Understanding the fundamental heat-transport mechanisms across interfaces comprised of two-dimensional (2D) materials is crucial for the further development of the next generation of optoelectronic devices based on 2D heterostructures for which one of the main factors affecting the device performance is their poor thermal management. Here we use systematic atomistic simulations to unravel the influence of anharmonicity in dictating the thermal boundary conductance across graphene and ${\mathrm{Mo}\mathrm{S}}_2$-based 2D and three-dimensional (3D) interfaces. Specifically, we conduct nonequilibrium molecular dynamics simulations on computational domains with graphene or ${\mathrm{Mo}\mathrm{S}}_2$ layers encapsulated between crystalline or amorphous silicon leads across a wide temperature range (of 50–600 K). We show that while the interfacial conductance across a graphene and crystalline silicon interface demonstrates considerable temperature dependence, the conductance across a graphene and amorphous silicon interface has no significant temperature dependence. In contrast, the thermal boundary conductance for the ${\mathrm{Mo}\mathrm{S}}_2$-based heterostructures with both the crystalline and amorphous leads demonstrate no significant temperature dependence. Our spectral energy-density calculations along with our spectrally resolved heat-flux accumulation calculations on the various interfaces show that anharmonic coupling across the entire vibrational spectrum as well as the strong hybridization of a broader spectrum of the flexural modes with substrate Rayleigh waves in graphene heterostructures give rise to the relatively higher and drastically different heat-transport mechanisms across these interfaces as compared to the ${\mathrm{Mo}\mathrm{S}}_2$-based heterostructures. Through these understandings, we show that one strategy to enhance heat conductance across 2D-3D interfaces is to increase the anharmonic coupling between the acoustic and optic modes in the 2D materials by inducing a stronger interaction strength with the substrates. Our findings elucidate the fundamental heat-transfer mechanisms dictating thermal-boundary conductances across 2D-3D interfaces and will be crucial for heat dissipation in the next generation of optoelectronic devices where the utilization of 2D materials are becoming ubiquitous.

Vulnerabilities of components used in quantum-key-distribution systems affect its implementation security and must be taken into consideration during system development and security analysis. In this paper, we investigate transmission of fiber optical elements, which are commonly used in quantum-key-distribution systems for designing countermeasures against Trojan-horse attacks, in the $1500$–$2100$-$\mathrm{nm}$ range. As a result, we find loopholes in their transmission spectra that open possibilities for eavesdropping. We also consider a simple passive countermeasure based on the violation of total internal reflection in a single-mode fiber, that provides additional insertion losses of at least 60 dB for double-pass Trojan-horse probe pulses for wavelengths longer than 1830 nm.

We develop a theoretical model for calibrating the absorption imaging of cold atoms under high magnetic fields. Particularly it also works when the hyperfine structure is decoupled and cannot provide good quantum numbers anymore. Compared with zero or low magnetic fields, for high magnetic fields the efficiency of the absorption imaging is lower, while an additional correction factor is required to obtain the absolute atom number under the Beer-Lambert law. Our model is based on the rate equations and can account for many experimental imperfections, such as Zeeman level crossing, off-resonant couplings, and low repumping efficiency. On the basis of this method, we can precisely calculate the correction factor for atom-number measurement without any empirical or fitting parameters. Meanwhile, we use a cold-atom apparatus based on ${}^{85}\mathrm{Rb}$ to experimentally verify our model. In addition, we find our work can also serve as a benchmark to measure the polarization impurity of a circularly polarized laser beam with high sensitivity. We believe this work will be beneficial for most cold-atom experiments using absorption imaging.

As integrated-circuit (IC) geometry and packaging become more sophisticated with ongoing fabrication and design innovations, the electrical-engineering community needs increasingly powerful failure-analysis (FA) methods to meet the growing troubleshooting challenges of multilayer (with multiple metal layers) and multichip components. In this work, we investigate an electronics FA method using a quantum diamond microscope (QDM) to image the magnetic fields from short-circuit faults. After quantifying the performance by detecting short-circuit faults in a multilayer silicon die, we assess how a QDM would detect faults in a heterogeneously integrated (HI) die stack. This work hopefully establishes QDM-based magnetic imaging as a competitive technique for electronics FA, offering high spatial resolution, high sensitivity, and robust instrumentation. We anticipate these advantages to be especially useful for finding faults deep within chip-stack ICs with many metal layers, optically opaque layers, or optically scattering layers.

Bilayer graphene is an attractive material that hosts a high-quality two-dimensional electron gas with a controllable band gap. By utilizing the band gap, electrical gate tuning of the carrier is possible and formation of nanostructures such as quantum dots has been reported. To probe the dynamics of the electronic states and realize applications for quantum bit devices, radio-frequency (rf) reflectometry, which enables high-speed electrical measurements, is important. Here we demonstrate rf reflectometry in bilayer graphene devices. We utilize a microscale graphite back-gate and an undoped $\mathrm{Si}$ substrate to reduce the parasitic capacitance, which degrades the rf reflectometry measurement. We measure the resonance properties including the matching condition of a tank circuit, which contains the bilayer graphene device. We construct a demodulated rf reflectometry setup and compare the result with dc measurement, and confirm their consistency. The wide-range frequency-dependent noise behavior is also analyzed. From the noise analysis, we calculate the readout error rate of our device for single-electron detection and demonstrate that a vertically integrated charge sensor has the potential to achieve a low error rate. We also measure Coulomb diamonds of quantum dots possibly formed by bubbles and confirm that rf reflectometry of quantum dots can be performed. This technique enables high-speed measurements of bilayer graphene quantum dots and contributes to the study of bilayer-graphene-based quantum devices by fast readout of the states.

The fabrication of superconducting circuits requires multiple deposition, etching, and cleaning steps, each possibly introducing material property changes and microscopic defects. In this work, we specifically investigate the process of argon milling, a potentially coherence-limiting step, using niobium and aluminum superconducting resonators as a proxy for the surface-limited behavior of qubits. We find that niobium microwave resonators exhibit an order of magnitude decrease in quality factors after surface argon milling, while aluminum resonators are resilient to the same process. Extensive analysis of the niobium surface shows no change in the suboxide composition due to argon milling, while two-tone spectroscopy measurements reveal an increase in two-level system electrical dipole moments, indicating a structurally altered niobium oxide. However, a short dry etch can fully recover the argon-milling-induced losses on niobium, offering a potential route towards state-of-the-art overlap Josephson junction qubits with niobium circuitry.

This paper develops a method for estimating the locations of laser-induced stray charges on surface-electrode ion traps. How the electric potential at the trapped-ion location changes in the presence of laser-induced stray charges is theoretically modeled, and the magnitude of the shift in the secular frequency of a trapped ion is derived from the model. This model can be used inversely to find the locations of stray charges from the secular frequency shift under certain conditions. Typically, this inverse relationship is not unique. However, in surface-electrode ion traps, the secular frequency shift caused by the stray charges can be measured at multiple ion positions, and therefore, the required number of secular-frequency-shift measurements can be performed to determine the stray-charge locations uniquely. This model is experimentally evaluated by intentionally producing electric charges at a single location by irradiating a laser onto that spot and by measuring the resulting secular frequency shift. The location of produced charges is estimated from the measured result, and the estimated location is consistent with where the laser is irradiating. The developed method can be applied to characterize the laser-induced stray charges, which in turn can be used to design experiments to avoid or reduce the occurrence of stray charges.

According to the Heisenberg uncertainty principle between time and energy fluctuation, a concept of the quantum speed limit (QSL) has been established to determine the minimum evolutionary time between quantum states. Considerable theoretical and experimental efforts are invested in obtaining the QSL time bounds in various scenarios. However, it remains a long-standing goal to derive a meaningful QSL bound for a general quantum problem. Here, we propose a geometrical approach to derive a QSL bound for closed and open quantum systems. By solving a quantum brachistochrone problem in the framework of the Riemannian metric, we show that the QSL between a given initial state and a final state is determined not only by the entire dynamics of the system but also by the individual dynamics of a critical parameter. We exemplify the utility of the bound in three representative scenarios, demonstrating a pronounced advantage in finding a tight and meaningful QSL bound of a general quantum evolution problem.

We propose a hetero-homodyne receiver for quantum illumination (QI) target detection. Unlike prior QI receivers, it uses a cascaded positive operator-valued measurement (POVM) that does not require a quantum interaction between QI’s returned radiation and its stored idler. When used without sequential detection its performance matches the 3-dB quantum advantage over optimum classical illumination (CI) that Guha and Erkmen’s [Phys. Rev. A 80, 052310 (2009)] phase-conjugate and parametric amplifier receivers enjoy. When used in a sequential detection QI protocol, the hetero-homodyne receiver offers a 9-dB quantum advantage over a conventional CI radar, and a 3-dB advantage over a CI radar with sequential detection. Our work is a significant step forward toward a practical quantum radar for the microwave region, and, more generally, emphasizes the potential offered by cascaded POVMs for quantum radar.

A self-driven hybrid atomic spin oscillator is demonstrated in theory and experiment with a vapor $\mathrm{Rb}$-$\mathrm{Xe}$ dual-spin system. The raw signal of $\mathrm{Rb}$ spin oscillation is amplified, phase-shifted, and sent back to drive the $\mathrm{Xe}$ spins coherently. By fine-tuning the driving-field strength and phase, a self-sustaining spin-oscillation signal with zero frequency shift is obtained. The effective coherence time is infinitely prolonged beyond the intrinsic coherence time of $\mathrm{Xe}$ spins, forming a hybrid atomic spin oscillator. Spectral analysis indicates that a frequency resolution of 13.1 nHz is achieved, increasing the detection sensitivity for a magnetic field. Allan-deviation analysis shows that the spin oscillator can operate in continuous-wave mode like a spin maser. The prototype spin oscillator can be easily incorporated into other hybrid spin systems and can increase the detection sensitivity of alkali-metal–noble-gas comagnetometers.

A label-free, low-photodamage, low-cost, and in vivo high-resolution microscopic imaging technique has been challenging to develop. Ghost imaging (GI) is a computational imaging protocol with the spatially nonlocal property, which enables low-light imaging. Considering the advantages of GI, we propose a method, called high-resolution microscopic ghost imaging (HRMGI), to achieve high-resolution imaging by simply demagnifying the signal beam. The analytical formulas for resolution and depth of field (DOF) of HRMGI are provided. Experimental results show that the HRMGI system can have 10.5 times higher resolution than the conventional GI, which can exceed the pixel size limit of multipixel detectors, and 58 times greater DOF than the traditional diffraction-limited optical (TDLO) system. To present the performance of HRMGI, blood vessels within an ovarian section are also imaged. The proposed method is expected to facilitate the development of bioimaging techniques and promote the practical application of GI.

Nonreciprocal quantum devices with low loss are indispensable in quantum information processing but challenging to be realized. Here we report an experiment to realize the nonreciprocal phase shift larger than $\pi$ with low loss in a cavity-free and magnetic-free cold atomic ensemble based on electromagnetically induced transparency. Furthermore, a four-port all-optical circulator is demonstrated with an average isolation ratio of 22.25 dB and transmission of 0.94 for each channel. At its optimal working point, the circulator’s total isolation ratio reaches 63.47 dB, transmission reaches 0.86, and fidelity at about 0.99. Our work can advance further investigation of nonreciprocity with cold atoms in quantum information processing applications.

A high-efficiency spin-photon interface is an essential piece of quantum hardware necessary for various quantum technologies. Self-assembled ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dots have excellent optical properties: if embedded in an optical microcavity, they can show near-deterministic spin-photon entanglement and spin readout. To address the individual spin states, an external magnetic field is required, and a superconducting magnet is usually used. Here we show a compact cryogenically compatible $\mathrm{Sm}$-$\mathrm{Co}$ magnet design that delivers a 475-mT in-plane (Voigt-geometry) magnetic field at $5\mathrm{K}$, which is suitable for lifting the energy degeneracy of the electron spin states and trion transitions of a single ($\mathrm{In}$,$\mathrm{Ga}$)$\mathrm{As}$ quantum dot. This quantum dot is embedded in a birefringent high-finesse optical microcavity that enables efficient collection of single photons emitted by the quantum dot. We demonstrate spin-state manipulation by addressing the trion transitions with a single laser field and two laser fields. The experimental data agree well with our model, which covers single- and two-laser cross-polarized resonance fluorescence, Purcell enhancement in a birefringent cavity, and variation of the laser powers.

Antiferromagnets that facilitate terahertz (THz) spin resonances have the potential to revolutionize high-speed electronics at the nanoscale. The electrical control of THz spin resonances is the key to such THz devices; however, experimental demonstration has remained elusive. In this work, we demonstrate electrically tunable THz spin resonance in an antiferromagnetic $\mathrm{Ni}\mathrm{O}/\mathrm{Pt}$ heterostructure by employing both low-wavenumber Raman and continuous-wave THz spectroscopy techniques. A redshift of over 100 GHz in the $\mathrm{Ni}\mathrm{O}$ spin resonance frequency of around 1 THz is observed by applying charge currents along the adjacent $\mathrm{Pt}$ layer. A control experiment with $\mathrm{Ni}\mathrm{O}/\mathrm{Cu}$ and temperature-dependent measurement confirm that the dominant tuning mechanism is Joule heating. Finally, a prototype device is designed to achieve an electrical control of THz transmission at dual channels of 0.96 and 1 THz, leading to a Q factor of 56. This work opens up the possibility for the implementation of tunable THz devices utilizing antiferromagnetic spin resonance.

An acoustic dimer composed of two electronically controlled electroacoustic resonators is presented with the view of exploring one-dimensional topological phenomena. Active control allows real-time manipulation of the metamaterial’s properties, including its mechanical mass, resistance, compliance, and internal coupling. The latter enables active tuning of the topological phase transition in the effective acoustic Su-Schrieffer-Heeger system. An analytical model of the unit cell as well as the electronic control scheme is provided. Band structures derived from the analytical model, the finite-element simulation, and the experimental data consistently demonstrate the realization of a tunable one-dimensional topological insulator.

We investigate the optimal deposition conditions for permalloy (${\mathrm{Ni}}_{81}\mathrm{Fe}$${}_{19}$) thin films fabricated via magnetron sputtering to achieve soft magnetic films for magnetoresistive sensing applications. The films are grown with different deposition techniques such as sputtering and molecular-beam epitaxy and parameters, including varying inert gas pressure and deposition power, and with different magnetic fields applied during the growth. Our approach enables sputtering of permalloy films with low coercivity while keeping high anisotropic magnetoresistance values. We develop a robust method to characterize the intrinsic magnetic anisotropy of the films that is not dominated by local defects and we demonstrate the possibility of magnetic anisotropy suppression via implementing a rotating magnetic field during sputtering.