We theoretically show that a magnet can be stably levitated on top of a punctured superconductor sheet in the Meissner state without applying any external field. The trapping potential created by such induced-only superconducting currents is characterized for magnetic spheres ranging from tens of nanometers to tens of millimeters. Such a diamagnetically levitated magnet is predicted to be extremely well isolated from the environment. We propose to use it as an ultrasensitive force and inertial sensor. A magnetomechanical readout of its displacement can be performed by using superconducting quantum interference devices. An analysis using current technology shows that force and acceleration sensitivities on the order of ${10}^{-23}\mathrm{N}/\sqrt{\mathrm{Hz}}$ (for a 100-nm magnet) and ${10}^{-14}\mathrm{g}/\sqrt{\mathrm{Hz}}$ (for a 10-mm magnet) might be within reach in a cryogenic environment. Such remarkable sensitivities, both in force and acceleration, can be used for a variety of purposes, from designing ultrasensitive inertial sensors for technological applications (e.g., gravimetry, avionics, and space industry), to scientific investigations on measuring Casimir forces of magnetic origin and gravitational physics.

Ensembles of nitrogen-vacancy centers in diamond are a highly promising platform for high-sensitivity magnetometry, whose efficacy is often based on efficiently generating and monitoring magnetic-field-dependent infrared fluorescence. Here, we report on an increased sensing efficiency with the use of a 532-nm resonant confocal cavity and a microwave resonator antenna for measuring the local magnetic noise density using the intrinsic nitrogen-vacancy concentration of a chemical-vapor deposited single-crystal diamond. We measure a near-shot-noise-limited magnetic noise floor of $200\mathrm{pT}/\sqrt{\mathrm{Hz}}$ spanning a bandwidth up to 159 Hz, and an extracted sensitivity of approximately $3\mathrm{nT}/\sqrt{\mathrm{Hz}}$, with further enhancement limited by the noise floor of the lock-in amplifier and the laser damage threshold of the optical components. Exploration of the microwave and optical pump-rate parameter space demonstrates a linewidth-narrowing regime reached by virtue of using the optical cavity, allowing an enhanced sensitivity to be achieved, despite an unoptimized collection efficiency of $<2\%$, and a low nitrogen-vacancy concentration of about 0.2 ppb.

We present a detailed numerical investigation of five nonlinear materials and their properties regarding photon-pair creation using parametric down-conversion. Periodic poling of ferroelectric nonlinear materials is a convenient way to generate collinearly propagating photon pairs. Most applications and experiments use the well-known potassium titanyl phosphate (${\mathrm{KTiOPO}}_4$, PPKTP) and lithium niobate (${\mathrm{LiNbO}}_3$, PPLN) crystals for this purpose. We provide a profound discussion on the family of KTP-isomorphic nonlinear materials, including KTP itself but also the much less common ${\mathrm{CsTiOAsO}}_4$, ${\mathrm{KTiOAsO}}_4$, ${\mathrm{RbTiOAsO}}_4$, and ${\mathrm{RbTiOPO}}_4$. We discuss the way in which these crystals can be used for the creation of spectrally pure down-conversion states and the generation of crystal-intrinsic polarization and frequency entanglement. The investigation of the new materials discloses an entirely different range of promising experimental setups, in some cases even outperforming the established materials PPLN and PPKTP.

We experimentally study the effects on the spin Hall angle due to systematic addition of Pt into the light metal Cu. We perform spin-torque ferromagnetic resonance measurements on a ${\mathrm{Py}/\mathrm{Cu}}_{1-x}{\mathrm{Pt}}_x$ bilayer and find that as the Pt concentration increases, the spin Hall angle of ${\mathrm{Cu}}_{1-x}{\mathrm{Pt}}_x$ increases. Moreover, only 28% Pt in ${\mathrm{Cu}}_{1-x}{\mathrm{Pt}}_x$ can give rise to a spin Hall angle close to that of Pt. We further extract the spin Hall resistivity of ${\mathrm{Cu}}_{1-x}{\mathrm{Pt}}_x$ for different Pt concentrations and find that the contribution of skew scattering is larger for lower Pt concentrations, while the side-jump contribution is larger for higher Pt concentrations. From a technological perspective, since ${\mathrm{Cu}}_{1-x}{\mathrm{Pt}}_x$ can sustain high processing temperatures, and Cu is the most common metallization element in the Si platform, it is easier to integrate the ${\mathrm{Cu}}_{1-x}{\mathrm{Pt}}_x$-based spintronic devices into existing Si fabrication technology.

We focus on a category of nanoantennas called magnetic nanoantennas made of a circular cluster of gold nanospheres that leads to enhanced local magnetic field oscillating at optical frequency. We elaborate on the magnetic field enhancement and the magnetic-to-electric-field ratio, i.e., the local field admittance, when the nanoantenna is illuminated by a single plane wave and by superposition of two plane waves to maximize the magnetic-only response. Single-dipole approximation is used to analyze magnetic nanoantennas and is verified by our findings with full-wave simulations. We derive a formula that estimates the natural frequency associated to the magnetic resonance of a circular plasmonic cluster with an arbitrary number of plasmonic nanospheres. Lastly, we classify clusters based on their quality factor and their ability to enhance the magnetic field and discuss the surface area with strong magnetic field provided by the plasmonic cluster.

The success of recently discovered absorber materials for photovoltaic applications has been generating increasing interest in systematic materials screening over the last years. However, the key for a successful materials screening is a suitable selection metric that goes beyond the Shockley-Queisser theory that determines the thermodynamic efficiency limit of an absorber material solely by its band-gap energy. In this work, we develop a selection metric to quantify the potential photovoltaic efficiency of a material. Our approach is compatible with detailed balance and applicable in computational and experimental materials screening. We use the complex refractive index to calculate radiative and nonradiative efficiency limits and the respective optimal thickness in the high mobility limit. We compare our model to the widely applied selection metric by Yu and Zunger [Phys. Rev. Lett. 108, 068701 (2012)] with respect to their dependence on thickness, internal luminescence quantum efficiency, and refractive index. Finally, the model is applied to complex refractive indices calculated via electronic structure theory.

Low-power, high-speed, and bright electrically driven true single-photon sources, which are able to operate at room temperature, are vital for the practical realization of quantum-communication networks and optical quantum computations. Color centers in semiconductors are currently the best candidates; however, in spite of their intensive study in the past decade, the behavior of color centers in electrically controlled systems is poorly understood. Here we present a physical model and establish a theoretical approach to address single-photon emission dynamics of electrically pumped color centers, which interprets experimental results. We support our analysis with self-consistent numerical simulations of a single-photon emitting diode based on a single nitrogen-vacancy center in diamond and predict the second-order autocorrelation function and other emission characteristics. Our theoretical findings demonstrate remarkable agreement with the experimental results and pave the way to the understanding of single-electron and single-photon processes in semiconductors.

The quantum computer has an amazing potential of fast information processing. However, the realization of a digital quantum computer is still a challenging problem requiring highly accurate controls and key application strategies. Here we propose a platform, quantum reservoir computing, to solve these issues successfully by exploiting the natural quantum dynamics of ensemble systems, which are ubiquitous in laboratories nowadays, for machine learning. This framework enables ensemble quantum systems to universally emulate nonlinear dynamical systems including classical chaos. A number of numerical experiments show that quantum systems consisting of 5–7 qubits possess computational capabilities comparable to conventional recurrent neural networks of 100–500 nodes. This discovery opens up a paradigm for information processing with artificial intelligence powered by quantum physics.

Discovering new electrodes for the sodium-ion battery requires a clear understanding of the material process during battery operation. Using first-principles calculations, we identify mechanisms of ionic diffusion and electronic transfer in newly developed cathode material, eldfellite ${\mathrm{Na}}_x\mathrm{Fe}{({\mathrm{SO}}_4)}_2$, reproducing the electrochemical properties in good agreement with experiment. The inserted sodium atom is suggested to diffuse along the two-dimensional pathway with preceding movement of the host sodium atom, and the activation energy is calculated to be reasonable for fast insertion. We calculate the electronic properties, showing the band insulating at a low composition of inserted sodium, for which the electron polaron formation and hopping are also suggested. Our results may contribute to opening an alternative way of developing innovative cathode materials based on iron and sulfate ion.

We present a phenomenological theory of bipolar filamentary resistive random-access memory describing the commonly observed features of their current-voltage characteristics. Our approach follows the approach of a thermodynamic theory developed earlier for chalcogenide memory and threshold switches and largely independent of their microscopic details. It explains, without adjustable parameters, such features as the domains of filament formation and switching, voltage-independent current in set and current-independent voltage in reset regimes, the relation between the set and reset voltages, filament resistance independent of its length, etc. Furthermore, it expresses the observed features through the material and circuitry parameters, thus paving the way to device improvements.

The $p\text{−}n$ junction dynamics induced in a graphene channel by stripe-domain nucleation, motion, and reversal in a ferroelectric substrate is explored using a self-consistent approach based on Landau-Ginzburg-Devonshire phenomenology combined with classical electrostatics. Relatively low gate voltages are required to induce the hysteresis of ferroelectric polarization and graphene charge in response to the periodic gate voltage. Pronounced nonlinear hysteresis of graphene conductance with a wide memory window corresponds to high amplitudes of gate voltage. Also, we reveal the extrinsic size effect in the dependence of the graphene-channel conductivity on its length. We predict that the top-gate–dielectric-layer–graphene-channel–ferroelectric-substrate nanostructure considered here can be a promising candidate for the fabrication of the next generation of modulators and rectifiers based on the graphene $p\text{−}n$ junctions.

Color centers in diamond provide a promising platform for quantum optics in the solid state, with coherent optical transitions and long-lived electron and nuclear spins. Building upon recent demonstrations of nanophotonic waveguides and optical cavities in single-crystal diamond, we now demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optical interface achieving $>90\%$ power coupling at visible wavelengths. We use this approach to demonstrate a bright source of narrow-band single photons based on a silicon-vacancy color center embedded within a waveguide-coupled diamond photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic interface results in a high flux (approximately 38 kHz) of coherent single photons (near Fourier limited at $<1\text{−}\mathrm{GHz}$ bandwidth) into a single-mode fiber, enabling possibilities for realizing quantum networks that interface multiple emitters, both on chip and separated by long distances.

We develop a modified two-temperature (2T) model of laser-matter interaction in dielectrics based on experimental insight from picosecond-pulsed high-frequency temperature-controlled second-harmonic (515 nm) generation in periodically poled stoichiometric ${\mathrm{LiTaO}}_3$ crystal and required for computational treatment of short-pulsed nonlinear optics and materials processing applications. We show that the incorporation of an extended set of recombination-kinetics-related energy-release and heat-exchange processes following short-pulsed photoionization by two-photon absorption of the second harmonic allows accurate simulation of the electron-lattice relaxation dynamics and electron-lattice temperature evolution in ${\mathrm{LiTaO}}_3$ crystal in nonlinear laser-frequency conversion. Our experimentally confirmed model and detailed simulation study show that two-photon ionization with the recombination mechanism via ion-electron-lattice interaction followed by a direct transfer of the recombination energy to the lattice is the main laser-matter energy-transfer pathway responsible for the majority of the crystal lattice heating (approximately 90%) continuing for approximately 50 ps after laser-pulse termination and competing with effect of electron-phonon energy transfer from the free electrons. This time delay is due to a recombination bottleneck which hinders faster relaxation to thermal equilibrium in photoionized dielectric crystal. Generally, our study suggests that in dielectrics photoionized by short-pulsed radiation with intensity range used in nonlinear laser-frequency conversion, the electron-lattice relaxation period is defined by the recombination-stage bottleneck of a few tens of picoseconds and not by the time of the electron-phonon energy transfer. This modification of the 2T model can be applied to a broad range of processes involving laser-matter interactions in dielectrics and semiconductors for charge density reaching the range of ${10}^{21}–{10}^{22}{\mathrm{cm}}^{-3}$.

This paper studies the room-temperature exchange-spring magnetic behavior of amorphous TbFe films subjected to an applied strain. The cross-sectional composition measurement of the sputter-deposited TbFe film shows a compositional gradient through the thickness. The gradient is near the compensation composition of amorphous TbFe film producing a Tb-dominant region and a Fe-dominant region. The as-deposited film shows a two-step switching behavior with a negative coercive field, while an applied compressive (or tensile) strain eliminates (or enhances) the two-step switching behavior. The strain influence is attributed to the TbFe composition gradient and relatively large magnetoelastic property of the Tb-dominant region as compared to the Fe-dominant one.

We calculate formation energies and transition levels for copper-related defects in silicon using the screened hybrid functional of Heyd, Scuseria, and Ernzerhof HSE06. We consider Cu siting on interstitial sites (${\mathrm{Cu}}_i$), a substitutional site (${\mathrm{Cu}}_{\mathrm{Si}}$), a ${\mathrm{Cu}}_{\mathrm{Si}}\text{−}{\mathrm{Cu}}_i$ pair, a complex formed from substitutional Cu and interstitial hydrogen (${\mathrm{Cu}}_{\mathrm{Si}}\text{−}{\mathrm{H}}_i$), and a complex formed from a substitutional Cu and three interstitial Cu atoms (${\mathrm{Cu}}_{\mathrm{Si}}\text{−}3{\mathrm{Cu}}_i$). We find that ${\mathrm{Cu}}_i$ is a fast diffuser, with a migration barrier of only 0.19 eV, in good agreement with experimental values. ${\mathrm{Cu}}_i$ is a shallow donor and its formation energy is lower than that of ${\mathrm{Cu}}_{\mathrm{Si}}$ for all Fermi-level positions in the band gap. ${\mathrm{Cu}}_{\mathrm{Si}}$, on the other hand, induces levels in the gap which are related to the occupation of antibonding states originating from the coupling between the Cu $3d$ states ($t_2^{(d)}$), resonant in the valence band, and the vacancy-induced gap states ($t_2^{(p)}$). The stable charge states of ${\mathrm{Cu}}_{\mathrm{Si}}$ in the gap are $+1$, 0, $-1$, and $-2$. The transition levels of ${\mathrm{Cu}}_{\mathrm{Si}}\text{−}{\mathrm{Cu}}_i$ and ${\mathrm{Cu}}_{\mathrm{Si}}\text{−}{\mathrm{H}}_i$ are closely related to the levels of isolated ${\mathrm{Cu}}_{\mathrm{Si}}$: a donor level ($+/0$) near the valence band, an acceptor level near midgap, and a double-acceptor level in the upper part of the gap. The calculated transition levels are in good agreement with experimental results, and the formation energies explain the observed solubility.

We propose to use graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum from visible to radio frequencies. Our approach takes advantage of the exceptionally low electronic heat capacity of monolayer graphene and its constricted thermal conductance to its phonon degrees of freedom. Such a system could provide high-sensitivity photon detection required for research areas including quantum information processing and radio astronomy. As an example, we present our device concepts for GJJ single-photon detectors in both the microwave and infrared regimes. The dark count rate and intrinsic quantum efficiency are computed based on parameters from a measured GJJ, demonstrating feasibility within existing technologies.

Reliable generation of single photons is of key importance for fundamental physical experiments and to demonstrate quantum protocols. Waveguide-based photon-pair sources have shown great promise in this regard due to their large spectral tunability, high generation rates, and long temporal coherence of the photon wave packet. However, integrating such sources with fiber-optic networks often results in a strong degradation of performance. We answer this challenge by presenting an alignment-free source of photon pairs in the telecommunications band that maintains heralding efficiency $>50\%$ even after fiber pigtailing, photon separation, and pump suppression. The source combines this outstanding performance in heralding efficiency with a compact, stable, and easy-to-use “plug-and-play” package: one simply connects a laser to the input and detectors to the output, and the source is ready to use. This high performance can be achieved even outside the lab without the need for alignment which makes the source extremely useful for any experiment or demonstration needing heralded single photons.

Acoustic traps are used to capture and handle suspended microparticles and cells in microfluidic applications. A particular simple and much-used acoustic trap consists of a commercially available, millimeter-sized, liquid-filled straight glass capillary actuated by a piezoelectric transducer. Here, we present a three-dimensional numerical model of the acoustic pressure field in the liquid coupled to the displacement field of the glass wall, taking into account mixed standing and traveling waves as well as absorption. The model explains the dynamical mechanism that leads to the formation of localized acoustic resonance modes in such a straight acoustic waveguide without any geometrical cavities in the axial direction of the capillary. The model further predicts that some of these modes are well suited for acoustic trapping, and it provides estimates for their frequencies and quality factors, the magnitude of the acoustic radiation force on a single test particle as a function of position, and the resulting acoustic retention force of the trap. We show that the model predictions are in agreement with published experimental results, and we discuss how improved and more-stable acoustic-trapping modes might be obtained using the model as a design tool.

Precise manipulations of nonspherical microparticles by shape have diverse applications in biology and biomedical engineering. Here, we study lateral migration of ellipsoidal paramagnetic microparticles in low-Reynolds-number flows under uniform magnetic fields. We show that magnetically induced torque alters the rotation dynamics of the particle and results in shape-dependent lateral migration. By adjusting the direction of the magnetic field, we demonstrate versatile control of the symmetric and asymmetric rotation of the particles, thereby controlling the direction of the particle’s lateral migration. The particle rotations are experimentally measured, and their symmetry or asymmetry characteristics agree well with the prediction from a simple theory. The lateral migration mechanism is found to be valid for nonmagnetic particles suspended in a ferrofluid. Finally, we demonstrate shape-based sorting of microparticles by exploiting the proposed migration mechanism.