The process of recombination of a quasifree electron into a bound state of an initially bare nucleus with the simultaneous creation of a bound-electron–free-positron pair is investigated. This process is called negative-continuum-assisted dielectronic recombination (NCDR). In a typical experimental setup, the initial electron is not free but bound in a light atomic target. In the present work, we study the effects of the atomic target on the single- and double-differential cross sections of positron production in the NCDR process. Calculations are performed within the relativistic framework based on QED theory, accounting for the electron-electron interaction to first order in perturbation theory. We demonstrate how the momentum distribution of the target electrons removes the nonphysical singularity of the differential cross section which occurs for the initially free and monochromatic electrons.

The manipulation of trapped charged particles by electric fields is an accurate, robust, and reliable technique for many applications or experiments in high-precision spectroscopy. The transfer of an ion sample between multiple traps allows the use of a tailored environment in quantum information, cold chemistry, or frequency metrology experiments. In this article, we experimentally study the transport of ion clouds of up to $80000$ ions over a distance of 20 mm inside a linear radio-frequency trap. Ion transport is controlled by a transfer function, which is designed taking into account the local electric potentials. We observe that the ion response is very sensitive to the details of the description of the electric potential. Nevertheless, we show that fast transport—with a total duration of 100 $\mu s$—results in transport efficiencies attaining values higher than 90% of the ion number, even with large ion clouds. For clouds smaller than 2000 ions, a 100% transfer efficiency is observed. Transport induced heating, which depends on the transport duration, is also analyzed.

We study the time evolution of two coupled many-body quantum systems, one of which is assumed to be Bose condensed. Specifically, we consider two ultracold atomic clouds each populating two localized single-particle states, i.e., a two-component bosonic Josephson junction. The cold atom cloud can retain its coherence when coupled to the condensate and displays synchronization with the latter, differing from usual entrainment. We term this effect among the ultracold and the condensed clouds as hybrid synchronization. The onset of synchronization, which we observe in the evolution of average properties of both gases when increasing their coupling, is found to be related to the many-body properties of the quantum gas, e.g., condensed fraction quantum fluctuations of the particle number differences. We discuss the effects of different initial preparations and the influence of unequal particle numbers for the two clouds, and we explore the dependence on the initial quantum state, e.g., coherent state, squeezed state, and Fock state, finding essentially the same phenomenology in all cases.

We present a perturbation technique to study the linear and nonlinear output characteristics of coherent photon transport in a parity-time ($\mathcal{PT}$)-symmetric double-microcavity system where one passive cavity contains a single quantum emitter. It is found that (i) for the linear transmission of a low-power input probe field, the output spectra of the proposed $\mathcal{PT}$-symmetric system exhibit a single transparent resonance dip and two symmetric, strongly amplifying sidebands, i.e., an inverted dipole-induced transparency; and (ii) for the nonlinear transmission of the input probe field, giant optical third-order nonlinearities with high linear transmission rate and vanishing nonlinear absorption can be achieved efficiently when the system parameters are tuned properly so that a $\mathcal{PT}$-symmetry phase transition occurs. The obtained results can be useful for quantum information processing, quantum nondemolition measurements of photons, and optical signal processing.

An important topic of interest in imaging is the construction of protocols that are not diffraction limited. This can be achieved in a variety of ways, including classical superresolution techniques or quantum entanglement-based protocols. Here, we consider superresolving imaging in the far field using higher-order intensity correlations. We show that third- and fourth-order correlations can improve upon the first- and second-order correlations that are traditionally used in classical optics and Hanbury Brown–Twiss-type experiments. The improvement is achieved entirely by post-processing of the data. As a demonstrator, we simulate the far field intensity distribution of a circular aperture that emits thermal light and use maximum likelihood estimation to determine the radius of the aperture. We compare the achieved precision to the Cramér-Rao lower bound and find that the variance of measurements for the third- and fourth-order correlation functions are indeed closer to the Cramér-Rao bound than that of the second-order correlation function. The method presented here is general, and can be used for all kinds of incoherent emitters, geometries, and types of noise.

The advent of attosecond techniques opens up the possibility to observe experimentally electron dynamics following ionization of molecules. Theoretical studies of pure electron dynamics at single fixed nuclear geometries in molecules have demonstrated oscillatory charge migration at a well-defined frequency but often neglecting the natural width of the nuclear wave packet. The effect on electron dynamics of the spatial delocalization of the nuclei is an outstanding question. Here, we show how the inherent distribution of nuclear geometries leads to dephasing. Using a simple analytical model, we demonstrate that the conditions for a long-lived electronic coherence are a narrow nuclear wave packet and almost parallel potential-energy surfaces of the states involved. We demonstrate with numerical simulations the decoherence of electron dynamics for two real molecular systems (paraxylene and polycyclic norbornadiene), which exhibit different decoherence time scales. To represent the quantum distribution of geometries of the nuclear wave packet, the Wigner distribution function is used. The electron dynamics decoherence result has significant implications for the interpretation of attosecond spectroscopy experiments since one no longer expects long-lived oscillations.

Continuous measurements play a pivotal role in the study of dynamical open quantum systems. “Dyne” detections are among the most widespread and efficient measurement schemes, and give rise to quantum diffusion of the conditioned state. In this work we study under what conditions the detector dependency of the conditional state of a quantum system subject to diffusive monitoring can be demonstrated experimentally, in the sense of ruling out any detector-independent pure-state dynamical model for the system. We consider an arbitrary number $L$ of environments to which the system is coupled, and an arbitrary number $K$ of different types of dyne detections. We prove that nontrivial necessary conditions for such a demonstration can be determined efficiently by semidefinite programming. To determine sufficient conditions, different physical environmental couplings and Hamiltonians for a qubit and different sets of diffusive monitorings are scrutinized. We compare the threshold efficiencies that are sufficient in the various cases, as well as cases previously considered in the literature, to suggest the most feasible experimental options.

We search for the optimal quantum pure states of identical bosonic particles for applications in quantum metrology, in particular, in the estimation of a single parameter for the generic two-mode interferometric setup. We consider the general case in which the total number of particles is fluctuating around an average $N$ with variance $\mathrm{\Delta }{N}^2$. By recasting the problem in the framework of classical probability, we clarify the maximal accuracy attainable and show that it is always larger than the one reachable with a fixed number of particles (i.e., $\mathrm{\Delta }N=0)$. In particular, for larger fluctuations, the error in the estimation diminishes proportionally to $1/\mathrm{\Delta }N$, below the Heisenberg-like scaling $1/N$. We also clarify the best input state, which is a quasi-NOON state for a generic setup and, for some special cases, a two-mode Schrödinger-cat state with a vacuum component. In addition, we search for the best state within the class of pure Gaussian states with a given average $N$, which is revealed to be a product state (with no entanglement) with a squeezed vacuum in one mode and the vacuum in the other.

Using geometric means, we first consider a density matrix decomposition of a multipartite quantum system of a finite dimension into two density matrices: a separable one, also known as the best separable approximation, and an essentially entangled one, which contains no product state components. We show that this convex decomposition can be achieved in practice with the help of a linear programming algorithm that scales in the general case polynomially with the system dimension. We illustrate the algorithm implementation with an example of a composite system of dimension 12 that undergoes a loss of coherence due to classical noise and we trace the time evolution of its essentially entangled component. We suggest a “geometric” description of entanglement dynamics and demonstrate how it explains the well-known phenomena of sudden death and revival of multipartite entanglements. For a statistical weight loss of the essentially entangled component with time, its average entanglement content is not affected by the coherence loss.

Direct evidence of excitation of the $5p_{3/2}\to 6p_{3/2}$ electric-dipole-forbidden transition in atomic rubidium is presented. The experiments were performed in a room-temperature rubidium cell with continuous-wave external cavity diode lasers. Optical-optical double-resonance spectroscopy with counterpropagating beams allows the detection of the nondipole transition free of Doppler broadening. The $5p_{3/2}$ state is prepared by excitation with a laser locked to the maximum $F$ cyclic transition of the $D_2$ line, and the forbidden transition is produced by excitation with a 911 nm laser. Production of the forbidden transition is monitored by detection of the 420 nm fluorescence that results from decay of the $6p_{3/2}$ state. Spectra with three narrow lines ($\approx 13$ MHz FWHM) with the characteristic $F-1, F$, and $F+1$ splitting of the $6p_{3/2}$ hyperfine structure in both rubidium isotopes were obtained. The results are in very good agreement with a direct calculation that takes into account the $5s\to 5p_{3/2}$ preparation dynamics, the $5p_{3/2}\to 6p_{3/2}$ nondipole excitation geometry, and the $6p_{3/2}\to 5s_{1/2}$ decay. The comparison also shows that the electric-dipole-forbidden transition is a very sensitive probe of the preparation dynamics.

The photoionization of ${\mathrm{H}}_2^{+}$ molecular ions is investigated for Bessel beams of twisted light. In particular, the angle-differential photoionization cross sections are evaluated for a macroscopic target of randomly distributed but initially aligned ions by using the nonrelativistic first-order perturbation theory. Detailed calculations of these cross sections and angular distributions are performed for different setups of the electron detectors and for selected opening angles of the Bessel beams and are compared with those for incident plane-wave radiation. It is shown that the modification in the angular distributions of the photoelectrons can be understood quite easily from the variations in the intensity pattern of the Bessel beams, relative to the size of the ${\mathrm{H}}_2^{+}$ molecular ions.

We report the long-time nonlinear dynamical evolution of ultracold atomic gases in the $P$ band of an optical lattice. A Bose-Einstein condensate is fast and efficiently loaded into the $P$ band at zero quasimomentum with a nonadiabatic shortcut method. For the first 1.5 ms, this momentum state undergoes oscillations due to coherent superposition of different bands, which are followed by oscillations up to 60 ms of a much longer period. Our analysis shows the dephasing from the nonlinear interaction is very conducive to the long-period oscillations induced by the variable force due to the harmonic confinement.

We study the attractive Bose-Hubbard model with a tunable, on-site three-body constraint. It is shown that the critical behavior of the system undergoing a phase transition from pair superfluid to superfluid at unit filling depends on the value of the three-body repulsion. In particular, we calculate critical exponents and the central charge governing the quantum phase transition.

We consider a few-boson system confined to one dimension with a single distinguishable particle of lesser mass. All particle interactions are modeled with $\delta$ functions, but due to the mass imbalance the problem is nonintegrable. Universal few-body binding energies, atom-dimer and atom-trimer scattering lengths, are all calculated in terms of two parameters, namely the mass ratio $m_{\text{L}}/m_{\text{H}}$, and ratio $g_{\text{HH}}/g_{\text{HL}}$ of the $\delta$-function couplings. We specifically identify the values of these ratios for which the atom-dimer or atom-trimer scattering lengths vanish or diverge. We identify regions in this parameter space in which various few-body inelastic processes become energetically allowed. In the Tonks-Girardeau limit ($g_{\text{HH}}\to \infty$), our results are relevant to experiments involving trapped fermions with an impurity atom.

Based on perturbation theory up to second order, analytical asymptotic expressions for the variation of the population of hyperfine component $\mu =0$ particles in the ground state of spin-1 condensates against a magnetic field $B$ have been derived. The ranges of $B$ in which the asymptotic expressions are applicable have been clarified via a comparison of the numerical results from the analytical expressions and from a diagonalization of the Hamiltonian in a complete spin space. It was found that, for ${}^{87}\mathrm{Rb}$, the two analytical expressions, one for a weak field and the other one for a strong field, together cover the whole range of $B$ from zero to infinity. For Na, the analytical expressions are valid only if $B$ is very weak or sufficiently strong.

We study the effect of spin-orbit coupling on both the zero-temperature and nonzero-temperature behavior of a two-dimensional Fermi gas. We include a generic combination of Rashba and Dresselhaus terms into the system Hamiltonian, which allows us to study both the experimentally relevant equal-Rashba-Dresselhaus (ERD) limit and the Rashba-only (RO) limit. At zero temperature, we derive the phase diagram as a function of the two-body binding energy and Zeeman field. In the ERD case, this phase diagram reveals several topologically distinct uniform superfluid phases, classified according to the nodal structure of the quasiparticle excitation energies. Furthermore, we use a momentum-dependent SU(2) rotation to transform the system into a generalized helicity basis, revealing that spin-orbit coupling induces a triplet pairing component of the order parameter. At nonzero temperature, we study the Berezinskii-Kosterlitz-Thouless (BKT) phase transition by including phase fluctuations of the order parameter up to second order. We show that the superfluid density becomes anisotropic due to the presence of spin-orbit coupling (except in the RO case). This leads both to elliptic vortices and antivortices, and to anisotropic sound velocities. The latter prove to be sensitive to quantum phase transitions between topologically distinct phases. We show further that at a fixed nonzero Zeeman field, the BKT critical temperature is increased by the presence of ERD spin-orbit coupling. Subsequently, we demonstrate that the Clogston limit becomes infinite: $T_{\mathrm{BKT}}$ remains nonzero at all finite values of the Zeeman field. We conclude by extending the quantum phase transition lines to nonzero temperature, using the nodal structure of the quasiparticle spectrum, thus connecting the BKT critical temperature with the zero-temperature results.

We consider an all-optical photon switch in an atomic gas which is gated via a so-called Rydberg spin wave, i.e., a single Rydberg excitation that is coherently shared by the whole ensemble. A current challenge is to preserve the coherence of the Rydberg spin wave during the operation of the switch, which would enable, for example, its coherent optical readout. By employing a simple model description we investigate how the coherence of the Rydberg spin wave is affected during the operation of the switch. The coherence becomes increasingly protected with growing interatomic interaction strength. For the strongly interacting limit we derive analytical expressions for the spin-wave fidelity as a function of the optical depth and bandwidth of the incoming photon.

We propose and demonstrate theoretically that vertical-external-cavity surface-emitting lasers (VECSELs) with external flat mirrors can be stabilized by applying a periodic spatiotemporal modulation of the pump current. Such pump modulation is shown to suppress the pattern-forming instabilities (modulation instabilities), which eventually results in stable beam emission. A modified Floquet linear stability analysis is used to characterize the dynamics of the modulated system and to evaluate its stabilization performance. Stability maps identify the regions in parameter space for complete and partial stabilization of VECSELs operating in different regimes depending on the external-cavity length. In particular, the stabilization method is shown to operate most efficiently in Class-A laser limit (for relatively long VECSEL resonators), while it becomes ineffective in Class-B laser limit (for relatively short resonators). The stabilization effect is further confirmed through direct integration of the dynamical equations.

Based on the Heisenberg-picture analog of the master equation, we develop a method for computing the exact time dependence of noise-averaged observables for general noninteracting fermionic systems with noisy fluctuations. Upon noise averaging, these fluctuations generate effective interactions, limiting analytical approaches. While the short-time dynamics can be studied with Langevin-type numerical simulations, the long-time limit is not amenable to such simulations. Our results provide access to this long-time limit. As a simple example, we examine the fate of the fractional charge in cold-atom emulations of polyacetylene after stochastic driving. We find that in a quantum quench to a fluctuating hopping Hamiltonian, the fractional charge remains robust for hopping between different sublattices, while it becomes unstable in the presence of noisy hopping on the same sublattice.