We excite spin waves with spatially inhomogeneous Ramsey pulses and study the resulting frequency shifts of a chip-scale atomic clock of trapped ${}^{87}\mathrm{Rb}$. The density-dependent frequency shifts of the hyperfine transition simulate the $s$-wave collisional frequency shifts of fermions, including those of optical lattice clocks. As the spin polarizations oscillate in the trap, the frequency shift reverses and it depends on the area of the second Ramsey pulse, exhibiting a predicted beyond mean-field frequency shift. Numerical and analytic models illustrate these observed behaviors.

A statistical analysis of optimal universal cloning shows that it is possible to identify an ideal (but nonpositive) copying process that faithfully maps all properties of the original Hilbert space onto two separate quantum systems, resulting in perfect correlations for all observables. The joint probabilities for noncommuting measurements on separate clones then correspond to the real parts of the complex joint probabilities observed in weak measurements on a single system, where the measurements on the two clones replace the corresponding sequence of weak measurement and postselection. The imaginary parts of weak measurement statics can be obtained by replacing the cloning process with a partial swap operation. A controlled-swap operation combines both processes, making the complete weak measurement statistics accessible as a well-defined contribution to the joint probabilities of fully resolved projective measurements on the two output systems.

We explicitly exhibit a set of four ququad-ququad orthogonal maximally entangled states that cannot be perfectly distinguished by means of local operations and classical communication. Before our work, it was unknown whether there is a set of $d$ locally indistinguishable $d\otimes d$ orthogonal maximally entangled states for some positive integer $d$. We further show that a $2\otimes 2$ maximally entangled state can be used to locally distinguish this set of states without being consumed, thus demonstrate a novel phenomenon of entanglement discrimination catalysis. Based on this set of states, we construct a new set $K$ consisting of four locally indistinguishable states such that $K^{\otimes m}$ (with $4^m$ members) is locally distinguishable for some $m$ greater than one. As an immediate application, we construct a noisy quantum channel with one sender and two receivers whose local zero-error classical capacity can achieve the full dimension of the input space but only with a multi-shot protocol.

The widely used density matrix renormalization group (DRMG) method often fails to converge in systems with multiple length scales, such as lattice discretizations of continuum models and dilute or weakly doped lattice models. The local optimization employed by DMRG to optimize the wave function is ineffective in updating large-scale features. Here we present a multigrid algorithm that solves these convergence problems by optimizing the wave function at different spatial resolutions. We demonstrate its effectiveness by simulating bosons in continuous space and study nonadiabaticity when ramping up the amplitude of an optical lattice. The algorithm can be generalized to tensor network methods and combined with the contractor renormalization group method to study dilute and weakly doped lattice models.

We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes ${}^6\mathrm{Li}$ and ${}^7\mathrm{Li}$ can be produced, the exact figures varying with the characteristics of the torus and reaching as much as ${10}^{-2}{M}_{\odot }$ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis.

We know very little about primordial curvature perturbations on scales smaller than about a Mpc. Measurements of the $\mu$ distortion of the cosmic microwave background spectrum provide the unique opportunity to probe these scales over the unexplored range from 50 to ${10}^4{\mathrm{Mpc}}^{-1}$. This is a very clean probe, in that it relies only on well understood linear evolution. Also, just the information about the low multipoles ($l\sim 100$) of $\mu$ is necessary. We point out that correlations between $\mu$ distortion and temperature anisotropies can be used to test Gaussianity at these very small scales. In particular the $\mu T$ two-point correlation is proportional to the very squeezed limit of the primordial bispectrum and hence measures $f_{\mathrm{NL}}^{\mathrm{loc}}$, while $\mu \mu$ is proportional to the primordial trispectrum and measures ${\tau }_{\mathrm{NL}}$. We present a Fisher matrix forecast of the observational constraints on $f_{\mathrm{NL}}^{\mathrm{loc}}$ and stress that a cosmic variance limited experiment could in principle reach $\Delta f_{\mathrm{NL}}^{\mathrm{loc}}\sim \mathcal{O}({10}^{-3})$.

We demonstrate that light propagating in an appropriately designed lattice can exhibit dynamics akin to that expected from massless relativistic particles as governed by the one-dimensional Dirac equation. This is accomplished by employing a waveguide array with alternating positive and negative effective coupling coefficients, having a band structure with two intersecting minibands. Through this approach optical analogues of massless particle-antiparticle pairs are experimentally realized. One-dimensional conical diffraction is also observed for the first time in this work.

This study explores how surface morphology affects the dynamics of contact line depinning of an evaporating sessile droplet on micropillared superhydrophobic surfaces. The result shows that neither a liquid-solid contact area nor an apparent contact line is a critical physical parameter to determine the depinning force. The configuration of a contact line on a superhydrophobic surface is multimodal, composed of both two phases (liquid and air) and three phases (liquid, solid, and air). The multimodal state is dynamically altered when a droplet recedes. The maximal three-phase contact line attainable along the actual droplet boundary is found to be a direct and linear parameter that decides the depinning force on the superhydrophobic surface.

We present an effective approach to determine the amount of energy absorbed by solid samples exposed to ultrashort laser pulses, thus, retrieving the maximum temperature attained by the ion lattice in the picosecond time scale. The method is based on the pyrometric detection of a slow temperature fluctuation on the rear side of a sample slab associated with absorption of the laser pulse on the front side. This approach, successfully corroborated by theoretical calculations, can provide a robust and practical diagnostic tool for characterization of laser-generated warm dense matter.

The understanding of metal nucleation on graphene is essential for promising future applications, especially of magnetic metals which can be used in spintronics or computer storage media. A common method to study the grown morphology is to measure the nucleated island density $n$ as a function of growth parameters. Surprisingly, the growth of Fe on graphene is found to be unusual because it does not follow classical nucleation: $n$ is unexpectedtly high, it increases continuously with the deposited amount $\theta$ and shows no temperature dependence. These unusual results indicate the presence of long range repulsive interactions. Kinetic Monte Carlo simulations and density functional theory calculations support this conclusion. In addition to answering an outstanding question in epitaxial growth, i.e., to find systems where long range interactions are present, the high density of magnetic islands, tunable with $\theta$, is of interest for nanomagnetism applications.

We investigate ground state properties of the half-filled staggered-flux Hubbard model on a square lattice. Energy gaps to charge and spin excitations and magnetic as well as dimer orders are calculated as a function of interaction strength $U/t$ by means of a constrained-path quantum Monte Carlo method. It is found that the system is a semimetal at $U/t\lesssim 5.6$ and a Mott insulator, with long-range antiferromagnetic order, at $U/t\gtrsim 6.6$. In the range $5.6\lesssim U/t\lesssim 6.6$, the ground state is a correlated insulator where both magnetic and dimer orders are absent. Furthermore, spin excitation in the intermediate phase appears to be gapless, and the measured spin-spin correlation function exhibits power-law decaying behavior. The data suggest that the nonmagnetic ground state is a possible candidate for the putative algebraic spin liquid.

We show that a continuous-wave (cw) pump beam at a wavelength of 532 nm produces substantial light-induced (LI) absorption in the visible range in initially transparent undoped ${\mathrm{LiNbO}}_3$ crystals. The LI absorption coefficient stays linear in the pump intensity $I_{\mathrm{p}}$ up to $I_{\mathrm{p}}^{\max }=48\mathrm{kW}/{\mathrm{cm}}^2$. Together with other features including long-term stretched-exponential relaxation of the LI absorption, it indicates that the present concept of LI electron processes in this important optical material must be revised: the amount of photoactive electrons increases already within the cw intensity range. A quantitative model is proposed that explains the experimental data and employs two-step excitations from filled localized states near the valence band via intermediate deep centers into the conduction band. The introduced localized states serve as a hidden reservoir of electrons.

The properties of metal crystals are governed by the electrons of the highest occupied states at the Fermi level and determined by Fermi surfaces, the Fermi energy contours in momentum space. Topological regulation of the Fermi surface has been an important issue in synthesizing functional materials, which we found to be realized at room temperature in nanometer-thick films. Reducing the thickness of a metal thin film down to its electron wavelength scale induces the quantum size effect and the electronic system changes from three to two-dimensional, transforming the Fermi surface topology. Such an ultrathin film further changes its topology through one-dimensional (1D) structural deformation of the film when it is grown on a 1D substrate. In particular, when the interface has 1D metallic bands, the system is additionally stabilized by forming an electron energy gap by hybridization between 1D states of the film and substrate.

We demonstrate a direct approach to investigate heat transport in the fractional quantum Hall regime. At a filling factor of $\nu =4/3$, we inject power at quantum point contacts and detect the related heating from the activated current through a quantum dot. The experiment reveals a chargeless heat transport from a significant heating that occurs upstream of the power injection point, in the absence of a concomitant electrical current. By tuning in situ the edge path, we show that the chargeless heat transport does not follow the reverse direction of the electrical current path along the edge. This unexpected heat conduction, whose mechanism remains to be elucidated, may play an important role in the physics of the fractional quantum Hall regime.

We report on infrared studies of charge dynamics in a prototypical pnictide system: the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ family. Our experiments have identified hallmarks of the pseudogap state in the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ system that mirror the spectroscopic manifestations of the pseudogap in the cuprates. The magnitude of the infrared pseudogap is in accord with that of the spin-density-wave gap of the parent compound. By monitoring the superconducting gap of both P- and Co-doped compounds, we find that the infrared pseudogap is unrelated to superconductivity. The appearance of the pseudogap is found to correlate with the evolution of the antiferromagnetic fluctuations associated with the spin-density-wave instability. The strong-coupling analysis of infrared data further reveals the interdependence between the magnetism and the pseudogap in the iron pnictides.

Polarized and unpolarized neutron scattering experiments on the frustrated ferromagnetic spin-$1/2$ chain ${\mathrm{LiCuVO}}_4$ show that the phase transition at $H_{\mathrm{Q}}$ of 8 T is driven by quadrupolar fluctuations and that dipolar correlations are short range with moments parallel to the applied magnetic field in the high-field phase. Heat-capacity measurements evidence a phase transition into this high-field phase, with an anomaly clearly different from that at low magnetic fields. Our experimental data are consistent with a picture where the ground state above $H_{\mathrm{Q}}$ has a next-nearest neighbor bond-nematic order along the chains with a fluidlike coherence between weakly coupled chains.

X-ray absorption spectroscopy studies of the magnetic-insulating ground state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at ambient pressure show a clear deviation from a strong spin-orbit (SO) limit $J_{\mathrm{eff}}=\frac{1}{2}$ state, a result of local exchange interactions and a nonzero tetragonal crystal field mixing SO split $J_{\mathrm{eff}}=\frac{1}{2}$, $\frac{3}{2}$ states. X-ray magnetic circular dichroism measurements in a diamond anvil cell show a magnetic transition at a pressure of $\sim 17\mathrm{GPa}$, where the “weak” ferromagnetic moment is quenched despite transport measurements showing insulating behavior to at least 40 GPa. The magnetic transition has implications for the origin of the insulating gap and the nature of exchange interactions in this SO coupled system. The expectation value of the angular part of the SO interaction, $⟨\mathbf{L}·\mathbf{S}⟩$, extrapolates to zero at $\sim 80–90\mathrm{GPa}$ where an increased bandwidth strongly mixes $J_{\mathrm{eff}}=\frac{1}{2}$, $\frac{3}{2}$ states and SO interactions no longer dominate the electronic ground state of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

Using photoelectron spectroscopy we demonstrate charge transfer at an interface between two well-known transition metal phthalocyanines, MnPc and ${\mathrm{F}}_{16}\mathrm{CoPc}$, resulting in charged ${\mathrm{MnPc}}^{\delta +}$ and ${\mathrm{F}}_{16}{\mathrm{CoPc}}^{\delta -}$ species. Moreover, the transferred charge is substantially confined to the two transition metal centers. Density functional theory calculations reveal that a hybrid state is formed between the two types of phthalocyanines, which causes this charge transfer. For the hybrid state the Mn $3d_{xz}$ interacts with the Co $3d_{z^2}$ orbital leading to a two-level system. As only the lower of the two hybrid states is occupied, the charge is directly transferred to the Co $3d_{z^2}$ orbital.

We present an ab initio determination of the shear viscosity $\eta$ of the unitary Fermi gas, based on finite temperature quantum Monte Carlo calculations and the Kubo linear-response formalism. We determine the temperature dependence of the shear viscosity-to-entropy density ratio $\eta /s$. The minimum of $\eta /s$ appears to be located above the critical temperature for the superfluid-to-normal phase transition with the most probable value being $(\eta /s{)}_{\min }\approx 0.2\hslash /k_B$, which is close the Kovtun-Son-Starinets universal value $\hslash /(4\pi k_B)$.

Entanglement entropy has become an important theoretical concept in condensed matter physics because it provides a unique tool for characterizing quantum mechanical many-body phases and new kinds of quantum order. However, the experimental measurement of entanglement entropy in a many-body system is widely believed to be unfeasible, owing to the nonlocal character of this quantity. Here, we propose a general method to measure the entanglement entropy. The method is based on a quantum switch (a two-level system) coupled to a composite system consisting of several copies of the original many-body system. The state of the switch controls how different parts of the composite system connect to each other. We show that, by studying the dynamics of the quantum switch only, the Rényi entanglement entropy of the many-body system can be extracted. We propose a possible design of the quantum switch, which can be realized in cold atomic systems. Our work provides a route towards testing the scaling of entanglement in critical systems as well as a method for a direct experimental detection of topological order.