Hard-core dipolar bosons trapped in a parallel stack of $N\ge 2$ one-dimensional optical lattices (tubes) can develop several phases made of composites of particles from different tubes: superfluids, supercounterfluids, and insulators as well as mixtures of those. Bosonization analysis shows that these phases are thresholdless with respect to the dipolar interaction, with the key “control knob” being filling factors in each tube, provided the intertube tunneling is suppressed. The effective ab initio quantum Monte Carlo algorithm capturing these phases is introduced and some results are presented.

We report on a systematic experimental study of the heralding efficiency and generation rate of telecom-band infrared photon pairs generated by spontaneous parametric down-conversion and coupled to single-mode optical fibers. We define the correlated-mode coupling efficiency, an inherent source efficiency, and explain its relation to heralding efficiency. For our experiment, we developed a reconfigurable computer-controlled pump-beam and collection-mode optical apparatus which we used to measure the generation rate and correlated-mode coupling efficiency. The use of low-noise, high-efficiency superconducting nanowire single-photon detectors in this setup allowed us to explore focus configurations with low overall photon flux. The measured data agree well with theory, and we demonstrated a correlated-mode coupling efficiency of $97\%\pm 2\%$, which is the highest efficiency yet achieved for this type of system. These results confirm theoretical treatments and demonstrate that very high overall heralding efficiencies can, in principle, be achieved in quantum optical systems. It is expected that these results and techniques will be widely incorporated into future systems that require, or benefit from, a high heralding efficiency.

We demonstrate a complete, probabilistic quantum dynamical simulation of the standard nonlinear Hamiltonian of optomechanics, including decoherence at finite temperatures. Robust entanglement of a photonic pulse with a mechanical oscillator is predicted. Our exact quantum simulations use the positive-$P$ technique, are scalable to large Hilbert spaces, and give excellent agreement with recent experiments. We predict the required conditions for the next stage in this research. Strong quantum steering between the photonic and mechanical systems is possible, depending on thermal occupation. This is more viable in optical than in electromechanical experiments.

We propose experimental methods to engineer reservoirs at arbitrary temperature which are feasible with current technology. Our results generalize to mixed states the possibility of quantum state engineering through controlled decoherence. Finite-temperature engineered reservoirs can lead to the experimental observation of thermal entanglement—the appearance and increase of entanglement with temperature—to the study of the dependence of finite-time disentanglement and revival with temperature, quantum thermodynamical effects, and others, enlarging the comprehension of temperature-dependent entanglement properties. Our proposal is discussed in detail in two model systems, consisting of different modes of a single photon and a trapped-ion system.

Recent and ongoing experimental work on the positronium spectrum motivates new efforts to calculate positronium energies at the level of $O\left({\alpha }^3\right)$ corrections to the lowest-order result. We have obtained results for one set of such corrections involving light-by-light scattering of the photons produced in a two-photon virtual annihilation process. Our result is an energy shift $1.58377\left(8\right)m{\alpha }^7/{\pi }^3$ for the $n=1$ singlet state, correcting the ground-state hyperfine splitting by $-6.95$ kHz. We also obtained a new and more precise result for the light-by-light scattering correction to the real decay of parapositronium into two photons.

We study strong-field atomic ionization driven by an XUV pulse with a nonzero displacement, the quantity defined as the time integral of the pulse vector potential taken over the pulse duration. The use of such pulses may lead to an extreme sensitivity of the ionization process to subtle changes of a driving XUV pulse, in particular, the ramp-on and off profile and the carrier envelope phase. We illustrate this sensitivity for atomic hydrogen and lithium driven by few-femtosecond XUV pulses with intensity in the ${10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ range. The observed effect is general and should modify strong-field ionization of any atom or molecule, provided the ionization rate is sufficiently high.

A surface-electrode decelerator and deflector for Rydberg atoms and molecules has been developed with the goal of performing collisional experiments. Translationally cold ${\mathrm{H}}_2$ molecules in a supersonic beam were excited to Rydberg-Stark states of principal quantum number $n=31$, loaded into electric traps moving at a predetermined speed above the surface of a bent printed circuit board, decelerated, and deflected from the original direction of the supersonic beam by an angle of ${10}^{\circ }$. The phase-space characteristics of the deflected beam were characterized by measuring the time-of-flight distribution and images of the Rydberg molecules and comparing them to the results of numerical particle-trajectory simulations. More than 1000 ${\mathrm{H}}_2$ molecules were deflected per experimental cycle at a repetition rate of 25 Hz. The phase-space characteristics of the deflector make it attractive to study ion-molecule reactions at low collision energies.

We perform an ab initio and accurate exploration of the subcycle transient multiphoton-ionization dynamics of atomic and molecular systems subject to intense near-infrared laser fields on the subfemtosecond time scale. Multiple-ionization bursts within a single optical cycle are found in the time-dependent ionization rates not only for diatomic molecules ${{\mathrm{H}}_2}^{+}$ and ${\mathrm{HHe}}^{2+}$, but also for the hydrogen atom. The analysis of the electron density reveals that several distinct density portions can be shaped and detached from the target within a half cycle of the laser field.

The polaronic system consisting of an impurity in a dilute Bose-Einstein condensate is considered in the presence of a narrow Feshbach resonance. For this purpose a coupled-channel model is used, which at the mean-field level predicts the formation of quasiparticles that are a superposition of the impurity and the molecular states. The impurity-boson interactions and the coupling between the open and closed channels are then considered weak and a perturbative calculation of the corrections to the mean-field results is presented. This allows us to examine the properties of the quasiparticles, such as the lifetime and the effective mass. The relation between the model parameters and the experimental parameters is determined by identifying the low-energy $T$ matrix and applying a proper renormalization scheme.

Selective laser addressing of a single atom or atomic ion qubit can be improved using narrow-band composite pulse sequences. We describe a Lie-algebraic technique to generalize known narrow-band sequences and introduce sequences related by dilation and rotation of sequence generators. Our method improves known narrow-band sequences by decreasing both the pulse time and the residual error. Finally, we experimentally demonstrate these composite sequences using ${}^{40}{\mathrm{Ca}}^{+}$ ions trapped in a surface-electrode ion trap.

Transition frequencies were determined for transitions in Ra in an atomic beam and for reference lines in ${\mathrm{Te}}_2$ molecules in a vapor cell. The absolute frequencies were calibrated against a GPS stabilized Rb clock by means of an optical frequency comb. The $7s^2{}^1{S}_0(F=1/2)–7s7p{}^1{P}_1(F=3/2)$ transition in ${}^{225}\mathrm{Ra}$ was determined to be $621042124\left(2\right)\mathrm{MHz}$. The measurements provide input for designing efficient and robust laser cooling of Ra atoms in preparation of a search for a permanent electric dipole moment in Ra isotopes.

In measurement-based quantum computing an algorithm is performed by measurements on highly entangled resource states. To date, several implementations were demonstrated, most of them assuming perfect noise-free environments. Here we consider measurement-based information processing in the presence of noise and demonstrate quantum error detection. We implement the protocol using a four-qubit photonic cluster state where we first encode a general qubit nonlocally such that phase errors can be detected. We then read out the error syndrome and analyze the output states after decoding. Our demonstration shows a building block for measurement-based quantum computing which is crucial for realistic scenarios.

Recently, great progress has been made in the creation of a solid-state quantum computer using superconducting qubits on Cooper pairs of charged electrons. However, this approach has met limitations due to decoherence effects caused by the strong Coulomb interaction of the superconducting qubit with the environment. Here, we propose the solution of this problem by switching to another Bose-Einstein condensate (BEC), uncharged long-lived magnons, wherein the magnon BEC qubit can be realized due to the magnon blockade isolating a pair of the magnon condensate energy levels in the mesoscopic and nanoscopic ferromagnetic dielectric sample. We demonstrate the single-qubit gates by operating quantum transition between these states in the external microwave field. We also consider implementation of the two-qubit gates by using the interaction between such magnon BEC qubits due to exchange by virtual photons in a microwave cavity. Finally, we discuss the condition for long-lived magnon BEC qubits, a scalable architecture, and promising advantages of the multiqubit quantum computer based on the magnon qubit.

We optimize a quantum walk of multiple fermions following a quench in a spin chain to generate near-ideal resources for quantum networking. We first prove a useful theorem mapping the correlations evolved from specific quenches to the apparently unrelated problem of quantum state transfer between distinct spins. This mapping is then exploited to optimize the dynamics and produce large amounts of entanglement distributed in very special ways. Two applications are considered: the simultaneous generation of many Bell states between pairs of distant spins (maximal block entropy) and high entanglement between the ends of an arbitrarily long chain (distance-independent entanglement). Thanks to the generality of the result, we study its implementation in different experimental setups using present technology: nuclear magnetic resonance, ion traps, and ultracold atoms in optical lattices.

We construct exact Kohn-Sham potentials for the ensemble density-functional theory (EDFT) from the ground and excited states of helium. The exchange-correlation (XC) potential is compared with the quasi-local-density approximation and both single-determinant and symmetry-eigenstate ghost-corrected exact exchange approximations. Symmetry-eigenstate Hartree exchange recovers distinctive features of the exact XC potential and is used to calculate the correlation potential. Unlike the exact case, excitation energies calculated from these approximations depend on ensemble weight, and it is shown that only the symmetry-eigenstate method produces an ensemble derivative discontinuity. Differences in asymptotic and near-ground-state behavior of exact and approximate XC potentials are discussed in the context of producing accurate optical gaps.

We report on the storage of light via the phenomenon of coherent population oscillation (CPO) in an atomic cesium vapor at room temperature. In the experiment the optical information of a probe field is stored in the CPO of two ground states of a $\Lambda$ three-level system formed by the Zeeman sublevels of the hyperfine transition $F=3\to F^{\prime }=2$ of the cesium $D_2$ line. We show directly that this CPO-based memory is very insensitive to stray magnetic field inhomogeneities and presents a lifetime which is limited mainly by atomic motion. A theoretical simulation of the measured spectra was also developed and is in very good agreement with the experiment.

To describe the photoionization of a hydrogen atom in a static electric field, we considerably rely on the properties of a global asymptotic of the Stark wave functions in momentum representation. The accurate analytical photoionization cross section is found in terms of the Stokes multipliers for the quartic oscillator.

We introduce an alternative method based on local quantum uncertainty to measure non-Markovianity for an open quantum system. The local quantum uncertainty proposed by Girolami $et al$. [Phys. Rev. Lett. 110, 240402 (2013)] is defined by the minimum skew information from a local measurement perspective. The measure effectively captures the characteristics of non-Markovianity and has a clearly physical interpretation: in correlated bipartite systems, the non-Markovian (Markovian) process of a considered open subsystem corresponds to the increase (decrease) of quantum uncertainty of local observables of another measured free subsystem. Then the local quantum uncertainty is further discussed by carefully handling the square root of the density operator for the bipartite $2\otimes d$ systems. Specifically, for a two-qubit system we give an explicit expression of the local quantum uncertainty. Furthermore, by applying the measure to three typical non-Markovian noise channels, we find that the conditions of detecting the non-Markovianity are consistent with the results of existing measures for non-Markovianity, i.e., information backflow and divisibility.