We study Hamiltonians which have Kitaev’s toric code as a ground state, and show how to construct a Hamiltonian which shares the ground space of the toric code, but which has gapless excitations with a continuous spectrum in the thermodynamic limit. Our construction is based on the framework of projected entangled pair states, and can be applied to a large class of two-dimensional systems to obtain gapless “uncle Hamiltonians.”

Distributed-phase-reference quantum key distribution stands out for its easy implementation with present day technology. For many years, a full security proof of these schemes in a realistic setting has been elusive. We solve this long-standing problem and present a generic method to prove the security of such protocols against general attacks. To illustrate our result, we provide lower bounds on the key generation rate of a variant of the coherent-one-way quantum key distribution protocol. In contrast to standard predictions, it appears to scale quadratically with the system transmittance.

A renormalization group approach is used to show that a one-dimensional system of bosons subject to a lattice quench exhibits a finite-time dynamical phase transition where an order parameter within a light cone increases as a nonanalytic function of time after a critical time. Such a transition is also found for a simultaneous lattice and interaction quench where the effective scaling dimension of the lattice becomes time dependent, crucially affecting the time evolution of the system. Explicit results are presented for the time evolution of the boson interaction parameter and the order parameter for the dynamical transition as well as for more general quenches.

The evolution of an initially smooth spatial inhomogeneity in the density of a one-dimensional Fermi gas is well described by classical mechanics. The classical evolution leads to the formation of a shock wave: the density develops kinks in its coordinate dependence. We show that quantum corrections to the shock wave produce density ripples which run off the kinks. Despite their quantum origin, the amplitude and period of the ripples are expressed only in terms of classical objects derived from a smooth density profile.

A high energy $ep$ collider, such as the proposed LHeC, possesses the unique facility of permitting direct measurement of the $HWW$ coupling without contamination from the $HZZ$ coupling. At such a machine, the fusion of two $W$ bosons through the $HWW$ vertex would give rise to typical charged current events accompanied by a Higgs boson. We demonstrate that azimuthal angle correlations between the observable charged current final states could then be a sensitive probe of the nature of the $HWW$ vertex and hence of the $CP$ properties of the Higgs boson.

The density dependence of nuclear symmetry energy remains poorly constrained. Starting from precise empirical values of the nuclear volume and surface symmetry energy coefficients and the nuclear saturation density, we show how in the ambit of microscopic calculations with different energy density functionals, the value of the symmetry energy slope parameter $L$ along with that for neutron skin can be put in tighter bounds. The value of $L$ is found to be $L=64\pm 5\mathrm{MeV}$. For ${}^{208}\mathrm{Pb}$, the neutron skin thickness comes out to be $0.188\pm 0.014\mathrm{fm}$. Knowing $L$, the method can be applied to predict neutron skin thicknesses of other nuclei.

The first measurement of the elastic scattering of the halo nucleus ${}^{11}\mathrm{Li}$ and its core ${}^9\mathrm{Li}$ on ${}^{208}\mathrm{Pb}$ at energies near the Coulomb barrier is presented. The ${}^{11}\mathrm{Li}+{}^{208}\mathrm{Pb}$ elastic scattering shows a strong reduction with respect to the Rutherford cross section, even at energies well below the barrier and down to very small scattering angles. This drastic change of the elastic differential cross section observed in ${}^{11}\mathrm{Li}+{}^{208}\mathrm{Pb}$ is the consequence of the halo structure of ${}^{11}\mathrm{Li}$, as it is not observed in the elastic scattering of its core ${}^9\mathrm{Li}$ at the same energies. Four-body continuum-discretized coupled-channels calculations, based on a three-body model of the ${}^{11}\mathrm{Li}$ projectile, are found to explain the measured angular distributions and confirm that the observed reduction is mainly due to the strong Coulomb coupling to the dipole states in the low-lying continuum of ${}^{11}\mathrm{Li}$. These calculations suggest the presence of a low-lying dipole resonance in ${}^{11}\mathrm{Li}$ close to the breakup threshold.

We investigate molecular dynamics of multiple ionization in ${\mathrm{N}}_2$ through multiple core-level photoabsorption and subsequent Auger decay processes induced by intense, short x-ray free electron laser pulses. The timing dynamics of the photoabsorption and dissociation processes is mapped onto the kinetic energy of the fragments. Measurements of the latter allow us to map out the average internuclear separation for every molecular photoionization sequence step and obtain the average time interval between the photoabsorption events. Using multiphoton ionization as a tool of the multiple-pulse pump-probe scheme, we demonstrate the modification of the ionization dynamics as we vary the x-ray laser pulse duration.

The high-resolution sub-Doppler Lamb dips of the ${\nu }_2$ fundamental band transitions of ${\mathrm{H}}_3^{+}$ have been observed using an extended negative glow discharge tube as an ion source and a periodically poled lithium niobate optical parametric oscillator as a radiation source. The absolute frequency of the R(1,0) transition was measured to be 81720371.550 MHz with an accuracy of 250 kHz using an optical frequency comb. In addition, we have investigated the linewidth of the Lamb-dip signal of the R(3,0) transition systematically and obtained its pressure-broadening parameter, which may shed some light on the reaction of ${\mathrm{H}}_3^{+}$ with ${\mathrm{H}}_2$. This is the first observation of the infrared saturated spectrum and the first determination of the pressure-broadening parameter of the ro-vibrational transitions of a molecular ion.

We scan the collision energy of two clouds of cesium atoms between 12 and $50\mu \mathrm{K}$ in an atomic fountain clock. By directly detecting the difference of $s$-wave scattering phase shifts, we observe a rapid variation of a scattering phase shift through a series of Feshbach resonances. At the energies we use, resonances that overlap at threshold become resolved. Our statistical phase uncertainty of 8 mrad can be improved in future precision measurements of Feshbach resonances to accurately determine the Cs-Cs interactions, which may provide stringent limits on the time variation of fundamental constants.

Optical frequency combs based on mode-locked lasers have proven to be invaluable tools for a wide range of applications in precision spectroscopy and metrology. A novel principle of optical frequency comb generation in whispering-gallery mode microresonators (“microcombs”) has been developed recently, which represents a promising route towards chip-level integration and out-of-the-lab use of these devices. Presently, two families of microcombs have been demonstrated: Combs with electronically detectable mode spacing that can be directly stabilized, and broadband combs with up to octave-spanning spectra but mode spacings beyond electronic detection limits. However, it has not yet been possible to achieve these two key requirements simultaneously, as will be critical for most microcomb applications. Here we present a route to overcome this problem by interleaving an electro-optic comb with the spectrum from a parametric microcomb. This allows, for the first time, direct control and stabilization of a microcomb spectrum with large mode spacing ($>140\mathrm{GHz}$) with no need for an additional mode-locked laser frequency comb. The attained residual 1-sec instability of the microcomb comb spacing is ${10}^{-15}$, with a microwave reference limited absolute instability of ${10}^{-12}$ at a 140 GHz mode spacing.

We present a new interferometer technique whereby multiple extreme ultraviolet light pulses are generated at different positions within a single laser focus (i.e., from successive sources) with a highly controllable time delay. The interferometer technique is tested with two generating media to create two extreme ultraviolet light pulses with a time delay between them. The delay is found to be a consequence of the Gouy phase shift. Ultimately the apparatus is capable of accessing unprecedented time scales by allowing stable and repeatable delays as small as 100 zs.

At impact of a liquid drop on a solid surface, an air bubble can be entrapped. Here, we show that two competing effects minimize the (relative) size of this entrained air bubble: for large drop impact velocity and large droplets, the inertia of the liquid flattens the entrained bubble, whereas for small impact velocity and small droplets, capillary forces minimize the entrained bubble. However, we demonstrate experimentally, theoretically, and numerically that in between there is an optimum, leading to maximal air bubble entrapment. For a 1.8 mm diameter ethanol droplet, this optimum is achieved at an impact velocity of $0.25\mathrm{m}/\mathrm{s}$. Our results have a strong bearing on various applications in printing technology, microelectronics, immersion lithography, diagnostics, or agriculture.

We report the chain conformations of polymer molecules accommodated at the solid-polymer melt interfaces in equilibrium. Polystyrene “Guiselin” brushes (adsorbed layers) with different molecular weights were prepared on Si substrates and characterized by using x-ray and neutron reflectivity. The results are intriguing to show that the adsorbed layers are composed of the two different nanoarchitectures: flattened chains that constitute the inner higher density region of the adsorbed layers and loosely adsorbed polymer chains that form the outer bulklike density region. In addition, we found that the lone flattened chains, which are uncovered by the additional prolonged solvent leaching ($\sim 120$ days), are reversibly densified with increasing temperature up to $150°\mathrm{C}$. By generalizing the chain conformations of bulks, we postulate that the change in probabilities of the local chain conformations (i.e., trans and gauche states) of polymer molecules is the origin of this densification process.

We present a systematic ab initio density functional theory–based study which demonstrates that even one of the simplest defects in single-wall carbon nanotubes, the reconstructed monovacancy (a pentagonal ring and a single dangling bond known as a 5-1db defect), leads to extraordinarily long-ranged structural distortions. We show that relaxation due to reconstruction can only be modeled accurately through a careful selection of boundary conditions and an appropriately long nanotube fragment.

Pressure exerted by crystallization of salts within porous materials contributes to damage in historic and modern construction. By unequivocally identifying the precipitating phase(s) while simultaneously determining solution supersaturation and associated crystallization pressure in subsurface pores, we show that the formation of a thermodynamically metastable salt phase (heptahydrate; ${\mathrm{Na}}_2{\mathrm{SO}}_4·7{\mathrm{H}}_2\mathrm{O}$) and the resulting transition to a less soluble stable phase (mirabilite; ${\mathrm{Na}}_2{\mathrm{SO}}_4·10{\mathrm{H}}_2\mathrm{O}$) is largely responsible for the high supersaturation and crystallization pressure developed during evaporative crystallization of sodium sulfate, the most damaging salt known. These results help to explain why salts with various (stable and metastable) hydrated phases are the most damaging. We also show that damage associated with metastable-stable phase transitions can be suppressed by the use of crystallization promoters. These results open new ways for the prevention of salt damage to building materials.

We report two-dimensional discrete dislocation dynamics simulations of combined dislocation glide and climb leading to “power-law” creep in a model aluminum crystal. The approach fully accounts for matter transport due to vacancy diffusion and its coupling with dislocation motion. The existence of quasiequilibrium or jammed states under the applied creep stresses enables observations of diffusion and climb over time scales relevant to power-law creep. The predictions for the creep rates and stress exponents fall within experimental ranges, indicating that the underlying physics is well captured.

A simulation scheme for performing first-principles molecular dynamics at a constant electrode potential is presented, opening the way for a more realistic modeling of voltage-driven devices. The system is allowed to exchange electrons with a reservoir at fixed potential, and dynamical equations for the total electronic charge are derived by using the potential energy of the extended system. In combination with a thermostat, this potentiostat scheme reproduces thermal fluctuations of the charge with the correct statistics, implying a realistic treatment of the potential as a control variable. Practically, the dynamics of the charge are decoupled from the electronic structure calculations, making the scheme easily implementable in existing first-principles molecular dynamics codes. Our approach is demonstrated on a test system by considering various test cases.

We adopt fringe counting from classical moiré interferometry on moiré patterns observed in scanning tunneling microscopy of strained thin films on single crystalline substrates. We analyze inhomogeneous strain distribution in islands of ${\mathrm{CeO}}_2(111)$ on Cu(111) and identify a generic source of strain in heteroepitaxy—a thickness-dependent lattice constant of the growing film. This observation is mediated by the ability of ceria to glide on the Cu substrate. The moiré technique we are describing has a potential of nanometer-scale resolution of inhomogeneous two dimensional strain in incommensurate layered systems, notably in supported graphene.

Scanning tunneling microscopy results reveal a pronounced site selectivity in the hole-mediated photooxidation of trimethyl acetate (TMA) on ${\mathrm{TiO}}_2(110)$, wherein the reaction readily occurs at regular Ti sites but is completely inhibited at oxygen vacancy ($V_{\mathrm{O}}$) defects. Utilizing electron energy loss spectroscopy and density functional theory, we show that the lack of reactivity of TMA groups adsorbed at $V_{\mathrm{O}}$’s cannot be attributed to either a less active adsorption conformation or electron transfer from the $V_{\mathrm{O}}$ defect. Instead, we propose that the excess unpaired electrons associated with the $V_{\mathrm{O}}$ promptly recombine with photoexcited holes approaching the surface, effectively “screening” TMA species at the $V_{\mathrm{O}}$ site. We also show that this screening effect is predominately localized at the $V_{\mathrm{O}}$, only mildly affecting TMA’s at adjacent Ti sites.