Multiplexed quantum memories capable of storing and processing entangled photons are essential for the development of quantum networks. In this context, we demonstrate and certify the simultaneous storage and retrieval of two entangled photons inside a solid-state quantum memory and measure a temporal multimode capacity of ten modes. This is achieved by producing two polarization-entangled pairs from parametric down-conversion and mapping one photon of each pair onto a rare-earth-ion-doped (REID) crystal using the atomic frequency comb (AFC) protocol. We develop a concept of indirect entanglement witnesses, which can be used as Schmidt number witnesses, and we use it to experimentally certify the presence of more than one entangled pair retrieved from the quantum memory. Our work puts forward REID-AFC as a platform compatible with temporal multiplexing of several entangled photon pairs along with a new entanglement certification method, useful for the characterization of multiplexed quantum memories.

Lorentz symmetry violations can be parametrized by an effective field theory framework that contains both general relativity and the standard model of particle physics called the standard-model extension (SME). We present new constraints on pure gravity SME coefficients obtained by analyzing lunar laser ranging (LLR) observations. We use a new numerical lunar ephemeris computed in the SME framework and we perform a LLR data analysis using a set of 20 721 normal points covering the period of August, 1969 to December, 2013. We emphasize that linear combination of SME coefficients to which LLR data are sensitive and not the same as those fitted in previous postfit residuals analysis using LLR observations and based on theoretical grounds. We found no evidence for Lorentz violation at the level of ${10}^{-8}$ for ${\overline{s}}^{TX}$, ${10}^{-12}$ for ${\overline{s}}^{XY}$ and ${\overline{s}}^{XZ}$, ${10}^{-11}$ for ${\overline{s}}^{XX}-{\overline{s}}^{YY}$ and ${\overline{s}}^{XX}+{\overline{s}}^{YY}-2{\overline{s}}^{ZZ}-4.5{\overline{s}}^{YZ}$, and ${10}^{-9}$ for ${\overline{s}}^{TY}+0.43{\overline{s}}^{TZ}$. We improve previous constraints on SME coefficient by a factor up to 5 and 800 compared to postfit residuals analysis of respectively binary pulsars and LLR observations.

The possible exotic meson $Z_c(3900)$, found in $e^{+}{e}^{-}$ reactions, is studied by the method of coupled-channel scattering in lattice QCD. The interactions among $\pi J/\psi$, $\rho {\eta }_c$, and $\overline{D}{D}^{*}$ channels are derived from ($2+1$)-flavor QCD simulations at $m_{\pi }=410–700\mathrm{MeV}$. The interactions are dominated by the off-diagonal $\pi J/\psi \text{−}\overline{D}{D}^{*}$ and $\rho {\eta }_c\text{−}\overline{D}{D}^{*}$ couplings, which indicates that the $Z_c(3900)$ is not a usual resonance but a threshold cusp. Semiphenomenological analyses with the coupled-channel interaction are also presented to confirm this conclusion.

The weakly bound exotic ${}^{11}\mathrm{Be}$ nucleus, famous for its ground-state parity inversion and distinct $n+{}^{10}\mathrm{Be}$ halo structure, is investigated from first principles using chiral two- and three-nucleon forces. An explicit treatment of continuum effects is found to be indispensable. We study the sensitivity of the ${}^{11}\mathrm{Be}$ spectrum to the details of the three-nucleon force and demonstrate that only certain chiral interactions are capable of reproducing the parity inversion. With such interactions, the extremely large $E1$ transition between the bound states is reproduced. We compare our photodisintegration calculations to conflicting experimental data and predict a distinct dip around the $3/2_1^{-}$ resonance energy. Finally, we predict low-lying $3/2^{+}$ and $9/2^{+}$ resonances that are not or not sufficiently measured in experiments.

Photoelectron spectra obtained from the ab initio solution of the time-dependent Schrödinger equation can be in striking disagreement with predictions by the strong-field approximation (SFA), not only at low energy but also around twice the ponderomotive energy where the transition from the direct to the rescattered electrons is expected. In fact, the relative enhancement of the ionization probability compared to the SFA in this regime can be several orders of magnitude. We show for which laser and target parameters such an enhancement occurs and for which the SFA prediction is qualitatively good. The enhancement is analyzed in terms of the Coulomb-corrected action along analytic quantum orbits in the complex-time plane, taking soft recollisions under the Coulomb barrier into account. These recollisions in complex time and space prevent a separation into sub-barrier motion up to the “tunnel exit” and subsequent classical dynamics. Instead, the entire quantum path up to the detector determines the ionization probability.

The photophysics of a deprotonated model chromophore for the green fluorescent protein is studied by femtosecond laser pulses in an electrostatic ion-storage ring. The laser-pulse duration is much shorter than the time for internal conversion, and, hence, contributions from sequential multiphoton absorption, typically encountered with ns-laser pulses, are avoided. Following single-photon excitation, the action-absorption maximum is shown to be shifted within the $S_0$ to $S_1$ band from its origin at about 490 to 450 nm, which is explained by the different photophysics involved in the detected action.

We experimentally study particle exchange in a dissipative double-well potential using laser-cooled atoms in a hybrid trap. We measure the particle hopping rate as a function of barrier height, temperature, and atom number. Single-particle resolution allows us to measure rates over more than 4 orders of magnitude and distinguish the effects of loss and hopping. Deviations from the Arrhenius-law scaling at high barrier heights occur due to cold collisions between atoms within a well. By driving the system periodically, we characterize the phenomenon of stochastic resonance in the system response.

We demonstrate effective control on the carrier-envelope phase and angular distribution as well as the peak intensity of a nearly single-cycle terahertz pulse emitted from a laser filament formed by two-color, the fundamental and the corresponding second harmonics, femtosecond laser pulses propagating in air. Experimentally, such control has been performed by varying the filament length and the initial phase difference between the two-color laser components. A linear-dipole-array model, including the descriptions of both the generation (via laser field ionization) and propagation of the emitted terahertz pulse, is proposed to present a quantitative interpretation of the observations. Our results contribute to the understanding of terahertz generation in a femtosecond laser filament and suggest a practical way to control the electric field of a terahertz pulse for potential applications.

We show that a 2D spin-polarized Fermi gas immersed in a 3D Bose-Einstein condensate constitutes a very promising system for realizing a $p_x+i{p}_y$ superfluid. The fermions attract each other via an induced interaction mediated by the bosons, and the resulting pairing is analyzed with retardation effects fully taken into account. This is further combined with Berezinskii-Kosterlitz-Thouless (BKT) theory to obtain reliable results for the superfluid critical temperature. We show that both the strength and the range of the induced interaction can be tuned experimentally, which can be used to make the critical temperature approach the maximum value allowed by general BKT theory. Moreover, this is achieved while keeping the Fermi-Bose interaction weak so that three-body losses are small. Our results show that realizing a topological superfluid with atomic Fermi-Bose mixtures is within experimental reach.

Discriminating between metallic ($M$) and semiconducting ($S$) single-walled carbon nanotubes (SWNTs) remains a fundamental challenge in the field of nanotechnology. We address this issue by studying the adsorption of the isotropic atoms Xe, Kr, and a highly anisotropic molecule $n$ heptane on $M$- and $S$-SWNTs with density functional theory that includes many-body dispersion forces. We find that the distinct polarizabilities of $M$- and $S$-SWNTs exhibit significantly different physisorption properties, which are also strongly controlled by the SWNT’s diameter, adsorption site, adsorbate coverage, and the adsorbate’s anisotropy. These findings stem from the wavelike nature of charge-density fluctuations in SWNTs. Particularly, these results allow us to rationalize the unusual $\sqrt{3}\times \sqrt{3}R30^0$ phase of Kr atoms on small gap $M$-SWNTs and the double desorption peak temperatures of $n$ heptane on $M$-SWNTs in experiments, and also propose the $n$ heptane as an effective sensor for experimentally discriminating $M$- and $S$-SWNTs.

GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a critical temperature $T_c$ well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above $T_c$. As the in-plane lattice transforms from a rectangle into a square at $T_c$, the electronic, spin, optical, and piezoelectric properties dramatically depart from earlier predictions. Indeed, the $Y$ and $X$ points in the Brillouin zone become effectively equivalent at $T_c$, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at $T_c$. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement is drawn by theoretical predictions of giant piezoelectricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about $3\times {10}^{-12}\mathrm{C}/\mathrm{K}\mathrm{m}$ here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides.

We use a surface-selective angle-resolved photoemission spectroscopy and unveil the electronic nature on the topmost layer of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ crystal, consisting of slightly rotated ${\mathrm{RuO}}_6$ octahedrons. The $\gamma$ band derived from the $4d_{xy}$ orbital is found to be about three times narrower than that for the bulk. This strongly contrasts with a subtle variation seen in the $\alpha$ and $\beta$ bands derived from the one-dimensional $4d_{xz/yz}$. This anomaly is reproduced by the dynamical mean-field theory calculations, introducing not only the on-site Hubbard interaction but also the significant Hund’s coupling. We detect a coherence-to-incoherence crossover theoretically predicted for Hund’s metals, which has been recognized only recently. The crossover temperature in the surface is about half that of the bulk, indicating that the naturally generated monolayer of reconstructed ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ is extremely correlated and well isolated from the underlying crystal.

Using relativistic first-principles calculations, we show that the chemical trend of the Dzyaloshinskii-Moriya interaction (DMI) in $3d\text{−}5d$ ultrathin films follows Hund’s first rule with a tendency similar to their magnetic moments in either the unsupported $3d$ monolayers or $3d\text{−}5d$ interfaces. We demonstrate that, besides the spin-orbit coupling (SOC) effect in inversion asymmetric noncollinear magnetic systems, the driving force is the $3d$ orbital occupations and their spin-flip mixing processes with the spin-orbit active $5d$ states control directly the sign and magnitude of the DMI. The magnetic chirality changes are discussed in the light of the interplay between SOC, Hund’s first rule, and the crystal-field splitting of $d$ orbitals.

We theoretically show that a nanoelectromechanical system can be mechanically actuated by a heat flow through it via an electron-electron interaction. In contrast to most known actuation mechanisms in similar systems, this new mechanism does not involve an electronic current nor external ac fields. Instead, the mechanism relies on deflection-dependent tunneling rates and a heat flow which is mediated by an electron-electron interaction while an electronic current through the device is prohibited by, for instance, a spin-valve effect. Therefore, the system resembles a nanoelectromechanical heat engine. We derive a criterion for the mechanical instability and estimate the amplitude of the resulting self-sustained oscillations. Estimations show that the suggested phenomenon can be studied using available experimental techniques.

In this Letter, both the manipulation of valley-polarized currents and the optical-like behaviors of Dirac fermions are theoretically explored in polycrystalline graphene. When strain is applied, the misorientation between two graphene domains separated by a grain boundary can result in a mismatch of their electronic structures. Such a discrepancy manifests itself in a strong breaking of the inversion symmetry, leading to perfect valley polarization in a wide range of transmission directions. In addition, these graphene domains act as different media for electron waves, offering the possibility to modulate and obtain negative refraction indexes.