We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian, and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can construct explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. New observables living in the bulk appear at the quantum level (analogous to spin in quantum mechanics) that are not present at the classical level and are associated with the discrete nature of the spin network states of loop quantum gravity. The resulting quantum space-times resolve the singularity present in the classical theory inside black holes.

We point out that current constraints on dark matter imply only that the majority of dark matter is cold and collisionless. A subdominant fraction of dark matter could have much stronger interactions. In particular, it could interact in a manner that dissipates energy, thereby cooling into a rotationally supported disk, much as baryons do. We call this proposed new dark matter component double-disk dark matter (DDDM). We argue that DDDM could constitute a fraction of all matter roughly as large as the fraction in baryons, and that it could be detected through its gravitational effects on the motion of stars in galaxies, for example. Furthermore, if DDDM can annihilate to gamma rays, it would give rise to an indirect detection signal distributed across the sky that differs dramatically from that predicted for ordinary dark matter. DDDM and more general partially interacting dark matter scenarios provide a large unexplored space of testable new physics ideas.

Quasiparticle dynamics on the topological surface state of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, and superconducting ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ are studied by 7 eV laser-based angle resolved photoemission spectroscopy. We find strong mode couplings in the Dirac-cone surface states at energies of $\sim 3$ and $\sim 15–20\mathrm{meV}$ associated with an exceptionally large coupling constant $\lambda$ of $\sim 3$, which is one of the strongest ever reported for any material. This result is compatible with the recent observation of a strong Kohn anomaly in the surface phonon dispersion of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, but it appears that the theoretically proposed “spin-plasmon” excitations realized in helical metals are also playing an important role. Intriguingly, the $\sim 3\mathrm{meV}$ mode coupling is found to be enhanced in the superconducting state of ${\mathrm{Cu}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

We utilize the fluid-gravity duality to investigate the large order behavior of hydrodynamic gradient expansion of the dynamics of a gauge theory plasma system. This corresponds to the inclusion of dissipative terms and transport coefficients of very high order. Using the dual gravity description, we calculate numerically the form of the stress tensor for a boost-invariant flow in a hydrodynamic expansion up to terms with 240 derivatives. We observe a factorial growth of gradient contributions at large orders, which indicates a zero radius of convergence of the hydrodynamic series. Furthermore, we identify the leading singularity in the Borel transform of the hydrodynamic energy density with the lowest nonhydrodynamic excitation corresponding to a ‘nonhydrodynamic’ quasinormal mode on the gravity side.

We use electromagnetically induced transparency (EIT) to probe the narrow electron-spin resonance of nitrogen-vacancy centers in diamond. Working with a multipass diamond chip at temperatures 6–30 K, the zero-phonon absorption line (637 nm) exhibits an optical depth of 6 and inhomogeneous linewidth of $\sim 30\mathrm{GHz}$ FWHM. Simultaneous optical excitation at two frequencies separated by the ground-state zero-field splitting (2.88 GHz) reveals EIT resonances with a contrast exceeding 6% and FWHM down to 0.4 MHz. The resonances provide an all-optical probe of external electric and magnetic fields with a projected photon-shot-noise-limited sensitivity of $0.2\mathrm{V}/\mathrm{cm}/\sqrt{\mathrm{Hz}}$ and $0.1\mathrm{nT}/\sqrt{\mathrm{Hz}}$, respectively. Operation of a prototype diamond-EIT magnetometer measures a noise floor of $\lesssim 1\mathrm{nT}/\sqrt{\mathrm{Hz}}$ for frequencies above 10 Hz and Allan deviation of $1.3\pm 1.1\mathrm{nT}$ for 100 s intervals. The results demonstrate the potential of diamond-EIT devices for applications ranging from quantum-optical memory to precision measurement and tests of fundamental physics.

From the analysis of a set of airborne images of ship wakes, we show that the wake angles decrease as $U^{-1}$ at large velocities, in a way similar to the Mach cone for supersonic airplanes. This previously unnoticed Mach-like regime is in contradiction with the celebrated Kelvin prediction of a constant angle of 19.47° independent of the ship’s speed. We propose here a model, confirmed by numerical simulations, in which the finite size of the disturbance explains this transition between the Kelvin and Mach regimes at a Froude number $\mathrm{Fr}=U/\sqrt{gL}\simeq 0.5$, where $L$ is the hull ship length.

We study the influence of electron puddles created by doping of a 2D topological insulator on its helical edge conductance. A single puddle is modeled by a quantum dot tunnel coupled to the helical edge. It may lead to significant inelastic backscattering within the edge because of the long electron dwelling time in the dot. We find the resulting correction to the perfect edge conductance. Generalizing to multiple puddles, we assess the dependence of the helical edge resistance on the temperature and doping level and compare it with recent experimental data.

To describe the microscopic properties of matter, quantum mechanics uses wave functions, whose structure and time dependence is governed by the Schrödinger equation. In atoms the charge distributions described by the wave function are rarely observed. The hydrogen atom is unique, since it only has one electron and, in a dc electric field, the Stark Hamiltonian is exactly separable in terms of parabolic coordinates ($\eta$, $\xi$, $\phi$). As a result, the microscopic wave function along the $\xi$ coordinate that exists in the vicinity of the atom, and the projection of the continuum wave function measured at a macroscopic distance, share the same nodal structure. In this Letter, we report photoionization microscopy experiments where this nodal structure is directly observed. The experiments provide a validation of theoretical predictions that have been made over the last three decades.

We obtain an analytical bound on the efficiency of wireless power transfer to a weakly coupled device. The optimal source is solved for a multilayer geometry in terms of a representation based on the field equivalence principle. The theory reveals that optimal power transfer exploits the properties of the midfield to achieve efficiencies far greater than conventional coil-based designs. As a physical realization of the source, we present a slot array structure whose performance closely approaches the theoretical bound.

We have observed the Bose-Einstein condensation of an atomic gas in the (quasi)uniform three-dimensional potential of an optical box trap. Condensation is seen in the bimodal momentum distribution and the anisotropic time-of-flight expansion of the condensate. The critical temperature agrees with the theoretical prediction for a uniform Bose gas. The momentum distribution of a noncondensed quantum-degenerate gas is also clearly distinct from the conventional case of a harmonically trapped sample and close to the expected distribution in a uniform system. We confirm the coherence of our condensate in a matter-wave interference experiment. Our experiments open many new possibilities for fundamental studies of many-body physics.

We demonstrate orbital angular momentum (OAM) transfer by chiral plasmonic nanostructures designed on both sides of a thin suspended metallic membrane. We show how far-field vortex beams with tunable OAM indices can be tailored through nanostructure designs. We reveal the crucial role played by the central aperture that connects the two sides of the membrane from which OAM selection rules are derived in perfect agreement with experimental data.

The vanadium oxyfluoride $[{\mathrm{NH}}_4{]}_2[{\mathrm{C}}_7{\mathrm{H}}_{14}\mathrm{N}][{\mathrm{V}}_7{\mathrm{O}}_6{\mathrm{F}}_{18}]$ (DQVOF) is a geometrically frustrated magnetic bilayer material. The structure consists of $S=1/2$ kagome planes of ${\mathrm{V}}^{4+}$ $d^1$ ions with $S=1$ ${\mathrm{V}}^{3+}$ $d^2$ ions located between the kagome layers. Muon spin relaxation measurements demonstrate the absence of spin freezing down to 40 mK despite an energy scale of 60 K for antiferromagnetic exchange interactions. From magnetization and heat capacity measurements we conclude that the $S=1$ spins of the interplane ${\mathrm{V}}^{3+}$ ions are weakly coupled to the kagome layers, such that DQVOF can be viewed as an experimental model for $S=1/2$ kagome physics, and that it displays a gapless spin liquid ground state.

A virial expansion of fluid pressure in powers of the density can be used to calculate a wealth of thermodynamic information, but the $N$th virial coefficient, which multiplies the $N$th power of the density in the expansion, becomes rapidly more complicated with increasing $N$. This Letter shows that the $N$th virial coefficient can be calculated using a method that scales exponentially with $N$ in computer time and memory. This is orders of magnitude more efficient than any existing method for large $N$, and the method is simple and general. New results are presented for $N=11$ and 12 for hard spheres, and $N=9$ and 10 for soft spheres.

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.

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.

We experimentally and numerically investigate the expansion of initially localized ultracold bosons in homogeneous one- and two-dimensional optical lattices. We find that both dimensionality and interaction strength crucially influence these nonequilibrium dynamics. While the atoms expand ballistically in all integrable limits, deviations from these limits dramatically suppress the expansion and lead to the appearance of almost bimodal cloud shapes, indicating diffusive dynamics in the center surrounded by ballistic wings. For strongly interacting bosons, we observe a dimensional crossover of the dynamics from ballistic in the one-dimensional hard-core case to diffusive in two dimensions, as well as a similar crossover when higher occupancies are introduced into the system.

We report anisotropic magnetoresistance in $\mathrm{Pt}|{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ bilayers. In spite of ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ being a very good electrical insulator, the resistance of the Pt layer reflects its magnetization direction. The effect persists even when a Cu layer is inserted between Pt and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, excluding the contribution of induced equilibrium magnetization at the interface. Instead, we show that the effect originates from concerted actions of the direct and inverse spin Hall effects and therefore call it “spin Hall magnetoresistance.”

The composite fermion formalism elegantly describes some of the most fascinating behaviors of interacting two-dimensional carriers at low temperatures and in strong perpendicular magnetic fields. In this framework, carriers minimize their energy by attaching two flux quanta and forming new quasiparticles, the so-called composite fermions. Thanks to the flux attachment, when a Landau level is half-filled, the composite fermions feel a vanishing effective magnetic field and possess a Fermi surface with a well-defined Fermi contour. Our measurements in a high-quality two-dimensional hole system confined to a GaAs quantum well demonstrate that a parallel magnetic field can significantly distort the hole-flux composite fermion Fermi contour.

Surface plasmons in metal hole arrays have been studied extensively in the context of extraordinary optical transmission, but so far these arrays have not been studied as resonators for surface plasmon lasing at optical frequencies. We experimentally study a metal hole array with a semiconductor (InGaAs) gain layer placed in close (20 nm) proximity of the metal hole array. As a function of increasing pump power, we observe an intense and spectrally narrow peak, with a clear threshold. This laser emission is donut shaped and radially polarized. Three experimental observations support that the system shows surface plasmon lasing. First, the full wavelength dispersion of the observed resonances can be understood by using a single surface plasmon mode of the system. Second, the polarization of these resonances is as expected for surface plasmons. Third, the magnitude of the avoided crossing, which results from mode coupling at the holes, has a similar magnitude as found in simulations using surface plasmons.

The energy released in a magnetic material by reversing spins as they relax toward equilibrium can lead to a dynamical instability that ignites self-sustained rapid relaxation along a deflagration front that propagates at a constant subsonic speed. Using a trigger heat pulse and transverse and longitudinal magnetic fields, we investigate and control the crossover between thermally driven magnetic relaxation and magnetic deflagration in single crystals of ${\mathrm{Mn}}_{12}$-acetate.