We have performed a kinematically complete experiment for transfer ionization, i.e., the capture of one target electron by the projectile accompanied by the ejection of a second electron, for 50–100 keV $p$-He collisions by means of a reaction microscope. It was found that the electron momentum distribution projected onto the scattering plane is consistent with the prediction of an independent two-step mechanism involving a binary encounter. The deviation of the electron momentum distribution from the ideal binary ridge is attributed to binding energy effects and the interaction between the electron and the residual recoil ion. The good agreement between our measurements with the dynamic-screening classical trajectory Monte-Carlo calculations strongly suggests that electron-electron correlations play an important role in this two-electron process.

The exothermic proton transfer reaction ${\mathrm{D}}^{-}+{\mathrm{H}}_2\to \text{HD}+{\mathrm{H}}^{-}$ is known to proceed over a barrier of about 0.33 eV. Here we investigate whether this reaction may occur at low temperatures via tunneling through this barrier. The experiments were carried out in a cryogenic 22-pole ion trap, which provides a high sensitivity for slow ion-molecule reactions. Our experiments show no sign of the tunneling reaction with an upper limit to the rate coefficient of $2.6\times {10}^{-18}$ ${\mathrm{cm}}^3/\mathrm{s}$ obtained from the decrease of the ${\mathrm{D}}^{-}$ signal and $9\times {10}^{-19}$ ${\mathrm{cm}}^3/\mathrm{s}$ from the absence of an increase of ${\mathrm{H}}^{-}$. Background impurities were identified to be the main limitation of the sensitivity.

We present mean-field calculations of azimuthally averaged retarded dipole-dipole interactions in a Bose-Einstein condensate induced by a laser, at both long and short wavelengths. Our calculations demonstrate that dipole-dipole interactions become significantly stronger at shorter wavelengths, by as much as 30-fold, due to retardation effects. This enhancement, along with the inclusion of the dynamic polarizability, indicate a method of inducing long-range interatomic interactions in neutral atom condensates at significantly lower intensities than previously realized.

In this work we theoretically investigate the stochastic resonance (SR) effect in an optomechanical membrane system subject to two weak signals (one optical field and one mechanical force). The quadratic optomechanical coupling allows us to find a region with tristability where the noise-activated stochastic switching among three stable states occurs and SR phenomena are observed at the cooperation of input signals and noise. We show that the mechanical force and the optical field respectively serve as an additive signal and a multiplicative signal to the membrane position, and they induce completely different SR behaviors. Moreover, when two signals coexist the SR effect can be enhanced, and the beating effect appears in the SR synchronization process with unsynchronized signals.

We perform a detailed analysis of the nonclassical properties of a damped two-level system. We compute and compare three different criteria of quantumness, the $l_1$ norm of coherence, the Leggett-Garg inequality, and a quantum witness based on the no-signaling in time condition. We show that all three quantum indicators decay exponentially in time as a result of the coupling to the thermal reservoir. We further demonstrate that the corresponding characteristic times are identical and given by the coherence half-life. These results quantify how violations of Leggett-Garg inequalities and nonzero values of the quantum witness are connected to the coherence of the two-level system.

Recent experiments using innovative optical detectors and techniques have strongly increased the capacity of testing the violation of the Bell's inequalities in Nature. Most of them have used the Eberhard’s inequality (EI) to close the “detection” loophole. Closing the “locality” loophole has been attempted by spacelike separated detections and fast changes of the bases of observation, driven by random number generators of new design. Also, pulsed pumping and time-resolved data recording to close the “time-coincidence” loophole, and sophisticated statistical methods to close the “memory” loophole, have been used. In this paper, the meaning of the EI is reviewed. A simple hidden variables theory based on a relaxation of the condition of “measurement independence,” which was devised long ago for the Clauser-Horne-Shimony and Holt inequality, is adapted to the EI case. It is used here to evaluate the significance of the results of the recent experiments, which are briefly described. A table summarizes the main results.

Multiparty quantum states are useful for a variety of quantum information and computation protocols. We define a multiparty entanglement measure based on local measurements on a multiparty quantum state and an entanglement measure averaged on the postmeasurement ensemble. Using the generalized geometric measure as the measure of multipartite entanglement for the ensemble, we demonstrate, in the case of several well-known classes of multipartite pure states, that the localized multipartite entanglement can exceed the entanglement present in the original state. We also show that measurement over multiple parties may be beneficial in enhancing localizable multipartite entanglement. We point out that localizable generalized geometric measure faithfully signals quantum critical phenomena in well-known quantum spin models even when considerable finite-size effect is present in the system.

We consider a system of finite number of particles collectively interacting with a single-mode coherent field inside a cavity. Depending on the strength of the initial field compared to the number of atoms, we consider three regimes of weak-, intermediate-, and strong-field interaction. The dynamics of multiparticle entanglement detected by quantum Fisher information and spin squeezing are studied in each regime. It is seen that in the weak-field regime, spin squeezing and quantum Fisher information coincide. However, by increasing the initial field population toward the strong-field regime, quantum Fisher information is more effective in detecting entanglement compared to spin squeezing. In addition, in the two-atom system, we also study concurrence. In this case, the quantum Fisher information as a function of time is in good agreement with concurrence in predicting entanglement peaks.

One limitation of the quantum computing capability of a continuous-variable system is determined by our ability to cool it to the ground state, because pure logical states, in which we accurately encode quantum information, are conventionally pure physical states that are constructed from the ground state. In this work, we present an alternative quantum computing formalism that encodes logical quantum information in mixed physical states. We introduce a class of mixed-state protocols that are based on a parity encoding, and propose an implementation of the universal logic gates by using realistic hybrid interactions. When compared with the conventional pure-state protocols, our formalism could relax the necessity, and hence the systemic requirements, of cooling. Additionally, the mixed-state protocols are inherently resilient to a wider class of noise processes and reduce the fundamental energy consumption in initialization. Our work broadens the range of candidates for continuous-variable quantum computers.

Research on ultracold molecules has seen a growing interest recently in the context of high-resolution spectroscopy and quantum computation. After forming weakly bound molecules from atoms in cold collisions, the preparation of molecules in low vibrational levels of the ground state is experimentally challenging, and typically achieved by population transfer using excited electronic states. Accurate potential energy surfaces are needed for a correct description of processes such as the coherent de-excitation from the highest and therefore weakly bound vibrational levels in the electronic ground state via couplings to electronically excited states. This paper is dedicated to the vibrational analysis of potentially relevant electronically excited states in the alkali-metal (Li, Na, K, Rb)– alkaline-earth metal (Ca,Sr) diatomic series. Graphical maps of Frank-Condon overlap integrals are presented for all molecules of the group. By comparison to overlap graphics produced for idealized potential surfaces, we judge the usability of the selected states for future experiments on laser-enhanced molecular formation from mixtures of quantum degenerate gases.

We report a combined experimental and theoretical study of the electron-impact ionization of water (${\mathrm{H}}_2\mathrm{O}$) at the relatively low incident energy of $E_0=81\mathrm{eV}$ in which either the $1b_1$ or $3a_1$ orbitals are ionized leading to the stable ${\mathrm{H}}_2{\mathrm{O}}^{+}$ cation. The experimental data were measured by using a reaction microscope, which can cover nearly the entire $4\pi$ solid angle for the secondary electron emission over a range of ejection energies. We present experimental data for the scattering angles of $6^{\circ }$ and ${10}^{\circ }$ for the faster of the two outgoing electrons as a function of the detection angle of the secondary electron with energies of 5 and 10 eV. The experimental triple-differential cross sections are internormalized across the measured scattering angles and ejected energies. The experimental data are compared with predictions from two molecular three-body distorted-wave approaches: one applying the orientation-averaged molecular orbital (OAMO) approximation and one using a proper average (PA) over orientation-dependent cross sections. The PA calculations are in better agreement with the experimental data than the OAMO calculations for both the angular dependence and the relative magnitude of the observed cross-section structures.

Ab initio calculations and normalized experimental measurements of the differential, momentum transfer and integral cross sections for vibrationally elastic scattering of low-energy electrons from orthoxylene, metaxylene, and paraxylene are presented. The calculated cross sections are obtained using the Schwinger multichannel method implemented with norm-conserving pseudopotentials. The differential cross sections are measured at incident energies from 1 to 30 eV and scattering angles from ${10}^{\circ }$ to ${130}^{\circ }$. These cross sections are compared to experimental results for toluene. The comparisons illuminate the role of molecular structure in determining the integral cross sections and the angular distributions of resonantly scattered electrons.

The theory of the long-range interaction of metastable excited atomic states with ground-state atoms is analyzed. We show that the long-range interaction is essentially modified when quasidegenerate states are available for virtual transitions. A discrepancy in the literature regarding the van der Waals coefficient $C_6(2S;1S)$ describing the interaction of metastable atomic hydrogen ($2S$ state) with a ground-state hydrogen atom is resolved. In the the van der Waals range $a_0\ll R\ll a_0/\alpha$, where $a_0=\hslash /\alpha mc$ is the Bohr radius and $\alpha$ is the fine-structure constant, one finds the symmetry-dependent result $E_{2S;1S}\left(R\right)\approx (-176.75\pm 27.98)E_h{(a_0/R)}^6$ ($E_h$ denotes the Hartree energy). In the Casimir-Polder range $a_0/\alpha \ll R\ll \hslash c/\mathcal{L}$, where $\mathcal{L}\equiv E\left(2S_{1/2}\right)-E\left(2P_{1/2}\right)$ is the Lamb shift energy, one finds $E_{2S;1S}\left(R\right)\approx (-121.50\pm 46.61)E_h{(a_0/R)}^6$. In the the Lamb shift range $R\gg \hslash c/\mathcal{L}$, we find an oscillatory tail with a negligible interaction energy below ${10}^{-36}\mathrm{Hz}$. Dirac-$\delta$ perturbations to the interaction are also evaluated and results are given for all asymptotic distance ranges; these effects describe the hyperfine modification of the interaction or, expressed differently, the shift of the hydrogen $2S$ hyperfine frequency due to interactions with neighboring $1S$ atoms. The $2S$ hyperfine frequency has recently been measured very accurately in atomic beam experiments.

The interaction of two excited hydrogen atoms in metastable states constitutes a theoretically interesting problem because of the quasidegenerate $2P_{1/2}$ levels that are removed from the $2S$ states only by the Lamb shift. The total Hamiltonian of the system is composed of the van der Waals Hamiltonian, the Lamb shift, and the hyperfine effects. The van der Waals shift becomes commensurate with the $2S-2P_{3/2}$ fine-structure splitting only for close approach ($R<100a_0$, where $a_0$ is the Bohr radius) and one may thus restrict the discussion to the levels with $n=2$ and $J=1/2$ to a good approximation. Because each $S$ or $P$ state splits into an $F=1$ triplet and an $F=0$ hyperfine singlet (eight states for each atom), the Hamiltonian matrix a priori is of dimension 64. A careful analysis of the symmetries of the the problem allows one to reduce the dimensionality of the most involved irreducible submatrix to 12. We determine the Hamiltonian matrices and the leading-order van der Waals shifts for states that are degenerate under the action of the unperturbed Hamiltonian (Lamb shift plus hyperfine structure). The leading first- and second-order van der Waals shifts lead to interaction energies proportional to $1/R^3$ and $1/R^6$ and are evaluated within the hyperfine manifolds. When both atoms are metastable $2S$ states, we find an interaction energy of order $E_h\chi {(a_0/R)}^6$, where $E_h$ and $\mathcal{L}$ are the Hartree and Lamb shift energies, respectively, and $\chi =E_h/\mathcal{L}\approx 6.22\times {10}^6$ is their ratio.

Signatures of the Coulomb corrections in the photoelectron momentum distribution during laser-induced ionization of atoms or ions in tunneling and multiphoton regimes are investigated analytically in the case of a one-dimensional problem. A high-order Coulomb-corrected strong-field approximation is applied, where the exact continuum state in the $S$ matrix is approximated by the eikonal Coulomb-Volkov state including the second-order corrections to the eikonal. Although without high-order corrections our theory coincides with the known analytical $R$-matrix (ARM) theory, we propose a simplified procedure for the matrix element derivation. Rather than matching the eikonal Coulomb-Volkov wave function with the bound state as in the ARM theory to remove the Coulomb singularity, we calculate the matrix element via the saddle-point integration method by time as well as by coordinate, and in this way avoiding the Coulomb singularity. The momentum shift in the photoelectron momentum distribution with respect to the ARM theory due to high-order corrections is analyzed for tunneling and multiphoton regimes. The relation of the quantum corrections to the tunneling delay time is discussed.

Multimode quantum states, such as multipartite quantum entanglement or quantum correlations, are important for both fundamental science and the future development of quantum technologies. Here we theoretically propose and experimentally realize a scheme that can fully exploit the multi-spatial-mode nature of the four-wave-mixing (FWM) process, i.e., spatial multiplexing, and thus integrates multiple FWM processes into a single cell at each stage of the cascaded process. The number of generated quantum-correlated beams $2^n$ is exponentially dependent on the number of vapor cells $n$. In addition, the quantum correlations between the multiple beams also increase as the number of vapor cell increases. For the case of $n=2$, we experimentally show that the degree of intensity-difference squeezing between the four quantum-correlated beams in our scheme is enhanced to $-8.2\pm 0.2$ dB from $-5.6\pm 0.3$ and $-6.5\pm 0.2$ dB of squeezing obtained with a single FWM process. Our system may find applications in quantum information and precision measurement.

We investigate the dynamics of single- and multiphoton emission from detuned strongly coupled systems based on the quantum-dot–photonic-crystal resonator platform. Transmitting light through such systems can generate a range of nonclassical states of light with tunable photon counting statistics due to the nonlinear ladder of hybridized light-matter states. By controlling the detuning between emitter and resonator, the transmission can be tuned to strongly enhance either single- or two-photon emission processes. Despite the strongly dissipative nature of these systems, we find that by utilizing a self-homodyne interference technique combined with frequency filtering we are able to find a strong two-photon component of the emission in the multiphoton regime. In order to explain our correlation measurements, we propose rate equation models that capture the dominant processes of emission in both the single- and multiphoton regimes. These models are then supported by quantum-optical simulations that fully capture the frequency filtering of emission from our solid-state system.

We calculate the Casimir–Polder frequency shift and decay rate for an atom in front of a nonreciprocal medium by using macroscopic quantum electrodynamics. The results are a generalization of the respective quantities for matter with broken time-reversal symmetry which does not fulfill the Lorentz reciprocity principle. As examples, we contrast the decay rates, the resonant and nonresonant frequency shifts of a perfectly conducting (reciprocal) mirror with those of a perfectly reflecting nonreciprocal mirror. We find different power laws for the distance dependence of all quantities in the retarded and nonretarded limits. As an example of a more realistic nonreciprocal medium, we investigate a topological insulator subject to a time-symmetry-breaking perturbation.

We investigate the coupled-system dynamics of two-level quantum dots placed on a vibrating nanomechanical resonator. The ensemble of quantum dots exhibits superradiance features which are transferred to the mechanical degrees of freedom representing fast quantum dynamics and enhanced phonon emission in a nanomechanical setup, resembling the superradiance effect.