Single-photon multiple ionization of the argon dimer van der Waals complex, ${\mathrm{Ar}}_2$, is studied by the photoelectron-photoion-photoion coincidence technique using synchrotron radiation in the photon-energy range of 255–340 eV, which covers the Ar 2$p$ and Ar 2$s$ ionization continua. Dissociation processes into ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$, ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{2+}$, ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{3+}$, and ${\mathrm{Ar}}^{2+}+{\mathrm{Ar}}^{2+}$ ion pair channels are observed. The ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{+}$ and ${\mathrm{Ar}}^{+}+{\mathrm{Ar}}^{2+}$ channels show the most intense ion-ion coincidences, compared to other observed dissociative channels. For the four observed channels the intensities are integrated and compared as functions of photon energy.

Pulsed supersonic beams of helium atoms in the high-field-seeking $52p$ Rydberg state have been guided while following helical trajectories in the electrostatic field of a charged wire suspended along the axis of a grounded cylindrical metallic tube. The operation of this electrostatic wire guide for samples in high-field-seeking Rydberg-Stark states has been investigated with dc and pulsed electric potentials applied to the wire while the guided atoms were detected by pulsed electric-field ionization. The acceptance of the guide has been determined through numerical particle trajectory simulations.

In this work, we propose a single-atom interferometer based on a fully two-dimensional spatial adiabatic passage process using a system of three identical harmonic traps in a triangular geometry. While the transfer of a single atom from the ground state of one trap to the ground state of the most distant one can successfully be achieved in a robust way for a broad range of parameter values, we point out the existence of a specific geometrical configuration of the traps for which a crossing of two energy eigenvalues occurs and the transfer of the atom fails. Instead, the wave function is robustly split into a coherent superposition between two of the traps. We show that this process can be used to construct a single-atom interferometer and discuss its performance in terms of the final population distribution among the asymptotic eigenstates of the individual traps. This interferometric scheme could be used to study space-dependent fields from ultrashort to relatively large distances, or the decay of the coherence of superposition states as a function of the distance.

We analyze density-density correlations of expanding clouds of weakly interacting two-dimensional Bose gases below and above the Berezinskii-Kosterlitz-Thouless transition, with particular focus on short-time expansions. During time-of-flight expansion, phase fluctuations of the trapped system translate into density fluctuations, in addition to the density fluctuations that exist in situ. We calculate the correlations of these fluctuations both in real space and in momentum space and derive analytic expressions in momentum space. Below the transition, the correlation functions show an oscillatory behavior, controlled by the scaling exponent of the quasicondensed phase, due to constructive interference. We argue that this can be used to extract the scaling exponent of the quasicondensate experimentally. Above the transition, the interference is rapidly suppressed when the atoms travel an average distance beyond the correlation length. This can be used to distinguish the two phases qualitatively.

We analyze the phase structure of mass- and spin-imbalanced unitary Fermi gases in harmonic traps. To this end, we employ density functional theory in the local density approximation. Depending on the values of the control parameters measuring mass and spin imbalance, we observe that three regions exist in the trap, namely, a superfluid region at the center, surrounded by a mixed region of resonantly interacting spin-up and spin-down fermions, and, finally, a fully polarized phase surrounding the previous two regions. We also find regimes in the phase diagram where the existence of a superfluid region at the center of the trap is not energetically favored. We point out the limitations of our approach at the present stage and call for more detailed (ab initio) studies of the equation of state of uniform, mass-imbalanced unitary Fermi gases.

In recent years, controlled dissipation has proven to be a useful tool for the probing of a quantum system in an ultracold setup. In this paper we consider the dynamics of bosons induced by a dissipative local defect. We address superfluid and supersolid phases close to half filling that are ground states of an extended Bose-Hubbard Hamiltonian. To this end, we solve the master equation using the Gutzwiller approximation and find that in the superfluid phase repulsive nearest-neighbor interactions can lead to enhanced dissipation processes. On the other hand, our mean-field approach indicates that the effective loss rates are significantly suppressed deep in the supersolid phase where repulsive nearest-neighbor interactions play a dominant role. Our numerical results are explained by analytical arguments and, in particular, in the limit of strong dissipation we recover the quantum Zeno effect.

We present an experimental investigation on the spectral characteristics of an artificial atom “transmon qubit” constituting a three-level cascade system ($\Xi$-system) in the presence of a pair of external driving fields. We observe two different types of Autler-Townes (AT) splitting: type I, where the phenomenon of two-photon resonance tends to diminish as the coupling field strength increases, and type II, where this phenomenon mostly stays constant. We find that the types are determined by the cooperative effect of the decay rates and the field strengths. Theoretically analyzing the density-matrix elements in the weak-field limit where the AT effect is suppressed, we single out events of pure two-photon coherence occurring owing to constructive quantum interference.

Previous numerical work on the spatiotemporal distributions of the atomic polarization and of the difference in levels population of a one-dimensional system of two-level atoms initially inverted strongly suggested that the dynamics of the slab atomic ensemble in the linear regime of superradiance may best be treated by an eigenmode analysis. This method proved effective in obtaining both the time dependence of the atomic polarization and the spectral distribution of the emitted radiation from the slab in a number of physically interesting problems in the linear regime. In the present paper, I use the eigenfunction expansion technique to reduce the Maxwell-Bloch system of partial differential equations into a system of coupled first-order ordinary differential equations whose solutions clearly detail the contributions of the different modes to the system dynamics in the nonlinear regime of superradiant emission. I compute also the delay in time of the peak of the superradiant burst and its magnitude as functions of the ratio of the longitudinal decay rate to the transverse decay rate of the system.

With combined electrostatic and shaped laser fields with a slow turn on and rapid turn off, laser-field-free orientation of asymmetric top iodobenzene molecules with higher degrees of orientation has been achieved. In order to further increase the degrees of orientation, state-selected molecules are used as a sample. It is confirmed that higher degrees of orientation are maintained in the laser-field-free condition for 5–10 ps, which is long enough to study femtosecond-attosecond dynamics in molecules, after the rapid turn off of the laser pulse. The observation of the slow dephasing time of 5–10 ps ensures future prospects in molecular orientation techniques. This accomplishment means not only that a unique molecular sample has become available in various applications but also that the present technique can be used as a spectroscopic technique to investigate ultrafast rotational dynamics of molecules.

We propose an interaction-based method for remote tomography and entanglement swapping. Alice arranges a von Neumann-Arthurs-Kelly interaction between a system particle $P$ and two apparatus particles $A_1,A_2$, and then transports the latter to Bob. Bob can reconstruct the unknown initial state of particle $P$ not received by him by quadrature measurements on $A_1,A_2$. Further, if another particle $P^{\prime }$ in Alice's hands is EPR entangled with $P$, it will be EPR entangled with the distant pair $A_1,A_2$. This method will be contrasted with the usual teleportation protocols.

We discuss a scalable scheme for the implementation of quantum-information processing in qubits formed by superconducting resonators and spin ensembles. The scheme is based on a hybrid dual-rail encoding, which allows one to perform both single- and two-qubit gates by shifting the resonator frequency. We estimate the quantum-gate fidelity by simulating the driven dynamics through a master-equation approach. High values of the fidelity can be achieved also in the presence of the main decoherence sources, namely, cavity-photon loss, and pure dephasing of the superconductive elements that are involved in the two-qubit gates. This result allows envisioning the scalability of such elements to a quantum-computing architecture made of an array of hybrid spin-photon qubits. Analogous results are obtained for a simpler, nonscalable setup, which we propose here in order to simplify the realization of the first proof-of-principle experiments.

We reexamine a non-Gaussian quantum error-correction code designed to protect optical coherent-state qubits against errors due to an amplitude-damping channel. We improve on a previous result [R. Wickert, N. K. Bernardes, and P. van Loock, Phys. Rev. A 81, 062344 (2010)] by providing a tighter upper bound on the performance attained when considering realistic assumptions, which constrain the operation of the gates employed in the scheme. The quantitative characterization is performed through measures of fidelity and concurrence, the latter obtained by employing the code as an entanglement distillation protocol. We find that, when running the code in fully deterministic error-correction mode, direct transmission can only be beaten for certain combinations of channel and input state parameters. In contrast, in error-detection mode, the usage of higher repetition encodings remains beneficial throughout, however, at the expense of diminishing success probabilities.

We consider bipartite LOCC, the class of operations implementable by local quantum operations and classical communication between two parties. Surprisingly, there are operations that can be approximated to arbitrary precision but are impossible to implement exactly if only a finite number of messages are exchanged. This significantly complicates the analysis of what can or cannot be approximated with LOCC. Toward alleviating this problem, we exhibit two scenarios in which allowing vanishing error does not help. The first scenario is implementation of projective measurements with product measurement operators. The second scenario is the discrimination of unextendable product bases on two three-dimensional systems.

We study conditions under which vortices in a highly oblate harmonically trapped Bose-Einstein condensate (BEC) can be stabilized due to pinning by a blue-detuned Gaussian laser beam, with particular emphasis on the potentially destabilizing effects of laser beam positioning within the BEC. Our approach involves theoretical and numerical exploration of dynamically and energetically stable pinning of vortices with winding number up to $S=6$, in correspondence with experimental observations. Stable pinning is quantified theoretically via Bogoliubov-de Gennes excitation spectrum computations and confirmed via direct numerical simulations for a range of conditions similar to those of experimental observations. The theoretical and numerical results indicate that the pinned winding number, or equivalently the winding number of the superfluid current about the laser beam, decays as a laser beam of fixed intensity moves away from the BEC center. Our theoretical analysis helps explain previous experimental observations and helps define limits of stable vortex pinning for future experiments involving vortex manipulation by laser beams.

We propose a method of entangling two spinor Bose-Einstein condensates using a geometric phase gate. The scheme relies upon only the ac Stark shift and a common controllable optical mode coupled to the spins. Our scheme allows for the creation of an $S^z{S}^z$-type interaction where $S^z$ is the total spin. The geometric phase gate can be executed in times of the order of $2\pi \hslash /G$, where $G$ is the magnitude of the Stark shift. In contrast to related schemes which relied on a fourth-order interaction to produce entanglement, this is a second-order interaction in the number of atomic transitions. Closed expressions for the entangling phase are derived and the effects of decoherence due to cavity decay, spontaneous emission, and incomplete de-entangling of the light to the Bose-Einstein condensates are analyzed.

After a quantum quench, a sudden change of parameters, generic many-particle quantum systems are expected to equilibrate. A few collisions of quasiparticles are usually sufficient to establish approximately local equilibrium. Reaching global equilibrium is, however, much more difficult as conserved quantities have to be transported for long distances to build up a pattern of fluctuations characteristic for equilibrium. Here we investigate the quantum quench of the one-dimensional bosonic Hubbard model from infinite to finite interaction strength $U$ using semiclassical methods for weak and exact diagonalization for strong quenches. Equilibrium is approached only slowly, as $t^{-1/2}$ with subleading corrections proportional to $t^{-3/4}$, consistent with predictions from hydrodynamics. We show that these long-time tails determine the relaxation of a wide range of physical observables.

Attractive interaction between fermions can lead to pairing and superfluidity in an optical lattice. In contrast to the “continuum,” on a lattice the trap-induced density variation can generate a non monotonic profile of the pairing amplitude and completely modify the spectral signatures of any possible pseudogap phase. Using a tool that fully captures the inhomogeneity and strong thermal fluctuations, we demonstrate how the crucial radio-frequency signatures of pairing in a trapped attractive fermion lattice are “inverted” compared to the traditional continuum case. These features would be central in interpreting any spectroscopic hint of fermion pairing and superfluidity.

Nonequilibrium dynamics and effective thermalization are studied in a resonant tunneling scenario via multilevel Landau-Zener crossings. Our realistic many-body system, composed of two energy bands, naturally allows a separation of degrees of freedom. This gives access to an effective temperature and single- and two-body observables to characterize the delocalization of eigenstates and the nonequilibrium dynamics of our paradigmatic complex quantum system.

We use a combination of theoretical analysis, numerical simulation, and experimental measurement to investigate the near-threshold phase and coherence properties of parametric optical frequency combs generated in low-loss dielectric microresonators. Our analysis reveals that near threshold the phases of the comb lines do not stabilize to a constant value across the spectrum, although well-defined phase relationships relative to the pump laser do exist. Our results are supported by numerical simulations of two different microresonator combs operated under varying conditions of input drive, dispersion, and detuning. These results are also experimentally confirmed through phase measurements of the individual comb lines. We also investigate the processes leading to the breakdown of the equidistant frequency spacing of the modes in a microresonator comb.