We introduce photonic kernel machines, a scheme for ultrafast spectral analysis of noisy radio-frequency signals from single-shot optical intensity measurements. The approach combines the versatility of machine learning and the speed of photonic hardware to reach unprecedented throughput rates. We theoretically describe some of the key underlying principles, and then numerically illustrate the performance achieved in a photonic lattice-based implementation. We apply the technique both to picosecond pulsed radio-frequency signals, on energy-spectral-density estimation and a shape-classification task, and to continuous signals, on a frequency-tracking task. The optical computing scheme presented is resilient to noise while requiring minimal control on the photonic-lattice parameters, making it readily implementable in realistic state-of-the-art photonic platforms.

Topological pumping allows higher-dimensional topological phenomena to be explored in low-dimensional systems using synthetic dimension with time modulation. Extensive studies have been devoted to the one-dimensional pumping lattice with singly modulated parameters, which was shown to exhibit the mechanical-wave analog of the quantum Hall effect. Here, we study the topological pumping in the lattice chain with doubly modulated parameters, and report the theoretical finding of two additional topological phases with the large-gap Chern number, which has not been well disclosed previously in singly modulated mechanical systems. The edge state protected by the large-gap Chern number exhibits the faster edge-to-edge energy transportation under the slow temporal variation of modulating parameters. The adiabatic approximation of doubly modulated pumping is also analyzed, and it would not be violated in the process of the fast-pumping evolution of edge states. The finding is expected to be used for fast and robust energy transportation of mechanical waves.

In this work, we report on the bias-tunable and highly sensitive irradiance-dependent focus-induced photoresponse (FIP) in amorphous silicon p-i-n diodes for enhanced three-dimensional imaging applications. We identify a nonlinear current breakdown at irradiance down to at least $0.6\mu \mathrm{W}/\mathrm{m}{\mathrm{m}}^2$ due to defect-induced electrical field screening, surpassing state-of-the-art device sensitivities by at least a factor of 16 in the visible range. We further demonstrate the tunability of FIP by systematic device and field engineering to enable both precise and long-range high-resolution distance measurements in just one device architecture. We evaluate distance measurements up to at least 15 m and achieve a depth resolution of 2.1 mm at 3.3 m. The reproducible low-temperature (300 °C or lower) plasma-enhanced chemical vapor-deposition technology enables systematic tailoring of the FIP for three-dimensional imaging applications and allows for back-end sensor integration on top of silicon or flexible electronics with fill factors up to 100%.

Coherent gate errors are a concern in many proposed quantum-computing architectures. Here, we show that certain coherent errors can be reduced by a local optimization that chooses between two forms of the same Hermitian and unitary quantum gate. We refer to this method as hidden inverses, and it relies on constructing the same gate from either one sequence of physical operations or the inverted sequence of inverted operations. We use parity-controlled $Z$ rotations as our model circuit and numerically show the utility of hidden inverses as a function of circuit width $n$. We experimentally demonstrate the effectiveness for $n=2$ and $n=4$ qubits in a trapped-ion quantum computer. We numerically compare the method to other gate-level compilations for reducing coherent errors.

The rapidly developing quantum technologies and thermodynamics have put forward a requirement to precisely control and measure the temperature of microscopic matter at the quantum level. Many quantum thermometry schemes have been proposed. However, precisely measuring low temperature is still challenging because the obtained sensing errors generally tend to diverge with decreasing temperature. Using a continuous-variable system as a thermometer, we propose non-Markovian quantum thermometry to measure the temperature of a quantum reservoir. A mechanism to make the sensing error $\delta T$ scale with the temperature $T$ as the Landau bound $\delta T\simeq T$ in the full-temperature regime is discovered. Our analysis reveals that it is the quantum criticality of the total thermometer-reservoir system that causes this enhanced sensitivity. Efficiently avoiding the error-divergence problem, our result gives an efficient way to precisely measure the low temperature of quantum systems.

Ultrafast vortex motion has recently become a subject of extensive investigations, triggered by the fundamental question regarding the ultimate speed limits for magnetic flux quanta and enhancements of single-photon detectors. In this regard, the current-biased quench of a dynamic flux-flow regime—flux-flow instability (FFI)—has turned into a widely used method for the extraction of information about the relaxation of quasiparticles (unpaired electrons) in a superconductor. However, the large relaxation times ${\tau }_{ϵ}$ deduced from FFI for many superconductors are often inconsistent with the fast relaxation processes implied by their single-photon counting capability. Here, we investigate FFI in $15$-nm-thick $182$-$\mu \mathrm{m}$-wide MoSi strips with rough and smooth edges produced by laser etching and milling by a focused ion beam. For the strip with smooth edges we deduce, from current-voltage ($I$-$V$) curve measurements, a factor of 3 larger critical currents $I_c$, a factor of 20 higher maximal vortex velocities of 20 km/s, and a factor of 20 shorter ${\tau }_{ϵ}$. We argue that for the deduction of the intrinsic ${\tau }_{ϵ}$ of the material from the $I$-$V$ curves, utmost care should be taken regarding the edge and sample quality and such a deduction is justified only if the field dependence of $I_c$ points to the dominating edge pinning of vortices.

Deterministic sources of high-fidelity entangled qubit pairs encoded in the dual-rail photonic basis, i.e., presence of a single photon in one of two orthogonal modes, are a key enabling technology of many applications of quantum information processing, including high-rate, high-fidelity quantum communications over long distances. The most popular and mature sources of such photonic entanglement, e.g., those that leverage spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing, generate an entangled (so-called continuous-variable) quantum state that contains contributions from high-order photon terms that lie outside the span of the dual-rail basis, which is detrimental to most applications. One often uses low pump power to mitigate the effects of those high-order terms. However, that reduces the pair generation rate, and the source becomes inherently probabilistic. We investigate a cascaded source that performs a linear-optical entanglement swap between two SPDC sources, to generate a heralded photonic entangled state that has a higher fidelity (to the ideal Bell state) compared to a free-running SPDC source. Furthermore, with the Bell swap providing a heralding trigger, we show how to build a multiplexed source, which despite reasonable switching losses and detector loss and noise, yields a fidelity versus success probability trade-off of a high-efficiency source of high-fidelity dual-rail photonic entanglement. We find, however, that there is a threshold of $1.5$ dB of loss per switch, beyond which multiplexing hurts the fidelity versus success probability trade-off.

Acoustic waves and strain pulses created by a piezoelectric or optical transducer represent an attractive tool for the generation of spin dynamics in magnetoelastic materials and heterostructures. The advantage of acoustic excitation consists in much lower power consumption compared with the classical excitation by a microwave magnetic field. The magnetization precession driven by elastic waves can be further used for spin pumping into normal metals and semiconductors. Here we describe theoretically the magnetization dynamics and spin flow induced by plane acoustic waves traversing trilayer structures involving two ferromagnetic ($F$) films separated by a layer of normal metal ($N$). The problem is solved by advanced micromagnetoelastic simulations, which allow for the two-way coupling between spins and strains in the $F$ films and their long-range dynamic interaction resulting from the spin flow across the $N$ spacer. To derive the expression for such an interaction, we solve the spin-diffusion equation in $N$ with the account of spin pumping created by dynamically strained $F$ films and spin backflow through the interfaces. Performing numerical simulations for the ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}{/\mathrm{Au}/\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ trilayers excited by longitudinal and transverse elastic waves, we quantify the inhomogeneous magnetization precession in the galfenol films and determine the spatial distributions of the oscillating spin current and spin accumulation in the Au spacers of different thickness. These distributions are further used to calculate the mean spin current and total spin accumulation in the spacer. It is found that both ac and dc parts of these quantities exhibit strong variations with the spacer thickness. Remarkably, the nonzero components of the total spin accumulation and mean spin current mostly have pronounced maxima at $\mathrm{Au}$ thicknesses amounting to about 0.25 and 0.75 of the wavelength ${\lambda }_N$ of the elastic wave in $\mathrm{Au}$, which is explained by an analytical model of strain distribution in the trilayer. Our theoretical results, which shed light on the acoustically excited generation of spin imbalance in magnetoelastic spin-valve structures, could be useful for the development of energy-efficient spin injectors into normal conductors with weak spin-flip scattering.

The mutual interaction between a chiral ferromagnetic disk with confined skyrmionic structures and a superconducting disk is studied. The interaction is accounted for by solving simultaneously the Brown and London equations. We find that for a superconductor under an applied uniform field, the stray field of the shielding currents can trigger switching between different skyrmionic states in the ferromagnet that is not possible without the superconductor. We discuss the effect of the different parameters on the results and some possible applications such as multivalued logic, storage, and skyrmionic metamaterials.

Low-dimensional materials provide a unique platform for exploring exotic properties that are otherwise unachievable in bulk solids. ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ are two graphene-derived two-dimensional (2D) ordered alloys that have attracted increasing research attention. These materials are best known for their remarkable stability and moderate band gaps, and thus, are suitable for a range of applications. Perhaps the most interesting feature of the electronic structures of ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ is the existence of nearly parallel valence and conduction bands across a large region of the Brillouin zone. In this work, using many-body perturbation theory within the GW and Bethe–Salpeter-equation approach, we predict that the primarily $p_z$-orbital-derived nearly parallel valence and conduction bands in monolayer ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$ give rise to a giant narrow-band absorption peak in their optical absorption spectra. More surprisingly, two degenerate excitonic states contribute to over 90% and 80% of the dipole absorption below 5 eV for ${\mathrm{C}}_3\mathrm{N}$ and ${\mathrm{C}}_3\mathrm{B}$, respectively. Detailed examinations of the exciton-binding energies unveil a unique shell-like distribution of the excitonic states, with each shell (series) converging to a different excitation edge. Such distinctive absorption properties are not observed in any other 2D materials. We further investigate the internal structure of the excitonic states using a multifaceted approach and reveal several important characteristics of the excitonic states in these 2D materials.

All physical systems employed for quantum information tasks must act as unbiased carriers of encoded quantum states. Ensuring such indistinguishability of information carriers is a major challenge in many quantum information applications, including advanced quantum communication protocols. For photons, the workhorses of quantum communication networks, it is difficult to obtain and maintain their indistinguishability because of environment-induced transformations and loss imparted by communication channels, especially in noisy scenarios. Conventional strategies to mitigate these transformations often require hardware or software overhead that is restrictive (e.g., adding noise), infeasible (e.g., on a satellite), or time-consuming for deployed networks. Here we propose and develop resource-efficient Bayesian optimization techniques to rapidly and adaptively calibrate the indistinguishability of individual photons for quantum networks using only information derived from their measurement. To experimentally validate our approach, we demonstrate the optimization of Hong-Ou-Mandel interference between two photons–a central task in quantum networking– finding rapid, efficient, and reliable convergence towards maximal photon indistinguishability in the presence of high loss and shot noise. We expect our resource-optimized and experimentally friendly methodology will allow fast and reliable calibration of indistinguishable quanta, a necessary task in distributed quantum computing, communications, and sensing, as well as for fundamental investigations.

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinskii-Moriya interaction is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. This is especially true for polycrystalline materials, which can be grown using scalable techniques, the scalability of which is promising for future device applications. One such material is sputter-deposited B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, which is actively investigated as the basis for low-power high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal $\mathrm{Fe}\mathrm{Ge}$, probing the magnetism of sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ is more challenging because the submicron crystal grains add confounding contrast. This is a more general problem for polycrystalline magnetic devices, where scattering from grain boundaries tends to hide comparably weaker signals from magnetism. We address the challenge of disentangling magnetic and grain contrast by applying four-dimensional Lorentz scanning transmission electron microscopy using an electron-microscope pixel-array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic-range detector becomes extremely important. Despite the small grain size in sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, using this approach, we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains, as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.

Materials with extremely low impedance like air can insulate efficiently waterborne sound, particularly at low frequency, however the poor resistance to load hinders further their applications. This work demonstrates that the elastic anisotropy of materials can not only achieve extremely low impedance, but also high enough stiffness for load bearing. A dimensionless quality factor is proposed to characterize integrately the performance on sound insulation and load bearing of anisotropic materials. It is found that this factor has an upper bound of unity and the corresponding conditions are derived for some particular anisotropic materials. By setting the quality factor to be unity as an objective function for microstructure optimization, a methodology based on topological optimization is proposed to design the microstructure of the corresponding materials with optimized integrated performance on waterborne sound insulation and load bearing. The obtained optimized material is validated by numerical simulation and by comparing with other similar waterborne sound insulation materials with low impedance. This study paves the way for designing load-bearing materials with extremely low impedance, and opens a route to control low-frequency underwater waves.

Write error rate (WER) in voltage-induced magnetization switching of a conically magnetized free layer with an elliptic cylinder shape is numerically calculated on the basis of the macrospin model. Simulations using the Langevin equation show that a WER lower than the order of ${10}^{-5}$, which enables practical WER with a single error-correction process, is theoretically feasible at zero-bias magnetic field and room temperature. We also show that WER at a given thermal stability factor and the volume of the free layer can be reduced by adopting the free layer with the larger in-plane shape anisotropy field and a moderate cone angle at zero-bias voltage.

Redox-based resistive random access memories (ReRAMs) are promising candidate devices for new memory and computing paradigms. However, the fundamental mechanisms that rule the conduction in these devices are still heavily debated. The present work focuses on studying one model for the conduction, the quantum point contact (QPC), and specifically a single-subband approximation (SSA) to this model. With this intent, $\mathrm{Pt}$(20 nm)/$\mathrm{Ta}$(15 nm)$\mathrm{Ta}$${}_2$$\mathrm{O}$${}_5$(5 nm)/$\mathrm{Pt}$(20 nm) resistive switching devices are fabricated and electrically characterized by measuring current-voltage ($I$-$V$) curves in both resistance states. The QPC model has been found to be hard to apply, as the starting parameters have a strong influence on the fitting results. On the other hand, the SSA has proved its ability to provide good fits to the data and to do so better than other typical conduction mechanisms considered. However, its physical basis is criticized and it is concluded that in the devices studied, multiple subbands likely contribute to the conduction, in direct opposition to the assumptions made in such an approximation. A reinterpretation of the parameters of the SSA is proposed, to reconcile the increased performance with greater physical accuracy. Beyond that, the main challenges and difficulties regarding the application of the QPC to the case of valence-change-based ReRAM are discussed.

Twisted moiré superlattices (TMSs) are emergent materials with exotic physical properties. Among their properties, higher-order topology is seldom realized and investigated in experiments. Here, we report on the experimental observation of a class of bilayer higher-order topological states in TMSs. We create the physical realization of acoustic TMSs using moiré twisting and ultrastrong interlayer couplings in bilayer honeycomb lattices of coupled acoustic cavities. Through such a design we reach an unexplored regime in TMSs that is not available in graphene TMSs and other solid-state TMS systems. We reveal that the ultrastrong interlayer couplings lead to a large acoustic band gap with unique higher-order band topology, which is characterized by unprecedented topological indices and layer-hybridized corner states. The higher-order topological edge and corner states are observed via acoustic pump-probe measurements, which show consistency with theory and simulations. Our study paves the way toward higher-order topological phenomena in TMSs.

We consider mediated interactions in an array of floating transmons, where each qubit capacitor consists of two superconducting pads galvanically isolated from ground. Each such pair contributes two quantum degrees of freedom, one of which is used as a qubit, while the other remains fixed. However, these extraneous modes can generate coupling between the qubit modes that extends beyond the nearest neighbor. We present a general formalism describing the formation of this coupling and calculate it for a one-dimensional chain of transmons. We show that the strength of coupling and its range (that is, the exponential falloff) can be tuned independently via circuit design to realize a continuum from nearest-neighbor-only interactions to interactions that extend across the length of the chain. We discuss how this coupling breaks integrability, allowing the system to locally equilibrate, and so enables exploration of quantum thermalization. We present designs with capacitance and microwave simulations showing that various interaction configurations can be achieved in realistic circuits. Such a coupling could be used in analog simulation of different quantum regimes or to increase connectivity in digital quantum systems. Thus the mechanism must also be taken into account in other types of qubits with extraneous modes.

One of the key trends in laser material processing is the usage of structured laser beams. Collimated and focused Gaussian beams are the most common tools; however, more exotic beams can be beneficial too. For instance, Bessel beams with elongated focal area and self-healing properties, or vortex beams with helical wave fronts and a dark area along the optical axis are being increasingly used. Here, we propose and experimentally demonstrate dynamical “optical drill” beams presenting nonstationary intensity distributions that resemble a spinning mechanical drill. Optical drills appear as the spatiotemporal interference of two Bessel-vortex beams of different topological charges and different carrier frequencies. By mixing a pair of high-order Bessel beams, synthesized using a liquid crystal spatial light modulator, optical drills of tuned helicities are experimentally observed, and the simplest cases of matter processing (fluorescence) with such beams are demonstrated. Optical drill beams are expected to be useful in material processing by light or in cell and particle manipulation in biomedical applications.