The formation and dissolution of silver nanowires plays a fundamental role in a broad range of resistive-switching devices, which fundamentally rely on the electrochemical-metallization phenomenon. It has been shown that resistive switching may also appear in pure metallic nanowires lacking any silver-ion-hosting embedding environment but this pure atomic switching mechanism differs fundamentally from the conventional electrochemical-metallization-based resistive switching. To facilitate the quantitative description of the former phenomenon, we investigate a broad range of $\mathrm{Ag}$ atomic junctions, with a special focus on the frequency dependence and the fundamentally stochastic cycle-to-cycle variation of the switching-threshold voltage. These devices are established in an ultrahigh-purity environment in which electrochemical metallization can be excluded. The measured characteristics are successfully described by a vibrational-pumping model, yielding consistent predictions for the weak frequency dependence and the large variance of the switching-threshold voltage. We also demonstrate that electrochemical-metallization-based resistive switching and pure atomic switching may appear in the same device structure and therefore the proper understanding of the pure atomic switching mechanism has a very relevant importance in silver-based electrochemical-metallization cells.

Functional imaging equipment based on miniaturized atomic magnetometers with array arrangement exhibits promising prospects in biomagnetic scenarios. However, crosstalk from the modulated magnetic field between adjacent sensors degrades the imaging accuracy. To address this issue, this study proposes an all-optical dual-axis zero-field atomic magnetometer using light-shift modulation. We utilize an acousto-optic modulator to modulate a detuned circularly polarized beam for pumping the atomic spin ensembles. This beam allows for the optical modulation of spin polarization and meanwhile generates a light-shift modulation, effectively replacing the conventional magnetic field modulation. By using a probe beam to detect the optical rotation angle perpendicular to the direction of the pump beam, we construct an all-optical configuration of a longitudinally modulated atomic magnetometer, enabling dual-axis magnetic field measurements. Experimental results demonstrate dual-axis sensitivities of 29 and $15\mathrm{fT}/{\mathrm{Hz}}^{1/2}$, respectively. This method eliminates the need for conventional coil-based magnetic field modulation, thereby paving the way for potential applications in magnetocardiography and magnetoencephalography.

Microwave-to-optical transducers are integral to the future of superconducting quantum computing, as they would enable scaling and long-distance communication of superconducting quantum processors through optical-fiber links. However, optically induced microwave noise poses a significant challenge in achieving quantum transduction between microwave and optical frequencies. In this work, we study light-induced microwave noise in an integrated electro-optical transducer harnessing the Pockels effect of thin-film lithium niobate. We reveal three sources of added noise with distinctive time constants ranging from sub-$100\mathrm{ns}$ to milliseconds. Our results provide insights into the mechanisms and corresponding mitigation strategies for light-induced microwave noise in superconducting microwave-optical transducers and pave the way toward realizing the ultimate goal of quantum transduction.

Magnetic-tunnel-junction- (MTJ) based spintronic devices have demonstrated significant potential in neuromorphic computing. Here, we report an artificial synapse, which can be modulated by rf signals directly based on the nanoscale MTJs with perpendicular magnetic anisotropy (PMA). To utilize multiple rf signals in parallel, we take an approach to change the resonance frequencies of MTJs by changing the PMA between the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ free layer and $\mathrm{Mg}\mathrm{O}$ barrier, which can expand the application range of rf signal processing. Moreover, we experimentally demonstrate that MTJs with PMA can serve as an rf synapse with adjustable positive and negative weights. We have achieved effective classification of rf signals with an accuracy exceeding 96% through experimental results as synaptic weights, comparable to that of equivalent software-based neural networks. This work may pave the way for the development of rf-oriented hardware artificial neural networks.

We report on the investigation of an electrically biased high efficiency green III-nitride light-emitting diode (LED) by electron emission microscopy (EEM) using a low-energy electron microscope (LEEM). The surface of the LED was activated to negative electron affinity via deposition of a submonolayer of Cs. With the illumination column of the LEEM turned off, upon electrical injection of the LED, we directly image the hot electrons generated by eeh Auger-Meitner nonradiative processes that diffuse through the top $p$-$\mathrm{Ga}\mathrm{N}$ layer and emit out the surface of the biased LED. By determining the source of emitted electrons using complementary electron emission spectroscopy measurements, EEM allows us to effectively map the carrier density within the LED. Using EEM, we observed nonelectron emitting regions with a density of approximately $3\times {10}^8\text{c}{\text{m}}^{-2}$, identified as V-shaped defects (V-defects). This is confirmed through the corresponding dark spots of panchromatic cathodoluminescence measurements of the same sample and by plan-view transmission electron microscopy. The absence of electron emission at the sidewall of the V-defects can be attributed to several factors, including reduced carrier density in the sidewall quantum wells due to carriers traveling fast through the semipolar sidewalls before being injected into the planar quantum wells, the reduced population of hot electrons surviving diffusion through the thicker $p$-$\mathrm{Ga}\mathrm{N}$ filling in the V-defect before emission onto vacuum, and a smaller Auger-Meitner coefficient for the low $\mathrm{In}$ content semipolar sidewall quantum wells. The stronger electron emission observed at the ridges of most V-defects compared to the planar quantum well regions indicates larger local injected carrier densities, confirming that V-defect sidewalls allow for strong lateral carrier injection when compared to the weaker vertical injection away from the V-defect as evidenced by the weaker electron emission intensity away from the V-defects.

Suspended optics in gravitational-wave (GW) observatories are susceptible to alignment perturbations, particularly slow drifts over time, due to variations in temperature and seismic levels. Such misalignments affect the coupling of the incident laser beam into the optical cavities, degrade both the circulating power and optomechanical photon squeezing, and thus decrease the astrophysical sensitivity to merging binaries. Traditional alignment techniques involve differential wave-front sensing using multiple quadrant photodiodes but are often bandwidth restricted and limited by the sensing noise. We present a successful implementation of neural-network–based sensing and control at a GW observatory and demonstrate low-frequency control of the signal-recycling mirror at the GEO 600 detector. Alignment information for three critical optics is simultaneously extracted from the interferometric dark-port camera images via a convolutional neural net–long short-term memory network architecture and is then used for multiple-input–multiple-output control using soft actor-critic-based deep reinforcement learning. The overall sensitivity improvement achieved using our scheme demonstrates the capabilities of deep learning as a viable tool for real-time sensing and control for current and next-generation GW interferometers.

We formulate a multiphysics model describing the nonlinear propagation of a femtosecond, near-infrared, tightly focused laser pulse in a wide-band dielectric. The application of our model to the case of bulk sapphire shows that even under extreme excitation conditions, ionization is rigidly clamped at about one tenth of the electron density in the upper valence band. The earlier estimate of approximately 10-$\mathrm{TPa}$ pressure that could be attainable through the internal excitation of transparent dielectrics by tightly focused ultrafast laser beams is shown to be off by 2 orders of magnitude. We outline the potential routes towards overcoming the clamping limit. The computed distribution of the absorbed electromagnetic energy, combined with the appropriate equation of state, can be further used as an input to a hydrodynamics code to simulate the dynamics of the void formation inside the bulk of the solid.

It is generally believed that intrinsic charge generation via an autoionization mechanism in pristine single-component organic semiconductors is impossible upon photoexcitation within the lowest excited singlet state due to the large exciton binding energy. However, we present measurements of thermally stimulated luminescence, light-induced ESR, and photocurrent in the carbazole-based molecule 3′,5-di(9H-carbazol-9-yl)-[1,1′-biphenyl]-3-carbonitrile (mCBP-CN) films, revealing that charge-carrier pairs are efficiently produced upon excitation near their absorption edges. The photocurrent measurements show a superlinear dependence on the cw-photoexcitation intensity even at very low excitation power (below $1\mathrm{mW}/{\mathrm{cm}}^2$), suggesting a bimolecular nature of the charge photogeneration process. The photocurrent measured over a broad temperature range of 5–300 K exhibits a prominent maximum at moderately low temperature around 170 K and rolls off significantly at higher temperatures. This correlates remarkably with the maximum of delayed fluorescence induced by bimolecular triplet-triplet annihilation (TTA), i.e., triplet fusion, in this material. This behavior implies that the photocurrent is governed mainly by the TTA-induced production of geminate pairs and only a little by their subsequent dissociation. Moreover, we find that the field-assisted dissociation probability of photogenerated charge pairs becomes almost temperature-independent at temperatures below 100 K. This can be quantitatively described using a charge dissociation model accounting for the energy disorder and the distribution of geminate-pair radii. The key conclusion of this study is that triplet fusion can promote the energy up-conversion (to 5.42 eV), thereby enabling the autoionization of a high-energy neutral excited state. This serves as the predominant mechanism of intrinsic photogeneration in this single-component heavy-atom-free system. We attribute the effect to efficient intersystem crossing in mCBP-CN, a high triplet energy level (2.71 eV), and very long-lived triplet excitations. A broader implication of this finding is that the so far neglected mechanism of TTA-facilitated charge-carrier generation might be relevant for organic long-persistent luminescence materials, and even for organic photovoltaics and potentially for photocatalytic water splitting processes.

Cloning refers to producing identical copies of existing objects. Here, we experimentally show how to clone acoustic scattering objects. We acquire a digital twin and bring it back to life—a simple two-step process. First, we use broadband speakers to illuminate the scattering object within a closed receiver aperture. From these recorded reverberative data, we retrieve the object’s scattering Green’s functions using multidimensional deconvolution. In the second step, the acoustic scatterer is holographically reconstructed using the acquired scattering Green’s functions. The hologram scatters any wavefield in real time exactly like the original object would. Low-latency feedback reproduces all orders of interactions between the physical wavefield and the numerically defined hologram. This two-step process is demonstrated by cloning and modifying several rigid scatterers in a two-dimensional acoustic waveguide. Applications range from fully realistic digital scattering models to efficient metamaterial experimentation.

This work provides a proof-of-concept for superresolution magnetic imaging using a single Josephson junction. The technique resembles digital holography: magnetic patterns are obtained via an inverse-problem solution from diffractionlike modulation of the junction’s critical current, $I_c(H)$. We demonstrate numerical reconstruction of complex two-dimensional patterns, verify the technique experimentally using Nb-based planar junctions, and fabricate an operational sensor on a cantilever. Our results show that Josephson holography allows for both high spatial resolution (approximately $20\mathrm{nm}$) and high field sensitivity (approximately ${10}^{-11}\mathrm{T}/\sqrt{\mathrm{Hz}}$), thus resolving the trade-off problem between resolution and sensitivity in magnetic scanning probe imaging.

Superconducting cavities have emerged as a key tool for measuring the spin states of quantum dots. So far, however, few experiments have explored longitudinal couplings between quantum dots and cavities, and no solid-state qubit experiments have explicitly probed the “adiabatic” regime, where the Purcell decay is strongly suppressed. Here we report measurements of a double-quantum-dot charge qubit coupled to a high-impedance resonator via a “flip-chip” design geometry. By applying an adiabatic ac drive to the qubit through two different channels, and studying the effects of qubit energy detuning, interdot tunneling, and driving strength, we are able to unequivocally confirm the presence of a longitudinal coupling between the qubit and the cavity, while the qubit remains in its ground state. Since this coupling is proportional to the driving amplitude, and is therefore switchable, it has the potential to become a powerful tool in qubit experiments.

Thermo-osmotic flow around a microparticle in a liquid is characterized by observing and analyzing the distribution of tiny particles, i.e., tracers, near the surface of the microparticle. First, an optical-trapping laser is used to localize the tracer motion along a circular path near the circumference of the microparticle. Then, upon creating an overall temperature gradient in the liquid, the tracers on the circular path, originally uniformly distributed, gather toward the hotter side of the microparticle, indicating a flow along the particle toward the heat. Analyzing the tracer distribution further, it is found that (i) the flow magnitude decreases with the distance from the surface and (ii) changing the surface property of the microparticle results in a change in the flow magnitude. These results show that the observed flow is a thermally induced slip flow along the surface of the microparticle. Then, assuming a simple slip boundary condition for a fluid equation, we evaluate the magnitude of the slip coefficient based on two kinds of experimental data: (i) the thermophoretic velocity of the microparticle and (ii) the thermo-osmotic flow around the microparticle. The results of the two approaches are in quantitative agreement. They are also compared with those of theoretical models for a slip flow in existing studies.

Many recent papers report on the interest in spin waves for applications. This paper revisits propagating-spin-wave spectroscopy when using inductive transceivers connected to a vector network analyzer. The spin-wave conduit can be made of a nonreciprocal material. The formalism offers a simple and direct method to understand, design, and optimize devices harnessing propagating spin waves, including when a unidirectional energy flow is desired. The concept of the mismatch of helicity between the spin wave and the magnetic field radiated by antennas is first clarified. Owing to the form of the susceptibility tensor reflecting the precession ellipticity, there exist specific orientations of the wave vector for which a perfect helicity mismatch is reached. The spin waves with this orientation and this direction of wave vector are “dark” in the sense that they do not couple with the inductive antenna. This leads to single-sided wave-vector generation, which should not be confused with a unidirectional emission of energy. A method to calculate the antenna-to-antenna transmission parameter is then provided. Analytical approximations are then applied on situations that illustrate the respective role of the direction of the spin-wave wave vector versus that of the group velocity. The often-encountered cases of spin waves possessing either a V-shaped or a flat dispersion relation are revisited. These reciprocal dispersion relations lead to amplitude nonreciprocity because of the helicity mismatch phenomenon. Conversely, for spin waves with a line-shaped dispersion relation, an unforeseen quasi-unidirectional emission of spin waves occurs, which is of interest for applications. This situation can be obtained when using the acoustic spin waves of synthetic antiferromagnets when the wave vector is close to parallel to the applied field. We finally show that this configuration can be harnessed to design reconfigurable frequency filters.

We employ nitrogen-vacancy ($\mathrm{N}$-$V$) magnetometry to measure the stray field dynamics of a ferromagnetic permalloy nanowire driven by spin-orbit torques. Specifically, we observe the optically detected magnetic resonance (ODMR) signatures of both spontaneous dc-driven magnetic oscillations and phase locking to a second harmonic drive, developing a simple macrospin model that captures the salient features. We also observe signatures of dynamics beyond the macrospin model, including an additional ODMR feature (associated with a second spin-wave mode) and one mode sapping power from another. Our results provide additional insight into $\mathrm{N}$-$V$-spin-wave coupling mechanisms, and represent a new modality for subwavelength $\mathrm{N}$-$V$ scanned probe microscopy of nanoscale magnetic oscillators.

On-chip technology based on acoustic waves is a strong asset in modern telecommunications, with the prospects of becoming a cornerstone of next-generation devices. In this context, mapping and manipulating acoustic waves through coherent scattering is pivotal for a nontrivial control of the flow of acoustic energy, which could consequently enable advanced information manipulation. To this end, here a technique for mapping acoustic fields is introduced and used for characterizing micrometric-sized phononic crystals defined in GaAs slabs and excited by gigahertz surface acoustic waves based on atomic force microscopy. It is shown that incoherent scattering excites a wide distribution of modes, which enables the mapping of the dispersion relation of the two-dimensional structures, while the phononic crystal symmetry directly correlates with coherent scattering effects. Enabling the use of acoustic atomic force microscopy and understanding the role of scattering are of paramount importance for the versatile use of gigahertz acoustic waves in technological applications, setting the baseline for advanced operations like hyperspectral filtering, beam steering, or spatial-division multiplexing.

Quantum key distribution (QKD) protocols have been proven to be secure on the basis of fundamental physical laws; however, the proofs consider a well-defined setting and encoding of the sent quantum signals only. Side channels, where the encoded quantum state is correlated with properties of other degrees of freedom of the quantum channel, allow an eavesdropper to obtain information unnoticeably, as demonstrated in a number of hacking attacks on the quantum channel. However, also classical radiation emitted by the devices may be correlated, leaking information on the potential key, especially when combined with novel data-analysis methods. We demonstrate here a side-channel attack using a deep convolutional neural network to analyze the recorded classical, radio-frequency electromagnetic emissions. Even at a distance of a few centimeters from the electronics of a QKD sender containing frequently used electronic components, we are able to recover virtually all information about the secret key. However, as shown here, countermeasures can enable a significant reduction of both the emissions and the amount of secret-key information leaked to the attacker. Our analysis methods are independent of the actual device and thus provide a starting point for assessing the presence of classical side channels in QKD devices.

Superconducting qubits typically use a dispersive readout scheme, where a resonator is coupled to a qubit such that its frequency is qubit-state dependent. Measurement is performed by driving the resonator, where the transmitted resonator field yields information about the resonator frequency and thus the qubit state. Ideally, we could use arbitrarily strong resonator drives to achieve a target signal-to-noise ratio in the shortest possible time. However, experiments have shown that when the average resonator photon number exceeds a certain threshold, the qubit is excited out of its computational subspace in a process we refer to as a measurement-induced state transition (MIST). These transitions degrade readout fidelity, and constitute leakage, which precludes further operation of the qubit in, for example, error correction. Here we study these transitions experimentally with a transmon qubit by measuring their dependence on qubit frequency, average resonator photon number, and qubit state, in the regime where the resonator frequency is lower than the qubit frequency. We observe signatures of resonant transitions between levels in the coupled qubit-resonator system that exhibit noisy behavior when measured repeatedly in time. We provide a semiclassical model of these transitions based on the rotating-wave approximation and use it to predict the onset of state transitions in our experiments. Our results suggest the transmon is excited to levels near the top of its cosine potential following a state transition, where the charge dispersion of higher transmon levels explains the observed noisy behavior of state transitions. Moreover, we show that occupation in these higher energy levels poses a major challenge for fast qubit reset.

We report electric detection of the spin Peltier effect (SPE) in a bilayer consisting of a $\mathrm{Pt}$ film and a ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ ($\mathrm{YIG}$) single crystal at the cryogenic temperature $T$ as low as 2 K based on a ${\mathrm{Ru}\mathrm{O}}_2\text{−}{\mathrm{Al}\mathrm{O}}_x$ on-chip thermometer film. By means of a reactive co-sputtering technique, we successfully fabricated ${\mathrm{Ru}\mathrm{O}}_2\text{−}{\mathrm{Al}\mathrm{O}}_x$ films having a large temperature coefficient of resistance of approximately $100\mathrm{\%}{\text{K}}^{-1}$ at around $2\text{K}$. By using the ${\mathrm{Ru}\mathrm{O}}_2\text{−}{\mathrm{Al}\mathrm{O}}_x$ film as an on-chip temperature sensor for a $\mathrm{Pt}$/$\mathrm{YIG}$ device, we observe a SPE-induced temperature change on the order of sub-$\text{μ}\text{K}$, the sign of which is reversed with respect to the external magnetic field $B$ direction. We found that the SPE signal gradually decreases and converges to zero by increasing $B$ up to $10\text{T}$. The result is attributed to the suppression of magnon excitations due to the Zeeman-gap opening in the magnon dispersion of $\mathrm{YIG}$, whose energy much exceeds the thermal energy at 2 K.

We investigate a system composed of an ensemble of room-temperature rare-earth ions embedded in a bulk crystal, intrinsically coupled to internal strain via their sensitivity to the surrounding crystal field. We evidence the generation of a mechanical response under resonant atomic excitation. We find this motion to be the sum of two fundamental, resonant optomechanical backaction processes: a conservative, piezo-orbital mechanism, resulting from the modification of the crystal field associated with the promotion of the ions to their excited state, and a dissipative, nonradiative photothermal process related to the phonons generated throughout the atomic population relaxation. Our work expands the horizons of research in hybrid optomechanics, and unveils unexplored interactions that may be key for understanding the dephasing dynamics of ultracoherent rare-earth ions.

Brownian motion imposes a hard limit on the overall precision of a nanomechanical measurement. Here, we present a combined experimental and theoretical study of the Brownian dynamics of a quintessential nanomechanical system, a doubly clamped nanomechanical beam resonator, in a viscous fluid. Our theoretical approach is based on the fluctuation-dissipation theorem of statistical mechanics: we determine the dissipation from fluid dynamics; we incorporate this dissipation into the proper elastic equation to obtain the equation of motion; and the fluctuation-dissipation theorem then directly provides an analytical expression for the position-dependent power spectral density (PSD) of the displacement fluctuations of the beam. We compare our theory to experiments on nanomechanical beams immersed in air and water and obtain excellent agreement. Within our experimental parameter range, the Brownian-force noise driving the nanomechanical beam has a colored PSD due to the “memory” of the fluid; the force noise remains mode independent and uncorrelated in space. These conclusions are not only of interest for nanomechanical sensing but also provide insight into the fluctuations of elastic systems at any length scale.