusing outside the cavity.