Quantum key distribution (QKD) is a quantum-proof key-exchange scheme which is fast approaching the communication industry. An essential component in QKD is the information reconciliation step, which is used for correcting the quantum-channel noise errors. The recently suggested blind-reconciliation technique, based on low-density parity-check codes, offers remarkable prospectives for efficient information reconciliation without an a priori quantum bit error rate estimation. We suggest an improvement of the blind-information-reconciliation protocol promoting a significant increase in the efficiency of the procedure and reducing its interactivity. The proposed technique is based on introducing symmetry in operations of parties, and the consideration of results of unsuccessful belief-propagation decodings.

Subwavelength structures (meta-atoms) with artificially engineered permittivity and permeability have shown promising applications for guiding and controlling the flow of electromagnetic energy on the nanoscale. Ultrafast laser nanoprinting emerges as a promising single-step, green and flexible technology in fabricating large-area arrays of meta-atoms through the translative or ablative modification of noble-metal thin films. Ultrafast laser energy deposition in noble-metal films produces irreversible, intricate nanoscale translative mass redistributions after resolidification of the transient thermally assisted hydrodynamic melt perturbations. Such mass redistribution results in the formation of a radially symmetric frozen surface with modified hidden nanofeatures, which strongly affect the optical response harnessed in plasmonic sensing and nonlinear optical applications. Here, we demonstrate that side-view electron microscopy and ion-beam cross sections together with low-energy electron x-ray dispersion microscopy provide exact information about such three-dimensional patterns, enabling an accurate acquisition of their cross-sectional mass distributions. Such nanoscale solidified structures are theoretically modeled, considering the underlying physical processes associated with laser-induced energy absorption, electron-ion energy exchange, acoustic relaxation, and hydrodynamic flows. A theoretical approach, separating slow and fast physical processes and combining hybrid analytical two-temperature calculations, scalable molecular-dynamics simulations, and a semianalytical thin-shell model is synergistically applied. These advanced characterization approaches are required for a detailed modeling of near-field electromagnetic response and pave the way to a fully automated noninvasive in-line control of a high-throughput and large-scale laser fabrication. This theoretical modeling provides an accurate prediction of scales and topographies of the laser-fabricated meta-atoms and metasurfaces.

Quantum sensors with solid-state spins have attracted considerable interest due to their advantages in high sensitivity and high spatial resolution. The robustness against environmental noise is a critical requirement for solid-state spin sensors. In this paper, we present a self-protected infrared high-sensitivity thermometry based on spin defects in silicon carbide. Based on the conclusion that the Ramsey oscillations of the spin sensor are robust against magnetic noise due to a self-protected mechanism from the intrinsic transverse strain of the defect, we experimentally demonstrate the Ramsey-based thermometry. The self-protected infrared silicon-carbide thermometry may provide a promising platform for high sensitivity and high-spatial-resolution temperature sensing in a practical noisy environment, especially in biological systems and microelectronics systems.

We report experiments and calculations investigating the pressure and temperature dependences of the optical phonons in ${\mathrm{BaZrS}}_3$, and the pressure dependence of its absorption edge. ${\mathrm{BaZrS}}_3$ is a chalcogenide perovskite in a novel class of materials being considered for photovoltaics. It is studied by Raman spectroscopy as functions of temperature (at 1 atm) and pressure (at 120 and 295 K), and by pressure-transmission spectroscopy at 295 K. Density-functional-theory calculations predict the allowed Raman lines, their intensities, their pressure shifts, and the band-gap pressure shift. Cooling shifts all but one of the phonon peaks to higher frequencies; the temperature coefficients are typical of semiconductors. A strong low-temperature peak at $392.3{\mathrm{cm}}^{-1}$ is attributed to resonant forbidden LO scattering; its shift with temperature has the opposite sign. The pressure coefficients of the phonon frequencies for all observed Raman peaks are positive, indicating no mode softening. The rates of pressure shift also are typical, and show the customary scaling with phonon frequency. Experiment and theory show good agreement on the pressure-induced frequency shifts. The ${\mathrm{BaZrS}}_3$ absorption edge moves to lower energy with pressure, reflecting a reduction of the band gap. The measured shift is $\sim -0.015\mathrm{eV}/\mathrm{GPa}$, slightly less than the density-functional-theory result of $-0.025\mathrm{eV}/\mathrm{GPa}$. We find no evidence that the perovskite structure of ${\mathrm{BaZrS}}_3$ undergoes any phase changes under hydrostatic pressure to at least 8.9 GPa. Our results indicate the robust structural stability of ${\mathrm{BaZrS}}_3$, and suggest cation alloying as a viable approach for band-gap engineering for photovoltaic and other applications.

We study the thermoelectric properties of As and Sb monolayers (arsenene and antimonene) using density-functional theory and the semiclassical Boltzmann transport approach. The materials show large band gaps combined with low lattice thermal conductivities. Specifically, the small phonon frequencies and group velocities of antimonene lead to an excellent thermoelectric response at room temperature. We show that $n$-type doping enhances the figure of merit.

It is known that the eye’s scotopic photodetectors, rhodopsin molecules, and their associated phototransduction mechanism leading to light perception, are efficient single-photon counters. We here use the photon-counting principles of human rod vision to propose a secure quantum biometric identification based on the quantum-statistical properties of retinal photon detection. The photon path along the human eye until its detection by rod cells is modeled as a filter having a specific transmission coefficient. Precisely determining its value from the photodetection statistics registered by the conscious observer is a quantum parameter estimation problem that leads to a quantum secure identification method. The probabilities for false-positive and false-negative identification of this biometric technique can readily approach ${10}^{-10}$ and ${10}^{-4}$, respectively. The security of the biometric method can be further quantified by the physics of quantum measurements. An impostor must be able to perform quantum thermometry and quantum magnetometry with energy resolution better than ${10}^{-9}\hslash$, in order to foil the device by noninvasively monitoring the biometric activity of a user.

In recent years, graphene growth optimization has been one of the key routes towards large-scale, high-quality graphene production. We measure in situ residual gas content during epitaxial-graphene growth on silicon carbide (SiC) to find detrimental factors of epitaxial-graphene growth. The growth conditions in high vacuum, in argon, purified argon, and the flow of argon are compared. The grown epitaxial graphene is studied by Raman-scattering mapping. We determine mechanical strain, number of graphene layers and the graphene quality. The surface topography is measured by atomic force microscopy. Charge density and carrier mobility are studied by Hall-effect measurements in van der Pauw configuration. We identify the major role of the chemical reaction of carbon and residual water. The rate of the reaction is lowered when purified argon is used. We also show that, according to time-varying gas content, it is preferable to grow graphene at higher temperatures and shorter times. Other sources of growth environment contamination are also discussed. The reaction of residual gas and SiC is discussed as one of the factors decreasing the lateral size of SiC atomically flat terraces and leading to their irregular shape. The importance of purified argon and its sufficient flow rate is concluded to be important for high-quality graphene growth as it reduces the rate of undesired chemical reactions and provides a more stable and defined growth ambient.

Imaging of surface topography and elasticity of living cells can provide insight into the roles played by the cells’ volumetric and mechanical properties and their response to external forces in regulating the essential cellular events and functions. Here, we report a unique technique of noncontact viscoelastic imaging of live cells using atomic force microscopy (AFM) with a long-needle glass probe. Because only the probe tip is placed in a liquid medium near the cell surface, the AFM cantilever in air functions well under dual-frequency modulation, retaining its high-quality resonant modes. The probe tip interacts with the cell surface through a minute hydrodynamic flow in the nanometer-thin gap region between them without physical contact. Quantitative measurements of the cell height, volume, and Young’s modulus are conducted simultaneously. The experiment demonstrates that the long-needle AFM has a wide range of applications in the study of cell mechanics.

A simple off-lattice method of deriving the Sanchez-Lacombe equation of state is presented. The Sanchez-Lacombe equation of state for mixtures is shown to be thermodynamically inconsistent for all mixing rules in such a way that fugacity coefficients, until now thought to correct for mixing inconsistencies, cannot make the theory consistent. The theory is consistent, however, for constant hole volumes and it is shown, for a sample mixture of polypropylene and carbon dioxide, that excellent agreement with experimental solubility results is achieved without changing the mixture parameters with temperature or pressure. To this end, pure-component Sanchez-Lacombe characteristic parameters for both branched and linear polypropylene are also provided. The agreement between theory and experiment for solubility using a constant hole volume for carbon dioxide mixtures with both branched and linear polypropylene is much better than for typical mixing rules for the Sanchez-Lacombe equation of state. Fair agreement with experimental swelling-ratio data at saturation is also achieved with no further free parameters, making this equation of state a good choice for predictions related to polymeric foaming. A consideration of the hole volume is given in terms of correlations, and evidence to support this perspective is presented in terms of the characteristic parameters regressed from different architectures of pure polypropylene. It is shown that only a single pure-polypropylene characteristic parameter is needed to characterize mixtures with carbon dioxide, and that an estimate of this parameter can be extracted from mixture solubility data. This example demonstrates the feasibility of applying the Sanchez-Lacombe equation of state to mixtures in which one of the pure components has not been independently characterized.

The pairing state and critical temperature ($T_C$) of a thin $s$-wave superconductor ($S$) on two or more ferromagnets ($F$) are controllable through the magnetization alignment of the $F$ layers. Magnetization misalignment can lead to spin-polarized triplet-pair creation, and since such triplets are compatible with spin-polarized materials, they are able to pass deeply into the $F$ layers and cause a decrease in $T_C$. Various experiments on $S/F_1/F_2$ “triplet spin valves” have been performed with the most pronounced suppression of $T_C$ reported in devices containing the half-metal ferromagnet (HMF) ${\mathrm{CrO}}_2$ ($F_2$) albeit using out-of-plane magnetic fields to tune magnetic noncollinearity [Singh et al., Phys. Rev. X 5, 021019 (2015)]. Routine transfer of spin-polarized triplets to HMFs is a major goal for superconducting spintronics so as to maximize triplet-state spin polarization. However, ${\mathrm{CrO}}_2$ is chemically unstable, and out-of-plane fields are undesirable for superconductivity. Here, we demonstrate low-field (3.3 mT) magnetization-tunable pair conversion and transfer of spin-polarized triplet pairs to the chemically stable mixed valence manganite ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ in a pseudo-spin-valve device using in-plane magnetic fields. The results match microscopic theory and offer full control over the pairing state.

Manipulation of oxygen vacancies ($V_{\mathrm{O}}$) in single oxide layers by varying the electric field can result in significant modulation of the ground state. However, in many oxide multilayers with strong application potentials, e.g., ferroelectric tunnel junctions and solid-oxide fuel cells, understanding $V_{\mathrm{O}}$ behavior in various layers under an applied electric field remains a challenge, owing to complex $V_{\mathrm{O}}$ transport between different layers. By sweeping the external voltage, a reversible manipulation of $V_{\mathrm{O}}$ and a corresponding fixed magnetic phase transition sequence in cobaltite/manganite (${\mathrm{SrCoO}}_{3-x}/{\mathrm{La}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_{3-y}$) heterostructures are reported. The magnetic phase transition sequence confirms that the priority of electric-field-induced $V_{\mathrm{O}}$ formation or annihilation in the complex bilayer system is mainly determined by the $V_{\mathrm{O}}$ formation energies and Gibbs free-energy differences, which is supported by theoretical analysis. We not only realize a reversible manipulation of the magnetic phase transition in an oxide bilayer but also provide insight into the electric-field control of $V_{\mathrm{O}}$ engineering in heterostructures.

Voltage control of perpendicular magnetic anisotropy (PMA) is very promising in the realization of high-density, lightweight, and energy-efficient information storage. However, it is hard to achieve $E$-field regulation of PMA in conventional multiferroic laminates since the voltage-induced magnetic anisotropy is relatively small. In this study, we demonstrate $E$-field control of PMA in ${(\mathrm{Co}/\mathrm{Pt})}_3/\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$ multiferroic heterostructures at room temperature using ferromagnetic resonance measurement (FMR) and a magnetic optical Kerr (MOKE) microscope. When a single Co layer $(t_{\mathrm{Co}})=1.0\mathrm{nm}$ PMA is obtained, a resonance shift up to 470 Oe is obtained with a $12\text{−}\mathrm{kV}{\mathrm{cm}}^{-1}$ $E$ field. Up to 75% perpendicular magnetic moments can be controlled with an applied $10\text{−}\mathrm{kV}{\mathrm{cm}}^{-1}$ electric field at $t_{\mathrm{Co}}=0.9\mathrm{nm}$ while little effect is obtained at $t_{\mathrm{Co}}=0.7\mathrm{nm}$. In addition, a multiferroic heterostructure with in-plane anisotropy is also studied at $t_{\mathrm{Co}}=1.4\mathrm{nm}$, and the resonance shifts are comparable to 1.0 nm. We relate this phenomenon to the instability of Co orbital moments near the critical transition area (0.9, 1.0, 1.4 nm). However, if $t_{\mathrm{Co}}$ is far from the transition point, the magnetic properties become insensitive to an external stimulus (0.7 nm). This voltage manipulation of PMA in ultrathin Co films should offer possibilities for realizing electronic devices and memories with great energy efficiency.

We propose and demonstrate a technique for coupling phonons out of an optomechanical crystal cavity. Using a perturbation that breaks a symmetry in the elastic structure, we induce phonon leakage without affecting the optical properties. It is shown experimentally via cryogenic measurements that the proposed cavity perturbation causes a loss of phonons into mechanical waves on the surface of silicon while leaving photon lifetimes unaffected. This result demonstrates that phonon leakage can be engineered in on-chip optomechanical systems. We experimentally observe large fluctuations in leakage rates that we attribute to fabrication disorder and verify this finding using simulations. Our technique opens the way to engineering more complex on-chip phonon networks utilizing guided mechanical waves to connect quantum systems.

High-entropy alloy (HEA) is a family of metallic materials with nearly equal partitions of five or more metals, which might possess mechanical and transport properties that are different from conventional binary or tertiary alloys. In this work, we demonstrate current-induced spin-orbit torque (SOT) magnetization switching in a Ta-Nb-Hf-Zr-Ti HEA-based magnetic heterostructure with perpendicular magnetic anisotropy. The maximum dampinglike SOT efficiency from this particular HEA-based magnetic heterostructure is further determined to be $|{\zeta }_{\mathrm{DL}}^{\mathrm{HEA}}|\approx 0.033$ by hysteresis-loop-shift measurements, while that for the Ta control sample is $|{\zeta }_{\mathrm{DL}}^{\text{Ta}}|\approx 0.04$. Our results indicate that HEA-based magnetic heterostructures can serve as an alternative group of potential candidates for SOT device applications due to the possibility of tuning buffer-layer properties with more than two constituent elements.

We develop and analyze an advanced concept for a domain-wall-based sensing of rotations. Moving domain walls in $n$ closed loops with $n-1$ intersecting convolutions by rotating fields, we are able to sense $n$ rotations. By combining loops with coprime numbers of rotations, we create a sensor system allowing for the total counting of millions of turns of a rotating applied magnetic field. We analyze the operation of the sensor and identify the intersecting cross structures as the critical component for reliable operation. Specifically, depending on the orientation of the applied field angle with the magnetization in the branches of the cross, a domain wall is found to propagate in an unwanted direction, yielding failures and counting errors in the device. To overcome this limiting factor, we introduce a specially designed syphon structure to the controlled pinning of the domain wall before the cross and depinning and propagation only for a selected range of applied field angles. By adjusting the syphon and the cross geometry, we find that the optimized combination of both structures prevents failures in the full sensor structure yielding robust operation. Our modeling results show that the optimized element geometry allows for the realization of the sensor with cross-shaped intersections and an operation that is tolerant to inaccuracies of the fabrication.

We study the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibit dephasing that is less sensitive to flux noise. This insight is used to fabricate qubits where dephasing due to flux noise is suppressed below other dephasing sources, leading to flux-independent dephasing times $T_2^{*}\sim 15\mu \mathrm{s}$ over a tunable range of approximately 340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and to fundamentally improve scalability for a quantum processor.

Noncontact atomic force microscopy images show that gold grows on the $(2\times 2)\text{−}{\mathrm{N}}_{\mathrm{ad}}$ reconstructed polar (0001) surface of AlN insulating films, in the form of large monatomic islands. High-resolution images and in situ reflection high-energy electron diffraction spectra reveal two moiré patterns from which an atomic model can be built. Density functional theory calculations confirm this model and give insight into the mechanisms that lead to the stabilization of the monolayer. Gold adsorption is accompanied, first, by a global vertical charge transfer from the AlN substrate that fulfills the electrostatic stability criterion for a polar material, and second, by lateral charge transfers that are driven by the local chemical properties of the $(2\times 2)\text{−}{\mathrm{N}}_{\mathrm{ad}}$ reconstruction. These results present alternative strategies to grow metal electrodes onto nitride compounds with a better controlled interface, a crucial issue for applications.

In this work, the $A$- and $B$-site cation migration pathways involving defect complexes in bulk ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_{3\pm \delta }$ (LSM) at $x=0.0–0.25$ are investigated based on density-functional-theory modeling for solid-oxide fuel-cell (SOFC) cathode applications. We propose a dominant $A$-site cation migration mechanism which involves an $A$-site cation (e.g., ${\mathrm{La}}_A^x$) hop into a $V_A^{\prime \prime \prime }$ of a $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ cluster, where ${\mathrm{La}}_A^x$, $V_A^{\prime \prime \prime }$, and, $V_B^{\prime \prime \prime }$ are ${\mathrm{La}}^{3+}$, $A$-site vacancy, and $B$-site vacancy in bulk LSM, respectively, and $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ is the first nearest-neighbor $V_A^{\prime \prime \prime }$ and $V_B^{\prime \prime \prime }$ pair. This hop exhibits an approximately 1.6-eV migration barrier as compared to approximately 2.9 eV of the ${\mathrm{La}}_A^x$ hop into a $V_A^{\prime \prime \prime }$. This decrease in the cation migration barrier is attributed to the presence of the $V_B^{\prime \prime \prime }$ relieving the electrostatic repulsion and steric constraints to the migrating $A$-site cations in the transition-state image configurations. The $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ interaction energy is predicted to be weakly repulsive (0.2–0.3 eV) in bulk LSM, which enables the $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$ cluster to readily form. The predicted apparent activation energy of $D_{\mathrm{La}}^{*}$ in ${\mathrm{LaMnO}}_{3\pm \delta }$ (LMO) for the $A$-site migration pathway is about 1.4 eV, in good agreement with the experimental $A$-site cation impurity diffusivity measurements. By examining the $A$-site cation migration barriers among different metal cations (${\mathrm{Zr}}^{4+}$, ${\mathrm{Y}}^{3+}$, ${\mathrm{Gd}}^{3+}$) relevant for SOFC applications, it is demonstrated that migration barriers of the cation impurity in bulk LSM correlate with the ionic charge and ionic radius at a given formal cationic charge. The $B$-site cation migration barrier takes place by an analogous mechanism that involves a ${\mathrm{Mn}}_B^x$ (${\mathrm{Mn}}^{3+}$ on the $B$ site) hop into a $V_B^{\prime \prime \prime }$ via the ${\mathrm{Mn}}_B^x\text{−}{V}_A^{\prime \prime \prime }\to V_B^{\prime \prime \prime }$ path with the same cation transport carrier of $V_A^{\prime \prime \prime }\text{−}{V}_B^{\prime \prime \prime }$. This diffusion pathway is found to have a barrier of approximately 1.6 eV, similar to the analogous $A$-site hop. However, hopping of the ${\mathrm{Mn}}_A^x$ antisite defect (${\mathrm{Mn}}^{3+}$ on the $A$ site) to a nearest-neighbor $V_A^{\prime \prime \prime }$ [${\mathrm{Mn}}_A^x$ ($V_A^{\prime \prime \prime }$) mechanism] has a barrier of only 0.5 eV. Such a low ${\mathrm{Mn}}_A^x$ ($V_A^{\prime \prime \prime }$) migration barrier opens the possibility to activate Mn transport in bulk LSM through the diffusion of the antisite ${\mathrm{Mn}}_A^x$ ($V_A^{\prime \prime \prime }$) pathway on the $A$-site lattice, particularly when the concentration of the Mn antisite defect can be altered upon varying the $A/B$ ratio and the activity of ${\mathrm{MnO}}_y$. The increase in ${\mathrm{Sr}}_A^{\prime }$ doping concentration in bulk ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_{3\pm \delta }$ ($x=0.0–0.25$) is found to influence primarily the formation energies of cation transport carriers (cation vacancies), whereas the cation migration barriers exhibit only a weak dependence on the ${\mathrm{Sr}}_A^{\prime }$ concentration.