${\mathrm{Cr}}^{2+}$ - and ${\mathrm{Fe}}^{2+}$ -doped $\mathrm{Zn}\mathrm{Se}$ crystals are laser materials with phonon-broadened absorption and emission spectra, which provide broadband laser gain in the 2- to 5-$\text{μ}\mathrm{m}$ wavelength range. While ${\mathrm{Cr}}^{2+}\text{:ZnSe}$ can be directly pumped with high-power Er, Ho, or Tm lasers, no such possibility exists for ${\mathrm{Fe}}^{2+}\text{:ZnSe}$. To this end, electronic excitation transfer between ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ ions in codoped ZnSe is investigated as an alternative excitation process in photo luminescence (PL) experiments with sub-10-ns temporal resolution. For a wide range of ion concentrations, we observe nonexponential decays of ${\mathrm{Cr}}^{2+}$ PL on a microsecond time scale, a 60-ns rise time of ${\mathrm{Fe}}^{2+}$ PL at high doping densities, and a prolonged decay of ${\mathrm{Fe}}^{2+}$ PL due to the temporal characteristics of excitation transfer over a range of interionic distances. Using a multirate equation model, the transfer process is analyzed on length scales up to 30 nm and compared to the established continuum model approach. The analysis reveals an unexpectedly efficient excitation transfer from ${\mathrm{Cr}}^{2+}$ to ${\mathrm{Fe}}^{2+}$ ions with an enhancement of the excitation transfer rates by up to a factor of 5 in comparison to resonant dipole-dipole coupling. The enhancement is assigned to (multi)phonon-assisted excitation transfer, in analogy to the phonon-mediated efficient radiationless decay of the excited ${\mathrm{Fe}}^{2+}$ state. As nonradiative losses and excitation transfer show different temperature scaling, a cryogenic temperature regime is found that promises overall efficiencies above 50%, making ${\mathrm{Fe}}^{2+}:{\mathrm{Cr}}^{2+}\text{:ZnSe}$ a much more viable alternative to parametric conversion schemes in the midinfrared range.

Dark-field imaging is an x-ray technique used to highlight subpixel, typically micrometer-scale, density fluctuations. It is often used alongside standard attenuation-based and also phase-contrast x-ray imaging, which both see regular use in tomography. We present x-ray dark-field computed tomography (CT) with a laboratory edge-illumination setup. The dark-field contrast is shown to increase linearly with the x-ray path length through the imaged object, a prerequisite for the use of standard tomographic reconstruction approaches. A multimaterial, custom-built phantom is used to show how dark-field contrast CT can complement attenuation contrast CT for the separation of materials based on their microstructure. As an example of a more complex, biological sample, we present a model rat heart. We show, by comparison with attenuation contrast tomography, that dark-field enables the identification of additional structures undetected through the attenuation contrast channel, as well as offering a consistently sharper reconstructed image.

Electromagnetic wave interaction with time-varying systems has gained a lot of research interest in recent years. The temporal modulation gives unprecedented control over the response, allowing us to go beyond the state of the art in passive systems. In this work, we use time variation and show it can enhance the direction-of-arrival sensitivity of a deeply subwavelength dimer, using the added to degrees of freedom. We formulate the problem, derive an analytical model, and discuss the various physical mechanisms responsible for the enhancement. We show that time modulation enables an alternative degree of control that can be used to optimize the response for various incident frequencies, allowing for wideband operation. Additionally, we show that incorporating the currents from higher generated harmonics into the sensing scheme allows us to extract more accurate information about the impinging wave.

We present a single-beam atomic magnetometer operating at room temperature for the measurement of ac magnetic fields. The magnetometer functions in the nonlinear regime of magneto-optical rotation of ${}^{85}\mathrm{Rb}$ atomic vapor without any buffer gases. We demonstrate a sensitivity of approximately $0.9\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at 2 kHz and a large bandwidth of 24 kHz. The dynamic range of measurement is ${10}^6$, making the sensor effective even in Earth’s field. We present the signal-to-noise and bandwidth characteristics of the system for both shielded and unshielded modes of operation. Moreover, we perform theoretical analysis for the atom-light system for the single-laser-beam configuration. The effect of light intensity and detuning on the magnetometer is studied theoretically as well as experimentally to understand the strengths and limitations of the technique.

Application of various electron-transport layers (ETLs) with desired morphology and dimensions is a key factor to be considered for the structural designs of perovskite solar cells (PSCs) in order to obtain enhanced optical and electrical properties. The metal-oxide ETLs composed of one-dimensional nanorod arrays are one of the most frequently used nanostructures for electron transporting in PSCs. In this work, computer-simulation methods are employed to investigate the optoelectrical properties of PSCs with planar and nanorod-based ${\mathrm{Sn}\mathrm{O}}_2$ ETLs. A two-dimensional optical model is used to determine the optical responses of devices and the standard drift-diffusion model is utilized to simulate their behavior and performance. The aspect ratio and the density of ${\mathrm{Sn}\mathrm{O}}_2$ nanorods are varied to examine their effect on the optical and the electrical properties of resulting devices and the findings are contrasted with those of planar devices. It is found that under optimum conditions, PSCs with thin planar ${\mathrm{Sn}\mathrm{O}}_2$ ETLs, which are referred to as reference planar devices, outperform PSCs with nanorod-based ${\mathrm{Sn}\mathrm{O}}_2$ ETLs even if the light-harvesting properties of the latter are improved with the implementation of structured ETLs. The underlying reasons for this phenomenon are analyzed and rationalized through a detailed analysis and comparison of electric field, current density, and carrier recombination distributions in PSCs. The findings of this work can be used as a theoretical guide for designing and fabrication of high-performance planar and structured PSCs using ${\mathrm{Sn}\mathrm{O}}_2$ ETLs.

Different from the traditional bulk-edge correspondence principle, the discovery of higher-order topological states has generated widespread interest. In a second-order, two-dimensional elastic wave topological insulator, the fluctuation information can be confined to the corners, with the state being topologically protected. To better apply the topological corner states, a cleverly designed mechanical structure is used to achieve the adjustment of the topological corner states on a two-dimensional elastic plate. The energy-band inversion can be achieved by switching the material of the inner and outer brackets of the unit cell of the honeycomb elastic plate. The topological corner states can be observed at the boundary of two different topological phase structures in the finite lattice using finite-element software. The robustness of the topological corner state is verified by setting up defective control groups at the corner. In addition, the topological corner states of this honeycomb elastic plate are discussed in terms of positional tunability. The honeycomb elastic plate can provide a reference for the design of devices for the local control of elastic waves and energy harvesting due to the possibility of modulation of topological corner states, which is beneficial for the application of topological corner states in practice.

Projective quantum measurement is fundamental for quantum technologies. Here we propose a method for construction of projective measurement of nonconserved quantity of nuclear spin via a weakly coupled auxiliary electron spin. The key idea is to apply suitable quantum control on the electron to construct a weak quantum nondemolition (QND) measurement on the nuclear spin and then cascade a sequence of such measurements into a projective one. We identify a set of tunable parameters to select the QND observables and control the strength of the weak QND measurement. We also find that the QND measurement can be stabilized against realistic experimental control errors. As a demonstration of our method, we design projective measurement on a ${}^{13}\mathrm{C}$ nuclear spin weakly coupled to a nitrogen-vacancy center electron spin in diamond.

A steerable parametric loudspeaker array is known for its directivity and narrow beam width. However, it often suffers from the grating lobes due to periodic array distributions. Here we propose the array configuration of hyperuniform disorder, which is short-range random while correlated at large scales, as a promising alternative distribution of acoustic antennas in phased arrays. Angle-resolved measurements reveal that the proposed array suppresses grating lobes and maintains a minimal radiation region in the vicinity of the main lobe for the primary frequency waves. These distinctive emission features benefit the secondary frequency wave in canceling the grating lobes regardless of the frequencies of the primary waves. Besides, the hyperuniform disordered array is duplicatable, which facilitates extra-large array design without any additional computational efforts.

Heralded single-photon sources are a fundamental building block for optical quantum technologies. These sources need to be unfiltered and integrated to have good scalability and avoid unnecessary losses. To attain this goal, it is necessary to control the effective nonlinearity seen by the fields as they mix and propagate in a waveguide source. In this paper, we introduce a method to design nonlinear waveguides with arbitrarily shaped effective nonlinearity profiles. The method takes advantage of the fact that the second-order nonlinear response is a tensor quantity and thus the local effective nonlinearity of a material depends on the propagation direction of the fields participating in the interaction. Thus, by locally changing the propagation direction of the fields we can modulate the wave-mixing process. Our method allows for the waveguide fabrication process to be significantly simplified. The material structure of the waveguide is made by a single crystal, no longer needing oriented patterning or periodic poling. We use our method to design waveguides with a nonlinearity profile that is Gaussian in the propagation length, allowing the generation of perfectly pure heralded single photons.

Defect-free atomic arrays have been demonstrated as a scalable and fully controllable platform for quantum simulations and quantum computations. To push the qubit size limit of this platform further, we design an integrated measurement and feedback system, based on field-programmable gate array (FPGA), to quickly assemble two-dimensional defect-free atomic array using maximum parallelisms. The total time cost of the rearrangement is first reduced by processing atom detection, atomic occupation analysis, rearrangement strategy formulation, and acousto-optic deflectors driving signal generation in parallel in time. Then, by simultaneously moving multiple atoms in the same row (column), we save rearrangement time by parallelism in space. To best utilize these parallelisms, we propose an alternative algorithm named the Tetris algorithm to reassemble atoms to arbitrary target array geometry from two-dimensional stochastically loaded atomic arrays. For an $L\times L$ target array geometry, the number of moves scales as $L$, and the total rearrangement time scales at most as $L^2$. Although in this work we do not test on actual atoms, we simulate the performance of our FPGA system experimentally with all components integrated except for the atoms. We present the overall performance for different target geometries, and demonstrate a dramatic boost in rearrangement time cost and the potential to scale up defect-free atomic array to 1000 atoms in room-temperature platform and 10 000 atoms in cryogenic environment.

Identifying clusters in data is an important task in many fields. In this paper, we consider situations in which data live in a physical world, so we first have to collect the images using sensors before clustering them. Using sensors enhanced by quantum entanglement, we can image surfaces more accurately than using purely classical strategies. However, it is not immediately obvious whether the advantage we gain is robust enough to survive data-processing steps such as clustering. It has previously been found that using quantum-enhanced sensors for imaging and pattern recognition can give an advantage for supervised learning tasks, and here we demonstrate that this advantage also holds for an unsupervised learning task, namely clustering.

Two types of errors can occur when discriminating pairs of quantum states. Asymmetric state discrimination involves minimizing the probability of one type of error, subject to a constraint on the other. We give explicit expressions bounding the set of achievable errors, using the trace norm, the fidelity, and the quantum Chernoff bound. The upper bound is asymptotically tight and the lower bound is exact for pure states. Unlike asymptotic bounds, our bounds give error values instead of exponents, so can give more precise results when applied to finite-copy state discrimination problems.

We propose a scheme for optical entanglement distribution in quantum networks based on a quasideterministic entangled photon-pair source. By combining heralded photonic Bell-pair generation with spectral mode conversion to interface with quantum memories, the scheme eliminates switching losses due to multiplexing in the source. We analyze this “zero-added-loss multiplexing” (ZALM) Bell-pair source for the particularly challenging problem of long-baseline entanglement distribution via satellites and ground-based memories, where it unlocks additional advantages: (i) the substantially higher channel efficiency $\eta$ of downlinks versus uplinks with realistic adaptive optics, and (ii) photon loss occurring before interaction with the quantum memory—i.e., Alice and Bob receiving rather than transmitting—improve entanglement generation rate scaling by $\mathcal{O}(\sqrt{\eta })$. Based on numerical analyses, we estimate our protocol to achieve $>10\text{ebit/s}$ at memory multiplexing of ${10}^2$ spin qubits for ground distance $>{10}^2\text{km}$, with the spin-spin Bell-state fidelity exceeding 99%. Our architecture presents a blueprint for realizing global-scale quantum networks in the near term.

Single-particle imaging (SPI) at x-ray free-electron lasers is a technique to determine the three-dimensional structure of nanoscale objects like biomolecules from a large number of diffraction patterns of copies of these objects in random orientations. The technique has been limited to relatively low resolution due to background noise and heterogeneity of the target particles. A recently introduced reference-enhanced holographic SPI methodology uses strongly scattering holographic references to improve background tolerance, and, thus, the achievable resolution, at the cost of additional latent variables beyond orientation. Here, we describe an improved reconstruction algorithm based on maximum likelihood estimation, which scales better, enabling fine sampling of latent parameters to reach high resolutions, and much better performance in the low signal limit. Furthermore, we show that structural variations within the target particle are averaged in real space, significantly improving robustness to conformational heterogeneity in comparison to conventional SPI. With these computational improvements, we believe reference-enhanced SPI is capable of reaching sub-nanometer resolution biomolecule imaging.

Using positive muons as local probes implanted at low energy enables gathering information about the material of interest with nanometer-depth resolution (low-energy muon-spin-rotation spectroscopy, LE $\mu \mathrm{SR}$). In this work, we leverage the capabilities of LE $\mu \mathrm{SR}$ to perform a detailed investigation of the widely studied yet poorly understood ${\mathrm{Si}\mathrm{O}}_2$-$\mathrm{Si}\mathrm{C}$ interface. Thermally oxidized samples are investigated before and after annealing in nitric oxide (NO) and argon ($\mathrm{Ar}$) environment. Thermal oxidation is found to result in structural changes both in the $\mathrm{Si}\mathrm{C}$ crystal close to the interface and at the interface itself, which severely degrade the transport properties of charge carriers. Annealing in NO environment is known to passivate the defects leading to a reduction of the density of interface traps ($D_{\mathrm{it}}$); LE $\mu \mathrm{SR}$ further reveals that the NO annealing results in a thin layer of high carrier concentration in $\mathrm{Si}\mathrm{C}$, extending to more than 50 nm depending on the annealing conditions. From our measurements, we see indications of $\mathrm{Si}$ vacancy ($V_{\mathrm{Si}}$) formation in $\mathrm{Si}\mathrm{C}$ after thermal oxidation. Following NO annealing, nitrogen occupies the $V_{\mathrm{Si}}$ sites, leading to the well-documented reduction in $D_{\mathrm{it}}$ and, at the same time, creating a charge-carrier-rich region near the interface. The LE $\mu \mathrm{SR}$ technique sheds light on the near-interface region in the ${\mathrm{Si}\mathrm{O}}_2$-$\mathrm{Si}\mathrm{C}$ system, which is challenging to access using other techniques. By comparing the LE $\mu \mathrm{SR}$ data from a sample with known doping density, we perform a high-resolution quantification of the free carrier concentration near the interface after NO annealing and discuss the origin of the observed near-surface variations. Finally, the depletion of carriers in a MOS capacitor in the region exactly below the interface is demonstrated using LE $\mu \mathrm{SR}$. The NO-annealed sample shows the narrowest depletion region, likely due to the reduced density of interface traps and the charge-carrier-rich region near the interface. Our findings demonstrate the many benefits of LE $\mu \mathrm{SR}$ to study critical regions of semiconductor devices that have been inaccessible with other techniques, while simultaneously retaining both nanoscale-depth resolution and a nondestructive approach.