Tiny violations of the Lorentz symmetry of relativity and the associated discrete $CPT$ symmetry could emerge in a consistent theory of quantum gravity such as string theory. Recent evidence for linear polarization in gamma-ray bursts improves existing sensitivities to Lorentz and $CPT$ violation involving photons by factors ranging from ten to a million.

It is generally believed that the spontaneous breaking of the Poincaré group by flux tubes (strings) generates only two zero modes localized on the string and associated with the spontaneous breaking of translational invariance (the so-called Low-Manohar argument). Being perfectly true in many instances this argument is nevertheless nonuniversal and has to be amended in the case of order parameters carrying spatial indices. We show that under certain circumstances additional zero (or quasizero) modes can appear due to spin symmetry.

We observe individual dark-state polaritons as they propagate through an ultracold atomic gas involving Rydberg states coupled via an electromagnetically induced transparency resonance. Strong long-range interactions between Rydberg excitations give rise to a blockade between polaritons, resulting in large optical nonlinearities and modified polariton number statistics. By combining optical imaging and high-fidelity detection of the Rydberg polaritons we investigate both aspects of this coupled atom-light system. We map out the full nonlinear optical response as a function of atomic density and follow the temporal evolution of polaritons through the atomic cloud. In the blockade regime, the statistical fluctuations of the polariton number drop well below the quantum noise limit. The low level of fluctuations indicates that photon correlations modified by the strong interactions have a significant backaction on the Rydberg atom statistics.

Theory predicts a distinct spectral shift between the near- and far-field optical response of plasmonic antennas. Here we combine near-field optical microscopy and far-field spectroscopy of individual infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations corroborate our experimental results. We furthermore discuss the implications of this effect in surface-enhanced infrared spectroscopy.

By applying the optical nanocircuit concepts to metasurfaces, we propose an effective route to locally control light transmission over a deeply subwavelength scale. This concept realizes the optical equivalent of a transmit-array, whose use is demonstrated for light bending and focusing with unprecedented efficiency over a subwavelength distance, with crucial benefits for nano-optics applications. These findings may lead to large improvements in the manipulation of optical transmission and processing of nanoscale optical signals over conformal and Si-compatible substrates.

A lattice of optical ring resonators can exhibit a topological insulator phase, with the role of spin played by the direction of propagation of light within each ring. Unlike the system studied by Hafezi et al., [Nat. Phys. 7, 907 (2011).] topological protection is achieved without fine-tuning the interresonator couplings, which are given the same periodicity as the underlying lattice. The topological insulator phase occurs for strong couplings, when the tight-binding method is inapplicable. Using the transfer matrix method, we derive the band structure and phase diagram, and demonstrate the existence of robust edge states. When gain and loss are introduced, the system functions as a diode for coupled resonator modes.

We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collisionless shock in a laser-driven laboratory experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching particle-in-cell simulation and theoretical considerations, we suggest that this process is analogous to ion reflection at supercritical collisionless shocks in supernova remnants.

Large-wave-vector phonons have an important role in determining the thermal and electronic properties of nanoscale materials. The small volumes of such structures, however, have posed significant challenges to experimental studies of the phonon dispersion. We show that synchrotron x-ray thermal diffuse scattering can be adapted to probe phonons with wave vectors spanning the entire Brillouin zone of nanoscale silicon membranes. The thermal diffuse scattering signal from flat Si nanomembranes with thicknesses from 315 to 6 nm, and a sample volume as small as $5\mu {\mathrm{m}}^3$, has the expected linear dependence on the membrane thickness and also exhibits excess intensity at large wave vectors, consistent with the scattering signature expected from low-lying large-wave-vector modes of the membranes.

Thermodynamic properties of the Hubbard model on the anisotropic triangular lattice at half filling are calculated by the finite-temperature Lanczos method. The charge susceptibility exhibits clear signatures of a Mott metal-insulator transition. The metallic phase is characterized by a small charge susceptibility, large entropy, large renormalized quasiparticle mass, and large spin susceptibility. The fluctuating local magnetic moment in the metallic phase is large and comparable to that in the insulating phase. These bad metallic characteristics occur above a relatively low coherence temperature, as seen in organic charge transfer salts.

Elastic backscattering of electrons moving along the helical edge is prohibited by time-reversal symmetry. We demonstrate, however, that an ensemble of magnetic impurities may cause time-reversal symmetry-preserving quasielastic backscattering, resulting in interference effects in the conductance. The characteristic energy transferred in a backscattering event is suppressed due to the Ruderman-Kittel-Kasuya-Yosida interaction of localized spins (the suppression is exponential in the total number of magnetic impurities). We predict the statistics of conductance fluctuations to differ from those in the conventional case of a one-dimensional system with quenched disorder.

We propose a minimal model describing magnetic behavior of Fe-based superconductors. The key ingredient of the model is a dynamical mixing of quasidegenerate spin states of ${\mathrm{Fe}}^{2+}$ ion by intersite electron hoppings, resulting in an effective local spin $S_{\mathrm{eff}}$. The moments $S_{\mathrm{eff}}$ tend to form singlet pairs and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. The long-range ordered part $m$ of $S_{\mathrm{eff}}$ varies widely, $0\le m\le S_{\mathrm{eff}}$, but magnon spectra are universal and scale with $S_{\mathrm{eff}}$, resolving the puzzle of large but fluctuating Fe moments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.

We use the sixteen-vertex model to describe bidimensional artificial spin ice. We find excellent agreement between vertex densities in 15 differently grown samples and the predictions of the model. Our results demonstrate that the samples are in usual thermal equilibrium away from a critical point separating a disordered and an antiferromagnetic phase in the model. The second-order phase transition that we predict suggests that the spatial arrangement of vertices in near-critical artificial spin ice should be studied in more detail in order to verify whether they show the expected space and time long-range correlations.

There exists compelling experimental evidence in numerous systems for logarithmically slow time evolution, yet its full theoretical understanding remains elusive. We here introduce and study a generic transition process in complex systems, based on nonrenewal, aging waiting times. Each state $n$ of the system follows a local clock initiated at $t=0$. The random time $\tau$ between clock ticks follows the waiting time density $\psi (\tau )$. Transitions between states occur only at local clock ticks and are hence triggered by the local forward waiting time, rather than by $\psi (\tau )$. For power-law forms $\psi (\tau )\simeq {\tau }^{-1-\alpha }$ ($0<\alpha <1$) we obtain a logarithmic time evolution of the state number $⟨n(t)⟩\simeq \log (t/t_0)$, while for $\alpha >2$ the process becomes normal in the sense that $⟨n(t)⟩\simeq t$. In the intermediate range $1<\alpha <2$ we find the power-law growth $⟨n(t)⟩\simeq t^{\alpha -1}$. Our model provides a universal description for transition dynamics between aging and nonaging states.

A common goal in the control of a large network is to minimize the number of driver nodes or control inputs. Yet, the physical determination of control signals and the properties of the resulting control trajectories remain widely underexplored. Here we show that (i) numerical control fails in practice even for linear systems if the controllability Gramian is ill conditioned, which occurs frequently even when existing controllability criteria are satisfied unambiguously, (ii) the control trajectories are generally nonlocal in the phase space, and their lengths are strongly anti-correlated with the numerical success rate and number of control inputs, and (iii) numerical success rate increases abruptly from zero to nearly one as the number of control inputs is increased, a transformation we term numerical controllability transition. This reveals a trade-off between nonlocality of the control trajectory in the phase space and nonlocality of the control inputs in the network itself. The failure of numerical control cannot be overcome in general by merely increasing numerical precision—successful control requires instead increasing the number of control inputs beyond the numerical controllability transition.

A search for Lorentz violation in electrodynamics was performed by measuring the resonant frequency difference between two counterpropagating directions of an optical ring cavity. Our cavity contains a dielectric element, which makes our cavity sensitive to the violation. The laser frequency is stabilized to the counterclockwise resonance of the cavity, and the transmitted light is reflected back into the cavity for resonant frequency comparison with the clockwise resonance. This double-pass configuration enables a null experiment and gives high common mode rejection of environmental disturbances. We found no evidence for odd-parity anisotropy at the level of $\delta c/c\lesssim {10}^{-14}$. Within the framework of the standard model extension, our result put more than 5 times better limits on three odd-parity parameters ${\tilde{\kappa }}_{o+}^{JK}$ and a 12 times better limit on the scalar parameter ${\tilde{\kappa }}_{\mathrm{tr}}$ compared with the previous best limits.

An asymptotic scaling theory is presented using the conceptual basis of trapping-free subspace (i.e., orthogonal subspace) to establish the generic mechanism of optimal efficiency of excitation energy transfer in light-harvesting systems. A quantum state orthogonal to the trap will exhibit noise-assisted transfer, clarifying the significance of initial preparation. For such an initial state, the efficiency is enhanced in the weak damping limit ($⟨t⟩\sim 1/\Gamma$), and suppressed in the strong damping limit ($⟨t⟩\sim \Gamma$), analogous to Kramers turnover in classical rate theory. An interpolating expression $⟨t⟩=A/\Gamma +B+C\Gamma$ quantitatively describes the trapping time over the entire range of the dissipation strength, and predicts the optimal efficiency at ${\Gamma }_{\mathrm{opt}}\sim J$ for homogenous systems. In the presence of static disorder, the scaling law of transfer time with respect to dephasing rate changes from linear to square root, suggesting a weaker dependence on the environment. The prediction of the scaling theory is verified in a symmetric dendrimer system by numerically exact quantum calculations. Though formulated in the context of excitation energy transfer, the analysis and conclusions apply in general to open quantum processes, including electron transfer, fluorescence emission, and heat conduction.

The analysis of a combined data set, totaling $3.6\times {10}^{14}$ stopped muons on target, in the search for the lepton flavor violating decay ${\mu }^{+}\to e^{+}\gamma$ is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and yield a new upper limit on the branching ratio of this decay of $5.7\times {10}^{-13}$ (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.

We report entanglement of a single atom’s hyperfine spin state with its motional state in a time scale of less than 3 ns. We engineer a short train of intense laser pulses to impart a spin-dependent momentum transfer of $\pm 2\hslash k$. Using pairs of momentum kicks, we create an atomic interferometer and demonstrate collapse and revival of spin coherence as the motional wave packet is split and recombined. The revival after a pair of kicks occurs only when the second kick is delayed by an integer multiple of the harmonic trap period, a signature of entanglement and disentanglement of the spin with the motion. Such quantum control opens a new regime of ultrafast entanglement in atomic qubits.

The idea of atoms defying ionization in ultrastrong laser fields has fascinated physicists for the last three decades. In contrast to extensive theoretical work on atoms stabilized in strong fields only few experiments limited to intermediate intensities have been performed. In this work we show exceptional stability of Rydberg atoms in strong laser fields extending the range of observation to much higher intensities. Corresponding field amplitudes of more than $1\mathrm{GV}/\mathrm{cm}$ exceed the thresholds for static field ionization by more than 6 orders of magnitude. Most importantly, however, is our finding that a surviving atom is tagged with a measure of the laser intensity it has interacted with. Reading out this information removes uncertainty about whether the surviving atom has really seen the high intensity. The experimental results allow for an extension of the investigations on the stabilization and interaction of a quasifree electron with a strong field into the relativistic regime.

Two-dimensional electron energy-loss spectra of acrylonitrile are presented in the incident energy range 0.095–0.9 eV, where the incoming electron is temporarily captured in the lowest ${\pi }^{*}$ orbital. The cross section is plotted as a function of the incident electron energy that determines which vibrational resonant anion state is populated, and of the electron energy loss, that shows into which final vibrational states each resonance decays. The striking result is that besides the expected horizontal (for given resonances) and vertical (for given final states) patterns of peaks, diagonal arrangements of peaks are observed. They reveal restrictive selectivity with respect to which vibrations are deexcited in the detachment process, with ${\nu }_4$ ($\mathrm{C}\mathrm{N}$ stretch) and ${\nu }_7$ ($\mathrm{C}\mathrm{H}$ rocking) dominating. Surprisingly, all spectral features could be assigned to vibrational levels of the valence ${\pi }^{*}$ resonance, and none to dipole-bound vibrational Feshbach resonance, although the latter must also exist in this energy range.