Effective low-energy theories represent powerful theoretical tools to reduce the complexity in modeling interacting quantum many-particle systems. However, common theoretical methods rely on perturbation theory, which limits their applicability to weak interactions. Here we introduce the Variational Adiabatic Gauge Transformation (VAGT), a nonperturbative hybrid quantum algorithm that can use nowadays quantum computers to learn the variational parameters of the unitary circuit that brings the Hamiltonian to either its block-diagonal or full-diagonal form. If a Hamiltonian can be diagonalized via a shallow quantum circuit, then VAGT can learn the optimal parameters using a polynomial number of runs. The accuracy of VAGT is tested through numerical simulations, as well as simulations on Rigetti and IonQ quantum computers.

Magnetic tunnel junctions (MTJs) have attracted strong research interest within the last decades due to their potential use as nonvolatile memory such as magnetoresistive random access memory as well as for magnetic logic applications. Half-metallic magnets (HMMs) have been suggested as ideal electrode materials for MTJs to achieve an extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode effect, which was achieved in a magnetic tunnel junction concept [E. Şaşıoğlu et al., ACS Appl. Electron. Mater. 1, 1552–9 (2019)] based on HMMs and type-II spin-gapless semiconductors (SGSs). The proposed concept has recently been experimentally demonstrated using Heusler compounds. In the present work, we investigate from first-principles MTJs based on type-II SGS and HMM quaternary Heusler compounds $\mathrm{Fe}\mathrm{V}\mathrm{Ta}\mathrm{Al}$, $\mathrm{Fe}\mathrm{V}\mathrm{Ti}\mathrm{Si}$, $\mathrm{Mn}\mathrm{V}\mathrm{Ti}\mathrm{Al}$, and $\mathrm{Co}\mathrm{V}\mathrm{Ti}\mathrm{Sb}$. Our ab initio quantum transport calculations based on a nonequilibrium Green’s function method have demonstrated that the MTJs under consideration exhibit current rectification with relatively high on:off ratios. We show that, in contrast to conventional semiconductor diodes, the rectification bias voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that the diode effect can be configured dynamically, i.e., depending on the relative orientation of the magnetization of the electrodes, the MTJ allows the electrical current to pass either in one or the other direction, which leads to an inverse TMR effect. The combination of nonvolatility, reconfigurable diode functionality, tunable rectification voltage window, and high Curie temperature of the electrode materials makes the proposed MTJs very promising for room-temperature spintronic applications and opens ways to magnetic memory and logic concepts as well as logic-in-memory computing.

We demonstrate an algorithm-dependent computational ghost imaging (GI) by proposing a pattern multiplexing method. Based on the characteristics of three typical GI algorithms (i.e., traditional second-order GI, differential GI, total variational regularization GI), we design a set of pseudorandom patterns to achieve different reconstructed images with different GI algorithms. With our designed patterns, total variational regularization GI can be used for imaging unknown scenes or prehidden images, while traditional second-order GI and differential GI are used for encrypt other different images. Not only being critical for imaging quality in GI, reconstruction algorithms in our simulations and experiments demonstrate their potential role in multiple-image reconstruction, providing an idea for the application of GI in image encryption.

We show that resonant plasmonic detection dramatically increases the sensitivity of the terahertz detectors based on a gated graphene $p$-$i$-$n$ (GPIN) field-effect transistor (FET) structure. In the proposed device, the gated $p$ and $n$ regions serve as the hole and electron reservoirs and the terahertz resonant plasma cavities. The current-voltage ($I$-$V$) characteristics are strongly nonlinear due to the Zener-Klein interband tunneling in the reverse-biased $i$ region between the gates. The terahertz signal rectification by this region enables the terahertz detection. The resonant excitation of the hole and electron plasmonic oscillations results in a substantial increase in the terahertz detector responsivity at the signal frequency close to the plasma frequency and its harmonics. Because of the transit-time effects, the GPIN-FET response at the higher plasmonic modes could be stronger than for the fundamental mode. Our estimates predict the detector responsivity up to a few ${10}^5$ V/W at room temperature, much larger than for other electronic terahertz detectors, such as Schottky diodes, $p$-$n$ junctions, Si CMOS, and III-V and III-N HEMTs.

The advantages of using quantum states of light for object detection are often highlighted in schemes that use simultaneous and optimal measurements. Here, we describe a theoretical but experimentally realizable quantum illumination scheme based on nonsimultaneous and nonoptimal measurements, which can maintain this advantage. In particular, we examine the multiclick-heralded two-mode-squeezed vacuum state as a probe signal in a quantum illumination process. The increase in conditioned signal intensity associated with multiclick heralding is greater than that from a single detector-heralded signal. Our results show, for lossy external conditions, the presence of the target object can be revealed earlier using multiclick measurements. We demonstrate this through sequential shot measurements based on Monte Carlo simulation.

$\mathrm{Mn}\mathrm{N}$ is a centrosymmetric collinear antiferromagnet belonging to the transition-metal-nitride family with a high Néel temperature ($660\mathrm{K}$), a low anisotropy field, and a large magnetic moment ($3.3{\mu }_B$ per $\mathrm{Mn}$ atom). Despite several recent experimental and theoretical studies, the spin symmetry (magnetic point group) and magnetic domain structure of the material remain unknown. In this work, we use optical second-harmonic generation (SHG) to study the magnetic structure of thin epitaxially grown single-crystal (001) $\mathrm{Mn}\mathrm{N}$ films. Our work shows that spin moments in $\mathrm{Mn}\mathrm{N}$ are tilted away from the [001] direction and the components of the spin moments in the (001) plane are aligned along one of the two possible in-plane symmetry axes ([100] or [110]), resulting in a magnetic-point-group symmetry of 2/m1’. Our work rules out magnetic-point-group symmetries 4/mmm1’ and mmm1’ that have been previously discussed in the literature. Four different spin domains consistent with the 2/m1’ magnetic-point-group symmetry are possible in $\mathrm{Mn}\mathrm{N}$. A statistical model based on the observed variations in the polarization-dependent intensity of the second-harmonic signal collected over large sample areas puts an upper bound of $0.65\mu \mathrm{m}$ on the mean domain size. Our results show that SHG can be used to probe the magnetic order in metallic antiferromagnets. This work is expected to contribute to the recent efforts in using antiferromagnets for spintronic applications.

The current-induced type-x spin-orbit-torque (SOT) switching configuration describes the orthogonal relationship between the magnetic easy axis (EA) of a ferromagnetic (FM) layer and the injected spin polarization, $\sigma$, from a heavy-metal (HM) layer, which has the potential to eclipse the conventional type-y scenario (EA parallel to $\sigma$) at the sub-nanosecond pulse regime. Here, we show that, in HM/FM bilayer heterostructures, current-driven differential planar Hall signals can serve as efficient means to probe type-x SOT switching in a simple measurement fashion. Through this approach, we demonstrate field-free type-x SOT switching in all devices with a canted EA, which are engineered via field-annealing processes. By analyzing the switching-phase diagrams, we further verify that such field-free switching stems from a z-direction effective field contributed to by both the dampinglike torque and the canted EA. Our work indicates that the canted EA, as engineered by a field-annealing process, can give rise to a robust field-free SOT switching and further reduce the critical switching current density.

Single rare-earth ions in solids show great potential for quantum applications, including single-photon emission, quantum computing, and high-precision sensing. However, the linewidths of single rare-earth ions are often broadened due to perturbations associated with the detection methods. Identifying the dominant broadening sources is key to reduce the linewidths for practical applications. We report a spectral broadening study on a single ${\mathrm{Er}}^{3+}$ ion in a $\mathrm{Si}$ nanotransistor. The single ion spectra display a Lorentzian lineshape at all light intensities considered. The linewidth remains nearly constant at 32±2 MHz in the low-intensity regime, and shows a monotonic increase with the intensity in the high-intensity regime. The power broadening does not persist over the microsecond time scales considered after resonant excitation. Nor does it depend on the resonant excitation intensity or the Zeeman shift. These observations and temperature-dependent measurements suggest that charge fluctuations are likely to be a dominant broadening source. Laser heating may also contribute to the power broadening. Charge suppression in the ${\mathrm{Er}}^{3+}$-doped region and coupling ${\mathrm{Er}}^{3+}$ ions to a confined optical mode could be implemented to reduce the spectral linewidth and to enhance the sensing precision of single ${\mathrm{Er}}^{3+}$ ions in $\mathrm{Si}$.

Charge injection from electrodes into doped organic films is a widespread technology used in the majority of state-of-the-art organic semiconductor devices. Although such interfaces are commonly considered to form Ohmic contacts via strong band bending, an experiment that directly measures the contact resistance has not yet been demonstrated. In this study, we use a simple metal/doped-organic-semiconductor/metal stack and study its voltage-dependent resistance. A transport-layer thickness variation proves that the experiment presented gains direct access to the contact resistance of the device. We can quantify that for an operating current density of $10{\text{mA/cm}}^2$, the investigated material system exhibits a voltage drop over the metal-organic interface of about $200\mathrm{mV}$, which can be reduced by more than one order of magnitude when employing an additional injection layer. The experiment presented proposes a simple strategy to measure the contact resistance between any metal and doped organic film without applying numerical tools or elaborate techniques. Furthermore, the simplistic device architecture allows for very high, homogeneous, and tunable electric fields within the organic layer, which enables a clear investigation of the Poole-Frenkel effect.

We present a classical algorithm to find approximate solutions to instances of quadratic unconstrained binary optimization. The algorithm can be seen as an analog of quantum annealing under the restriction of a product-state space, where the dynamical evolution in quantum annealing is replaced with a gradient-descent-based method. This formulation is able to quickly find high-quality solutions to large-scale problem instances and can naturally be accelerated by dedicated hardware such as graphics processing units. We benchmark our approach for large-scale problem instances with tunable hardness and planted solutions. We find that our algorithm offers a similar performance to current state-of-the-art approaches within a comparably simple gradient-based and nonstochastic setting.

As society paves its way towards device miniaturization and precision medicine, microscale actuation and transport become increasingly prominent research fields with high impact in both technological and clinical contexts. In order to accomplish movement of micron-sized objects towards specific target sites, active biohybrid transport systems, such as motile living cells that act as smart biochemically powered microcarriers, have been suggested as an alternative to synthetic microrobots. Inspired by the motility of leukocytes, we propose the amoeboid crawling of eukaryotic cells as a promising mechanism for transport of micron-sized cargoes and present an in-depth study of this type of composite active matter. Its transport properties result from the interactions of an active element (cell) and a passive one (cargo) and reveal an optimal cargo size that enhances the locomotion of the load-carrying cells, even exceeding their motility in the absence of cargo. The experimental findings are rationalized in terms of a biohybrid active particle model that describes the emergent cell-cargo dynamics and enables us to derive the long-time diffusive transport of amoeboid microcarriers. As amoeboid locomotion is commonly observed for mammalian cells such as leukocytes, our results lay the foundations for the study of transport performance of other medically relevant cell types and for extending our findings to more advanced transport tasks in complex environments, such as tissues.

Recently, the ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ (LAO/STO) interface has been highlighted as a major platform for spintronics, and its fundamental control of spin properties, therefore, becomes a key issue for application of this system. Here, we present a study showing the modulation of magnetic two-dimensional electron gases (2DEGs) with simultaneously enhanced spin-orbit interaction at the interface of LAO/STO by inserting ${\mathrm{La}\mathrm{Co}\mathrm{O}}_3$ submonolayers. At first, transport experiments provide evidence that Kondo behavior can be well controlled below about 13 K with the interlayer $\mathrm{Co}$ ions contributing as scattering centers. In addition, the systematic variation of the anomalous Hall effect obtained with increasing fraction of interfacial $\mathrm{Co}$ concentration below 10 K reveals that the spin polarization of 2DEGs is enhanced via submonolayer insertion. Simultaneously, we also observe an enlarged spin-orbit interaction at the buffered LAO/STO interface, resulting in a remarkably strong field-orientation-dependent magnetoresistance. Such tailored LAO/STO interfaces could potentially contribute to stronger spin-to-charge conversion responses and spin-torque measurements. Our observations indicate the subunit-cell insertion of functional layers to be a suitable route to tailor spin, orbital, and lattice interactions of the interfacial 2DEG, which makes the LAO/STO system an intriguing platform for spintronic applications.

Phase measurement constitutes a key task in many fields of science, both in the classical and quantum regime. The higher precision of such measurement offers significant advances, and can also be utilized to achieve finer estimates for quantities such as distance, the gravitational constant, electromagnetic field amplitude, etc. Here we theoretically model the use of a quantum network, composed of a randomly coupled set of two-level systems, as a processing device for phase measurement. An incoming resource state carrying the phase information interacts with the quantum network, whose emission is trained to produce a desired output signal. We demonstrate phase-precision scaling following the standard quantum limit and Heisenberg limit. This can be achieved using quantum resource states such as NOON states or other entangled states, however, we also find that classically correlated mixtures of states are alone sufficient, provided that they exhibit quantum coherence. Our proposed setup does not require conditional measurements, and is compatible with many different types of coupling between the quantum network and the phase-encoding state, hence making it attractive to a wide range of possible physical implementations.

We propose how to engineer longitudinal coupling to accelerate the measurement of a qubit longitudinally coupled to a cavity, motivated by the concept of shortcuts to adiabaticity. Different modulations are inversely designed from inverse engineering, counter-diabatic driving, and genetic algorithm, for achieving optimally large values of the signal-to-noise ratio (SNR) at a nanosecond scale. By comparison, we demonstrate that our protocols outperform the usual periodic modulations on pointer state separation and SNR. Finally, we show a possible implementation considering state-of-the-art circuit quantum electrodynamics architecture, estimating the minimal time allowed for the measurement process.

Superconducting quantum computing is experiencing a tremendous growth. Although major milestones have already been achieved, useful quantum-computing applications are hindered by a variety of decoherence phenomena. Decoherence due to two-level systems (TLSs) hosted by amorphous dielectric materials is ubiquitous in planar superconducting devices. We use high-quality quasilumped-element resonators as quantum sensors to investigate TLS-induced loss and noise. We perform two-tone experiments with a probe and pump electric fields; the pump is applied at different power levels and detunings. We measure and analyze time series of the quality factor and resonance frequency for very long time periods, up to 1000 h. We additionally carry out simulations based on the TLS interacting model in the presence of a pump field. We find that loss and noise are reduced at medium and high power, matching the simulations, but not at low power.

Efforts to make the International System of Units (SI) more accessible have been growing since the global redefinition of the SI in 2019. Presently, the ohm is defined through the quantum Hall effect (QHE). However, quantum Hall devices require strong magnetic fields for a robust expression of the QHE. Magnetically doped topological insulators offer an opportunity to circumvent this limitation, as they require no magnetic field to provide a quantized resistance plateau via the quantum anomalous Hall effect (QAHE). Here, we detail experimental methodology and noise-analysis techniques for high-precision measurements of QAHE devices.

Composite optical systems can show compelling collective dynamics. For instance, the cooperative decay of quantum emitters into a common radiation mode can lead to super-radiance, where the emission rate of the ensemble is larger than the sum of the rates of the individual emitters. Here, we report experimental evidence of super spontaneous four-wave mixing, an analogous effect for the generation of photon pairs in a parametric nonlinear process on an integrated photonic device. We study this phenomenon in an array of microring resonators on a silicon photonic chip coupled to bus waveguides. We measure a cooperative pair-generation rate that always exceeds the incoherent sum of the rates of the individual resonators. We investigate the physical mechanisms underlying this collective behavior, clarify the impact of loss, and address the aspects of fundamental and technological relevance of our results.

This paper presents a theoretical and experimental investigation into the electrostatic coupling mechanism between different nonlinear modes inside a single resonator. Nonlinear intermodal coupling allows an arbitrary mode to be used as a modulator for the resonance of the coupled mode, presenting an efficient method to tune the frequency of a specific vibration mode. Here, a capacitive micromechanical resonator is developed, in which different modes exhibit different nonlinear characteristics. Inside this resonator, interactions between different nonlinear modes induced by electrostatic fields are observed and accurately modeled. It demonstrates that resonance modulation control can be achieved by activating the coupled modes based on the dispersive parametric coupling effect. Meanwhile, the resonance modulation is jointly determined by coupled modes, providing a theoretical basis and approach for modal manipulation technology. The dispersive coupling between intrinsic modes enables the probe resonance to be tuned by nearly 100 times its bandwidth, and its range and polarity can also be controlled by selecting the resonance of the pump mode. It is proven that resonance modulation control induced by the modal coupling effect can efficiently tune the resonator’s resonance frequency over a wide range, which presents a promising voltage-frequency transduction scheme with high sensitivity and low noise for the precision instrument. Its pull range exhibits a great potential of more than 20% as a voltage-controlled oscillator and can be customized to satisfy different requirements, paving the way toward advanced mechanics.