Silicon photonics offers a stable, miniaturized platform for quantum key distribution (QKD). As such, the chip-based system is vital for a wider adoption of QKD in the near future. However, the practical security issues presented in this alternative platform have not been fully studied. Here, we theoretically and experimentally evaluate the security of a chip-based QKD system under a powerful source attack, namely, Trojan-horse attack. We analyze the finite-key security of two prominent QKD protocols—BB84 and measurement-device-independent QKD—with the information leakage from both the polarization modulator and the intensity modulator integrated on a silicon photonic chip. Furthermore, we prove that our chip-based measurement-device-independent QKD system is feasibly resistant to the Trojan-horse attack by adding 232-dB isolation to the chip. In this sense, our work provides a guideline for the secure implementation of a leaky transmitter chip in the QKD system.

Significant photoconductive effects are reported in emergent two-dimensional (2D) ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$. In this work, we investigate the layer-dependent photoresponse properties and photovoltaic effects of 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ (X = $\mathrm{S}$, $\mathrm{Se}$, and $\mathrm{Te}$) by first-principles calculations and quantum-transport simulation. The absorbance per layer increases with the decreasing layer number for high-frequency light, so the absorbance density of 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ can be elevated by decreasing the layer number. An outstanding open-circuit voltage (1.08 V) among 2D materials is found for the monolayer (ML) ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ p-n junction. The computed responsivities of ML black phosphorous, ${\mathrm{Mo}\mathrm{S}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$ p-n junctions through our methods are in good agreement with experiments. The ML ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Te}$ p-n junctions show responsivities of 16.8 and 13.6 mA/W, respectively, under AM1.5 sunlight; these values are higher than those of their extensively studied ML ${\mathrm{Mo}\mathrm{S}}_2$ (8.6) and ${\mathrm{W}\mathrm{Se}}_2$ (8.8) counterparts. The ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ film and ${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{S}$ p-n junctions also show higher responsivities than those of commercial $\mathrm{Si}$ and $\mathrm{Ga}\mathrm{As}$. Therefore, the 2D ${\mathrm{Bi}}_2{\mathrm{O}}_{2}X$ p-n junctions have prospective applications in photovoltaic devices.

Postdeposition treatment (PDT) with alkali metals has profoundly improved the performance of $\mathrm{Cu}(\mathrm{In},\mathrm{Ga})(\mathrm{S},\mathrm{Se}{)}_2$ solar cells. Several mechanisms have been proposed to explain the improved performance, but the exact origin is still under debate. Here, using first-principles calculations, we demonstrate that alkali metals tend to accumulate at the interface, which significantly improves the band offset between the absorber and the alkali-metal-doped ${\mathrm{Cu}}_{1-x}{\mathrm{Na}}_x(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_2$ buffer layer. Our results show that, as the fraction of $\mathrm{Na}$ increases, the valence-band maximum (VBM) of the alloy layer decreases, while the conduction-band maximum (CBM) increases. The drop of the VBM introduces a hole barrier, repelling holes away from the interface, and a moderately higher CBM forms a benign spikelike conduction-band offset (CBO) against the absorber layer independent of the $\mathrm{Ga}$ concentration. Both effects are beneficial for the solar-cell efficiency and exhibit extra advantages over the ordered vacancy compound (OVC), which commonly exists before the PDT, because, although the OVC can introduce the hole barrier, it also introduces a detrimental clifflike CBO, especially at a high $\mathrm{Ga}$ concentration. Such understandings of band offsets introduced by PDT can further be extended to other alkali metals, such as $\mathrm{K}$, $\mathrm{Rb}$, and $\mathrm{Cs}$, and the design principles can also be extended to other types of thin-film solar cells.

Nonradiative losses to the open-circuit voltage are a primary factor in limiting the power-conversion efficiency of organic photovoltaic devices. The dominate nonradiative loss in the bulk is intrinsic to the active layer and can be determined from the quasi-Fermi-level splitting (QFLS) and the radiative thermodynamic limit of the photovoltage. Quantification of the QFLS in thin-film devices with low mobility is challenging due to the excitonic nature of photoexcitation and additional sources of nonradiative loss associated with the device structure. This work outlines an experimental approach based on electromodulated photoluminescence, which can be used to directly measure the intrinsic nonradiative loss to the open-circuit voltage, thereby quantifying the QFLS. Drift-diffusion simulations are carried out to show that this method accurately predicts the QFLS in the bulk of the device regardless of device-related nonradiative losses. State-of-the-art PM6:Y6-based organic solar cells are used as a model to test the experimental approach and the QFLS is quantified and shown to be independent of device architecture. This work provides a method to quantify the QFLS of organic solar cells under operational conditions, fully characterizing the different contributions to the nonradiative losses of the open-circuit voltage. The reported method will be useful not only in characterizing and understanding losses in organic solar cells but also in other device platforms such as light-emitting diodes and photodetectors.

Quantum cryptography provides the inherent security for transmitting confidential information across free space or a fiber link. However, a high secure-key rate is still a challenge for a quantum-cryptography system. High-dimensional quantum cryptography, which can tolerate much higher channel noise, is a prospective way to share a higher secure-key rate between legitimate users, and has received substantial attention over the last decade. In particular, orbital angular momentum (OAM) can provide an abundant resource for high-dimensional quantum cryptography. Furthermore, combining spin angular momentum (SAM) with OAM can increase the encoding alphabet. Here we verify a prepare-and-measure quantum-cryptography scheme based on four-dimensional SAM-OAM hybrid states over kilometer-scale ring-core fibers. The measured quantum-bit error rates are $4.3\mathrm{\%}$ for 4 km of fiber and $16.3\mathrm{\%}$ for 25 km of fiber. The scheme simplifies the process of state preparation and measurement, with a compact and scalable setup.

X-ray microcomputed tomography (micro-CT) is a common tool for the study of porous media structures and properties. High-quality micro-CT data are required to accurately capture pore structures. Acquiring high-quality micro-CT data, however, is not always possible, owing to application limitations and experimental constraints. Therefore, we propose a geometrical-based generative adversarial network (GAN) to rapidly restore noisy micro-CT images to their clean counterparts. The training data and related ground-truth (GT) data are scanned for 7 min and 9.5 h, respectively. To evaluate the performance of the geometrical-based GAN, a ${600}^3$ voxel image that has never been used for training is reconstructed and compared with the corresponding GT image. Histogram matching and linear normalization are implemented to adjust the histogram of the reconstructed image to that of the GT image. A watershed-based segmentation method is then applied to delineate pore and solid phases. Lastly, we measure the Minkowski functionals and petrophysical properties, including absolute permeability, pore size distribution, drainage capillary pressure-saturation curve, and imbibition relative permeability, to estimate the physical accuracy of the denoised image. The results show that the proposed geometrical-based GAN can accurately restore noisy micro-CT data. By reducing the scanning time from 9.5 h to 7 min, the expenditure of collecting micro-CT can be decreased significantly. This is particularly important for applications where time-lapse images of a dynamic process are required, high-throughput imaging is necessary for real-time data analysis, or where the quantification of large sample volumes is required.

Acoustic tweezers are gaining increasing attention due to their excellent biological compatibility. Recently, the concept of topology has been expanded from condensed-matter physics into acoustics, giving rise to robust wave manipulation against various defects. So far, topological acoustics have not been experimentally realized at an on-chip level to act as tweezers for microparticle manipulations. Here, we achieve topological on-chip acoustic tweezers based on the topologically protected phononic mode. These tweezers consist of one-dimensional arrays of Helmholtz resonant air cavities. Strong microfluidic oscillations induced by acoustic waves are experimentally observed at water-air surfaces of Helmholtz resonant air cavities at the topological interface. Acoustic radiation forces and streaming induced by these microfluidic oscillations capture microparticles, the sizes of which are up to 20 µm, and make them undergo orbital rotations. Our topological on-chip acoustic tweezers realize noncontact label-free microparticle manipulations in microfluidics and exhibit enormous application potential in the biomedical field.

Electron source coherence has a very important influence on the imaging capabilities of modern electron microscopes. However, conventional electron source models that are based on geometrical electron optics implicitly assume that the emission from the source surface is fully incoherent, which can complicate the treatment of highly coherent field-emission sources. In an attempt to treat the wave-optical properties of electron sources, models inspired by light optics treatments of (partially) coherent sources, which assume a planar source and free wave propagation, have been developed. In this case the underlying assumptions are problematic, because the source surface of a field emitter can have a radius of curvature on the nanometer scale, and the emitted electrons are accelerated by a strong, inhomogeneous electrostatic field following emission. We introduce a model based on wave-mechanical electron optics that draws on a quantum mechanical description of electron emission and propagation to obtain a physically consistent treatment of the wave-mechanical properties of electron sources. We apply the model to investigate spatial resolution limits in low-energy electron holography and microscopy, where it is shown that aberrations and coherence properties of the electron source are crucial and interrelated. The wave-mechanical electron-optical model can, furthermore, be readily generalized to assess and improve electron source performance in other scenarios and techniques where spatial and temporal coherence, and electron-optical aberrations, are relevant.

Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number $\overline{n}$ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing $\overline{n}$ is generally observed to also monotonously increase these transition rates. Here we present a fluxonium artificial atom in which, despite the fact that the measured transition rates show nonmonotonous fluctuations within a factor of 6, for photon numbers up to $\overline{n}\approx 200$, the signal-to-noise ratio continuously improves with increasing $\overline{n}$. Even without the use of a parametric amplifier, at $\overline{n}=74$, we achieve fidelities of 99% and 93% for feedback-assisted ground and excited state preparations, respectively. At higher $\overline{n}$, leakage outside the qubit computational space can no longer be neglected and it limits the fidelity of quantum state preparation.

Fast discrimination between quantum states of superconducting artificial atoms is an important ingredient for quantum information processing. In circuit quantum electrodynamics, increasing the signal-field amplitude in the readout resonator, dispersively coupled to the artificial atom, improves the signal-to-noise ratio and increases the measurement strength. Here, we employ this effect over 2 orders of magnitude in readout power, made possible by the unique combination of a dimer-Josephson-junction-array amplifier with a large dynamic range and the fact that the readout of our granular aluminum fluxonium artificial atom remains quantum nondemolition (QND) at relatively large photon numbers in the readout resonator, up to $\overline{n}=110$. Using Bayesian inference, this allows us to detect quantum jumps faster than the readout-resonator response time $2/\kappa$, where $\kappa$ is the bandwidth of the readout resonator.

Ensembles of erbium dopants can realize quantum memories and frequency converters that operate in the minimal-loss wavelength band of fiber optical communication. Their operation requires the initialization, coherent control, and readout of the electronic spin state. In this work, we use a split-ring microwave resonator to demonstrate such control in both the ground and optically excited state. The presented techniques can also be applied to other combinations of dopant and host and may facilitate the further development of quantum memory protocols and sensing schemes.

Noise in quantum operations often negates the advantage of quantum computation. However, most classical simulations of quantum computers calculate the ideal probability amplitudes by either storing full state vectors or using sophisticated tensor-network contractions. Here we investigate sampling-based classical simulation methods for noisy quantum circuits. Specifically, we characterize the simulation costs of two major schemes, stabilizer-state sampling of magic states and Heisenberg propagation, for quantum circuits subject to stochastic Pauli noise, such as depolarizing and dephasing noise. To this end, we introduce several techniques for the stabilizer-state sampling to reduce the simulation costs under such noise. It is revealed that in the low-noise regime, stabilizer-state sampling results in a smaller sampling cost, while Heisenberg propagation is better in the high-noise regime. Furthermore, for a high depolarizing noise rate of approximately 10%, these methods provide better scaling than that given by the low-rank stabilizer decomposition. We believe that this knowledge of classical simulation costs is useful to achieve possible quantum advantage on near-term noisy quantum devices as well as efficient classical simulation methods.

Nonradiative-recombination-related defects are significant for optoelectronic semiconductor devices. Here, we analyze nonradiative-recombination processes in ionic semiconductors using first-principles density-functional theory. In ionic group II-VI semiconductors, we find that large lattice relaxations of anion vacancies caused by strong Coulomb interactions between different charged defect states can significantly enhance recombination processes through a two-level recombination mechanism. Specifically, we show that the defect level of the 2+ charged anion vacancy $\mathrm{(}{V}_{\mathrm{Se}}^{2+})$ in group II-VI $\mathrm{Zn}\mathrm{Se}$ is close to the conduction-band minimum and easily captures an electron to form a metastable 1+ charged state $\mathrm{(}{V}_{\mathrm{Se}}^{+})$; then, the large lattice relaxation, on account of the change in Coulomb interactions locally in the different charged states, rapidly changes this metastable state to a stable one and simultaneously move the defect level of $V_{\mathrm{Se}}^{+}$ closer to that valence-band maximum, and thus, increases the hole-capture rate. Compared with the Shockley-Read-Hall nonradiative-recombination theory based on a single defect level, this two-level recombination mechanism involving anion vacancies can greatly increase the nonradiative-recombination rate in ionic group II-VI semiconductors. This understanding is expected to be useful for the study of the nonradiative-recombination process in ionic semiconductors for applications in the field of optoelectronic devices.

Ferrimagnetic materials represent unique systems where the ease of manipulating the spins with applied magnetic fields is combined with exchange-driven acceleration of the internal spin dynamics. Of particular interest is the temperature range around the magnetic and spin compensation points, finely balancing both magnetic moment and angular momentum of the system, and leading to a very particular character of magnetic switching by the domain wall motion. Here we present our studies of the temperature-dependent domain wall dynamics in the temperature range covering both angular momentum and magnetization compensation points in garnet film, and reaching up to the Curie temperature. Drastic difference in the domain wall mobility and maximum achievable velocity in the vicinities of these two compensation points is demonstrated. Also a remarkably high mobility in weak applied magnetic fields is indicated.

Decreasing the thermal conductivity of a thermoelectric material is always a prerequisite for its potential application. Using first-principle calculations, we examine the magnetism-induced change in lattice thermal transport in bismuth telluride. The source of magnetic moment, $\mathrm{Cr}$ in the doped system, weakly magnetizes the coordinated $\mathrm{Te}$ atoms to make the latter’s phonon softer than that in the pure compound. Although the transition metal dopants do not participate directly in the heat conduction process, the anharmonicity induced by them favors reducing the lattice thermal conductivity (${\kappa }_{\mathrm{ph}}$). Large anharmonicity in thermodynamically stable $({\mathrm{Bi}}_{0.67}{\mathrm{Cr}}_{0.33}{)}_2{\mathrm{Te}}_3$ reduces the in-plane room temperature ${\kappa }_{\mathrm{ph}}$ by approximately $79\mathrm{\%}$. However, the latter is found to lie above Cahill’s limit (${\kappa }_{\min }$) of the material which is an indication of the ability to achieve such a significant reduction of ${\kappa }_{\mathrm{ph}}$. The thermal conductivity, strictly, does not vary monotonically with doping concentration. For any particular doping level, the thermal conductivity is different for different configurations which is related to the internal energy of the system. We find that the internal energy variance of 0.03 eV would reduce the in-plane thermal conductivity of the room temperature lattice by at least 60% for 50% doping.

Silicon carbide is a promising host material for spin-defect-based quantum sensors owing to its commercial availability and established techniques for electrical and optical microfabricated device integration. The negatively charged silicon vacancy is one of the leading spin defects studied in silicon carbide owing to its near-telecom photoemission, high spin number, and nearly temperature-independent ground-state zero-field splitting. We report the realization of nanotesla shot-noise-limited ensemble magnetometry based on optically detected magnetic resonance with the silicon vacancy in $4H$ silicon carbide. By coarsely optimizing the anneal parameters and minimizing power broadening, we achieve a sensitivity of $50\mathrm{nT}/\sqrt{\mathrm{Hz}}$ and a theoretical shot-noise-limited sensitivity of $3.5\mathrm{nT}/\sqrt{\mathrm{Hz}}$. This is accomplished without utilizing complex photonic engineering, control protocols, or applying excitation powers greater than a watt. This work demonstrates that the silicon vacancy in silicon carbide provides a low-cost and simple approach to quantum sensing of magnetic fields.

An exceptional point of degeneracy (EPD) is induced in a system made of a linear electron beam interacting with an electromagnetic (EM) guided mode in a vacuum tube made of a corrugated circular metallic waveguide with distributed output ports. This scheme enables an exceptional synchronous regime in backward-wave oscillators (BWOs) where the electron beam provides distributed gain to the EM mode with distributed power extraction. Particle-in-cell (PIC) simulation results demonstrate that the proposed EPD BWO has a starting-oscillation current that scales quadratically with BWO length to a nonvanishing value, which does not occur in standard BWOs and demonstrates the occurrence of the EPD and hence the exceptional synchronism operational regime. The degeneracy of two interactive hot modes is also verified by observing the coalescence of their complex-valued wave numbers at the EPD frequency. Observations on the kinetic energy distribution of the electrons along the BWO demonstrate that the proposed EPD-BWO regime is capable of achieving higher power conversion efficiency at higher levels of power generation due to its ability to maintain the synchronism for longer BWO lengths compared to the standard BWO regime of operation.

Spontaneous parametric down-conversion (SPDC) in a laser pumped optical nonlinear medium can produce heralded single photons with a high purity but a very low yield. Improving the yield by increasing the pump power in SPDC inevitably reduces the purity due to excitation of multiphoton events. We propose a scheme to overcome this purity-yield trade-off by suppressing multiphoton events in a cavity-enhanced SPDC via the photon-blockade effect. By introducing a strong photon-photon interaction into the intracavity medium and increasing the pump power, we can improve the available single-photon yield to larger than $90\mathrm{\%}$, while keeping the purity high, towards on-demand generation of single photons through the SPDC process. Our quasi-on-demand SPDC sources may boost single-photon-based quantum-information technology.

A fundamental understanding of the interfacial magnetic properties in ferromagnetic heterostructures is essential for utilizing ferromagnetic materials for spintronic device applications. Here, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3(\mathrm{LAO})/{\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{Mn}\mathrm{O}}_3$(LSMO)/$\mathrm{Nb}:{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$($\mathrm{Nb}$:STO) heterostructures with varying LSMO layer thicknesses, in which the magnetic anisotropy strongly changes with the LSMO thickness due to the delicate balance between strains originating from both the $\mathrm{Nb}$:STO and LAO layers, using x-ray magnetic circular dichroism and photoemission spectroscopy. We successfully detect the clear change of the magnetic behavior of the $\mathrm{Mn}$ ions concomitant with the thickness-dependent metal-insulator transition. Our results suggest that the double-exchange interaction induces ferromagnetism in the metallic LSMO film under tensile strain caused by the $\mathrm{STO}$ substrate, while the superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. The change in strain, depending on LSMO layer thickness, is confirmed by scanning transmission electron microscopy. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via $\mathrm{Mn}$ $3d_{3z^2-r^2}$ orbitals under compressive strain and the double-exchange interaction via the $3d_{x^2-y^2}$ orbitals. These findings provide key aspects of ferromagnetic oxide heterostructures for the development of spintronic device applications.