We propose a quantum charging scheme fueled by measurements on ancillary qubits serving as disposable chargers. A stream of identical qubits are sequentially coupled to a quantum battery of $N+1$ levels and measured by projective operations after joint unitary evolutions of optimized intervals. If charger qubits are prepared in excited state and measured on ground state, then their excitations (energy) can be near perfectly transferred to battery by iteratively updating the optimized measurement intervals. Starting from its ground state, the battery could be constantly charged to an even higher energy level. Starting from a thermal state, the battery could also achieve a near-unit ratio of ergotropy and energy through less than $N$ measurements, when a population inversion is realized by measurements. If charger qubits are prepared in ground state and measured on excited state, useful work extracted by measurements alone could transform the battery from a thermal state to a high-ergotropy state before the success probability vanishes. Our operations in charging are more efficient than those without measurements and do not invoke the initial coherence in both battery and chargers. Particularly, our finding features quantum measurement in shaping nonequilibrium systems.

We report experimental studies of a driven spin-mechanical system, in which a nitrogen-vacancy ($\mathrm{N}\text{−}$V) center couples to out-of-plane vibrations of a diamond cantilever through the excited-state deformation potential. Photoluminescence excitation studies show that in the unresolved sideband regime and under strong resonant mechanical driving, the excitation spectra of a $\mathrm{N}\text{−}$V optical transition feature two spectrally sharp peaks, corresponding to the two turning points of the oscillating cantilever. In the limit that the strain-induced frequency separation between the two peaks far exceeds the $\mathrm{N}\text{−}$V zero-phonon linewidth, the spectral position of the individual peak becomes sensitive to minute detuning between the mechanical resonance and the external driving force. For a fixed optical excitation frequency near the $\mathrm{N}\text{−}$V transition, $\mathrm{N}\text{−}$V fluorescence as a function of mechanical detuning features resonances with a linewidth that can be orders of magnitude smaller than the intrinsic linewidth of the mechanical mode. This enhanced sensitivity to mechanical detuning can potentially provide an effective mechanism for mechanical sensing, for example, mass sensing via measurements of induced changes in the mechanical oscillator frequency.

Predictive design and optimization methods for controlled quantum systems depend on the accuracy of the system model. Any distortion of the input fields in an experimental platform alters the model accuracy and eventually disturbs the predicted dynamics. These distortions can be nonlinear with a strong frequency dependence so that the field interacting with the microscopic quantum system has limited resemblance to the input signal. We present an effective method for estimating these distortions, which is suitable for nonlinear transfer functions of arbitrary lengths and magnitudes provided the available training data has enough spectral components. Using a quadratic estimation, we have successfully tested our approach for a numerical example of a single Rydberg atom system. The transfer function estimated from the presented method is incorporated into an open-loop control optimization algorithm allowing for high-fidelity operations in quantum experiments.

Fluctuations of the qubit frequencies are one of the major problems to overcome on the way to scalable quantum computers. Of particular importance are fluctuations with a correlation time that exceeds the decoherence time due to decay and dephasing by fast processes. The statistics of the fluctuations can be characterized by measuring the correlators of the outcomes of periodically repeated Ramsey measurements. We describe qubit dynamics during repeated measurements and evaluate the two-time correlator for noise from two-level systems and two- and three-time correlators for Gaussian noise. The explicit expressions for the correlators are compared with simulations. We find that, even though the three-time correlators for noise from two-level systems and for Gaussian noise are generally significantly different, already ten two-level systems can mimic Gaussian noise provided they are symmetric. This is not the case for asymmetric systems.

Wave-based analog computation platforms hold the promise of realizing intelligent information processing with high speed and low computational cost, but current design principles face intractable problems in processing low-frequency wave information end to end at the physical layer. Here we theoretically demonstrate a concept of a metamaterial-based analog recurrent neural network (Meta-RNN) for intelligent classification of physical information carried by mechanical vibrations. With the spatiotemporal recurrence relation induced by coupled local resonators of the simulated metamaterial model, the Meta-RNN is capable of memorizing the internal hidden state in the dynamic displacement field. The trained coupling and synergy of local resonators lead to desirable vibration energy localization with selective frequency extraction capability, which can distinguish the intrinsic characteristics of vibration information. The proposed Meta-RNN provides a foundation for implementing a promising mechanical analog processor for intelligent vibration information processing and classification, and paves the way to efficient machine learning platforms for machine intelligence.

Quadrupolar topological semimetals with a bulk quadrupole moment have been intensively investigated recently. Of special interest is the quadrupolar surface topological semimetal, which has gapped bulk but surface Dirac nodes attached to each other by hinge states. However, this novel state remains to be observed in condensed matter. Here, we implement the quadrupolar surface topological semimetal in acoustic crystals. By tuning the symmetry of the structure, the hinge states at a specific hinge of the quadrupolar topological semimetal can be engineered, thus allowing the coexistence of different hinge dispersions in a single system. The surface Dirac nodes and hinge states are revealed by momentum-space spectroscopy and real-space visualization. Our findings may provide alternative opportunities to manipulate acoustic waves.

Quantum-cascade-laser (QCL) frequency combs are compact semiconductor light sources operating in the mid-IR and terahertz frequencies. Achieving subpicosecond laser pulses with high peak power is of vital importance for performing nonlinear time-resolved spectroscopy as well for exploring nonlinear phenomena. Therefore, investigation and characterization of time-resolved free-running laser emission is a key for further improvement and optimization of these intersubband devices. In this work, we demonstrate a direct electric field measurement of a free-running terahertz QCL frequency comb using electro-optic sampling in combination with computational phase correction, where we retrieve the electric field profile and access the comb parameters. The demonstrated method is of high interest for time-resolved low-power laser emission characterization, typical for ring terahertz QCL frequency combs and investigation of phase-compensated emission via waveguide dispersion engineering for broadband comb operation.

The characterization of externally applied signals is essential in order to improve performance and design appropriate engineering strategies in cold-atom experiments. Here, we present a practical technique to detect the artificially introduced electric signals by an exquisite sensor made of a single ${}^{40}$$\mathrm{Ca}$${}^{+}$ ion behaving as the phonon laser. The sensor can work as an oscilloscope, detecting spectra of the signals in the time domain, and also as a frequency spectrograph for the signal probe with respect to the frequency. The high precision of such an atomic sensor relies on the unique characteristics of the phonon laser. In particular, the sensor is practical since it works without the prerequisite of sideband cooling and the performance of the sensor can be further improved depending on better trapping conditions and a higher collection efficiency of the spontaneous emission fluorescence.

Bioprinting of living cells can cause major shape deformations, which may severely affect cell survival and functionality. While the shear stresses occurring during cell flow through the printer nozzle have been quantified to some extent, the extensional stresses occurring as cells leave the nozzle into the free printing strand have been mostly ignored. Here we use lattice Boltzmann simulations together with a finite-element based cell model to study cell deformation inside the nozzle and at its exit. Our simulation results are in good qualitative agreement with experimental microscopy images. We show that, for cells flowing in the center of the nozzle, extensional stresses can be significant while, for cells flowing off-center, their deformation is dominated by the shear flow inside the nozzle. From the results of these simulations, we develop two simple methods that only require the printing parameters (nozzle diameter, flow rate, bioink rheology) to (i) accurately predict the maximum cell stress occurring during the three-dimensional bioprinting process and (ii) approximately predict the cell strains caused by the elongational flow at the nozzle exit.

High-quality nanomechanical oscillators can sensitively probe force, mass, or displacement in experiments bridging the gap between the classical and quantum domain. Dynamics of these stochastic systems is inherently determined by the interplay between acting external forces, viscous dissipation, and random driving by the thermal environment. The relevance of inertia then dictates that both position and momentum must, in principle, be known to fully describe the system, which makes its quantitative experimental characterization rather challenging. We introduce a general method of Bayesian inference of the force field and environmental parameters in stochastic inertial systems that operates solely on the time series of recorded noisy positions of the system. The method is first validated on simulated trajectories of model stochastic harmonic and anharmonic oscillators with damping. Subsequently, the method is applied to experimental trajectories of particles levitating in tailored optical fields and used to characterize the dynamics of particle motion in a nonlinear Duffing potential, a static or time-dependent double-well potential, and a nonconservative force field. The presented inference procedure does not make any simplifying assumptions about the nature or symmetry of the acting force field and provides robust results with trajectories 2 orders of magnitude shorter than those typically required by alternative inference schemes. In addition to being a powerful tool for quantitative data analysis, it can also guide experimentalists in choosing appropriate sampling frequency (at least 20 measured points per single characteristic period) and length of the measured trajectories (at least 10 periods) to estimate the force field and environmental characteristics with a desired accuracy and precision.

The subthreshold leakage current in transistors has become a critical limiting factor for realizing ultralow-power transistors. The leakage current is predominantly dictated by the long thermal tail of the charge carriers. We propose a solution to this problem by using narrow-bandwidth semiconductors to limit the thermionic leakage current by filtering out the high-energy carriers. We specifically demonstrate this solution in transistors with a laterally confined ${\text{MoSi}}_2{\text{N}}_4$ monolayer with different passivation serving as channel material. Remarkably, we find that the proposed narrow-bandwidth devices can achieve a large on:off current ratio with an ultralow-power supply voltage of $\sim 0.1\text{V}$, even for devices with $\sim 5\mathrm{nm}$ gate length. We also show that several other materials share the unique electronic properties of narrow-bandwidth conduction and valence bands in the same series.

Efficient optical interfacing of spin-bearing quantum emitters is a crucial ingredient for quantum networks. A promising route therefore is to incorporate host materials as minimally processed membranes into open-access microcavities: it enables significant emission enhancement and efficient photon collection, minimizes deteriorating influence on the quantum emitter, and allows for full spatial and spectral tunability. Here, we study the properties of a high-finesse fiber Fabry-Pérot microcavity with integrated single-crystal diamond membranes by scanning cavity microscopy. We observe the spatially resolved effects of the diamond-air interface on the cavity-mode structure: a strong correlation of the cavity finesse and mode structure with the diamond thickness and surface topography, prevalent transverse-mode mixing under diamondlike conditions, and mode-character-dependent polarization-mode splitting. Our results reveal the influence of the diamond surface on the achievable Purcell enhancement, which helps to clarify the route towards optimized spin-photon interfaces.

The Harrow-Hassidim-Lloyd quantum algorithm was proposed to solve linear systems of equations $A\overrightarrow{x}=\overrightarrow{b}$ and it is the core of various applications. However, there is no explicit quantum circuit for the subroutine that maps the inverse of the problem matrix $A$ into an ancillary qubit. This makes implementation in current quantum devices challenging, forcing us to use hybrid approaches. Here, we propose a systematic method to implement this subroutine, which can be adapted to other functions $f(A)$ of matrix $A$, we present a co-designed quantum processor that reduces the depth of the algorithm, and we introduce its digital-analog implementation. The depth of our proposal scales with precision $ϵ$ as $\mathcal{O}({ϵ}^{-1})$, which is bounded by the number of samples allowed for a certain experiment. The co-design of the Harrow-Hassidim-Lloyd algorithm leads to a “kitelike” architecture, which allows us to reduce the number of required swap gates. Finally, merging a co-design quantum processor architecture with a digital-analog implementation contributes to the reduction of noise sources during the experimental realization of the algorithm.

The reverse intersystem-crossing (RISC) channels from triplet to singlet charge-transfer states (such as exciplex or electroplex states) frequently occur in donor:acceptor heterojunction organic light-emitting diodes (OLEDs) to achieve high external quantum efficiency. Although exciplexes have been extensively investigated, there are few reports on the physical microscopic processes for electroplex-based devices. Herein, three kinds of donor:acceptor heterojunction OLEDs are fabricated: two coexisting electroplex and exciton devices and one pure exciplex-based device. Amazingly, via the fingerprint magnetoelectroluminescence measurement at room temperature, we observe an abnormal current-dependent RISC, which is enhanced with increasing bias current (I) in the coexisting electroplex and exciton but electroplex-dominated device; the conversion from ISC to RISC in the coexisting electroplex and exciton but exciton-dominated device; and only the normal I-dependent ISC, which weakens with increasing I in the pure exciplex-based device. This is because low-energy-triplet electroplexes are well confined by the high triplet energies of electron donors and acceptors, promoting the occurrence of the RISC process in electroplexes, and Dexter energy transfer from triplet excitons to triplet electroplexes enhances the RISC process with increasing I, causing the abnormal I-dependent RISC and the conversion from ISC to RISC. Moreover, as the operational temperature rises from 10 to 300 K, these two coexisting electroplex and exciton devices present the conversion from ISC to RISC at different temperatures, but the pure exciplex-based device shows only the normal temperature-dependent ISC, which is enhanced with increasing temperature. This is because the RISC channel is an endothermic process and temperature-dependent electroluminescence measurements show that the electroplex emission weakens as the temperature is reduced, causing the weakened RISC process and even the presence of the ISC process at low temperature. These experimental results demonstrate that the electroplex emission plays an important role in the occurrence of strong RISC processes. Our work deepens our microscopic understanding of the charge-transfer states in donor:acceptor heterojunction devices but also paves the way for utilizing electroplex states to design highly efficient OLEDs.

Alternative types of artificial cooling techniques are of large interest for multiple applications. Here, we develop a framework for studying the role of electro-optical coupling in the analysis of solid-state refrigerators based on electroluminescent cooling (ELC) by combining device measurements with optical simulations. The studied device consists of a light-emitting diode (LED) epitaxially connected to a photodetector (PD) in a double-diode structure (DDS). Previous results of the DDS have indicated that the LED side already operates at conditions corresponding to ELC, but Ohmic losses and imperfect photodetection of the LED light in the PD have prevented observing the effect directly. Here, to break down the detection losses of the DDS, we report on the electro-optical response of the LED and the PD in detail, as well as the role of the spectral coupling from the LED to the PD. We present a detailed framework for combining measurements and simulations of the DDS to gain quantitative insight of the electro-optical response of the LED and PD, as well as the coupling between them, including the analysis of effects that are not directly accessible by standard measurements. The developed approach allows identifying the different photodetection loss mechanisms from the current-voltage and electroluminescence measurements and thereby gives guidance for designs toward a direct demonstration of ELC at practically relevant cooling powers. Somewhat surprisingly, the results show that an imperfect spectral absorption efficiency of the PD, in addition to its below unity quantum efficiency, are together required to explain the previously observed low photodetection efficiency of the DDS even for several microns thick PD structures. In comparison, the LED top mirror introduces only a minuscule drop in photodetection efficiency. Put in plain numbers, our analysis reveals that in the current DDS designs, there is headroom by 14% in the spectral matching between the LED and the PD, 5% in the charge collection efficiency of the PD, and 4% in the efficiency at which photons emitted from the LED reach the PD.

An understanding of acoustic spin, energy flux, and reactive power is crucial for manipulating and directing near-field sound. However, despite their importance, the relationship between these physical quantities has not been fully explored. In this paper, we aim to address this gap in knowledge by presenting a proof of some universal geometric relations between the acoustic spin, the cross product of the time-averaged energy flux and reactive power, and the local vorticity of the energy flux. These relations are independent of the specific wave structure and are verified for various acoustic systems. Moreover, our study demonstrates the mechanical torques and forces on small lossy particles generated by three different types of acoustic near-field sources, namely Janus, Huygens, and spin sources. These findings reveal hidden locking relations beyond the simple spin-momentum locking of near-field waves and provide valuable insights into the fundamental principles underlying the relationship between acoustic spin, radiation torque, energy flux, and reactive power in the field of acoustics.