We present experimental results for the annihilation rate of ortho-positronium in gaseous xenon at 250 kPa using a digital-oscilloscope-based positron annihilation lifetime spectrometer as a function of temperature (300–623 K). Owing to the Zeeman mixing of positronium in a magnetic field, the annihilation rate is divided into two components: one is due to ortho-para postronium spin conversion induced by spin-orbit interaction, while the other is due to pick-off annihilation. The spin-conversion annihilation rate is proportional to $T^{2.1}$, where $T$ is the temperature. We attributed this dependence to the fact that the spin conversion occurs only in $p$-wave scattering during positronium–xenon collisions and to the fact that, after thermalization of the ortho-positronium atoms, their velocity profile follows a Maxwell-Boltzmann distribution. The results also show that the pick-off annihilation rate is almost linear in temperature. This increase is a dependence exceptionally stronger than that observed in other gases.

We consider collisions of electric and magnetic polar molecules, taking the OH radical as an example, subject to combined electric and magnetic static fields. We show that the relative orientation of the fields has an important effect on the collision processes for different fields' magnitudes at different collision energies. This is due to the way the molecules polarize in the combined electric and magnetic fields and hence the way the electric dipole-dipole interaction rises. If OH molecules are confined in magnetic quadrupole traps and if an electric field is applied, molecular collisions will strongly depend on the position as well as the velocity of the molecules. Consequences on the molecular dynamics are discussed.

We investigate Zeeman relaxation in cold Sb(${}^4{S}_{3/2}^{\circ }$)–He collisions in a magnetic field. Ensembles of $>$${10}^{13}$ laser-ablated Sb atoms are cooled in cryogenic ${}^4$He buffer gas to 800 mK and inelastic collisions are observed to equilibrate the $m_J$-state distribution to the translational temperature. The ratio $\gamma$ of momentum transfer to inelastic collision rates is measured to be $⩽9.1\times {10}^2$. We also perform quantum scattering calculations of Sb–${}^4$He collisions, based on ab initio interaction potentials, that demonstrate significant anisotropy of the ground state induced by the spin-orbit interaction. Agreement is obtained between theory and experiment with a $\approx$10$\%$ increase in the ab initio potential depth. This work suggests that buffer-gas-cooled pnictogen atoms lighter than Sb can be loaded into a magnetic trap.

Dissociative electron attachment to carbon monoxide, $e^{-}$ $+$ CO → C $+$ O${}^{-}$, at 9.5, 10.0, and 10.6 eV is investigated by using the anion velocity time-sliced map imaging technique. The completely backward scattering distributions of the fast O${}^{-}$ fragment are observed at 10.0 and 10.6 eV. The single electron-molecule resonance model fails in interpretation to these unusual angular distributions, while a quantum interference model including two (at 10.0 eV) or three (at 10.6 eV) dissociative outgoing waves is consequently proposed and provides the satisfying results about the experimental data fittings. Moreover, coherent interference among the ${}^2$Π, ${}^2$Δ, and ${}^2$Φ resonant states of CO${}^{-}$ at 10.6 eV could be further established, based on the fact that the sum of the phase-shift fitting values equals π rad.

The ionization of a highly excited Rydberg atom subjected to a periodic sequence of electric field impulses, or “kicks,” is chaotic. We focus on the dynamics of a single kicking period in order to isolate the ionization mechanism. Potassium Rydberg atoms, prepared in a quasi-one-dimensional state, are exposed to a sequence of ionization kicks, and the total fraction of ionized atoms is then measured. These experimental data are compared to a one-dimensional classical model. The classical analysis reveals that the ionization process is governed by a phase space turnstile—a geometric structure associated with chaotic transport in diverse systems. The turnstile geometry is reflected in the experimental data. Previous work explored the dependence of the turnstile geometry on the kicking period. The present work explores the dependence on the kicking strength. In particular, increasing the kicking strength allows us to observe the stretching of the turnstile lobe as it penetrates the region of phase space occupied by the electronic state, leading to a sharp rise in the total ionization fraction. This work thus highlights the importance of phase space geometry in organizing chaotic transport in atomic systems.

It is widely believed that the hyperfine states must be resolved in one way or the other to measure the degree of nuclear-spin polarization of atoms and ions. We theoretically show that the angle-resolved photoelectron signals produced by resonant multiphoton ionization by short and hence broadband two-color laser pulses, which cover all fine-structure and hyperfine manifold of the ground and resonant excited states, can serve as a new type of nuclear-spin polarimeter. Because of the use of time-delayed short laser pulses the proposed method has very high (subnanosecond) time resolution, and does not suffer from the Doppler broadening. Specific results are presented for the two-photon resonant three-photon ionization of hydrogen atoms using 205- and 410-nm laser pulses with picosecond to femtosecond pulse durations. The proposed scheme is immediately applicable to other unstable elements such as ${}^{11}$Be${}^{+}$, ${}^{27}$Mg${}^{+}$, and ${}^{35}$Ca${}^{+}$, etc.

We investigate the physical mechanisms responsible for fine structure in the photoelectron angular distributions from atoms subject to intense midinfrared laser fields by solving the time-dependent Schrödinger equation in the integral form. By restricting the ionization to a half cycle of the laser field and then propagating the liberated electron wave packet during the laser pulse, we show conclusively that low-energy-momenta structure in the photoelectron angular distribution originates from multiple scatterings of the tunnel-ionized electron with the ion. We also show that two conditions must be satisfied simultaneously in order to observe prominent low-energy features. First, multiple scattering of the tunnel-ionized electron wave packet is necessary. Second, tunnel ionization must dominate over multiphoton ionization. While the first condition is generally satisfied for all laser wavelengths, the second condition is satisfied only for longer laser wavelengths.

We study the lifetime of an atom trapped in an optical vertical lattice in proximity of a massive surface using a complex-scaling approach. We analyze how the presence of the surface modifies the known lifetimes of Wannier-Stark states associated with Landau-Zener tunneling. We also investigate how the existence of a hypothetical short-distance deviation from Newton's gravitational law could affect these lifetimes. Our study is relevant in order to discuss the feasibility of any atomic-interferometry experiment performed near a surface. Finally, the difficulties encountered in applying the complex-scaling approach to the atom-surface Casimir-Polder interaction are addressed.

Photoexcitation of neutral ethylene to its $V$ state can initiate conformational changes of the molecule. The migration of one hydrogen atom to another carbon site occurs via a nonadiabatic passage and leads to the formation of a stable and asymmetric ethylidene isomer. We present the carbon-$K$-shell molecular-frame photoelectron angular distributions (MFPADs) calculated at relevant geometries in the isomerization of ethylene. The theoretical results are compared with available experimental data at the ground-state geometry of ethylene. Despite the complexity of this system and the presence of two heavy atomic centers, the main features of the photoisomerization of ethylene can be traced through the shape of the MFPADs.

We study composite solitons, consisting of domain walls and vortex lines attaching to the walls in two-component Bose-Einstein condensates. When the total density of the two components is homogeneous, the system can be mapped to the O(3) nonlinear $\sigma$ model for the pseudospin representing the two-component order parameter, and the analytical solutions of the composite solitons can be obtained. Based on the analytical solutions, we discuss the detailed structure of the composite solitons in two-component condensates by employing the generalized nonlinear $\sigma$ model, where all degrees of freedom of the original Gross-Pitaevskii theory are active. The domain wall pulled by a vortex is logarithmically bent as a membrane pulled by a pin. It bends more flexibly than the domain wall in the $\sigma$ model, because the density inhomogeneity results in a reduction of the domain wall tension from that in the $\sigma$ model limit. We find, however, that the curvature of the wall bending pulled by a vortex is still greater than that expected from the reduced tension due to only the density inhomogeneity. Finally, we study the composite soliton structure for actual experimental situations with trapped immiscible condensates under rotation, through numerical simulations of the coupled Gross-Pitaevskii equations.

We revisit ground states of spinor Bose-Einstein condensates with a Rashba spin-orbit coupling, and find that various types of vortices show up without rotation as a direct consequence of spontaneous symmetry breaking into a combined gauge, spin, and space rotation symmetry. The combined symmetry also determines the vortex-core spin state at the rotating center. For the continuous combined symmetry, the spin rotates by $2\pi$ around the rotating axis, whereas for the discrete combined symmetry, we further need $2F$ quantum numbers to determine how much the spin rotates for the spin-$F$ system. For lattice phases we find that in the ground state the topological charge for each unit cell vanishes. However, we find two types of highly symmetric lattices with nontrivial topological charges for the spin-$\frac{1}{2}$ system based on the symmetry classification, and show that they are Skyrmion crystals.

The Chern number $\nu$, as a topological invariant that identifies the winding of the ground state in the particle-hole space, is a definitive theoretical signature that determines whether a given superconducting system can support Majorana zero modes. Here we show that such a winding can be faithfully identified for any superconducting system ($p$ wave or $s$ wave with spin-orbit coupling) through a set of time-of-flight measurements, making it a diagnostic tool also in actual cold-atom experiments. As an application, we customize the measurement scheme for a chiral topological model of spinless fermions. The proposed model only requires the experimentally accessible $s$-wave pairing and staggered tunneling that mimics spin-orbit coupling. By adiabatically connecting this model to Kitaev's honeycomb lattice model, we show that it gives rise to $\nu =\pm 1$ phases, where vortices bind Majorana fermions, and $\nu =\pm 2$ phases that emerge as the unique collective state of such vortices. Hence, the preparation of these phases and the detection of their Chern numbers provide an unambiguous signature for the presence of Majorana modes. Finally, we demonstrate that our detection procedure is resilient against most inaccuracies in experimental control parameters as well as finite temperature.

We present an analysis of the excitation spectrum for a two-component quasi-two-dimensional Bose-Einstein condensate (BEC). We study how excitations change character across the miscible to immiscible phase transition. We find that the bulk excitations are typical of a single-component BEC with the addition of interface bending excitations. We study how these excitations change as a function of the interaction strength.

A simple algorithm is presented based on a type of partial reverse engineering that generates an unlimited number of exact analytical solutions to the Schrödinger equation for a general time-dependent two-level Hamiltonian. I demonstrate this method by deriving exact solutions corresponding to fast control pulses that contain arbitrarily many tunable parameters. It is shown that the formalism is naturally suited to generating analytical control protocols that perform precise nonadiabatic rapid passage and Landau-Zener interferometry near the quantum speed limit. A general, exact formula for Landau-Zener interference patterns is derived.

Strong cross-Kerr nonlinearities have been long sought after for quantum information applications. Recent work has shown that they are intrinsically unreliable in traveling-wave configurations: cavity configurations avoid this, but require knowledge of both the nonlinearity and the loss. Here we present a detailed systematic study of cross-phase modulation and absorption in an Rb vapor confined within a hollow-core photonic crystal fiber. Using a two-photon transition, we observe phase modulations of up to $\pi$ rad with a signal power of 25 $\mu$W, corresponding to a nonlinear Kerr coefficient, $n_2$, of $0.8\times {10}^{-6}$ cm${}^2$/W, or $1.3\times {10}^{-6}$ rad per photon.

The light-matter interaction described by the Rabi model and Jaynes-Cummings (JC) model under local bias fields is investigated by parity breaking as well as the scaling behavior of the ground state. We show that the parity breaking leads to different scaling behaviors in the two models, where the biased Rabi model demonstrates scaling invariance, but the biased JC model behaves in a cusp-like way. Our study helps further understand the rotating-wave approximation and could present more subtle physics than other characteristic parameters for the difference between the two models. More importantly, the scaling behavior is observable using currently available techniques in light-matter interaction.

In this paper, we propose an open template method (OTM) that can make a completely invisible open tunnel for any closed cylindrical metamaterial device to allow the exchange of inner and outer substances. Such structure is realized by coating a well-designed negative index material (NIM) shell on the surface of a sectorial cylinder. The open tunnel is concealed by the complementary media that consists of four pairs of NIM and vacuum. With no approximation, OTM can provide theoretically complete invisibility for these open tunnels of any size as desired. Analytical expression of OTM can be deduced based on the concept of complementary media. Ideal performance of three open metamaterial devices is demonstrated by numerical simulations.

We present a simple method to reconstruct the mode distribution of multimode classical and nonclassical optical fields using a single measurement of higher-order photon number correlation functions. Knowing the underlying number and structure of occupied modes of a light field plays a crucial role in minimizing loss and decoherence of quantum information. Typically, full characterization of the mode structure involves a series of several separate measurements in spatial, temporal, frequency, and polarization domains. We experimentally demonstrate reconstruction of up to three modes with excellent agreement and study the robustness of our method in experimentally realizable regimes.

We study the effects of four-wave mixing (4WM) in a quantum memory scheme based on electromagnetically induced transparency (EIT). We treat the problem of field propagation on the quantum mechanical level, which allows us to calculate the fidelity of propagation for a quantum light pulse such as a single photon. While 4WM can be beneficial for classical, all-optical information storage, the quantum noise associated with the signal amplification and idler generation is, in general, detrimental for a quantum memory. We identify a range of parameters where 4WM makes a single-photon quantum memory impossible.

We develop efficient algorithms for compiling single-qubit unitary gates into circuits over the universal $V$ basis. The $V$ basis is an alternative universal basis to the more commonly studied basis consisting of Hadamard and $\pi /8$ gates. We propose two classical algorithms for quantum circuit compilation: the first algorithm has expected polynomial time [in precision $\log (1/\epsilon )$] and produces an $\epsilon$ approximation to a single-qubit unitary with a circuit depth $\le 12{\log }_5(2/\epsilon )$. The second algorithm performs optimized direct search and yields circuits a factor of 3 to 4 times shorter than our first algorithm, but requires time exponential in $\log (1/\epsilon )$; however, we show that in practice the runtime is reasonable for an important range of target precisions. Decomposing into the $V$ basis may offer advantages when considering the fault-tolerant implementation of quantum circuits.