The Casimir-Polder interaction of ground-state and excited atoms with graphene is investigated with the aim to establish whether graphene systems can be used as a shield for vacuum fluctuations of an underlying substrate. We calculate the zero-temperature Casimir-Polder potential from the reflection coefficients of graphene within the framework of the Dirac model. For both doped and undoped graphene we show limits at which graphene could be used effectively as a shield. Additional results are given for AB-stacked bilayer graphene.

We generalize the analysis of the normal modes for a rotating ionic Coulomb crystal in a Penning trap to allow for inhomogeneities in the system. Our formal developments are completely general, but we choose to examine a crystal of Be${}^{+}$ ions with BeH${}^{+}$ defects to compare with current experimental efforts. We examine the classical phonon modes (both transverse and planar) and we determine the effective spin-spin interactions when the system is driven by an axial spin-dependent optical dipole force. We examine situations with up to approximately 15% defects. We find that most properties are not strongly influenced by the defects, indicating that the presence of a small number of defects will not significantly affect experiments.

Based on the frequency-domain theory, we investigate the above-threshold ionization (ATI) process of an atom in a two-color laser field with infrared (IR) and extreme ultraviolet (XUV) frequencies, where the photon energy of the XUV laser is close to or larger than the atomic ionization threshold. By using channel analysis, we find that the two laser fields play different roles in an ionization process, where the XUV laser determines the ionization probability by the photon number that the atom absorbs from it, while the IR laser accelerates the ionized electron and hence widens the electron kinetic energy spectrum. As a result, the ATI spectrum presents a terracelike structure. By using the saddle-point approximation, we obtain a classical formula which can predict the cutoff of each plateau in the terracelike ATI spectrum. Furthermore, we find that the difference of the heights between two neighboring plateaus in the terracelike structure of the ATI spectrum increases as the frequency of the XUV laser increases.

We discuss nonlinear optical processes in the presence of quantum gases. We show that, in contrast to a normal gas, where the medium “passively” participates in nonlinear optical processes, quantum gases “actively” interact with the wave mixing process by enforcing the properties arising from condensed matter physics on the generation and propagation of new light fields. This manifestation of condensed matter physics leads to intriguing suppression and enhancement effects in directional wave generation and propagation processes that have no counterpart in a normal gas. This opens a new chapter on light-matter-wave mixing and scattering: nonlinear optics with quantum gases.

Motivated by the recent experimental observation of negative absolute temperature states in systems of ultracold atomic gases in optical lattices [Braun et al., Science 339, 52 (2013)], we investigate theoretically the formation of these states. More specifically, we consider the relaxation after a sudden inversion of the external parabolic confining potential in the one-dimensional inhomogeneous Bose-Hubbard model. First, we focus on the integrable hard-core boson limit, which allows us to treat large systems and arbitrarily long times, providing convincing numerical evidence for relaxation to a generalized Gibbs ensemble at negative temperature $T<0$, a notion we define in this context. Second, going beyond one dimension, we demonstrate that the emergence of negative temperature states can be understood in a dual way in terms of positive temperatures, which relies on a dynamic symmetry of the Hubbard model. We complement the study by exact diagonalization simulations at finite values of the on-site interaction.

The dispersion cancellation observed in Hong-Ou-Mandel (HOM) interference between frequency-entangled photon pairs has been the basis of quantum optical coherence tomography and quantum clock synchronization. Here we explore the effect of phase dispersion on ultranarrow HOM dips. We show that the higher-order dispersion, the linewidth of the pump laser, and the spectral shape of the parametric fluorescence have a strong effect on the dispersion cancellation in the high-resolution regime with several experimental verifications. Perfect dispersion cancellation with a linewidth of 3 $\mu$m is also demonstrated through 25 mm of water.

We quantize the macroscopic electromagnetic field in a system of nondispersive polarizable bodies moving at constant velocities possibly exceeding the Cherenkov threshold. It is shown that in general the quantized system is unstable and neither has a ground state nor supports stationary states. The quantized Hamiltonian is written in terms of quantum harmonic oscillators associated with both positive and negative frequencies, such that the oscillators associated with symmetric frequencies are coupled by an interaction term that does not preserve the quantum occupation numbers. Moreover, in the linear regime the amplitudes of the fields may grow without limit provided the velocity of the moving bodies is enforced to be constant. This requires the application of an external mechanical force that effectively pumps the system.

Third- and higher-order quantum virial coefficients require the solution of the corresponding quantum many-body problem. Nevertheless, in an earlier paper [Phys. Rev. Lett. 108, 260402 (2012)], we proposed that the higher-order cluster integrals of a dilute unitary fermionic gas may be approximated in terms of the two-body cluster together with an appropriate suppression factor. Although not exact, this ansatz gave a fair agreement up to fugacity $z\approx 6$ with the experimentally obtained equation of state. The objective of the present Brief Report is to give some physical arguments in favor of this ansatz.

We demonstrate that a non-self-adjoint Hamiltonian of harmonic-oscillator type defined on a two-dimensional noncommutative space can be diagonalized exactly by making use of pseudobosonic operators. The model admits an antilinear symmetry and is of the type studied in the context of $\mathcal{P}\mathcal{T}$-symmetric quantum mechanics. Its eigenvalues are computed to be real for the entire range of the coupling constants and the biorthogonal sets of eigenstates for the Hamiltonian and its adjoint are explicitly constructed. We show that despite the fact that these sets are complete and biorthogonal, they involve an unbounded metric operator and therefore do not constitute (Riesz) bases for the Hilbert space ${\mathcal{L}}^2\left({\mathbb{R}}^2\right)$, but instead only $\mathcal{D}$ quasibases. As recently proved by one of us, this is sufficient to deduce several interesting consequences.

With the advent of physical implementations of quantum walks, a general theoretical and efficient numerical framework is required for the study of their interactions with defects and disorder. In this paper, we derive analytic expressions for the eigenstates of a one-dimensional continuous-time quantum walk interacting with a single defect, before investigating the effects of multiple diagonal defects and disorder, with emphasis on its transmission and reflection properties. Complex resonance behavior is demonstrated, showing alternating bands of zero and perfect transmission for various defect parameters. Furthermore, we provide an efficient numerical method to characterize quantum walks in the presence of diagonal disorder, paving the way for selective control of quantum walks via the optimization of position-dependent defects. The numerical method can be readily extended to higher dimensions and multiple interacting walkers.

We report a method to determine time evolution of ortho-positronium ($o$-Ps) kinetic energy with higher accuracy and sensitivity than conventional methods. Our method utilizes the fact that, during Ps-Xe collisions, an $o$-Ps atom can undergo a spin-conversion reaction induced by spin-orbit interaction to decay into two gamma-ray photons and the fact that the reaction rate strongly depends on the $o$-Ps kinetic energy because the reaction occurs only in $p$-wave scattering; thus a small change in the energy leads to a large change in the two-photon annihilation rate. Utilizing this reaction as a “lens” to magnify the $o$-Ps kinetic energy, we obtain its time evolution by measuring the time-resolved two-photon annihilation rate with an age-momentum correlation spectrometer. The time evolution of $o$-Ps kinetic energy can be explained by a classical model that assumes elastic collisions in a time range greater than 20 ns and an energy range of 40–60 meV. By applying this method, the Ps-Xe momentum-transfer cross section is found to be $12\left(2\right)\times {10}^{-16}$ cm${}^2$.

We investigate three-boson recombination of equal mass systems as function of (negative) scattering length, mass, finite energy, and finite temperature. An optical model with an imaginary potential at short distance reproduces experimental recombination data and allows us to provide a simple parametrization of the recombination rate as function of scattering length and energy. Using the two-body van der Waals length as unit we find that the imaginary potential range and also the potential depth agree to within 30% for lithium and cesium atoms. As opposed to recent studies suggesting universality of the threshold for bound-state formation, our results suggest that the recombination process itself could have universal features.

We measured the kinetic energy distributions of the fragment ions of doubly and quadruply ionized argon dimers using 3000 eV electron impact. For the dissociation of (Ar${}_2$)${}^{2+}$, the peak that indicates radiative charge transfer is observed, where the outer-shell ionization (dominant in highly charged ion collision) and the inner-shell ionization (preferential in x-ray experiments) have approximately equal contributions. For the dissociation of (Ar${}_2$)${}^{4+}$, the interatomic Coulombic decay and electron-transfer-mediated decay are first observed in the electron-impact process.

We present angular distributions for the electron-impact single ionization of sodium and magnesium at intermediate incident electron energies. The results are obtained from a full-dimensionality solution of the two-active-electron time-dependent Schrödinger equation using the time-dependent close-coupling method. We compare calculated angular distributions with existing measurements. We find good overall agreement with measurements over a range of incident electron energies in both cases. We also calculate angular distributions for ejection configurations in which no measurements are currently available.

The intermediate molecular states in the double ionization of molecules by laser-driven electron recollision are encoded into the angular-dependent signature of the process. We identify through this signature the specific ionic states participating in the double-ionization mechanism. Using CO${}_2$ as an example we employ a 1350-nm strong field at $2\times {10}^{14}$ W cm${}^{-2}$ to ensure that nonsequential double ionization (NSDI) is the exclusive mechanism in CO${}_2^{2+}$ formation. Under these conditions we obtain two main results. First, the shape of the recolliding electron wave packet is inferred from measurements of the ellipticity dependence of the signal, and second, the inelastic recollision cross section in the molecular frame is extracted from the variation of the signal as a function of the angle between strong-field probe polarization and molecular axis. These results provide an angularly resolved electron-ion collision measurement in CO${}_2$ and demonstrate contributions to NSDI from both ground and first excited ionic states.

A surface-electrode decelerator consisting of 44 electrodes has been used to decelerate supersonic beams of helium Rydberg atoms moving with an initial velocity of 1200 m/s. Prior to the deceleration, the helium atoms were excited from the $1s2s{}^1{S}_0$ metastable state to selected Rydberg-Stark states with a narrow-band tunable UV laser. Complete deceleration could be achieved over a distance of 36 mm and in 60 $\mu$s, corresponding to an acceleration of $-2.0\times {10}^7$ m/s${}^2$. After deceleration, the atoms were held in stationary electric traps above the chip surface, reaccelerated off the chip, and detected by pulsed electric-field ionization. The decelerator was also used to generate helium Rydberg-atom beams with a final velocity tunable between 0 and 1200 m/s. Transitions between low- and high-field-seeking Rydberg-Stark states were observed during trap loading and are attributed to adiabatic Landau-Zener dynamics at electric fields exceeding the Inglis-Teller field. By comparing the experimental results with the results of particle-trajectory simulations, the velocity distribution of the decelerated atoms was found to be characterized by temperatures ranging between 50 and 200 mK, depending on the magnitude of the electric dipole moment of the Rydberg-Stark states selected prior to the deceleration.

We present numerical calculations of the snake instability in a Fermi superfluid within the Bogoliubov-de Gennes theory of the Bose-Einstein condensate (BEC) to BCS crossover using the random-phase approximation complemented by time-dependent simulations. We examine the snaking behavior across the crossover and quantify the time scale and length scale of the instability. While the dynamics shows extensive snaking before eventually producing vortices and sound on the BEC side of the crossover, the snaking dynamics is preempted by decay into sound due to pair breaking in the deep BCS regime. At the unitarity limit, hydrodynamic arguments allow us to link the rate of snaking to the experimentally observable ratio of inertial to physical mass of the soliton. In this limit we witness an unresolved discrepancy between our numerical estimates for the critical wave number of suppression of the snake instability and recent experimental observations with an ultracold Fermi gas.

We extend our earlier study of the ground state of a bosonic quasiparticle Hamiltonian by investigating the effect of a constant external velocity field. Below a critical velocity the ground state is a quasiparticle vacuum, corresponding to a pure superfluid phase at zero temperature. Beyond the critical velocity energy minimization leads to a macroscopic condensation of quasiparticles at a nonzero wave vector ${\mathbf{k}}_{\mathbf{v}}$ parallel to the velocity $\mathbf{v}$. Simultaneously, physical particles also undergo a condensation at ${\mathbf{k}}_{\mathbf{v}}$ and, to a smaller extent, at $-{\mathbf{k}}_{\mathbf{v}}$. Together with the Bose-Einstein condensation at $\mathbf{k}=\mathbf{0}$, the three coexisting condensates give rise to density modulations of wave vectors ${\mathbf{k}}_{\mathbf{v}}$ and $2{\mathbf{k}}_{\mathbf{v}}$. For larger $\left|\mathbf{v}\right|$ our model predicts a bifurcation of ${\mathbf{k}}_{\mathbf{v}}$ with corresponding two pure condensates and no density modulation.

An asymmetric atom trap is investigated as a means to implement a “battery” that supplies ultracold atoms to an atomtronic circuit. The battery model is derived from a scheme for continuous loading of a nondissipative atom trap proposed by Roos et al. [C. F. Roos, P. Cren, D. Guery-Odelin, and J. Dalibard, Europhys. Lett. 61, 187 (2003)]. The trap is defined by longitudinal and transverse trap frequencies $f_z$ and $f_{\perp }$ and corresponding trap energy heights $U_z$ and $U_{\perp }$. The battery's ability to supply power to a load is evaluated as a function of an input atom flux and power, $I_{\mathrm{in}}$ and $P_{\mathrm{in}}=I_{\mathrm{in}}(1+\epsilon )U_z$, respectively, where $\epsilon$ is an excess fractional energy. For given trap parameters, the battery is shown to have a resonantly optimum value of $\epsilon$. The battery behavior can be cast in terms of an equivalent circuit model; specifically, for fixed input flux and power the battery is modeled in terms of a Thévenin equivalent chemical potential and internal resistance. The internal resistance establishes the maximum power that can be supplied to a circuit, the heat that will be generated by the battery, and the noise that will be imposed on the circuit. We argue that any means of implementing a battery for atomtronics can be represented by a Thévenin equivalent and that its performance will likewise be determined by an internal resistance.

The response of matter to an optical excitation consists essentially of absorption and emission. Traditional spectroscopy accesses the frequency-resolved and time-integrated response, while the temporal evolution stays concealed. However, we will demonstrate here that the temporal evolution of a virtual single-photon transition can be mapped out by a second pulsed electromagnetic field. The resulting optical signal shows optical gain and loss, which can be gated and controlled via the relative delay of the electromagnetic fields.