We surrounded a rotating torsion pendulum containing $9.8\times {10}^{22}$ polarized electrons by 2 or 4 stationary sources, each with a net spin of $6.0\times {10}^{24}$ polarized electrons. Multiple source configurations gave sensitivity to hypothetical dipole-dipole, spin-dot-spin, and spin-cross-spin exchange interactions mediated by bosons with masses up to $20\mu \mathrm{eV}$. For bosons with masses $\le 0.1\mu \mathrm{eV}$ our null results for the dipole-dipole, spin-dot-spin, and spin-cross-spin forces imply $1\sigma$ upper limits on $(g_P^e{)}^2/(\hslash c)$, $(g_A^e{)}^2/(\hslash c)$ and $(g_V^e{g}_A^e)/(\hslash c)$ of $2.2\times {10}^{-16}$, $3.8\times {10}^{-40}$, and $1.2\times {10}^{-28}$, respectively. We also constrain, for the first time, any possible linear combination of static spin-spin interactions. In this case our upper limits relax to $5.6\times {10}^{-16}$, $9.8\times {10}^{-40}$, and $1.2\times {10}^{-28}$, respectively.

When molecules move, their nuclei flow. The corresponding quantum observable, i.e., the nuclear flux density, was introduced by Schrödinger in 1926, but until now, it has not been measured. Here the first experimental results are deduced from high-resolution pump-probe measurements of the time-dependent nuclear densities in a vibrating diatomic molecule or molecular ion. The nuclear densities are converted to flux densities by means of the continuity equation. The flux densities are much more sensitive to time-dependent quantum effects than the densities. Applications to the sodium molecule and the deuterium molecular ion unravel four new effects; e.g., at the turns from bond stretch to compression, the flux of the nuclei exhibits multiple changes of directions, from small to large bond lengths, a phenomenon that we call the “quantum accordion.”

The generation of light endowed with orbital angular momentum, frequently termed optical vortex light, is commonly achieved by passing a conventional beam through suitably constructed optical elements. This Letter shows that the necessary phase structure for vortex propagation can be directly produced through the creation of twisted light from the vacuum. The mechanism is based on optical emission from a family of chromophore nanoarrays that satisfy specific geometric and symmetry constraints. Each such array can support pairs of electronically delocalized doubly degenerate excitons whose azimuthal phase progression is responsible for the helical wave front of the emitted radiation. The exciton symmetry dictates the maximum magnitude of topological charge; detailed analysis secures the conditions necessary to deliver optical vortices of arbitrary order.

We report a measurement of the large optical transmission matrix (TM) of a complex turbid medium. The TM is acquired using polarization-sensitive, full-field interferometric microscopy equipped with a rotating galvanometer mirror. It is represented with respect to input and output bases of optical modes, which correspond to plane wave components of the respective illumination and transmitted waves. The modes are sampled so finely in angular spectrum space that their number exceeds the total number of resolvable modes for the illuminated area of the sample. As such, we investigate the singular value spectrum of the TM in order to detect evidence of open transmission channels, predicted by random-matrix theory. Our results comport with theoretical expectations, given the experimental limitations of the system. We consider the impact of these limitations on the usefulness of transmission matrices in optical measurements.

Prompted by intriguing events observed in certain particle-physics searches for rare events, we study light and acoustic emission simultaneously in some inorganic scintillators subject to mechanical stress. We observe mechanoluminescence in ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$, ${\mathrm{CdWO}}_4$, and ${\mathrm{ZnWO}}_4$, in various mechanical configurations at room temperature and ambient pressure. We analyze the temporal and amplitude correlations between the light emission and the acoustic emission during fracture. A novel application of the precise energy calibration of ${\mathrm{Bi}}_4{\mathrm{Ge}}_3{\mathrm{O}}_{12}$ provided by radioactive sources allows us to deduce that the fraction of elastic energy converted to light is at least $3\times {10}^{-5}$.

We present a general theory of the singularity in the London penetration depth at symmetry-breaking and topological quantum critical points within a superconducting phase. While the critical exponents and ratios of amplitudes on the two sides of the transition are universal, an overall sign depends upon the interplay between the critical theory and the underlying Fermi surface. We determine these features for critical points to spin density wave and nematic ordering, and for a topological transition between a superconductor with ${\mathbb{Z}}_2$ fractionalization and a conventional superconductor. We note implications for recent measurements of the London penetration depth in ${\mathrm{BaFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$ [K. Hashimoto et al., Science 336, 1554 (2012)].

Spin exchange between a single-electron charged quantum dot and itinerant electrons leads to an emergence of Kondo correlations. When the quantum dot is driven resonantly by weak laser light, the resulting emission spectrum allows for a direct probe of these correlations. In the opposite limit of vanishing exchange interaction and strong laser drive, the quantum dot exhibits coherent oscillations between the single-spin and optically excited states. Here, we show that the interplay between strong exchange and nonperturbative laser coupling leads to the formation of a new nonequilibrium quantum-correlated state, characterized by the emergence of a laser-induced secondary spin screening cloud, and examine the implications for the emission spectrum.

Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mössbauer transition of ${}^{61}\mathrm{Ni}$. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mössbauer factor increases from 33 meV at ambient pressure to 60 meV at 100 GPa. The change of this energy over volume compression is well described by a Grüneisen parameter of 2.09.

We extend a model for the morphology and dynamics of a crawling eukaryotic cell to describe cells on micropatterned substrates. This model couples cell morphology, adhesion, and cytoskeletal flow in response to active stresses induced by actin and myosin. We propose that protrusive stresses are only generated where the cell adheres, leading to the cell’s effective confinement to the pattern. Consistent with experimental results, simulated cells exhibit a broad range of behaviors, including steady motion, turning, bipedal motion, and periodic migration, in which the cell crawls persistently in one direction before reversing periodically. We show that periodic motion emerges naturally from the coupling of cell polarization to cell shape by reducing the model to a simplified one-dimensional form that can be understood analytically.

This Letter proposes an answer to a challenge posed by Bell on the lack of clarity in regards to the dividing line between the quantum and classical regimes in a measurement problem. To this end, a generalized logarithmic nonlinear Schrödinger equation is proposed to describe the time evolution of a quantum dissipative system under continuous measurement. Within the Bohmian mechanics framework, a solution to this equation reveals a novel result: it displays a time constant that should represent the dividing line between the quantum and classical trajectories. It is shown that continuous measurements and damping not only disturb the particle but compel the system to converge in time to a Newtonian regime. While the width of the wave packet may reach a stationary regime, its quantum trajectories converge exponentially in time to classical trajectories. In particular, it is shown that damping tends to suppress further quantum effects on a time scale shorter than the relaxation time of the system. If the initial wave packet width is taken to be equal to $2.8\times {10}^{-15}\mathrm{m}$ (the approximate size of an electron), the Bohmian time constant is found to have an upper limit, i.e., ${\tau }_{B\max }={10}^{-26}\mathrm{s}$.

We present a scheme for linear optical quantum computing using time-bin-encoded qubits in a single spatial mode. We show methods for single-qubit operations and heralded controlled-phase (cphase) gates, providing a sufficient set of operations for universal quantum computing with the Knill-Laflamme-Milburn [Nature (London) 409, 46 (2001)] scheme. Our protocol is suited to currently available photonic devices and ideally allows arbitrary numbers of qubits to be encoded in the same spatial mode, demonstrating the potential for time-frequency modes to dramatically increase the quantum information capacity of fixed spatial resources. As a test of our scheme, we demonstrate the first entirely single spatial mode implementation of a two-qubit quantum gate and show its operation with an average fidelity of $0.84\pm 0.07$.

Measurements of the variation of inclusive jet suppression as a function of relative azimuthal angle, $\Delta \varphi$, with respect to the elliptic event plane provide insight into the path-length dependence of jet quenching. ATLAS has measured the $\Delta \varphi$ dependence of jet yields in $0.14{\mathrm{nb}}^{-1}$ of $\sqrt{s_{NN}}=2.76\mathrm{TeV}$ $\mathrm{Pb}+\mathrm{Pb}$ collisions at the LHC for jet transverse momenta $p_T>45\mathrm{GeV}$ in different collision centrality bins using an underlying event subtraction procedure that accounts for elliptic flow. The variation of the jet yield with $\Delta \varphi$ was characterized by the parameter, $v_2^{\mathrm{jet}}$, and the ratio of out-of-plane ($\Delta \varphi \sim \pi /2$) to in-plane ($\Delta \varphi \sim 0$) yields. Nonzero $v_2^{\mathrm{jet}}$ values were measured in all centrality bins for $p_T<160\mathrm{GeV}$. The jet yields are observed to vary by as much as 20% between in-plane and out-of-plane directions.

In one-dimensional optical setups, light-matter interaction is drastically enhanced by the interference between the incident and scattered fields. Particularly, in the impedance-matched $\Lambda$-type three-level systems, a single photon deterministically induces the Raman transition and switches the electronic state of the system. Here, we show that such a $\Lambda$ system can be implemented by using dressed states of a driven superconducting qubit and a resonator. The input microwave photons are perfectly absorbed and are down-converted into other frequency modes in the same waveguide. The proposed setup is applicable to the detection of single microwave photons and the swapping of the photon and matter qubits.

Time-bin encoding is a robust form of optical quantum information, especially for transmission in optical fibers. To readout the information, the separation of the time bins must be larger than the detector time resolution, typically on the order of nanoseconds for photon counters. In the present work, we demonstrate a technique using a nonlinear interaction between chirped entangled time-bin photons and shaped laser pulses to perform projective measurements on arbitrary time-bin states with picosecond-scale separations. We demonstrate a tomographically complete set of time-bin qubit projective measurements and show the fidelity of operations is sufficiently high to violate the Clauser-Horne-Shimony-Holt-Bell inequality by more than 6 standard deviations.

We describe a scheme for controlling electron injection into the quasilinear wakefield driven by a guided drive pulse via ionization of a dopant species by a collinear injection laser pulse with a short Rayleigh range. The scheme is analyzed by particle-in-cell simulations which show controlled injection and acceleration of electrons to an energy of 370 MeV, a relative energy spread of 2%, and a normalized transverse emittance of $2.0\mu \mathrm{m}$.

We propose that the structure of amorphous metal oxides can be regarded as a dual-dense-random-packing structure, which is a superposition of the dense random packing of metal atoms and that of oxygen atoms. Our ab initio molecular dynamics simulations show that the medium-range order of amorphous ${\mathrm{HfO}}_2$, ${\mathrm{ZrO}}_2$, ${\mathrm{TiO}}_2$, ${\mathrm{In}}_2{\mathrm{O}}_3$, ${\mathrm{Ga}}_2{\mathrm{O}}_3$, ${\mathrm{Al}}_2{\mathrm{O}}_{3,}$, and ${\mathrm{Cu}}_2\mathrm{O}$ is characterized by the pentagonal-bipyramid arrangement of metal atoms and that of oxygen atoms, and prove the validity of our dual-random-sphere-packing model. In other words, we find that the pentagonal medium-range order is universal independent of type of metal oxide.

The level structures of the very neutron-rich nuclei ${}^{128}\mathrm{Pd}$ and ${}^{126}\mathrm{Pd}$ have been investigated for the first time. In the $r$-process waiting-point nucleus ${}^{128}\mathrm{Pd}$, a new isomer with a half-life of $5.8(8)\mu \mathrm{s}$ is proposed to have a spin and parity of $8^{+}$ and is associated with a maximally aligned configuration arising from the $g_{9/2}$ proton subshell with seniority $\upsilon =2$. For ${}^{126}\mathrm{Pd}$, two new isomers have been identified with half-lives of 0.33(4) and $0.44(3)\mu \mathrm{s}$. The yrast $2^{+}$ energy is much higher in ${}^{128}\mathrm{Pd}$ than in ${}^{126}\mathrm{Pd}$, while the level sequence below the $8^{+}$ isomer in ${}^{128}\mathrm{Pd}$ is similar to that in the $N=82$ isotone ${}^{130}\mathrm{Cd}$. The electric quadrupole transition that depopulates the $8^{+}$ isomer in ${}^{128}\mathrm{Pd}$ is more hindered than the corresponding transition in ${}^{130}\mathrm{Cd}$, as expected in the seniority scheme for a semimagic, spherical nucleus. These experimental findings indicate that the shell closure at the neutron number $N=82$ is fairly robust in the neutron-rich Pd isotopes.

The best-known property of superfluid helium is the vanishing viscosity that objects experience while moving through the liquid with speeds below the so-called critical Landau velocity. This critical velocity is generally considered a macroscopic property as it is related to the collective excitations of the helium atoms in the liquid. In the present work we determine to what extent this concept can still be applied to nanometer-scale, finite size helium systems. To this end, atoms and molecules embedded in helium nanodroplets of various sizes are accelerated out of the droplets by means of optical excitation, and the speed distributions of the ejected particles are determined. The measurements reveal the existence of a critical velocity in these systems, even for nanodroplets consisting of only a thousand helium atoms. Accompanying theoretical simulations based on a time-dependent density functional description of the helium confirm and further elucidate this experimental finding.

We show that retardation in adjusting an electronic state to an instantaneous internuclear configuration caused by the finiteness of the electron’s velocity breaks the validity of the Born-Oppenheimer (BO) approximation at large electron-nuclei distances. This applies even to the ground state. As a result, the BO approximation in the theory of tunneling ionization of molecules breaks down at sufficiently weak fields. We also show that to account for nuclear motion the weak-field asymptotic expansion for the tunneling ionization rate must be restructured. The predictions for the rate using the BO approximation and the asymptotic expansion are compared with numerical results for a one-dimensional three-body system modeling a diatomic molecule, with both electronic and nuclear motions treated exactly.

Dynamic control of the direction of radiation of the light emanating from a subwavelength slit carved out of a thin metal film is experimentally demonstrated. This is achieved by selective excitation of the individual guided modes in the slit by setting the phase of three coherent laser beams. By changing the voltage across a piezoelement, we obtain unprecedented directional steering, without relying on any mechanical alignment of optical elements. The angular range over which this maximum can be swept is determined by the intensity setting of one of the incident beams. Through simulations, we show that this method can also be applied to steer the radiation from a square hole in two independent directions. Our method can be applied to create a directional nanoemitter which can selectively address one or more detectors, or as an optical switch in photonic circuits.