We propose a method for ultrasensitive infrared (IR) vibrational spectroscopy of molecules with nanoscale footprints by combining the tip enhancement of a scattering-type scanning near-field optical microscope (s-SNOM) and the plasmon enhancement of breathing-mode (BM) plasmon resonances of graphene nanodisks (GNDs). To demonstrate this, we develop a quantitative model that is capable of computing accurately the s-SNOM signals of nanoscale samples. With our modeling, we show that the s-SNOM tip can effectively excite gate-tunable BM plasmonic resonances in GNDs with strong field enhancement and sensitive dependence on the size of GND. Moreover, we demonstrate that the intense electric field of tip-excited plasmonic BMs can strongly enhance the IR vibrational modes of molecules. As a result, IR vibrational signatures of individual molecular particles with sizes down to 1–2 nm are readily observable by s-SNOM. Our study sheds light on future ultrasensitive IR biosensing that takes advantage of both the tip and plasmon enhancement.

We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride–trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the phonon spectrum when the atoms are pulled away from their equilibrium positions by the electric field. Our finding provides an efficient way to manipulate the thermal conductivity by tuning external fields in ferroelectric materials.

One of the key challenges for the future development of efficient and stable metal-halide perovskite solar cells is related to the migration of ions in these materials. Mobile ions have been linked to the observation of hysteresis in the current-voltage characteristics, shown to reduce device stability against degradation and act as recombination centers within the band gap of the active layer. In the literature, one finds a broad spread of reported ionic defect parameters (e.g., activation energies) for seemingly similar perovskite materials, rendering the identification of the nature of these species difficult. In this work, we perform temperature-dependent deep-level transient spectroscopy (DLTS) measurements on methylammonium-lead-iodide perovskite solar cells and develop a extended regularization algorithm for inverting the Laplace transform. Our results indicate that mobile ions form a distribution of emission rates (i.e., a distribution of diffusion constants) for each observed ionic species, which may be responsible for the differences in the previously reported defect parameters. Importantly, different DLTS modes such as optical and current DLTS yield the same defect distributions. Finally, the comparison of our results with conventional boxcar DLTS and impedance spectroscopy verifies our evaluation algorithm.

Many quantum key distribution systems employ a laser followed by an optical attenuator to prepare weak coherent states in the source. Their mean photon number must be precalibrated to guarantee the security of key distribution. Here we experimentally show that this calibration can be broken with a high-power laser attack. We test four fiber-optic attenuator types used in quantum key distribution systems, and find that two of them exhibit a permanent decrease in attenuation after laser damage. This results in higher mean photon numbers in the prepared states and may allow an eavesdropper to compromise the key.

Magnetoelectric composites are an important class of multiferroic materials that are paving the way towards a new generation of multifunctional devices directly integrable into data-storage technology and spintronics. This study focuses on strain-mediated electrical manipulation of magnetization in an extrinsic multiferroic. The composite used includes 5- or 60-nm ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ thin films coupled to a piezoelectric (011) $\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3$-$\mathrm{Pb}({\mathrm{Zr}}_x{\mathrm{Ti}}_{1-x}){\mathrm{O}}_3$ (PMN-PZT) material. A magnetization-reversal study reveals a converse magnetoelectric coefficient ${\alpha }_{\mathrm{CME},max}\approx 2.7\times {10}^{-6}\mathrm{s}{\mathrm{m}}^{-1}$ at room temperature. This reported value of ${\alpha }_{\mathrm{CME}}$ is among the highest so far, compared with previous reports on single-phase multiferroics and on composites. An angular dependence of ${\alpha }_{\mathrm{CME}}$ is also shown, arising from the intrinsic magnetic anisotropy of $\mathrm{Fe}\text{−}\mathrm{Ga}$. The highly efficient magnetoelectric composite $\mathrm{Fe}\text{−}\mathrm{Ga}$/PMN-PZT demonstrates drastic modifications of the in-plane magnetic anisotropy, with an almost ${90}^{\circ }$ rotation of the preferential anisotropy axis in thinner films under an electric field $E=10.8\mathrm{kV}{\mathrm{cm}}^{-1}$. Also, the influence of thermal strain on the bilayer’s magnetic coercivity is compared with that for a reference $\mathrm{Fe}\text{−}\mathrm{Ga}$/glass bilayer at cryogenic temperatures. A different evolution is observed as a function of temperature, revealing a thermomechanical influence of the substrate which has not yet been reported in $\mathrm{Fe}\text{−}\mathrm{Ga}$ thin films coupled to a piezoelectric material.

For a better understanding of multistimuli-responsive caloric materials with a first-order transition and for optimization of their functional properties, it is necessary to predict the behavior of the material under changes of both magnetic field and pressure. Here, we design and build a special device that can provide a self-consistent set of parameters needed for the comprehensive characterization of multistimuli-responsive functional magnetic materials. Using this scientific instrument, a data set of simultaneously measured magnetization, ${M(T)}_H$, and volume magnetostriction, $\omega (T{)}_H$, values are obtained for ${\mathrm{LaFe}}_{11.4}{\mathrm{Si}}_{1.6}$ with a first-order transition. Furthermore, based on simultaneously measured M(T) and ω(T) dependencies obtained at ambient pressure, we develop an approach that allows the behavior of magnetization under different pressures, ${M(T)}_P$, to be described analytically. Additional parameters, such as compressibility, κ(T); thermal expansion coefficient, α(T); and magnetoelastic interaction or effective magnetovolume coupling constant, $C_{MV}$, are determined. For verification of our developed model, direct measurements of magnetization under external pressure (up to P = 1 GPa) are carried out on the same sample as that used for simultaneous measurement of magnetization and magnetovolume effect. A comparison of simulated ${M(T)}_P$ dependencies with experimental $M(T{)}_P$ confirms that our approach provides a more realistic behavior of transition temperature under pressure, $T_C(P)$, than that of the $T_C(P)$ predicted by the Bean-Rodbell model; thus, this approach is more suitable for predicting the behavior of multistimuli-responsive caloric materials with first-order transitions under changes of both magnetic field and pressure.

Time-bin qubits, where information is encoded in a single photon at different times, have been widely used in optical-fiber- and waveguide-based quantum communications. With the recent developments in distributed quantum computation, it is logical to ask whether time-bin encoded qubits may be useful in that context. We have recently realized a time-bin qubit controlled-phase (C-phase) gate using a $2\times 2$ optical switch based on a lithium-niobate waveguide, with which we demonstrated the generation of an entangled state. However, the experiment was performed with only a pair of input states and thus the functionality of the C-phase gate was not fully verified. In this research, we use quantum process tomography to establish a process fidelity of $97.1\mathrm{\%}$. Furthermore, we demonstrate the controlled-not gate operation with a process fidelity greater than $94\mathrm{\%}$. This study confirms that typical two-qubit logic gates used in quantum computational circuits can be implemented with time-bin qubits and thus it is a significant step forward for the realization of distributed quantum computation based on time-bin qubits.

We propose a nonvolatile memory element based on a lateral ferromagnetic Josephson junction with spin-orbit coupling and out-of-plane magnetization. The interplay between the latter and the intrinsic exchange field of the ferromagnet leads to a magnetoelectric effect that couples the charge current through the junction and its magnetization, such that by applying a current pulse the direction of the magnetic moment in $F$ can be switched. The two memory states are encoded in the direction of the out-of-plane magnetization. With the aim to determine the optimal working temperature for the memory element, we explore the noise-induced effects on the averaged stationary magnetization by taking into account thermal fluctuations affecting both the Josephson phase and the magnetic moment dynamics. We investigate the switching process as a function of intrinsic parameters of the ferromagnet, such as the Gilbert damping and strength of the spin-orbit coupling, and propose a nondestructive readout scheme based on a dc superconducting quantum interference device. Additionally, we analyze a way to protect the memory state from external perturbations by voltage gating in systems with a both linear-in-momentum Rashba and Dresselhaus spin-orbit coupling.

Point defects exist widely in engineering materials and are known to scatter vibrational modes, resulting in reduction in thermal conductivity. The Klemens description of point-defect scattering is the most-prolific analytical model for this effect. This work reviews the essential physics of the model and compares its predictions with first-principles results for isotope and alloy scattering, demonstrating the model to be a useful metric of material design. A treatment of the scattering parameter ($\mathrm{\Gamma }$) for a multiatomic lattice is recommended and compared with other treatments presented in the literature, which have been at times misused to yield incomplete conclusions about the system’s scattering mechanisms. Additionally, we demonstrate a reduced sensitivity of the model to the full phonon dispersion and discuss its origin. Finally, a simplified treatment of scattering in alloy systems with vacancies and interstitial defects is demonstrated to suitably describe the potent scattering strength of these off-stoichiometric defects.

Thanks to their versatility, nitrogen-vacancy (N-$V$) centers in nanodiamonds have been widely adopted as nanoscale sensors. However, their sensitivities are limited by their short coherence times relative to N-$V\mathrm{s}$ in bulk diamond. A more complete understanding of the origins of decoherence in nanodiamonds is critical to improving their performance. Here we present measurements of fast spin relaxation on qutrit transitions between the energy eigenstates composed of the $m_s=|\pm 1⟩$ states of the N-$V^{-}$ electronic ground state in approximately 40-nm nanodiamonds under ambient conditions. For frequency splittings between these states of 20 MHz or less the maximum theoretically achievable coherence time of the N-$V$ spin is approximately 2 orders of magnitude shorter than would be expected if the N-$V$ spin is treated as a qubit. We attribute this fast relaxation to electric field noise. We observe a strong falloff of the qutrit relaxation rate with the splitting between the states, suggesting that, whenever possible, measurements with N-$V\mathrm{s}$ in nanodiamonds should be performed at moderate axial magnetic fields ($>60$ G). We also observe that the qutrit relaxation rate changes with time. These findings indicate that surface electric field noise is a major source of decoherence for N-$V\mathrm{s}$ in nanodiamonds.

Techniques for the manipulation of nanodroplets are central to many miniaturized systems in chemical and biological research and applications. In this study, we demonstrate that the dispersed nanodroplets composed of perfluorocarbon can be manipulated to form a micron-sized elliptical aggregation via a nonuniform focused acoustic vortex, and the aggregation location can be further flexibly regulated. In addition, it is unveiled theoretically and experimentally that the fundamental mechanism for the elliptical aggregation is the nonuniform distribution of the pressure, which makes the acoustic radiation forces acting on the dispersed nanodroplets converge at a certain point. Moreover, the required acoustic droplet vaporization threshold for the nanodroplets is significantly lowered, while the generated microbubbles remain similar in size after aggregation. The findings of this study provide an alternative field of opportunities for targeted drug delivery and imaging using nanodroplet emulsions.

Unconditionally secure communication, which has been pursued for thousands of years hasn’t, however, yet been reached due to a continuous competition between encryption and hacking. Quantum key distribution (QKD), harnessing the quantum mechanical nature of superposition and noncloning, may promise unconditional security by incorporating the one-time pad algorithm rigorously proved by Claude Shannon. Massive efforts have been made in building practical and commercial QKD systems; in particular, decoy states have been employed to detect photon-number splitting attacks against a single-photon source loophole, and measurement-device-independent (MDI) QKD has further closed all loopholes on the detection side, which leads to a seemingly real-life application. Here, we propose and experimentally demonstrate a MDI-QKD hacking strategy on the trusted-source assumption by using an injection-locking technique. An eaves-dropper (Eve) injects near off-resonance photons with randomly chosen polarization into a sender’s laser, where injection locking in a shifted frequency can happen only when Eve’s choice matches the sender’s state. By setting a shifted window and switching the frequency of photons back afterwards, Eve in principle can obtain all the keys without terminating the real-time QKD. We observe the dynamics of a semiconductor laser with injected photons, and obtain a hacking success rate reaching $60.0\mathrm{\%}$ of raw keys. Our results suggest that the spear-and-shield competition of unconditional security may continue until all potential loopholes are discovered and closed ultimately.

Single-photon sources are of great interest because they are key elements in different promising applications of quantum technologies. Here we demonstrate a highly efficient tunable on-demand microwave single-photon source based on a transmon qubit with an intrinsic emission efficiency above $98\mathrm{\%}$. The high efficiency ensures a negligible pure dephasing rate and the necessary conditions for generation of indistinguishable photons. We provide an extended discussion and analysis of the efficiency of the photon generation. To further experimentally confirm the single-photon property of the source, correlation functions of the emission field are also measured using linear detectors with a GPU-enhanced signal-processing technique. Our results experimentally demonstrate that frequency tunability and negligible pure dephasing rate can be achieved simultaneously and show that such a tunable single-photon source can be good for various practical applications in quantum communication, simulations and information processing in the microwave regime.

Metasurfaces composed of elementary cells with elaborated tailoring of amplitude and phase responses have promising applications for the manipulation of electromagnetic waves. In this work, spatial energy distribution is manipulated to control an incident plane wave and focus it to any desired spatial position. A holographic metamirror implementing proper phase-profile distributions necessary to dynamically control the focal spot is designed. Near-field measurements are performed on a fabricated prototype to demonstrate energy focusing at predicted focal points. Furthermore, the reconfigurability mechanism implemented in the metasurface allows us to validate an adaptive focusing by modifying both the number and the position of the focal spots through the electronic tuning of the phase profile. Such a spatial energy manipulation concept paves the way to flat and efficient devices for wireless-power-transfer applications in the radio-frequency domain as well as for imaging and holography functionalities at much higher frequencies.

The emerging technique of photonic doping endows epsilon-near-zero (ENZ) media with a broadly tunable effective magnetic permeability. Here, we theoretically and experimentally demonstrate that a finite-size doped ENZ region counterintuitively behaves as a lumped circuit element, modeled as a controllable series reactance. Based on this concept, a general matching network is constructed to match a load with arbitrary complex impedance, while, interestingly, its operating bandwidth can also be modified by fine-tuning the dopants’ properties. To demonstrate the universality of the concept, different kinds of loads are matched, including microwave circuits, antennas, and absorbing particles. Since this general impedance matching technique is not limited to a specific type of load, nor a specific geometry, and can be readily transplanted from microwave to optical regimes, the proposed methodology facilitates impedance matching for maximum usage of power in quite general scenarios, and thus, exhibits promising potential for broad applications.

Based on the transformation acoustic methodology, an innovative approach for designing arbitrary shape concentrators is proposed. Unlike previous works, which utilized inhomogeneous and anisotropic materials to concentrate the incident acoustic waves in an arbitrary domain, the same functionality is attained by introducing only one homogeneous anisotropic medium, which is called the acoustic null medium (ANM). The other advantage of this method is that the obtained materials are not dependent on the shape of the concentrator. That is, regardless of the device geometry, a constant ANM will be used for each new shape and the output results do not alter. This will circumvent the conventional transformation acoustics’ sophisticated and tedious calculations and could be easily implemented in real-life scenarios. To show these competencies, several numerical full-wave simulations are performed. In addition to the bulk material simulations, an arbitrary shape concentrator is realized via effective medium theory and it is shown that an ANM, as a kind of extreme medium, could be implemented with natural materials.

Computation of drag force due to oscillation of a beam or any other solid structure near or away from the fixed surface is very important in controlling the performance of most microelectromechanical and nanoelectromechanical systems. Although there exist many formulations for drag-force computations near and away from the fixed bottom surface, they are limited to a single element. In this paper, we present a systematic formulation for arriving at a modified formula for drag force based on experimental and numerical simulations that can be used in computing forces in an array of microelectromechanical and nanoelectromechanical system beams. To develop the model, we first obtain an approximate analytical model for uniform as well as nonuniform motion of a cantilever beam. After validating the numerical model with experimental and analytical results for a single beam, we modify the formula to consider different boundary effects and then apply it to compute drag forces in an array of beams. Finally, we also modify the formulas to include the nearby wall effect. The formula developed works excellently when the interbeam gaps are small. However, the percentage error increases with interbeam gap and is found to be between 5% and 14%.

A powerful control method in experimental quantum computing is the use of spin echoes, employed to select a desired term in the system’s internal Hamiltonian, while refocusing others. Here, we address a more general problem, describing a method to not only turn on and off particular interactions but also to rescale their strengths so that we can generate any desired effective internal Hamiltonian. We propose an algorithm based on linear programming for achieving time-optimal rescaling solutions in fully coupled systems of tens of qubits, which can be modified to obtain near-time-optimal solutions for rescaling systems with hundreds of qubits.

In laser-solid interactions, electrons may be generated and subsequently accelerated to energies of the order of magnitude of the ponderomotive limit, with the underlying process dominated by direct laser acceleration. The breaking of this limit, realized here by a radially polarized laser pulse incident upon a wire target, can be associated with several novel effects. Three-dimensional particle-in-cell simulations show that a relativistic intense laser pulse can extract electrons from the wire and inject them into the accelerating field. Antidephasing, resulting from collective plasma effects, is shown here to enhance the accelerated electron energy by $2$ orders of magnitude compared to the ponderomotive limit. It is demonstrated that ultrashort radially polarized pulses produce superponderomotive electrons more efficiently than pulses of the linear and circular polarization varieties.