Topological states of light open up exciting possibilities in quantum photonics, promising the topological protection of quantum entanglement. Here, we put forward an approach to realizing the topological states of photon pairs mediated by the effective density-dependent coupling, which is manifested as the dependence of the tunneling amplitude on the number of photons. As a specific platform, we investigate the arrays of nearest-neighbor coupled transmon qubits, where the effective density-dependent coupling is engineered by inserting auxiliary frequency-detuned resonators. We prove the topological origin of the designed model by the direct evaluation of the Zak phase, highlighting the feasibility of our proposal for state-of-the-art fabrication technologies.

The molten-pool flow, particularly the keyhole effect, plays a critical role in the formation of defects in additive-manufacturing and welding processes. In this study, we derive an evaporation model for metal alloys considering the gas-flow structure and material composition and implement it in a multiphysics thermal-fluid flow model, which utilizes the volume of fluid (VOF) in the finite volume method (FVM) to capture free surfaces and the ray-tracing method to track multireflections of laser within the keyhole. The current derived evaporation model is validated against in situ x-ray imaging results via multiple cases: (1) stationary laser melting of a $\mathrm{Ti}\text{−}6\mathrm{Al}\text{−}4\mathrm{V}$ base plate under 1-atm ambient pressure, (2) stationary laser melting of a 304L stainless-steel base plate under 0.0002-atm ambient pressure, (3) laser scanning of a $\mathrm{Ti}\text{−}6\mathrm{Al}\text{−}4\mathrm{V}$ base plate under 1-atm ambient pressure. The simulation results indicate that our evaporation model is applicable for both common and near-vacuum environment, while Anisimov’s evaporation model, which is widely used in keyhole simulation, is unsuitable in near-vacuum keyhole simulation. Moreover, the absorbed energy distribution, recoil pressure, $z$-direction recoil force and keyhole growth are analyzed in the simulations.

We propose a postselection technique, based on quantum error detection, for quantum key distribution (QKD) systems that run over quantum repeaters with encoding. In such repeaters, quantum error-correction techniques are used for entanglement distillation. By developing an analytical approach to study such quantum repeaters, we show that, in the context of QKD, it is often more efficient to use the error-detection, rather than the error-correction, capability of the underlying code to sift out cases where an error has been detected. We implement our technique for three-qubit repetition codes by modeling different sources of error in crucial components of the system. We then investigate in detail the impact of such imperfections on the secret key generation rate of the QKD system, and how one can use the information obtained during entanglement swapping and decoding stages to maximize the rate. For benchmarking purposes, we specify the maximum allowed error rates in different components of the setup below which positive key rates can be obtained.

Angle-resolved photoluminescence spectroscopy is a powerful tool for investigation of the properties of light-emitting thin-film structures. For instance, it can reveal information about the orientation of the transition dipole moment in organic emission layers, which is one of the most important parameters defining the efficiency of organic light-emitting diodes. In this work, a refinement of one of the main experimental configurations utilized for such orientation measurements is presented. The technique is based on a step-by-step rotation of the optically excited layer with respect to the detector. By attaching the sample to a large glass half cylinder, the light initially trapped in the substrate can be extracted. It is shown that by inserting two additional optical lenses between the half cylinder and the detector, the signal-to-noise ratio can be improved by about one order of magnitude. A ray-optics model is derived that describes the light propagation in the macroscopic setup and enables us to characterize the impact of deviations from theoretical ideal measurement configurations. The numerical predictions are experimentally validated analyzing quantum-dot emission layers as an isotropic reference and established organic emitter molecules with well-known orientation values.

We demonstrate highly sensitive photon counting in the infrared region based on two-photon absorption (2PA) in a silicon avalanche photodiode, where the required photon energy for inducing effective conductivity is provided by an intense midinfrared (MIR) field at 3 $\mu \mathrm{m}$. The used MIR pumping scheme can not only benefit from the enhanced 2PA coefficient in the nondegenerate regime, but also eliminate the detrimental background noises due to the pump harmonic excitation of the pump. Consequently, the enhancement factor for the signal counting rate unprecedentedly reached about ${10}^5$ with input infrared pulses at the femtojoule level. Additionally, the noise equivalent power is substantially improved by 2 orders of magnitude in comparison to conventional schemes with near-infrared pumping. Therefore, the presented configuration might provide an alternative to realize sensitive infrared detection and imaging with desirable features of room-temperature operation, no phase-matching requirement, and broadband responding window, which can find a variety of applications, including remote ranging, sensitive sensing, biochemical imaging, and trace spectroscopy.

Recently, thermal manipulation has gained the attention of the scientific community due to its several applications. In this paper, based on a transformation-thermodynamics methodology, a special type of material, called a thermal-null medium (TNM), is proposed that leads to the design of various thermal functionalities such as thermal bending devices, arbitrarily shaped heat concentrators, and omnidirectional thermal cloaks. In contrast to the conventional transformation-thermodynamics-based conductivities, which are inhomogeneous and anisotropic, TNMs are homogeneous and easy to realize. It is shown that the TNMs obtained are independent of the desired device shape, meaning that if the geometry of the desired device is changed, there is no need to recalculate the conductivities required. This consequently makes the designed devices suitable for scenarios where reconfigurability is of the utmost importance. Several numerical simulations are carried out to demonstrate the capability of TNMs and their application in directional bending devices, concentrators, and cloaks. In order to validate the concept, by using effective-medium theory, a TNM with an aluminum-air multilayer structure is designed and fabricated. The structure realized is then used to implement an elliptically shaped thermal concentrator. It is observed that the experimental results exhibit good agreement with the results obtained from the numerical simulations, which corroborates the effectiveness of the proposed materials.

Metamaterial resonant structures made from arrays of superconducting lumped-circuit elements can exhibit microwave-mode spectra with left-handed dispersion for the standing-wave resonances, resulting in a high density of modes in the same frequency range where superconducting qubits are typically operated, as well as a bandgap at lower frequencies that extends down to dc. Using this regime for multimode circuit quantum electrodynamics, we perform a series of measurements of such a superconducting metamaterial resonator coupled to a flux-tunable transmon qubit. Through microwave measurements of the metamaterial, we observe the coupling of the qubit to each of the modes that it passes through. Using a separate readout resonator, we probe the qubit dispersively and characterize the qubit energy relaxation as a function of frequency, which is strongly affected by the Purcell effect in the presence of the dense mode spectrum. Additionally, we investigate the ac Stark shift of the qubit as the photon number in the various metamaterial modes is varied. Through numerical simulations, we explore designs based on this scheme with enhanced coupling so that the coupling energy between the qubit and the metamaterial modes can exceed the intermode spacing in future devices with achievable circuit parameters. The ability to tailor the dense mode spectrum through the choice of circuit parameters and manipulate the photonic state of the metamaterial through interactions with qubits makes this a promising platform for analog quantum simulation with microwave photons and quantum memories.

We present an algorithm that utilizes data generated by cosmic-ray muon-scattering tomography for passive nondestructive detection of dense objects. Our clustering-based approach uses a nonparametric statistical test based on a reference case to determine the presence of high-density high-$Z$ material, such as illicit nuclear material hidden inside a shipping canister. The algorithm outputs a single decision value for the absence or presence of illicit material without the need to perform a detailed visual tomographic reconstruction and/or the need to rely on human interpretation, in contrast to many other muon-based imaging techniques. The performance of the algorithm is demonstrated using experimental data obtained with the Cosmic-Ray Inspection and Passive Tomography detector from two setups consisting of a lead flask, a 55-gallon drum filled with sand, and 2 kg of metallic depleted uranium (DU). The results of these experiments illustrate that the high-density lead flask can be automatically distinguished from the background, with an area under the receiver-operating-characteristic curve (AUC) of 0.90 using less than 90 s of data; the lead flask with 2 kg of DU inside can be detected within the 55-gallon drum containing sand in under 5 min with an AUC of 0.91. Our experimental results illustrate the efficacy of this algorithm for the identification of dense objects using reference background measurements. Practical applications of this algorithm are discussed.

In this work, we explore the feasibility of performing satellite-to-earth quantum key distribution (QKD) using the orbital angular momentum (OAM) of light. Because of the fragility of OAM states the conventional wisdom is that turbulence would render OAM QKD nonviable in a satellite-to-earth channel. However, based on detailed phase screen simulations of the anticipated atmospheric turbulence we find that OAM QKD is viable in some system configurations, especially if quantum channel information is utilized in the processing of postselected states. More specifically, using classically entangled light as a probe of the quantum channel, and reasonably sized transmitter-receiver apertures, we find that nonzero QKD key rates are achievable on sea-level ground stations. We also determine the turbulence conditions under which the QKD key rate becomes effectively zero. Without using classical light probes, OAM QKD is relegated to high-altitude ground stations with large receiver apertures. Our work uses a quantitative assessment of the performance of OAM QKD from satellites to determine under what circumstances the much-touted higher dimensionality of OAM can be utilized in the context of secure communications.

Discrete-modulated continuous-variable quantum key distribution protocols are promising candidates for large-scale deployment due to the large technological overlap with deployed modern optical communication devices. The security of discrete modulation schemes has previously been analyzed in the ideal detector scenario in the asymptotic limit. In this work, we calculate asymptotic key rates against collective attacks in the trusted detector noise scenario. Our results show that we can thus cut out most of the effect of detector noise and obtain asymptotic key rates similar to those had we access to ideal detectors.

The intriguing electronic properties of graphene and the strong light-matter interaction in layered transition-metal dichalcogenide (TMDC) make them a natural partner for hybrid devices, not only for optoelectronic device applications, but also to understand the conversion of light to electricity in this atomic scale prototype of a donor-acceptor complex. Here, we describe graphene-on-${\mathrm{WS}}_2$ binary heterostructure FET device, displaying gate-tunable persistent photoconductivity. Our time-dependent photovoltage relaxation experiments suggest that the charge-transfer time scale in this heterostructure is dependent on the input optical power, contrary to what is observed for the bare TMDC and is orders of magnitude slower than that observed for various other vdW hybrids. The optoelectronic responsivity of this device at low optical power is found to be as high as ${10}^{10}\mathrm{V/W}$, and thus shows the potential to be one of the most sensitive visible range photodetectors, while the gate tunability of the persistent photoconductivity can be utilized in the memory device applications. We identify that the photoresponse is the outcome of a photogating mechanism, due to the exciton dissociation under optical excitation, followed by the trapping of holes in ${\mathrm{WS}}_2$ and subsequent electron transfer to graphene.

We theoretically formulate and experimentally demonstrate the design of metagratings (MGs) composed of periodic rectangular grooves in a metallic medium, intended for perfect anomalous reflection. Using mode matching, a semianalytical scheme for analysis and synthesis of such MGs, containing multiple, arbitrarily arranged grooves per period, is derived. Following the typical MG design approach, we use this formalism to identify the relevant Floquet-Bloch modes and conveniently formulate constraints for suppression of spurious scattering, directly tying the structure’s geometrical degrees of freedom to the desired functionality. Solving this set of constraints, in turn, yields a detailed fabrication-ready MG design, without any full-wave optimization. Besides providing means to realize highly efficient beam deflection with all-metallic formations, we show that the rectangular (two-dimensional) groove configuration enables simultaneous manipulation of both transverse-electric (TE) and transverse-magnetic (TM) polarized fields, unavailable to date with common, printed-circuit-board-based microwave MGs. In addition, we highlight a physical limitation on the TE-polarization performance, preventing the ability to achieve perfect anomalous reflection in any desired angle. These capabilities are verified using three MG prototypes, produced with standard computer numerical control (CNC) machines, demonstrating both single- and dual-polarized control of multiple diffraction modes. These results enable the use of MGs for a broader range of applications, where dual-polarized control is required, or all-metallic devices are preferable (e.g., spaceborne systems or at high operating frequencies).

The coexistence of a low magnetic Gilbert damping constant and large perpendicular magnetic anisotropy (PMA) in ferromagnetic thin films is critical for high-speed and energy-efficient spintronics devices. To clarify the spin-resolved contributions of damping and PMA, a flat, lattice-matched interface is developed in an $\mathrm{Fe}$(0.7 nm)/${\mathrm{Mg}\mathrm{Al}}_2{\mathrm{O}}_4$(oxide)(3 nm) bi-layer film by varying the ex situ annealing temperature, which acts as a catalyst to vary the PMA energy densities according to the oxidation degree. Here, the optimized procedure for interface engineering is implemented to achieve an extremely low magnetic Gilbert damping constant (0.013) and a strong interfacial PMA energy $(0.8\mathrm{MJ}{\mathrm{m}}^{-3})$ for epitaxial thin films. By employing different interfacial atomic configurations in a first-principles calculation, this study explains the origin of the PMA energy and damping constant. The d(yz) and d(zx) orbitals of the interfacial $\mathrm{Fe}$ atoms in the minority-spin states play a role in the orbital moment and its anisotropy. Furthermore, the matrix elements between these two orbitals in the non-spin-flip term predominately contribute to damping. These detailed findings provide a clear insight into the development of materials with significantly improved PMA energies and low damping characteristics, thereby facilitating promising future spintronic applications.

Quantum hypothesis testing is one of the most fundamental problems in quantum information theory, with crucial implications in areas like quantum sensing, where it has been used to prove quantum advantage in a series of binary photonic protocols, e.g., for target detection or memory cell readout. In this work, we generalize this theoretical model to the multipartite setting of barcode decoding and pattern recognition. We start by defining a digital image as an array or grid of pixels, each pixel corresponding to an ensemble of quantum channels. Specializing each pixel to a black and white alphabet, we naturally define an optical model of a barcode. In this scenario, we show that the use of quantum entangled sources, combined with suitable measurements and data processing, greatly outperforms classical coherent-state strategies for the tasks of barcode data decoding and classification of black and white patterns. Moreover, introducing relevant bounds, we show that the problem of pattern recognition is significantly simpler than barcode decoding, as long as the minimum Hamming distance between images from different classes is large enough. Finally, we theoretically demonstrate the advantage of using quantum sensors for pattern recognition with the nearest-neighbor classifier, a supervised learning algorithm, and numerically verify this prediction for handwritten digit classification.

Using samples of ${\mathrm{Co}}_{100-x}{\mathrm{Tb}}_x/\mathrm{Cu}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, we investigate the heat-to-charge current conversion phenomena in amorphous ${\mathrm{Co}}_{100-x}{\mathrm{Tb}}_x$ alloys. We find an enhancement of the inverse spin Hall voltage owing to the $\mathrm{Tb}$ impurity by means of the spin Seebeck effect. The spin Hall angle of ${\mathrm{Co}}_{68}{\mathrm{Tb}}_{32}$ is 3.5 times larger than that of $\mathrm{Co}$, and mainly originates from the extrinsic spin Hall effect. Meanwhile, we detect an anomalous Nernst voltage owing to the magnetic properties of ${\mathrm{Co}}_{100-x}{\mathrm{Tb}}_x$ alloy. The sign of the inverse spin Hall voltage is observed to be the same for all ${\mathrm{Co}}_{100-x}{\mathrm{Tb}}_x$ samples, whereas the anomalous Nernst voltage changes its sign around the magnetization compensation composition. Our finding proves ${\mathrm{Co}}_{100-x}{\mathrm{Tb}}_x$ is an efficient spin-to-charge current convertor, and it has advantages for manipulating not only the inverse spin Hall effect but also the anomalous Nernst effect by means of adjusting the $\mathrm{Tb}$ concentration.

Solid-state qubits incorporating quantum dots can be read out dispersively. Here, we theoretically describe physical mechanisms that render such reflectometry-based readout schemes imperfect. We discuss charge qubits, singlet-triplet spin qubits, and Majorana qubits. In our model, we account for readout errors due to slow charge noise, and due to overdriving, when the large amplitude of the probe field generates errors. A key result is that for charge and spin qubits, the readout fidelity saturates at large probe strengths, whereas for Majorana qubits, there is an optimal probe strength which provides a maximized readout fidelity. We also point out the existence of severe readout errors appearing in a resonancelike fashion as the pulse strength is increased, and show that these errors are related to probe-induced multiphoton transitions. Besides providing practical guidelines toward optimized readout, our study might also inspire ways to use gate reflectometry for device characterization.

An atom-light hybrid quantum gyroscope (ALHQG) is proposed due to its high rotation sensitivity. It consists of an optical Sagnac loop to couple rotation rate and an atomic ensemble as quantum beam splitter (recombiner) [QBS(C)] based on atomic Raman-amplification process to realize the splitting and recombination of the optical wave and the atomic spin wave. The rotation sensitivity can be enhanced by the quantum correlation between Sagnac loop and QBS(C). The optimal working condition is investigated to achieve the best sensitivity. The numerical results show that the rotation sensitivity can beat the standard quantum limit (SQL) in ideal condition. Even in the presence of the attenuation under practical condition, the best sensitivity of the ALHQG can still beat the SQL and is better than that of a classical fiber optic gyroscope. Such an ALHQG could be practically applied for modern inertial navigation system.

The readout technique of dispersive gate sensing (DGS) uses an electrode embedded in a microwave resonator to detect the quantum capacitance of a mesoscopic device arising from single-electron tunneling. Here, we extend DGS from the detection of discrete tunnel events to the open regime, observing jumps in the capacitance of a quantum point contact (QPC) that arise when the one-dimensional sub-bands populate. We compare the signal from DGS to transport measurements for various QPC geometries, including measurements at finite bias. Unlike traditional charge sensing, which is limited by screening at high density, our results suggest that DGS can also probe the charge configuration of open quantum devices, where electrons are delocalized and multiple sub-bands are occupied.