We study experimentally and theoretically a beam splitter setup for guided atomic matter waves. The matter wave is a guided atom laser that can be tuned from quasimonomode to a regime where many transverse modes are populated, and propagates in a horizontal dipole beam until it crosses another horizontal beam at 45°. We show that depending on the parameters of this $X$ configuration, the atoms can all end up in one of the two beams (the system behaves as a perfect guide switch), or be split between the four available channels (the system behaves as a beam splitter). The splitting regime results from a chaotic scattering dynamics. The existence of these different regimes turns out to be robust against small variations of the parameters of the system. From numerical studies, we also propose a scheme that provides a robust and controlled beam splitter in two channels only.

Quantum effects, besides offering substantial superiority in many tasks over classical methods, are also expected to provide interesting ways to establish secret keys between remote parties. A striking scheme called “counterfactual quantum cryptography” proposed by Noh [Phys. Rev. Lett. 103, 230501 (2009).] allows one to maintain secure key distributions, in which particles carrying secret information are seemingly not being transmitted through quantum channels. We have experimentally demonstrated, for the first time, a faithful implementation for such a scheme with an on-table realization operating at telecom wavelengths. To verify its feasibility for extension over a long distance, we have furthermore reported an illustration on a 1 km fiber. In both cases, high visibilities of more than 98% are achieved through active stabilization of interferometers. Our demonstration is crucial as a direct verification of such a remarkable application, and this procedure can become a key communication module for revealing fundamental physics through counterfactuals.

Motivated by the quantum adiabatic algorithm (QAA), we consider the scaling of the Hamiltonian gap at quantum first-order transitions, generally expected to be exponentially small in the size of the system. However, we show that a quantum antiferromagnetic Ising chain in a staggered field can exhibit a first-order transition with only an algebraically small gap. In addition, we construct a simple classical translationally invariant one-dimensional Hamiltonian containing nearest-neighbor interactions only, which exhibits an exponential gap at a thermodynamic quantum first-order transition of essentially topological origin. This establishes that (i) the QAA can be successful even across first-order transitions but also that (ii) it can fail on exceedingly simple problems readily solved by inspection, or by classical annealing.

We present the exact analytical expression for the spectrum of a sparse non-Hermitian random matrix ensemble, generalizing two standard results in random-matrix theory: this analytical expression constitutes a non-Hermitian version of the Kesten-McKay measure as well as a sparse realization of Girko’s elliptic law. Our exact result opens new perspectives in the study of several physical problems modelled on sparse random graphs, which are locally treelike. In this context, we show analytically that the convergence rate of a transport process on a very sparse graph depends in a nonmonotonic way upon the degree of symmetry of the graph edges.

The nucleus ${}^{49}\mathrm{Sc}$, having a single $f_{7/2}$ proton outside doubly magic ${}^{48}\mathrm{Ca}$ ($Z=20$, $N=28$), is one of the very few isotopes which makes possible testing of the fundamental theory of nuclear magnetism. The magnetic moment has been measured by online $\beta$ NMR of nuclei oriented at milli-Kelvin temperatures to be $(+)5.616(25){\mu }_N$. The result is discussed in terms of a detailed theory of the structure of the magnetic moment operator, showing excellent agreement with calculated departure from the $f_{7/2}$ Schmidt limit extreme single-particle value. The measurement completes the sequence of moments of Sc isotopes with even numbers of $f_{7/2}$ neutrons: the first such isotopic chain between two major shells for which a full set of moment measurements exists. The result further completes the isotonic sequence of ground-state moments of nuclei with an odd number of $f_{7/2}$ protons coupled to a closed subshell of $f_{7/2}$ neutrons. Comparison with a recent shell-model calculation of the latter sequence is made.

We report on a search for neutrinoless double-beta decay of ${}^{136}\mathrm{Xe}$ with EXO-200. No signal is observed for an exposure of 32.5 kg yr, with a background of $\sim 1.5\times {10}^{-3}{\mathrm{kg}}^{-1}{\mathrm{yr}}^{-1}{\mathrm{keV}}^{-1}$ in the $\pm 1\sigma$ region of interest. This sets a lower limit on the half-life of the neutrinoless double-beta decay $T_{1/2}^{0\nu \beta \beta }({}^{136}\mathrm{Xe})>1.6\times {10}^{25}\mathrm{yr}$ (90% C.L.), corresponding to effective Majorana masses of less than 140–380 meV, depending on the matrix element calculation.

We report the experimental realization of an optical trap that localizes single Cs atoms $\simeq 215\mathrm{nm}$ from the surface of a dielectric nanofiber. By operating at magic wavelengths for pairs of counterpropagating red- and blue-detuned trapping beams, differential scalar light shifts are eliminated, and vector shifts are suppressed by $\approx 250$. We thereby measure an absorption linewidth $\Gamma /2\pi =5.7\pm 0.1\mathrm{MHz}$ for the Cs $6S_{1/2}$, $F=4\to 6P_{3/2}$, $F^{\prime }=5$ transition, where ${\Gamma }_0/2\pi =5.2\mathrm{MHz}$ in free space. An optical depth $d\simeq 66$ is observed, corresponding to an optical depth per atom $d_1\simeq 0.08$. These advances provide an important capability for the implementation of functional quantum optical networks and precision atomic spectroscopy near dielectric surfaces.

The zero-phonon transition rate of a nitrogen-vacancy center is enhanced by a factor of $\sim 70$ by coupling to a photonic crystal resonator fabricated in monocrystalline diamond using standard semiconductor fabrication techniques. Photon correlation measurements on the spectrally filtered zero-phonon line show antibunching, a signature that the collected photoluminescence is emitted primarily by a single nitrogen-vacancy center. The linewidth of the coupled nitrogen-vacancy center and the spectral diffusion are characterized using high-resolution photoluminescence and photoluminescence excitation spectroscopy.

We present experimental observations of a nonresonant dynamic Stark shift in strongly coupled microcavity quantum well exciton polaritons—a system which provides a rich variety of solid-state collective phenomena. The Stark effect is demonstrated in a $\mathrm{GaAs}/\mathrm{AlGaAs}$ system at 10 K by femtosecond pump-probe measurements, with the blueshift approaching the meV scale for a pump fluence of $2\mathrm{mJ}{\mathrm{cm}}^{-2}$ and 50 meV red detuning, in good agreement with theory. The energy level structure of the strongly coupled polariton Rabi doublet remains unaffected by the blueshift. The demonstrated effect should allow generation of ultrafast density-independent potentials and imprinting well-defined phase profiles on polariton condensates, providing a powerful tool for manipulation of these condensates, similar to dipole potentials in cold-atom systems.

We put in evidence the unexpected behavior of Leidenfrost droplets at the later stage of their evaporation. We predict and observe that, below a critical size $R_l$, the droplets spontaneously take off due to the breakdown of the lubrication regime. We establish the theoretical relation between the droplet radius and its elevation. We predict that the vapor layer thickness increases when the droplets become smaller. A satisfactory agreement is found between the model and the experimental results performed on droplets of water and of ethanol.

We calculate the linear and nonlinear conductance of spinless fermions in clean, long quantum wires, where short-ranged interactions lead locally to equilibration. Close to the quantum phase transition, where the conductance jumps from zero to one conductance quantum, the conductance obtains a universal form governed by the ratios of temperature, bias voltage, and gate voltage. Asymptotic analytic results are compared to solutions of a Boltzmann equation which includes the effects of three-particle scattering. Surprisingly, we find that for long wires the voltage predominantly drops close to one end of the quantum wire due to a thermoelectric effect.

The muon anomalous magnetic moment exhibits a $3.6\sigma$ discrepancy between experiment and theory. One explanation requires the existence of a light vector boson, $Z_d$ (the dark $Z$), with mass 10–500 MeV that couples weakly to the electromagnetic current through kinetic mixing. Support for such a solution also comes from astrophysics conjectures regarding the utility of a $U(1{)}_d$ gauge symmetry in the dark matter sector. In that scenario, we show that mass mixing between the $Z_d$ and ordinary $Z$ boson introduces a new source of “dark” parity violation, which is potentially observable in atomic and polarized electron scattering experiments. Restrictive bounds on the mixing $(m_{Z_d}/m_Z)\delta$ are found from existing atomic parity violation results, ${\delta }^2<2\times {10}^{-5}$. Combined with future planned and proposed polarized electron scattering experiments, a sensitivity of ${\delta }^2\sim {10}^{-6}$ is expected to be reached, thereby complementing direct searches for the $Z_d$ boson.

From numerical simulation and analytical modeling it is shown that fast ions can resonate with plasma waves at fractional values of the particle drift-orbit transit frequency when the plasma wave amplitude is sufficiently large. The fractional resonances, which are caused by a nonlinear interaction between the particle orbit and the wave, give rise to an increased density of resonances in phase space which reduces the threshold for stochastic transport. The effects of the fractional resonances on spatial and energy transport are illustrated for an energetic particle geodesic acoustic mode but they apply equally well to other types of MHD activity.

We report measurements of the superfluid fraction ${\rho }_s/\rho$ and specific heat $c_p$ near the superfluid transition of ${}^4\mathrm{He}$ when confined in an array of $(2\mu \mathrm{m}{)}^3$ boxes at a separation of $S=2\mu \mathrm{m}$ and coupled through a 32.5 nm film. We find that $c_p$ is strongly enhanced when compared with data where coupling is not present. An analysis of this excess signal shows that it is proportional to the finite-size correlation length in the boxes $\xi (t,L)$, and it is measurable as far as $S/\xi \sim 30-50$. We obtain $\xi (0,L)$ and the scaling function (within a constant) for $\xi (t,L)$ in an $L^3$ box geometry. Furthermore, we find that ${\rho }_s/\rho$ of the film persists a full decade closer to the bulk transition temperature $T_{\lambda }$ than a film uninfluenced by proximity effects. This excess in ${\rho }_s/\rho$ is measurable even when $S/\xi >100$, which cannot be understood on the basis of mean field theory.

Freezing of water in hydrophilic nanopores ($D=1.2\mathrm{nm}$) is probed at the microscopic scale using x-ray diffraction, Raman spectroscopy, and molecular simulation. A freezing scenario, which has not been observed previously, is reported; while the pore surface induces orientational order of water in contact with it, water does not crystallize at temperatures as low as 173 K. Crystallization at the surface is suppressed as the number of hydrogen bonds formed is insufficient (even when including hydrogen bonds with the surface), while crystallization in the pore center is hindered as the curvature prevents the formation of a network of tetrahedrally coordinated molecules. This sheds light on the concept of an ubiquitous unfreezable water layer by showing that the latter has a rigid (i.e., glassy) liquidlike structure, but can exhibit orientational order.

We report on the experimental observation of the nonlinear analogue of the optical spin Hall effect under highly nonresonant circularly polarized excitation of an exciton-polariton condensate in a $\mathrm{GaAs}/\mathrm{AlGaAs}$ microcavity. The circularly polarized polariton condensates propagate over macroscopic distances, while the collective condensate spins coherently precess around an effective magnetic field in the sample plane performing up to four complete revolutions.

We report magnetotransport measurements in ballistic graphene mesoscopic wires where the charge carrier mean free path is comparable to the wire width $W$. Magnetoresistance curves show characteristic peak structures where the peak field scales with the ratio of cyclotron radius $R_{\mathrm{c}}$ and wire width $W$ as $W/R_{\mathrm{c}}=0.9\pm 0.1$, due to diffusive boundary scattering. The obtained proportionality constant between $R_{\mathrm{c}}$ and $W$ differs from that of a classical semiconductor two-dimensional electron system in which $W/R_{\mathrm{c}}=0.55$.

An intrinsic thermoelectric coupling effect in the linear response regime of magnetic tunneling junctions (MTJ) is reported. In the dc response, it leads to a nonlinear correction to Ohm’s law. Dynamically, it enables a novel Seebeck rectification and second harmonic generation, which apply for a broad frequency range and can be magnetically controlled. A phenomenological model on the footing of the Onsager reciprocal relation and the principle of energy conservation explains very well the experimental results obtained from both dc and frequency-dependent transport measurements performed up to GHz frequencies. Our work refines previous understanding of magnetotransport and microwave rectification in MTJs. It forms a new foundation for utilizing spin caloritronics in high-frequency applications.

We use a first-principles rational-design approach to demonstrate the potential of semiconducting half-Heusler compounds as a previously unrecognized class of piezoelectric materials. We perform a high-throughput scan of a large number of compounds, testing for insulating character and calculating structural, dielectric, and piezoelectric properties. Our results provide guidance for the experimental realization and characterization of high-performance materials in this class that may be suitable for practical applications.

Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay $K_1$, twist $K_2$, and bend $K_3$ elastic constants for the lyotropic chromonic liquid crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. $K_1$ and $K_3$ are comparable to each other and are an order of magnitude higher than $K_2$. At higher concentrations and lower temperatures, $K_1$ and the ratios $K_1/K_3$ and $K_1/K_2$ increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.