We observe optical bistability as well as oscillations in the upper bistable branch when cold ytterbium atoms are dispersively coupled to a high finesse optical cavity. Comparable previous observations were explained by atomic density oscillations in case of a Bose-Einstein condensate or optical pumping in case of a cold cloud of cesium. Both explanations do not apply in our case of thermal atoms with only a single ground state. We propose a simple two-level model including saturation to describe our experimental results. This paper introduces the experimental setup, derives the mentioned model, and compares it to our experimental data. Good quantitative agreement supports our model.

Parity-violation experiment in Yb is made on the strongly forbidden $M1$ transition $6s^2{}^{1}S_0\to 5d6s{}^{3}D_1$. The hyperfine mixing of the $5d6s{}^{3}D_1$ and $5d6s{}^{3}D_2$ levels opens the $E2$ channel, whose amplitude differs for $F$ sublevels of the ${}^{3}D_1$ level. This effect may be important for the experimental search for the nuclear-spin-dependent parity-violation effects predominantly caused by the nuclear anapole moment.

The cooling of anthracene dications ${\left({\mathrm{C}}_{14}{\mathrm{H}}_{10}\right)}^{2+}$ has been studied in a time range of 10 ms. Intact ${\left({\mathrm{C}}_{14}{\mathrm{H}}_{10}\right)}^{2+}$ ions were produced in a Nanogan electron cyclotron resonance (ECR) ion source and stored in a compact electrostatic ion storage ring, the Mini-Ring. The internal energy distribution of the stored dications was probed every millisecond during the storage time using optical parametric oscillator (OPO) nanosecond laser pulses. The wavelength of the laser was tuned at 560 nm (2.214 eV), which is the center of a broad resonance peak attributed to the $S_2\text{−}{S}_0$ transition of ${\left({\mathrm{C}}_{14}{\mathrm{H}}_{10}\right)}^{2+}$. The fast decay of neutral fragment counts due to laser induced dissociation was recorded for several probing times. These experimental neutral decay curves were reproduced by a model that accounted for the slow evolution of the internal energy distribution of the stored ions due to dissociation and radiative cooling processes. From the simulated internal energy distributions, we have estimated an energy shift rate varying from 140 to 30 eV/s in the time range from 1 to 9 ms, which was attributed mainly to the radiative cooling due to delayed electronic transitions. The electronic fluorescence rate was estimated to $331{\mathrm{s}}^{-1}$ for an internal energy of 7 eV, which was similar to the electronic fluorescence rate of an anthracene cation at the same energy. We conclude that, for anthracene, the fluorescence cooling process is not sensitive to the charge of the molecule for low charge states, cation and dication.

The transient-absorption spectrum of a $V$-type three-level system is investigated, when this is periodically excited by a train of equally spaced, $\delta$-like pump pulses as, e.g., from an optical-frequency-comb laser. We show that, even though the probe pulse is not assumed to be much shorter than the pump pulses, light-induced states appear in the absorption spectrum. The frequency- and time-delay-dependent features of the absorption spectra are investigated as a function of several laser control parameters, such as the number of pump pulses used, their pulse area, and the pulse-to-pulse phase shift. We show that the frequencies of the light-induced states and the time-delay-dependent features of the spectra contain information on the action of the intense pulses exciting the system, which can thus complement the information on light-imposed amplitude and phase changes encoded in the absorption line shapes.

We consider ultracold polar molecules trapped in a unit-filled one-dimensional chain in real space created with an optical lattice or a tweezer array and illuminated by microwaves that resonantly drive transitions within a chain of rotational states. We describe the system by a two-dimensional lattice model, with the first dimension being a lattice in real space and the second dimension being a lattice in a synthetic direction composed of rotational states. We calculate this system's ground-state phase diagram. We show that as the dipole interaction strength is increased, the molecules undergo a phase transition from a two-dimensional gas to a phase in which the molecules bind together and form a string that resembles a one-dimensional object living in the two-dimensional (i.e., one real and one synthetic dimensional) space. We demonstrate this with two complementary techniques: numerical calculations using matrix product state techniques and an analytic solution in the limit of infinitely strong dipole interaction. Our calculations reveal that the string phase at infinite interaction is effectively described by emergent particles living on the string and that this leads to a rich spectrum with excitations missed in earlier mean-field treatments.

Bell's inequality was originally derived under the assumption that experimenters are free to select detector settings independently of any local “hidden variables” that might affect the outcomes of measurements on entangled particles. This assumption has come to be known as “measurement independence” (also referred to as “freedom of choice” or “settings independence”). For a two-setting, two-outcome Bell test, we derive modified Bell inequalities that relax measurement independence, for either or both observers, while remaining locally causal. We describe the loss of measurement independence for each observer using the parameters $M_1$ and $M_2$, as defined by Hall in 2010, and also by a more complete description that adds two new parameters, which we call $\widehat{M_1}$ and $\widehat{M_2}$, deriving a modified Bell inequality for each description. These “relaxed” inequalities subsume those considered in previous work as special cases, and quantify how much the assumption of measurement independence needs to be relaxed in order for a locally causal model to produce a given violation of the standard Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) inequality. We show that both relaxed Bell inequalities are tight bounds on the CHSH parameter by constructing locally causal models that saturate them. For any given Bell inequality violation, the new two-parameter and four-parameter models each require significantly less mutual information between the hidden variables and measurement settings than previous models. We conjecture that the new models, with optimal parameters, require the minimum possible mutual information for a given Bell violation. We further argue that, contrary to various claims in the literature, relaxing freedom of choice need not imply superdeterminism.

There is a debate ongoing about the quantification of entanglement of indistinguishable particles, and recently a new method due to Lo Franco and Compagno (LFC) [Sci. Rep. 6, 20603 (2016)] has appeared with the aim of settling the controversies of this debate. In this work we show that the LFC approach can be recovered using the second quantization formalism and a local single-particle basis to perform the trace over a one-particle state. The use of the second quantization formalism has the advantage of being well known in many-body physics. We also compare the LFC and second quantization approaches or, equivalently, the effect of the partial trace over one particle depending on the local and nonlocal character of the single-particle basis. Additionally, we study the role played by the exchange correlations in the quantification of entanglement of indistinguishable particles.

The fragmentation of methane by impact of electrons and protons is studied experimentally to reveal the mechanisms behind fragmentation by projectiles with opposite charges. Coincidence measurements are used to separate the single from double ionization, in the case of protons, and the DETOF (delayed extraction time-of-flight) technique is used, in the case of electrons, to untangle kinematic signatures of the single vacancy from that of the one-electron–two-vacancy satellite states populated during the collision. The substantial differences in the fragmentation cross sections observed between the two projectiles, when several hydrogen bonds are broken, are here attributed essentially to single ionization and interpreted as due to the interference term between the mechanisms of shakeup and the excitation ionization by double impact that appears in the perturbative expansion of the ionization cross section.

Two-body fragmentation of ${\mathrm{N}}_2{\mathrm{O}}^{q+}$ ($q=2,3$) induced by electron-capture collision of $5.7-\mathrm{keV}/\mathrm{u}\mathrm{X}{\mathrm{e}}^{15+}$ is studied. Through the triply coincident measurement on ion-pair fragments with the scattered projectile and the correlation analysis on the ion-pair time of flight and momentum conservation, we have clearly identified 12 reaction channels for the formation and dissociation of ${\mathrm{N}}_2{\mathrm{O}}^{2+}$ and ${\mathrm{N}}_2{\mathrm{O}}^{3+}$. The fraction ratios for these channels and the corresponding kinetic energy release (KER) distributions for the ion-pair products have been obtained. Calculations of the potential energy curves of ${\mathrm{N}}_2{\mathrm{O}}^{3+}$ for the N-N and N-O bond stretches are performed using the complete active space self-consistent field method. The KER spectra for the two-body fragmentation of ${\mathrm{N}}_2{\mathrm{O}}^{2+}\to {\mathrm{N}}^{+}+\mathrm{N}{\mathrm{O}}^{+}$ and $\mathrm{N}{{}_2}^{+}+{\mathrm{O}}^{+}$ can be explained by the decay via the $X{}^3\mathrm{\Sigma }^{-}$ and $1{}^3\mathrm{\Pi }$ states, and the major peaks or structures observed in the KER spectra for ${\mathrm{N}}_2{\mathrm{O}}^{3+}\to {\mathrm{N}}^{+}+\mathrm{N}{\mathrm{O}}^{2+}$ can be attributed to the $1{}^2\mathrm{\Pi }, 2{}^2\mathrm{\Pi }$, and $2{}^2\mathrm{\Sigma }^{-}$ states, whereas those in the KER spectra for ${\mathrm{N}}_2{\mathrm{O}}^{3+}\to {\mathrm{O}}^{+}+{\mathrm{N}}_2^{2+}$ are mainly contributed from the $1{}^2\mathrm{\Pi }, 3{}^2\mathrm{\Pi }$, and $4{}^2\mathrm{\Pi }$ states. In addition, we found that the KER structures for the same ion-pair products are not sensitive to the number of electrons stabilized at the projectile, but the KER intensities are clearly dependent on it. The mechanism of multielectron captures of the projectile to form the transient multicharged molecular ions and the following projectile stabilization with or without autoionizing cascades is proposed to explain it.

Following a strong analogy with two-dimensional physics, the three-body pseudopotential in one dimension is derived. The Born approximation is then considered in the context of ultracold atoms in a linear harmonic waveguide. The equivalence between this model and the three-body wave equation obtained directly from the Schrödinger equation, including the confinement potential of the waveguide, is demonstrated in the vicinity of the dimer threshold.

We theoretically study the photoelectron momentum distributions (PMDs) of ${{\mathrm{H}}_2}^{+}$ in elliptically polarized laser fields by numerically solving the time-dependent Schrödinger equation. Depending on the internuclear distance of ${{\mathrm{H}}_2}^{+}$, the PMDs reveal different asymmetries with respect to the major and minor axes of the laser ellipse. At an internuclear distance corresponding to the enhanced ionization region, the asymmetry of the PMDs is opposite to that of the companion atom. We attribute this phenomenon to the effect of the internal scattered electrons, which are mainly released at an earlier ionization moment with respect to the field maximum within each half cycle. We further show that the asymmetry of the PMDs in the elliptical laser fields can be used to identify the instant when the electron wave packet is released from the molecule.

Laser-induced high harmonic generation (HHG) in solids strongly depends on the band structure of the target. In this work, we studied the band structure of bulk wurtzite ZnO under different strains. The changes of band structure induced by shear, uniaxial, biaxial, and isotropic strains are compared. It is found that the band gap can be narrowed or widen by modulating the external conditions, which offers an efficient way to engineer the wide-gap crystals to produce HHG. The strain effects on HHG are investigated by numerically solving the semiconductor Bloch equations. Our calculations show that the HHG yield depends sensitively on the change of band structure caused by stress- and temperature-induced strains. In addition, we found that the emission times of HHG and the ratio between inter and intraband contributions to the total solid HHG spectra can be controlled by the magnitude of strain imposed on the solid targets.

Emitted electronic densities from an Al surface have been computed for various temporal shapes of near-infrared femtosecond laser pulses (less than or equal to 20 fs) over a wide range of low and intermediate laser intensities. At low intensities photoemission originates from the high-energy tail of the pulse spectrum leading to linear absorption in all cases, even if several photons are necessary to eject an electron at higher intensities. We propose an analytical method to calculate the transition intensity in which the behavior of emitted densities evolves from the single-photon $I_m$ slope to the multiphoton $I_m^n$ one (with $I_m$ the maximum intensity and $n$ the number of absorbed photons). We show that the use of a flat-top profile or at least of a super-Gaussian envelope enables strong electron emission at very low intensities for realistic pulse durations and should allow experimental observation of the transition between these two regimes of photoemission.

An experimental group in Beijing [Yueyang Zhai et al., Phys. Rev. A 87, 063638 (2013)] introduced the method of the standing-wave pulse sequence for efficiently preparing ultracold bosonic atoms into a specific excited band in a one-dimensional optical lattice. Here, we report on a theoretical extension of their work to the problem of one-dimensional bichromatic superlattices in order to understand a wave packet shaping process in higher bands. Varying the lattice parameters leads to the so-called Dirac point where a pair of excited bands crosses. This paper thus discusses simultaneously efficient excitation of the wave packet to the proximity of the Dirac point and its subsequent dynamics in the force field of a parabolic trap. We numerically explore optimal pulse-sequence parameters and find an optimized sequence which excites more than 99% of the atoms in an ideal situation to the first and second excited bands within 100 $\mu \mathrm{s}$. Our main finding is that the system permitting the Dirac point possesses a region of parameters where the excited energy bands become nearly parabolic, conducive to a robust wave packet with high coherence and isochronicity. We also provide an appropriate data set for future experimentation, including effects of the atom-atom interaction by way of the mean-field nonlinear term.

We demonstrate modulation of the effective interaction between the magnetic sublevels of the hyperfine spin $F=1$ in a ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate by Rabi coupling with radio-frequency (rf) field. The use of the $F=1$ manifold enables us to observe the long-term evolution of the system owing to the absence of inelastic collisional losses. We observe that the evolution of the density distribution reflects the change in the effective interaction between atoms due to rf coupling. We also realize a miscibility-to-immiscibility transition in the magnetic sublevels of $m=\pm 1$ by quenching the rf field. Rf-induced interaction modulation in long-lived states as demonstrated here will facilitate the study of out-of-equilibrium quantum systems.

A quantum model is considered for $N$ bosons condensed in two orthogonal single-particle modes with tunable energy separation in the presence of flavor-changing contact interaction, which can be readily solved by direct diagonalization. Despite its elementary character, this model possesses chiral quantum phases with finite orbital angular momentum arising as the result of spontaneous breaking of time-reversal symmetry. The phase diagram is explored on a mean-field and full quantum level. Thermal and quantum fluctuations are characterized with respect to regions of universal scaling behavior. The nonequilibrium dynamics shows a sharp transition between a self-trapping and a pair-tunneling regime. A recently realized experimental implementation is discussed with bosonic atoms condensed in the two inequivalent energy minima $X_{\pm }$ of the second band of a bipartite two-dimensional optical lattice.

Diffraction of orbital-angular-momentum–carrying Laguerre-Gauss vortex (LGV) beams by $N$-fold rotationally symmetric regular polygons is studied analytically and experimentally. The structure, symmetry, and dependence of the diffraction pattern on the angular momentum index of the LGV beam and the number of sides in the polygon are systematically investigated, and features and trends are identified. The evolution of the diffraction pattern and its symmetry with the aperture position relative to the waist is also studied, leading to a generalized Friedel's law for diffraction of LGV beams.

We study a superposed weak coherent state that can fundamentally mimic an ideal single photon not only with respect to the number of photons but also in terms of an indeterminate phase. It is close to the single-photon state with high fidelity and exhibits fundamental features of single photons such as antibunching and Hong-Ou-Mandel interference. The emergence and vanishing of single-photon characteristics can be directly observed by changing two parameters, i.e., the mean photon number and number of phases. Our result shows that the uncertainty between the photon number and phase indeed constitutes the characteristics of single photons. Finally, we apply the superposed weak coherent state to quantum key distribution and demonstrate that it outperforms the typical approach using phase-randomized weak coherent states.

Decoherence in quantum searches, and in the Grover search, in particular, has already been extensively studied, leading very quickly to the loss of the quadratic speedup over the classical case, when searching for some target (marked) element within a set of size $N$. The noise models used were, however, almost always global. In this paper, we study Grover search under the influence of localized partially dephasing noise of rate $p$. We find that, in the case when the size $k$ of the affected subspace is much smaller than $N$ and the target is unaffected by the noise, namely when $kp\ll \sqrt{N}$, the quadratic speedup is retained. Once these restrictions are not met, the quadratic speedup is lost. If the target is affected by the noise, the noise rate needs to scale as $1/\sqrt{N}$ to keep the speedup. We also observe an intermediate region, where if $k\sim N^{\mu }$ and the target is unaffected, the speedup seems to obey $N^{\mu }$, which for $\mu >0.5$ is worse than the quantum, but better than the classical case. We also put obtained results for quantum searches into perspective of quantum walks and searches on graphs.