Dynamical decoupling can enforce a symmetry on the dynamics of an open quantum system. Here we develop an efficient dynamical-decoupling-based strategy to create the decoherence-free subspaces for a set of qubits by optimally transferring the states of these qubits. Our method is to transfer the state of each qubit to all of the other qubits in an optimal and efficient manner, so as to produce an effective collective-type noise for all qubits. This collective pseudonoise is essentially derived from arbitrarily independent real baths and needs only nearest-neighbor state transfers for its implementation. Moreover, our scheme requires only $N$ steps to produce the effective collective-type noise for a system of $N$ qubits. It provides an experimentally feasible and efficient approach to achieving the decoherence-free subspaces for encoding the protected logical qubits.

We find an unusually optimal near-field heat transfer, where the maximum heat transfer is reached at experimentally feasible gap separation. We attribute this to the localized zero-energy electronic edge states, which also substantially change the near-field behaviors. We demonstrate these anomalous behaviors in two typical carbon-based nanostructures: zigzag single-walled carbon nanotubes and graphene nanotriangles. For the system of carbon nanotubes, the maximal heat flux in this work surpasses all the previous results reported so far. The underlying mechanism for the peculiar effects is uncovered from a simple Su-Schrieffer-Heeger model. Our findings also offer a route to achieve an active near-field thermal switch, where the heat flux can be modulated through tuning the presence or absence of edge states.

The development of alternative mm-wave high-gradient, >200 MV/m, accelerating structures offers a promising path to reduce the cost and footprint of future TeV-scale linear colliders, as well as linacs for industrial, medical, and security applications. The major factor limiting accelerating gradient is vacuum rf breakdown. The probability of such breakdowns increases with pulse length. For reliable operation, millimeter-wave structures require nanoseconds-long pulses at the megawatt level. This power is available from gyrotrons, which have a minimum pulse length on the order of microseconds. To create shorter pulses and to reliably detect rf breakdowns, we developed the following devices: a laser-based rf switch capable of selecting 10 ns long pulses out of the microseconds long gyrotron pulses, thus enabling the use of the gyrotrons as power sources for mm-wave high-gradient linacs, and a shot-to-shot sub-THz spectrometer with high-frequency resolution, capable of detecting pulse shortening due to rf breakdowns. We will describe the principle of operation of these devices and their achieved parameters. We also report on the experimental demonstration of these devices with the high-power gyrotron at the Massachusetts Institute of Technology. In the experiments, we demonstrate nanosecond rf power modulation, shot-to-shot measurements of the pulse spectra, and detection of rf breakdowns.

Wavelengths in the telecommunication window (approximately 1.25–1.65 $\mu \mathrm{m}$) are ideal for quantum communication due to low transmission loss in fiber networks. To realize quantum networks operating at these wavelengths, long-lived quantum memories that couple to telecom-band photons with high efficiency need to be developed. We propose coupling neutral ytterbium atoms, which have a strong telecom-wavelength transition, to a silicon photonic crystal cavity. Specifically, we consider the ${}^3{P}_0{\leftrightarrow }^3{D}_1$ transition in neutral ${}^{171}\mathrm{Yb}$ to interface its long-lived nuclear spin in the metastable ${}^3{P}_0$ “clock” state with a telecom-band photon at $1.4\mu \mathrm{m}$. We show that $\mathrm{Yb}$ atoms can be trapped using a short-wavelength (approximately $470$ nm) tweezer at a distance of 350 nm from the silicon photonic crystal cavity. At this distance, due to the slowly decaying evanescent cavity field at a longer wavelength, we obtain a single-photon Rabi frequency of $g/2\pi \approx 100$ MHz and a cooperativity of $C\approx 47$ while maintaining a high photon collection efficiency into a single mode fiber. The combination of high system efficiency, telecom-band operation, and long coherence times makes this platform well suited for quantum optics on a silicon chip and long-distance quantum communication.

We present a spin-exchange relaxation-free vector magnetometer with suppressed $1/f$ probe noise, achieved by applying a small dc bias field and a comb of magnetic dc $\pi$ pulses along the pump direction. This results in a synchronous orthogonal ac response for each of its two sensitive axes. The magnetometer is particularly well suited to applications such as biomagnetism in which the signal to be measured carries a dominant component of its power at low frequencies. The magnetometer reaches a technical noise floor of 8.4 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{x}}$) and 11 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{y}}$) at 0.01 Hz. A single-axis dc spin-exchange relaxation-free (SERF) magnetometer sharing the same experimental apparatus attains 61 fT ${\mathrm{Hz}}^{-1/2}$ at the same frequency. A noise minimum of 1.1 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{x}}$) and 2.0 fT ${\mathrm{Hz}}^{-1/2}$ ($\hat{\mathbf{y}}$) is reached by the magnetometer at 10 Hz, compared to 0.7 fT ${\mathrm{Hz}}^{-1/2}$ at 25 Hz for a dc SERF magnetometer.

In this paper, we develop an approximate wide-bandwidth upper bound to the absorption enhancement in arrays of metaparticles, applicable to general wave-scattering problems and motivated here by ocean-buoy energy extraction. We show that general limits, including the well-known Yablonovitch result in solar cells, arise from reciprocity conditions. The use of reciprocity in the stochastic regime leads us to a corrected diffusion model from which we derive our main result: an analytical prediction of optimal array absorption that closely matches exact simulations for both random and optimized arrays under angle and frequency averaging. This result also enables us to propose and quantify approaches to increase performance through careful particle design and/or using external reflectors. We show, in particular, that the use of membranes on the water’s surface allows substantial enhancement.

The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated by a combined experimental and theoretical approach. On the experimental side, $\mathrm{Ga}\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ resonant tunneling diodes (RTDs) are grown by molecular beam epitaxy. In situ electron diffraction is used to monitor the number of monolayers incorporated into each tunneling barrier of the RTD active region. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current density as a function of barrier thickness. At the same time, both the peak voltage and the characteristic threshold bias exhibit a dependence on barrier thickness as a result of the intense electric fields present within the polar heterostructures. To get further insight into the asymmetric tunneling injection originating from the polar active region, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current that includes contributions from coherent and sequential tunneling processes is introduced. After the application of this theory to the case of $\mathrm{Ga}\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities, with tunneling currents spanning several orders of magnitude. This agreement allows us to elucidate the effect of the internal polarization fields on the magnitude of the tunneling current and broadening of the resonant tunneling line shape. Under reverse bias, we identify new tunneling features originating from highly attenuated resonant tunneling phenomena, which are completely captured by our model. The analytical form of our quantum transport model provides a simple expression that reveals the connection between the design parameters of a general polar RTD and its current-voltage characteristics. This new theory paves the way for the design of polar resonant tunneling devices exhibiting efficient resonant current injection and enhanced tunneling dynamics as required in various practical applications.

Magnetic skyrmions are chiral spin structures recently observed at room temperature in multilayer films. Their topological stability will enable high scalability in confined geometries—a sought-after attribute for device applications. Despite numerous theoretical studies examining sub-100-nm Néel skyrmions in nanostructures, in practice their ambient stability and evolution with confinement and their magnetic parameters remain to be established. Here we present the zero-field stabilization of sub-100-nm room-temperature Néel-textured skyrmions confined in $\mathrm{Ir}$/$\mathrm{Fe}$$(x)$/$\mathrm{Co}$$(y)$/$\mathrm{Pt}$ nanodots over a wide range of magnetic and geometric parameters. The zero-field skyrmion size, here as small as approximately 50 nm, can be tailored by a factor of 4 with variation of dot size and magnetic interactions. Crucially, skyrmions with differing thermodynamic stability exhibit an unexpected dichotomy in confinement phenomenologies. These results establish skyrmion phenomenology in multilayer nanostructures, and prompt the synergistic use of magnetic and geometric parameters to achieve desired properties in devices.

Ferroelectric materals are an important platform for the realization of nonvolatile memories. So far, existing ferroelectric memory devices have utilized out-of-plane polarization in ferroelectric thin films. In this paper, we propose a type of random-access memory (RAM) based on ferroelectric thin films with in-plane polarization, called an “in-plane ferroelectric tunnel junction.” Apart from nonvolatility, lower power usage, and a faster writing operation compared with traditional dynamic RAMs, our proposal has the advantage of a faster reading operation and a nondestructive reading process, thus overcoming the write-after-read problem that exists widely in current ferroelectric RAMs. The recent discovered room-temperature ferroelectric IV-VI semiconductor thin films are a promising material platform for the realization of our proposal.

We present a procedure for addressing extrinsic defects in amorphous oxides, in which the most stable defect configurations are identified through ab initio molecular dynamics in various charge states and studied through hybrid functional calculations. The protocol is further complemented with an electron counting scheme based on maximally localized Wannier functions, which allows one to identify the nominal charge state and the composition of the defect core unit. Here, we apply this approach to the study of hydrogen, carbon and nitrogen impurities in amorphous alumina $(a{\text{−}\mathrm{Al}}_2{\mathrm{O}}_3)$, a highly relevant material for technological applications. Hydrogen is found to be amphoteric with a thermodynamic $+1$/$-1$ charge transition level lying at $\sim 4.6$ eV above the valence band maximum, in qualitative agreement with results obtained with crystalline models, but at a substantially different energy level. Hydroxyl groups are further shown to lead to the same defect states as observed for hydrogen. Application of our procedure to carbon and nitrogen impurities leads to structural configurations that are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively. This indicates that neither carbon nor nitrogen give charge transition levels in the band gap, in strong contrast with results achieved for crystalline models. In addition, the defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy depends on the oxygen chemical potential ${\mu }_O$. In oxygen-poor conditions, both the carbon and the nitrogen impurities favor bonding to $\mathrm{Al}$ atoms, while they tend to form single or double bonds with oxygen atoms as ${\mu }_O$ increases. Based on the band alignment of $a{\text{−}\mathrm{Al}}_2{\mathrm{O}}_3$ with three technologically relevant semiconductors ($\mathrm{Ga}\mathrm{As}$, $\mathrm{Ga}\mathrm{N}$ and $\alpha$-${\mathrm{Fe}}_2{\mathrm{O}}_3$), we discuss the possible role of point defects in degrading the performance of electronic devices and in favoring hole transport across the oxide in water-splitting setups.

Two reciprocity theorems are important for fundamental understanding of the solar cell operation and applications to device evaluation: (1) the carrier-transport reciprocity connecting the dark-carrier injection with the short-circuit photocarrier collection and (2) the optoelectronic reciprocity connecting the electroluminescence with the photovoltaic quantum efficiency at short circuit. These theorems, however, fail in devices with thick depletion regions such as p-i-n junction solar cells. By properly linearizing the carrier-transport equation in such devices, we report that the dark-carrier injection is related to the photocarrier collection efficiency at the operating voltage, not at short circuit as suggested in the original theorem. This leads to the general form of the optoelectronic reciprocity relation connecting the electroluminescence with the voltage-dependent quantum efficiency, providing a correct interpretation of the optoelectronic properties of p-i-n junction devices. We also discuss the validity of the well-known relation between the open-circuit voltage and the external luminescence efficiency. The impact of illumination intensity and device parameters on the validity of the reciprocity theorems is quantitatively investigated.

Recent advances in metasurfaces have shown the importance of controlling the bianisotropic response of the constituent meta-atoms for maximum efficiency wave-front transformation. By carefully designing the bianisotropic response of the metasurface, full control of the local transmission and reflection properties is enabled, opening new design avenues for creating reciprocal metasurfaces. Despite recent advances in the highly efficient transformation of both electromagnetic and acoustic plane waves, the importance of bianisotropic metasurfaces for transforming cylindrical waves is still unexplored. Motivated by the possibility of arbitrarily controlling the angular momentum of cylindrical waves, we develop a design methodology of a bianisotropic cylindrical metasurface that enables transformation of cylindrical waves for both acoustic and electromagnetic waves with theoretically 100% power efficiency. This formalism is further validated in the acoustic scenario where an experimental demonstration of highly efficient angular momentum transformation is shown.

Mask-based pattern generation is a crucial step in microchip production. The next-generation extreme-ultraviolet- (EUV) lithography instruments with a wavelength of 13.5 nm are currently under development. In principle, this should allow patterning down to a resolution of a few nanometers in a single exposure. However, there are many technical challenges, including those due to the very high energy of the photons. Lithography with metastable atoms has been suggested as a cost-effective, less-complex alternative to EUV lithography. The great advantage of atom lithography is that the kinetic energy of an atom is much less than that of a photon for a given wavelength. However, until now no method has been available for making masks for atom lithography that can produce arbitrary, high-resolution patterns. Here we present a solution to this problem. First, traditional binary holography is extended to near-field binary holography, based on Fresnel diffraction. By this technique, we demonstrate that it is possible to make masks that can generate arbitrary patterns in a plane in the near field (from the mask) with a resolution down to the nanometer range using a state-of-the-art metastable-helium source. We compare the flux of this source with that of an established EUV source (NXE:3100, ASML) and show that patterns can potentially be produced at comparable speeds. Finally, we present an extension of the grid-based holography method for a grid of hexagonally shaped subcells. Our method can be used with any beam that can be modeled as a scalar wave, including other matter-wave beams such as helium ions, electrons, or acoustic waves.

Magnetic sensors are important for detecting nuclear magnetization signals in nuclear magnetic resonance (NMR). As a complementary analysis tool to conventional high-field NMR, zero- to ultralow-field (ZULF) NMR detects nuclear magnetization signals in the submicrotesla regime. Current ZULF NMR systems are always equipped with high-quality magnetic shields to ensure that ambient magnetic-field noise does not dwarf the magnetization signal. An alternative approach is to separate the magnetization signal from the noise based on their differing spatial profiles, as can be achieved using a magnetic gradiometer. Here, we present a gradiometric ZULF NMR spectrometer with a magnetic-field-gradient noise of $17\mathrm{fT}/\mathrm{cm}{\mathrm{Hz}}^{1/2}$ in the frequency ranging from 100 to 400 Hz, based on a single vapor cell ($0.7\times 0.7\times 1.0{\mathrm{cm}}^3$). With applied white magnetic-field noise, we show that the gradiometric spectrometer achieves 13-fold enhancement in the signal-to-noise ratio (SNR) compared to the single-channel configuration. By reducing the influence of the common-mode magnetic-field noise, this work enables the use of compact and low-cost magnetic shields. Gradiometric detection also proves to be beneficial for eliminating systematic errors in ZULF-NMR experiments searching for exotic spin-dependent interactions and molecular parity violation.

Oocyte activation deficiency is considered to be the principal factor underlying fertilization failure after intracytoplasmic sperm injection (ICSI), and consequently exhibits a lack of cytosolic calcium (${\mathrm{Ca}}^{2+}$) oscillations in oocytes. Traditional artificial activation methods are usually proposed to induce an intracellular ${\mathrm{Ca}}^{2+}$ transient in oocytes. However, these current methods either cannot mimic the precise physiological processes of mammalian oocyte activation or there still exists controversy concerning their activation mechanisms. Here, we present a report on the potential of using nanosecond pulsed electric fields (nsPEFs) with a 10-ns duration not only to achieve a high activation rate of mouse oocytes, but also to significantly improve the developmental ability of parthenogenetic embryos. More importantly, in the exploration of activation mechanisms, we find that nsPEFs can regulate three different patterns of ${\mathrm{Ca}}^{2+}$ oscillation via mobilizing both ${\mathrm{Ca}}^{2+}$ influx from the outside and efflux from the endoplasmic reticulum (ER), and can even induce spontaneous ${\mathrm{Ca}}^{2+}$ oscillations. Furthermore, a nsPEFs-mediated ${\mathrm{Ca}}^{2+}$-channel kinetic model is introduced, and the simulation results agree well with our experiments. Our model predicts that nsPEFs could regulate ${\mathrm{Ca}}^{2+}$ oscillations via the direct effects on the openness of ${\mathrm{Ca}}^{2+}$ channels by simultaneously mobilizing extracellular and intracellular ${\mathrm{Ca}}^{2+}$. Therefore, as an effective and controllable method, nsPEFs provide a valuable tool for potential artificial activation treatment in the future.

Flexible electronic systems such as roll-up displays and wearable devices promise exciting possibilities that could change the way humans interact with the environment. However, they suffer from poor reliability of interconnects and devices. Interconnects on flex are prone to open-circuit failures due to mechanical stress, electrostatic discharge, and environmental degradation. Passive approaches such as the use of stretchable conductors and novel geometries improve their response to mechanical stress but cannot salvage the interconnect if a fault were to occur. Active approaches using self-healing techniques can repair a fault and have been demonstrated with use of methods that use relatively rare materials, change conventional interconnect-fabrication processes, address only faults due to mechanical stress, or do not permit stretching. In this work we discuss a self-healing technique that overcomes these limitations and demonstrate heals having metallic conductivity and nearly plastic stretchability. This is achieved by dispersion of conductive particles in an insulating fluid encapsulated over the interconnect. Healing is automatically triggered by the electric field appearing in the open gap of a failed interconnect, irrespective of the cause of failure. The field polarizes the conductive particles, causing them aggregate and chain up to bridge the gap and repair the fault. Using dispersions of copper microspheres in silicone oil, we show self-healing interconnects with the stretchable heal having conductivity of about $5\times {10}^5$ S/m and allowing strains from 12 to 60. Previously, stretchable interconnects used materials other than copper. Here we effectively show self-healing, stretchable copper. This work promises high-speed, self-healing, and stretchable interconnects on flex, thereby improving system reliability.

In the past decade, it has been demonstrated that surface-enhanced infrared absorption (SEIRA) is a powerful method to enhance vibrational signals of thin molecular layers. Much less attention has so far been given to the possibility of using SEIRA for the detection and characterization of nanometer-sized particles, such as ultrafine dust particles. Here, we report on SEIRA measurements demonstrating that even one single particle with a deeply subwavelength dimension of less than 100 nm can be detected and chemically characterized with standard infrared microspectroscopy. Our approach is based on plasmonic resonances of bowtie-shaped $\mathrm{Au}$ apertures that are designed to extraordinarily enhance the material-specific phononic excitations of a nanometer-sized silica particle. We show that the bowtie geometry is especially suited for single-particle spectroscopy, as it combines the advantage of an intense electromagnetic hot spot, the size of which can be adjusted to the particle dimension, with easy positioning of ultrafine dust particles inside that hot spot. In agreement with numerical calculations, we show that a detection limit in terms of a particle diameter of less than 20 nm can be achieved, which corresponds to a ratio of the diameter to the vacuum wavelength below 0.002. Our approach offers the possibility of analyzing infrared bands from tiniest particles and thus paves the way toward SEIRA-based devices that can sense ultrafine dust.

We present an extensive formulation of the inverse grating problem for flexural waves in which the energy of each diffracted mode is selected and the grating configuration is then obtained by our solving a linear system of equations. The grating is designed as a lineal periodic repetition of a unit cell comprising a cluster of resonators attached at points whose physical properties are directly derived by inversion of a given matrix. Although both active and passive attachments can be required in the most-general case, it is possible to find configurations with only passive (i.e., damped) solutions. This inverse design approach is an alternative to the design of metasurfaces for flexural waves, overcoming the limitations of gradient phase metasurfaces, which require a continuous variation of the surface’s impedance. When the grating is designed in such a way that all the energy is channeled into a single diffracted mode, it behaves as an anomalous refractor or reflector. The negative refractor is analyzed in depth, and it is shown that with only three scatterers per unit cell is it possible to build such a device with unitary efficiency.

We investigate a nonadiabatic holonomic operation that enables us to entangle two fixed-frequency superconducting transmon qubits attached to a common bus resonator. Two coherent microwave tones are applied simultaneously to the two qubits and drive transitions between the first excited resonator state and the second excited state of each qubit. The cyclic evolution within this effective three-level $\mathrm{\Lambda }$-type system gives rise to a holonomic operation entangling the two qubits. Two-qubit states with 95% fidelity, limited mainly by charge noise of the current device, are created within 213 ns. This scheme is a step toward implementation of a swap-type gate directly in an all-microwave controlled hardware platform. By extending the available set of two-qubit operations in the fixed-frequency qubit architecture, the proposed scheme may find applications in near-term quantum applications using variational algorithms to efficiently create problem-specific trial states. We illustrate this point by computing the ground state of molecular hydrogen using the holonomic operation.

Magnetic skyrmions are promising information carriers for building future high-density and high-speed spintronic devices. However, to achieve a current-driven high-speed skyrmion motion, the required driving-current density is usually very large, which could be energy inefficient and even destroy the device due to Joule heating. The mechanism of voltage-driven skyrmion motion through equidistant identical electrodes on a uniformly thick nanowire is studied. The high-speed skyrmion motion is realized by utilizing a voltage shift, and the average skyrmion velocity reaches up to 259 m/s under a 0.45-V applied voltage. In comparison with the widely studied vertical current-driven model, the energy dissipation is three orders of magnitude lower in our voltage-driven model for the same speed motion of skyrmions. Our approach uncovers valuable opportunities for building skyrmion racetrack memories and logic devices with both ultra-low power consumption and ultra-high processing speed, which are appealing features for future spintronic applications.