We report experimental results on correlated double ionization of magnesium (Mg) by near-infrared (0.8- and $1.03\text{−}\mu \mathrm{m}$) circularly polarized laser fields. With $0.8\mu \mathrm{m}$, we confirm the recollision interpretation of the observed “knee” structure of Mg [Gillen et al., Phys. Rev. A 64, 043413 (2001)], even though the experiments are performed in the multiphoton regime. Our experiments show that, even in the multiphoton regime, which is normally thought to be a purely quantum-mechanical territory, some ionization phenomena can still be understood classically.

We investigate intense, ultrafast laser-induced isomerization and two-body fragmentation of acetylene monocations and dications using coincidence three-dimensional momentum imaging. Whereas the vast majority of previous work on strong-field isomerization and fragmentation of acetylene has necessarily involved ionization, by focusing solely on dissociation of ion-beam targets, we ensure that the dynamics ensue within a single molecular ion species, potentially simplifying interpretation. We demonstrate the rich information that can be extracted from such a measurement and discuss advantages and disadvantages of this approach.

Laser-induced tunneling ionization is one of the most fundamental and ubiquitous quantum processes and it initiates various ultrafast phenomena in intense laser-atom and laser-molecule interactions. Accurately resolving tunneling ionization is essential for understanding these phenomena. Based on the advanced attosecond technologies, such as high-order harmonic spectroscopy and strong-field photoelectron holography, previous studies have resolved the temporal properties of the tunneling process for rescattering electron. Here, as a complement, we theoretically demonstrate the retrieval of the time information of the tunneling process for the direct electron with the temporal double-slit interferometer. In this scheme, a weak second harmonic parallel to the fundamental field is added. By solving the time-dependent Schrödinger equation, the photoelectron momentum distribution (PEMD) from strong-field tunneling ionization is obtained. Varying the relative phase of the parallel two-color field, the path of the ionized electrons periodically changes, leading to a shift of the time double-slit interference fringes in PEMDs. By analyzing the response of the interference fringes to the perturbation, the time information of direct electron in strong-field tunneling ionization is reconstructed. Interestingly, our results present excellent agreement with the complex time obtained from the quantum-orbit model.

We report the design and experimental demonstration of a compact, reconfigurable Penning ion trap constructed with rare-earth permanent magnets placed outside of a trap vacuum enclosure. We describe the observation of Doppler laser cooling of ions in a permanent magnet Penning trap. We detail a method for quantifying and optimizing the trap magnetic-field uniformity in situ using a thermal beam of neutral ${}^{40}\mathrm{Ca}$ precursor atoms. Doppler laser cooling of ${}^{40}\mathrm{Ca}^{+}$ is carried out at 0.65 T, and side-view images of trapped ion fluorescence show crystalline order for both two- and three-dimensional arrays. Measured ${}^{40}\mathrm{Ca}^{+}$ trap frequencies confirm the magnetic-field characterization with neutral ${}^{40}\mathrm{Ca}$. The compact trap described here enables a variety of cold ion experiments with low size, weight, power, and cost requirements relative to traditional electromagnet-based Penning traps.

We apply the functional path-integral approach to analyze how the presence of a spin-orbit coupling (SOC) affects the basic properties of a BCS-type paired state in a two-component Bose gas. In addition to a mean-field theory that is based on the saddle-point approximation for the intercomponent pairing, we derive a Ginzburg-Landau theory by including the Gaussian fluctuations on top, and use them to reveal the crucial roles played by the momentum-space structure of an arbitrary SOC field in the stability of the paired state at finite temperatures. For this purpose, we calculate the critical transition temperature for the formation of paired bosons, and that of the gapless quasiparticle excitations for a broad range of interaction and SOC strengths. In support of our results for the many-body problem, we also benchmark our numerical calculations against the analytically tractable limits, and provide a full account of the two-body limit including its nonvanishing binding energy for arbitrarily weak interactions and the anisotropic effective mass tensor.

We theoretically investigate the thermodynamic properties of a strongly correlated two-dimensional Fermi gas with a confinement-induced negative effective range of interactions, which is described by a two-channel model Hamiltonian. By extending the many-body $T$-matrix approach by Nozières and Schmitt-Rink to the two-channel model, we calculate the equation of state in the normal phase and present several thermodynamic quantities as functions of temperature, interaction strength, and effective range. We find that there is a nontrivial dependence of thermodynamics on the effective range. In the experiment, where the effective range is set by the tight axial confinement, the contribution of the effective range becomes nonnegligible as the temperature decreases down to the degenerate temperature. We compare our finite-range results with recent measurements on the density equation of state, and show that the effective range has to be taken into account for the purpose of a quantitative understanding of the experimental data.

Trapped atoms are among the most promising platforms for quantum technologies. They play a remarkable role in quantum simulation as well as high-precision quantum metrology, which exploits the quantum coherence of a single electronic or motional degree of freedom of an atom or an ensemble. However, future high-precision quantum metrology will require the use of entangled states of several degrees of freedom. Here we propose a protocol capable of generating high-$N$ NOON states where the entanglement is shared between the motion of a trapped atom and an electromagnetic cavity mode, a so-called “hybrid” configuration. We explore the feasibility of the proposal in a platform consisting of an optically trapped neutral atom excited to its circular-Rydberg-state manifold, coupled to the modes of a high-$Q$ microwave cavity. This compact hybrid architecture has the advantage that it can couple to signals of very different nature, which modify either the atom's motion or the cavity modes. Moreover, the exact same setup can be used right after the state-preparation phase to implement the interferometer required for quantum metrology.

We study the effect of a weakly driven atomic cloud's polarization distribution on its photon scattering line shape in the limit where the atoms can be treated as stationary and randomly distributed. We find three distinct polarization regimes. First, for dilute clouds, the polarization magnitude is relatively constant. Second, for denser clouds, polarization builds at the front of the cloud for near-resonant light. Third, when the cloud condenses to the point where its dimensions become comparable to the wavelength, light focuses towards the back of the cloud for red detuning. For these regimes, we show which “mean-field” frameworks accurately describe the differing photon-scattering line shapes. Finally, for even denser clouds, mean-field models become inaccurate and necessitate the full point dipole model that includes atom-atom correlations.

On a lossless periodic dielectric structure sandwiched between two homogeneous media, bound states in the continuum (BICs) with real frequencies and real Bloch wave vectors may exist, and they decay exponentially in the surrounding homogeneous media and do not couple with propagating plane waves with the same frequencies and wave vectors. The BICs are of significant current interest because they give rise to high-$Q$ resonances when the structure or the Bloch wave vector is slightly perturbed. In this paper, the effect of a small material loss on the BICs is analyzed by a perturbation method and illustrated by numerical results. It is shown that bound states with complex frequencies near the continuum appear, but they behave differently depending on whether the BIC is symmetry protected or not. The Bloch wave vector of a bound state with a complex frequency can be real if the original BIC is symmetry protected, and it is usually complex if the original BIC is not symmetry protected. Our study improves the theoretical understanding of BICs and provides useful insight for their practical applications.

Below-threshold harmonic generation (BTHG) is one of few potential methods for the generation of novel vacuum-ultraviolet (VUV) light sources. However, most previous studies have focused on the atomic BTHG; a comprehensive understanding of the molecular BTHG is valuable for the advancement of this field. Here, we present an ab initio study of the role of nuclear symmetry on BTHG of both homonuclear and heteronuclear diatomic molecules. We reveal very distinctive electronic dynamical behaviors in molecular BTHG by identifying subtle multiple-channel contributions. In particular, it is found that the channels associated with the electron-nuclear coupling from light nuclear core to heavy nuclear core of the asymmetric molecules play an important role in the constructive interference of the harmonic emission, leading to a significant enhancement of the BTHG. Our results suggest that asymmetric molecular systems can be used for substantial improvement of the molecular BTHG-based VUV light sources.

Several versions of quantum theory assume some form of localized collapse. If measurement outcomes are indeed defined by localized collapses, then a loophole-free demonstration of Bell nonlocality needs to ensure spacelike separated collapses associated with the measurements of the entangled systems. This collapse-locality loophole remains largely untested, with one significant exception probing Diosi's and Penrose's gravitationally induced collapse hypotheses. I distinguish two versions of the loophole, and focus on the more strongly motivated version, in which local hidden variables may depend causally on collapse outcomes but may not independently depend on measurement settings. I describe here techniques that allow much stronger experimental tests. These apply to all the well-known types of collapse postulate, including gravitationally induced collapse, spontaneous localization models, and Wigner's consciousness-induced collapse.

We study the computational complexity of quantum-mechanical expectation values of single-particle operators in bosonic and fermionic multiparticle product states. Such expectation values appear, in particular, in full-counting-statistics problems. Depending on the initial multiparticle product state, the expectation values may be either easy to compute (the required number of operations scales polynomially with the particle number) or hard to compute (at least as hard as a permanent of a matrix). However, if we only consider full counting statistics in a finite number of final single-particle states, then the full-counting-statistics generating function becomes easy to compute in all the analyzed cases. We prove the latter statement for the general case of the fermionic product state and for the single-boson product state (the same as used in the boson-sampling proposal). This result may be relevant for using multiparticle product states as a resource for quantum computing.

Quantum teleportation with a two-qubit state can be suitably characterized in terms of maximal fidelity and fidelity deviation—the former is the maximal value of the average fidelity achievable within the standard protocol and local unitary operations and the latter is the standard deviation of fidelity over all input states. In this paper, we consider the problem of characterizing two-qubit states that are optimal for quantum teleportation for a given value of some state property. The optimal states are defined as those states that, for a given value of the state property under consideration, achieve the largest maximal fidelity and also exhibit zero fidelity deviation. We provide a complete characterization of optimal states for a given linear entropy, maximum mean value of the Bell–Clauser-Horne-Shimony-Holt (CHSH) observable, and concurrence, respectively. We find that for a given linear entropy or Bell-CHSH violation the largest maximal fidelity states are optimal, but for a given concurrence the optimal states form a strict subset of the largest maximal fidelity states.

The present work discusses the impact of vacancy defects in solid high-order harmonic generation. The total energy cutoff of the high-order harmonic spectrum increases as a function of concentration of vacancy defects, and the total spectrum gradually turns into a single slanted spectrum without having an abrupt transition between primary and secondary plateaus. The spectral intensity of the below-band-gap harmonics in a solid with vacancy defects is enhanced significantly in comparison to the harmonics in a pristine solid. The changes in the harmonic spectra are understood in terms of their effective band structures. The presence of vacancy defects breaks the translational symmetry of the unit cell locally. As a consequence of this, new defect states appear, which open additional paths for the electron dynamics. The ill-resolved electron trajectories in the Gabor profile confirm the interference of additional paths. Moreover, the single slanted high-order harmonic spectrum carries a unique signature of vacancy defects in comparison to the high-order harmonic spectrum corresponding to solids with defects such as underdoping or overdoping.

We theoretically investigate strong-coupling properties of an ultracold Fermi gas in the BCS-BEC crossover regime in the nonequilibrium steady state, being coupled with two fermion baths. By developing a nonequilibrium strong-coupling theory based on the combined $T$-matrix approximation with the Keldysh Green's function technique, we show that the chemical potential bias applied by the two baths gives rise to the anomalous enhancement of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing fluctuations (although the system has no spin imbalance), resulting in the re-entrant behavior of the nonequilibrium superfluid phase transition in the Bardeen-Cooper-Schrieffer unitary regime. These pairing fluctuations are also found to anomalously enhance the pseudogap phenomenon. Since various nonequilibrium phenomena have recently been measured in ultracold Fermi gases, our nonequilibrium strong-coupling theory would be useful to catch up with this experimental development in this research field.

In self-induced-transparency (SIT) mode locking, 2π SIT solitons with a duration much smaller than the polarization relaxation time $T_2$ in the gain and absorber are formed in a laser cavity. This is in contrast to standard passive mode-locking schemes based on gain and absorption saturation. Despite the great promise, up to now SIT mode locking with 2π pulses was mainly studied theoretically. In this paper, a stable self-starting passive mode locking is demonstrated experimentally in a Ti:sapphire laser with a Rb vapor cell. We show that the mode locking indeed appears to be due to SIT in the Rb cell; that is, the pulse in the Rb cell is a 2π SIT soliton. We also confirm self-starting of the SIT mode locking. Self-starting takes place via a set of intermediate regimes, including the one containing zero-area pulses.

We show that nonlinear problems including nonlinear partial differential equations can be efficiently solved by variational quantum computing. We achieve this by utilizing multiple copies of variational quantum states to treat nonlinearities efficiently and by introducing tensor networks as a programming paradigm. The key concepts of the algorithm are demonstrated for the nonlinear Schrödinger equation as a canonical example. We numerically show that the variational quantum ansatz can be exponentially more efficient than matrix product states and present experimental proof-of-principle results obtained on an IBM Q device.

In state-of-the-art quantum computing platforms, including superconducting qubits and trapped ions, imperfections in the two-qubit entangling gates are the dominant contributions of error to systemwide performance. Recently, a novel two-qubit parametric gate was proposed and demonstrated with superconducting transmon qubits. This gate is activated through rf modulation of the transmon frequency and can be operated at an amplitude where the performance is first-order insensitive to flux noise. In this work we experimentally validate the existence of this ac sweet spot and demonstrate its dependence on white-noise power from room-temperature electronics. With these factors in place, we observe entangling-gate fidelity with coherence-limited performance. An ensemble of repeated observations has a median fidelity of 98.8%, with roughly 10% of observations above 99%.