We demonstrate active control of inhomogeneous dephasing and rephasing for single collective atomic spin excitations (spin waves) created by spontaneous Raman scattering in a quantum memory based on cold ${}^{87}\mathrm{Rb}$ atoms. The control is provided by a reversible external magnetic field gradient inducing an inhomogeneous broadening of the atomic hyperfine levels. We demonstrate experimentally that active rephasing preserves the single photon nature of the retrieved photons. Finally, we show that the control of the inhomogeneous dephasing enables the creation of time-separated spin waves in a single ensemble followed by a selective read-out in time. This is an important step towards the implementation of a functional temporally multiplexed quantum repeater node.

We study the impact of the wetting properties on the immiscible displacement of a viscous fluid in disordered porous media. We present a novel pore-scale model that captures wettability and dynamic effects, including the spatiotemporal nonlocality associated with interface readjustments. Our simulations show that increasing the wettability of the invading fluid (the contact angle) promotes cooperative pore filling that stabilizes the invasion and that this effect is suppressed as the flow rate increases, due to viscous instabilities. We use scaling analysis to derive two dimensionless numbers that predict the mode of displacement. By elucidating the underlying mechanisms, we explain classical yet intriguing experimental observations. These insights could be used to improve technologies such as hydraulic fracturing, ${\mathrm{CO}}_2$ geosequestration, and microfluidics.

We report the detailed electronic structure of ${\mathrm{WTe}}_2$ by high resolution angle-resolved photoemission spectroscopy. We resolved a rather complicated Fermi surface of ${\mathrm{WTe}}_2$. Specifically, there are in total nine Fermi pockets, including one hole pocket at the Brillouin zone center $\mathrm{\Gamma }$, and two hole pockets and two electron pockets on each side of $\mathrm{\Gamma }$ along the $\mathrm{\Gamma }\text{−}X$ direction. Remarkably, we have observed circular dichroism in our photoemission spectra, which suggests that the orbital angular momentum exhibits a rich texture at various sections of the Fermi surface. This is further confirmed by our density-functional-theory calculations, where the spin texture is qualitatively reproduced as the conjugate consequence of spin-orbital coupling. Since the spin texture would forbid backscatterings that are directly involved in the resistivity, our data suggest that the spin-orbit coupling and the related spin and orbital angular momentum textures may play an important role in the anomalously large magnetoresistance of ${\mathrm{WTe}}_2$. Furthermore, the large differences among spin textures calculated for magnetic fields along the in-plane and out-of-plane directions also provide a natural explanation of the large field-direction dependence on the magnetoresistance.

We use ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES), temperature- and field-dependent resistivity, and thermoelectric power (TEP) measurements to study the electronic properties of ${\mathrm{WTe}}_2$, a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the $X\text{−}\mathrm{\Gamma }\text{−}X$ direction. Using detailed ARPES temperature scans, we find a rare example of a temperature-induced Lifshitz transition at $T\simeq 160\mathrm{K}$, associated with the complete disappearance of the hole pockets. Our electronic structure calculations show a clear and substantial shift of the chemical potential $\mu (T)$ due to the semimetal nature of this material driven by modest changes in temperature. This change of Fermi surface topology is also corroborated by the temperature dependence of the TEP that shows a change of slope at $T\approx 175\mathrm{K}$ and a breakdown of Kohler’s rule in the 70–140 K range. Our results and the mechanisms driving the Lifshitz transition and transport anomalies are relevant to other systems, such as pnictides, 3D Dirac semimetals, and Weyl semimetals.

Concurrent remote entanglement of distant, noninteracting quantum entities is a crucial function for quantum information processing. In contrast with the existing protocols which employ the addition of signals to generate entanglement between two remote qubits, the continuous variable protocol we present is based on the multiplication of signals. This protocol can be straightforwardly implemented by a novel Josephson junction mixing circuit. Our scheme would be able to generate provable entanglement even in the presence of practical imperfections: finite quantum efficiency of detectors and undesired photon loss in current state-of-the-art devices.

Theoretical analysis and fully atomistic molecular dynamics simulations reveal a Brownian ratchet mechanism by which thermal fluctuations drive the net displacement of immiscible liquids confined in channels or pores with micro- or nanoscale dimensions. The thermally driven displacement is induced by surface nanostructures with directional asymmetry and can occur against the direction of action of wetting or capillary forces. Mean displacement rates in molecular dynamics simulations are predicted via analytical solution of a Smoluchowski diffusion equation for the position probability density. The proposed physical mechanisms and derived analytical expressions can be applied to engineer surface nanostructures for controlling the dynamics of diverse wetting processes such as capillary filling, wicking, and imbibition in micro- or nanoscale systems.

We show that by modifying the setup of the recent experiment that creates a “Dirac string” one can engineer a quasi-2D spinor Bose-Einstein condensate on a cylindrical surface, with a synthetic magnetic field normal to the surface. Because of the muticonnectivity of the surface, there are two types of vortices (called $A$ and $B$) with the same vorticity. This is very different from the planar case, which only has one kind of vortex for fixed circulation. As the strength of the synthetic gauge field increases, the ground states will form a necklace of alternating $AB$ vortices surrounding the lateral midpoint of the cylinder, and will split into two $A$ and $B$ necklaces at higher synthetic gauge fields. The fact that even the basic vortex structure of a Bose-Einstein condensate is altered in a cylindrical surface implies that richer phenomena are in store for quantum gases in other curved surfaces.

Raman scattering experiments on stoichiometric, Mott-insulating ${\mathrm{LaTiO}}_3$ over a wide range of excitation energies reveal a broad electronic continuum which is featureless in the paramagnetic state, but develops a gap of $\sim 800{\mathrm{cm}}^{-1}$ upon cooling below the Néel temperature $T_N=146\mathrm{K}$. In the antiferromagnetic state, the spectral weight below the gap is transferred to well-defined spectral features due to spin and orbital excitations. Low-energy phonons exhibit pronounced Fano anomalies indicative of strong interaction with the electron system for $T>T_N$, but become sharp and symmetric for $T<T_N$. The electronic continuum and the marked renormalization of the phonon lifetime by the onset of magnetic order are highly unusual for Mott insulators and indicate liquidlike correlations between spins and orbitals.

${\mathrm{SmB}}_6$ was recently proposed to be both a strong topological insulator and a topological crystalline insulator. For this and related cubic topological Kondo insulators, we prove the existence of four different topological phases, distinguished by the sign of mirror Chern numbers. We characterize these phases in terms of simple observables, and we provide concrete tight-binding models for each phase. Based on theoretical and experimental results for ${\mathrm{SmB}}_6$ we conclude that it realizes the phase with ${\mathcal{C}}_{k_z=0}^{+}=+2$, ${\mathcal{C}}_{k_z=\pi }^{+}=+1$, ${\mathcal{C}}_{k_x=k_y}^{+}=-1$, and we propose a corresponding minimal model.

The recent discovery of topological Kondo insulators has triggered renewed interest in the well-known Kondo insulator samarium hexaboride, which is hypothesized to belong to this family. In this Letter, we study the spin texture of the topologically protected surface states in such a topological Kondo insulator. In particular, we derive close relationships between (i) the form of the hybridization matrix at certain high-symmetry points, (ii) the mirror Chern numbers of the system, and (iii) the observable spin texture of the topological surface states. In this way, a robust classification of topological Kondo insulators and their surface-state spin texture is achieved. We underpin our findings with numerical calculations of several simplified and realistic models for systems like samarium hexaboride.

The Goos-Hänchen (GH) shift and the Imbert-Fedorov (IF) shift are optical phenomena which describe the longitudinal and transverse lateral shifts at the reflection interface, respectively. Here, we predict the GH and IF shifts in Weyl semimetals (WSMs)—a promising material harboring low energy Weyl fermions, a massless fermionic cousin of photons. Our results show that the GH shift in WSMs is valley independent, which is analogous to that discovered in a 2D relativistic material—graphene. However, the IF shift has never been explored in nonoptical systems, and here we show that it is valley dependent. Furthermore, we find that the IF shift actually originates from the topological effect of the system. Experimentally, the topological IF shift can be utilized to characterize the Weyl semimetals, design valleytronic devices of high efficiency, and measure the Berry curvature.

We generalize a semiclassical theory and use the argument of angular momentum conservation to examine the ballistic transport in lightly doped Weyl semimetals, taking into account various phase-space Berry curvatures. We predict universal transverse shifts of the wave-packet center in transmission and reflection, perpendicular to the direction in which the Fermi energy or velocities change adiabatically. The anomalous shifts are opposite for electrons with different chirality, and they can be made imbalanced by breaking inversion symmetry. We discuss how to utilize local gates, strain effects, and circularly polarized lights to generate and probe such a chirality-dependent Hall effect.

The electrical response of two diffusive metals is studied when they are linked by a magnetic insulator hosting a topologically stable (superfluid) spin current. We discuss how charge currents in the metals induce a spin supercurrent state, which in turn generates a magnetoresistance that depends on the topology of the electrical circuit. This magnetoresistance relies on phase coherence over the entire magnet and gives direct evidence for spin superfluidity. We show that driving the magnet with an ac current allows coherent spin transport even in the presence of U(1)-breaking magnetic anisotropy that can preclude dc superfluid transport. Spin transmission in the ac regime shows a series of resonance peaks as a function of frequency. The peak locations, heights, and widths can be used to extract static interfacial properties, e.g., the spin-mixing conductance and effective spin Hall angle, and to probe dynamic properties such as the spin-wave dispersion. Thus, ac transport may provide a simpler route to realizing nonequilbrium coherent spin transport and a useful way to characterize the magnetic system, serving as a precursor to the realization of dc superfluid spin transport.

Despite the formal analogy with the Higgs particle, the amplitude fluctuations of the order parameter in weakly coupled superconductors do not identify a real mode with a Lorentz-invariant dynamics. Indeed, its resonance occurs at $2{\mathrm{\Delta }}_0$, which coincides with the threshold $2E_{\mathrm{gap}}$ for quasiparticle excitations that spoil any relativistic dynamics. Here we investigate the fate of the Higgs mode in the unconventional case where $2E_{\mathrm{gap}}$ becomes larger than $2{\mathrm{\Delta }}_0$, as due to strong coupling or strong disorder. We show that also in this situation, the amplitude fluctuations never identify a real mode at $2{\mathrm{\Delta }}_0$, since such a “bosonic” limit is always reached via strong mixing with the phase fluctuations, which dominate the low-energy part of the spectrum. Our results have direct implications for the interpretation of the subgap optical absorption in disordered superconductors.

Double-barrier heterostructures are model systems for the study of electron tunneling and discrete energy levels in a quantum well (QW). Until now resonant tunneling phenomena in metallic QWs have been observed for limited thicknesses (1–2 nm) under which electron phase coherence is conserved. In the present study we show evidence of QW resonance states in Fe QWs up to 12 nm thick and at room temperature in fully epitaxial double ${\mathrm{MgAlO}}_x$ barrier magnetic tunnel junctions. The electron phase coherence displayed in this QW is of unprecedented quality because of a homogenous interface phase shift due to the small lattice mismatch at the $\mathrm{Fe}\text{−}{\mathrm{MgAlO}}_x$ interface. The physical understanding of the critical role of interface strain on QW phase coherence will greatly promote the development of spin-dependent quantum resonant tunneling applications.

Colloidal crystal structures with complexity and diversity rivaling atomic and molecular crystals have been predicted and obtained for hard particles by entropy maximization. However, thus far homochiral colloidal crystals, which are candidates for photonic metamaterials, are absent. Using Monte Carlo simulations we show that chiral polyhedra exhibiting weak directional entropic forces self-assemble either an achiral crystal or a chiral crystal with limited control over the crystal handedness. Building blocks with stronger faceting exhibit higher selectivity and assemble a chiral crystal with handedness uniquely determined by the particle chirality. Tuning the strength of directional entropic forces by means of particle rounding or the use of depletants allows for reconfiguration between achiral and homochiral crystals. We rationalize our findings by quantifying the chirality strength of each particle, both from particle geometry and potential of mean force and torque diagrams.

By using a model based on the second-order time-dependent perturbation theory, we show that the nonsequential two-photon double ionization of He can be understood in a virtual sequential picture: to excite the final double continuum state $|{\mathbf{k}}_1,{\mathbf{k}}_2⟩$ by absorbing two photons from the ground state $|1s^2,{}^1{S}_0⟩$, the single continuum states $|1s,{\mathbf{k}}_1⟩$ and $|1s,{\mathbf{k}}_2⟩$ serve as the dominant intermediate states. This virtual sequential picture is verified by the perfect agreement of the total ionization cross section, respectively, calculated by this model and by the sophisticated numerical solution to the full-dimensional time-dependent Schrödinger equation. This model, without the consideration of the electron correlation in the final double continuum state, works well for a wide range of laser parameters extending from the nonsequential to the sequential regime. The present Letter demonstrates that the electron correlation in the final double continuum state is not important in evaluating the total cross section, while it is indispensable for an accurate computation of a triply differential cross section. In addition, the virtual sequential picture bridges the sequential and nonsequential two-photon double ionization and reveals connections and distinctions between them.