We present an experimental study of cavity-assisted Rydberg-atom electromagnetically induced transparency (EIT) using a high-finesse optical cavity ($F\sim 28000$). Rydberg atoms are excited via a two-photon transition in a ladder-type EIT configuration. A three-peak structure of the cavity transmission spectrum is observed when Rydberg EIT is generated inside the cavity. The two symmetrically spaced side peaks are caused by bright-state polaritons, while the central peak corresponds to a dark-state polariton. Anticrossing phenomena and the effects of mirror adsorbate electric fields are studied under different experimental conditions. We determine a lower bound on the coherence time for the system of $7.26\pm 0.06\mu \mathrm{s}$, most likely limited by laser dephasing. The cavity-Rydberg EIT system can be useful for single-photon generation using the Rydberg blockade effect, studying many-body physics, and generating novel quantum states among many other applications.

We explore the feasibility of a fiber clock, i.e., a compact, high-precision, optical lattice atomic clock based on atoms trapped inside a hollow-core optical fiber. Such a setup offers an intriguing potential both for a substantially increased number of interrogated atoms (and thereby an improved clock stability) and for miniaturization. We evaluate the sensitivity of the ${{}^{1}S}_0\text{−}{{}^{3}P}_0$ clock transition in Hg and other divalent atoms to the fiber inner core surface at nonzero temperatures. The Casimir-Polder interaction induced ${{}^{1}S}_0{\text{−}}^3{P}_0$ transition frequency shift is calculated for the atom inside the hollow capillary as a function of atomic position, capillary material, and geometric parameters. For $\mathrm{Hg}$ atoms on the axis of a silica capillary with inner radius $\ge 15\mu \mathrm{m}$ and optimally chosen thickness $d\sim 1\mu \mathrm{m}$, the atom-surface interaction induced ${{}^{1}S}_0{\text{−}}^3{P}_0$ clock transition frequency shift can be kept on the level $\delta \nu /{\nu }_{\mathrm{Hg}}\sim {10}^{-19}$. We also estimate the atom loss and heating due to collisions with the buffer gas, lattice intensity noise induced heating, spontaneous photon scattering heating, and residual birefringence induced frequency shifts.

We propose and demonstrate a method for generating propagation-invariant spatially stationary fields in a controllable manner. Our method relies on producing incoherent mixtures of plane waves using planar primary sources that are spatially completely uncorrelated. The strengths of the individual plane waves in the mixture determine the exact functional form of the generated coherence function. We use light-emitting diodes as the primary incoherent sources and experimentally demonstrate the effectiveness of our method by generating several spatially stationary fields, including a type that we refer to as the regionwise spatially stationary field. We also experimentally demonstrate the propagation invariance of these fields, which is an extremely interesting and useful property of such fields. Our work should have important implications for applications that exploit the spatial coherence properties either in a transverse plane or in a propagation-invariant manner, such as correlation holography, wide-field optical coherence tomography, and imaging through turbulence.

Rabi oscillations typify the inherent nonlinearity of optical excitations in quantum dots. Using an integral kernel formulation to solve the three-dimensional Maxwell-Bloch equations in ensembles of up to ${10}^4$ quantum dots, we observe features in Rabi oscillations due to the interplay of nonlinearity, nonequilibrium excitation, and electromagnetic coupling between the dots. This approach allows us to observe the dynamics of each dot in the ensemble without resorting to spatial averages. Our simulations predict synchronized multiplets of dots that exchange energy, dots that dynamically couple to screen the effect of incident external radiation, localization of the polarization due to randomness and interactions, as well as wavelength-scale regions of enhanced and suppressed polarization.

We report on the fragmentation of the water molecule by 1 MeV ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{+}$ and 650 keV ${\mathrm{N}}^{+}$ ion impact. The fragment-ion energy spectra were measured by an electrostatic spectrometer at different observation angles. The obtained double-differential fragmentation cross sections for ${\mathrm{N}}^{+}$ are found to be more than an order of magnitude higher than that for ${\mathrm{H}}^{+}$. The relative ratios of the fragmentation channels are also different for the three projectiles. Additional fragmentation channels were observed in the spectra for ${\mathrm{He}}^{+}$ and for ${\mathrm{N}}^{+}$ impact, which are missing in the case of ${\mathrm{H}}^{+}$. From the analysis of the kinetic energy of the fragments, the maximum observed degree of ionization was found to be $q_{max}=3$, 4, and 5 for $\mathrm{H}{}^{+}$, $\mathrm{He}{}^{+}$, and $\mathrm{N}{}^{+}$ impact, respectively. Absolute multiple-ionization cross sections have been determined. They are compared with the predictions of the classical trajectory Monte Carlo and continuum-distorted-wave eikonal-initial-state theories. At lower degrees of ionization, theories provide reasonable agreement with experiment. The systematic overestimation of the cross section by the theories towards higher degrees of ionization indicates the failure of the independent particle model.

In this paper we report the L-shell dielectronic recombination measurement in highly charged $3d^n$ ions of tungsten by employing a fast electron beam-energy scanning technique at Shanghai EBIT. The studies of the LMM DR resonance strengths of Ar-like up to Mn-like tungsten were implemented through the experiment as well as a fully relativistic configuration interaction method in the flexible atomic code. In the analysis of DR spectrum, an isolated resonant peak was discovered coming from the DR processes via an extremely-long-lived metastable level in a Ti-like system. This work indicated the population of this Ti-like metastable level in the present EBIT plasma condition was as large as 45%.

The electron-nuclear joint energy spectrum allows one to unambiguously count the total number of photons absorbed by the electrons and nuclei of a molecule. Driven by phase-controlled, linearly polarized two-color femtosecond laser pulses, we experimentally demonstrate that the asymmetric bond breaking of a singly ionized ${\mathrm{H}}_2$ depends on the total number of photons absorbed by the molecule in the ionization and dissociation processes. The accessibilities of different dissociation pathways and their interference-induced asymmetric electron localization as a function of the absorbed photons are retrieved. Our results strengthen the understanding of the directional bond breaking of a molecule from the aspect of the correlated electron-nuclear dynamics.

We experimentally investigate ${\sigma }^{+}\text{−}{\sigma }^{-}$ polarization gradient cooling (PGC) of a single ${}^{87}\mathrm{Rb}$ atom in a tightly focused dipole trap and show that the cooling limit strongly depends on the polarization of the trapping field. For optimized cooling light power, the temperature of the atom reaches $10.4\left(6\right)\mu \mathrm{K}$ in a linearly polarized trap, approximately five times lower than in a circularly polarized trap. The inhibition of PGC is qualitatively explained by the fictitious magnetic fields induced by the trapping field. We further demonstrate that switching the trap polarization from linear to circular after PGC induces only minor heating.

We discuss how to perform the continuum limit of the $d$-dimensional Hubbard model in the presence of a harmonic confining potential at zero temperature and fixed particle number. While for $d\ge 3$ the system can be mapped into a noninteracting two-component Fermi gas in a harmonic trap, for $d=1$ and 2 the two-body interaction is described by a Dirac $\delta$. We show that the properties of this continuum limit can be put in one-to-one correspondence with those obtained by applying the trap-size scaling formalism to the confined Hubbard model in the so called dilute regime (fixed number of particles and weak confinement). The correspondence for $d<3$ has been tested, in the case of two particles with opposite spin, by comparing numerical exact-diagonalization results of the lattice system with those obtained in the continuum limit.

In the present article the snake instability mechanism for dark solitons in superfluid Fermi gases is studied in the context of a recently developed effective field theory [S. N. Klimin et al., Eur. Phys. J. B 88, 122 (2015)]. This theoretical treatment has proven to be suitable to study stable dark solitons in quasi-one-dimensional setups across the BEC-BCS crossover. In this paper the nodal plane of the stable soliton solution is perturbed by adding a transverse modulation. The numerical solution of the system of coupled nonlinear differential equations describing the amplitude of the perturbation leads to an estimate of the growth rate and characteristic length scale of the instability, which are calculated for a wide range of interaction regimes and compared to other theoretical predictions. The behavior of the maximum transverse size that the atomic cloud can have in order to preserve the stability is described across the BEC-BCS crossover. The analysis of the effects of spin imbalance on this critical length reveals a stabilization of the soliton with increasing imbalance and therefore provides the experimental community with a method to achieve the realization of stable solitons in real three-dimensional configurations, without reducing the system dimensionality.

We report on a theoretical study of cavity-free lasing of neutral nitrogen molecules in femtosecond laser filaments with a nonadiabatic Maxwell-Bloch model, compared with recent pump-seed experiments. The nonadiabaticity of the lasing process is revealed and it is found that electron-neutral collisions dominate the dipole dephasing rate. Moreover, we show that the asymmetry between forward and backward lasing not only depends on the different amplification lengths but also on the temporal dynamics of electron-neutral collisions. The comparison of the nonadiabatic model with simulations based on the adiabatic approximation (such as radiative transfer equations) explicitly sets a bound on the validity of the latter model for cavity-free nitrogen lasing phenomenon, which holds a unique potential in optical remote sensing applications.

We present a phase-induced transparency-based scheme to generate structured-beam patterns in a closed four-level atomic system. We employ a phase-structured probe beam and a transverse magnetic field (TMF) to create phase-dependent medium susceptibility. We show that such phase-dependent modulation of absorption holds the key to formation of a structured beam. We use a full density matrix formalism to explain the experiments of Radwell et al. [Phys. Rev. Lett. 114, 123603 (2015)] at weak probe limits. Our numerical results on beam propagation confirms that the phase information present in the absorption profile gets encoded on the spatial probe envelope, which creates petal-like structures even in the strong field limit. The contrast of the formed structured beam can be enhanced by changing the strength of TMF as well as of the probe intensity. In weak field limits an absorption profile is solely responsible for creating a structured beam, whereas in the strong probe regime, both dispersion and absorption profiles facilitate the generation of a high-contrast structured beam. Furthermore, we find the rotation of structured beams owing to strong-field-induced nonlinear magneto-optical rotation.

A formula for the ionization rate in an extremely intense electromagnetic field is proposed and used for numerical study of QED cascades in noble gases in the field of two counterpropagating laser pulses. It is shown that the number of the electron-positron pairs produced in the cascade increases with the atomic number of the gas, where the gas density is taken to be inversely proportional to the atomic number. While most of the electrons produced in the laser pulse front are expelled by the ponderomotive force from the region occupied by the strong laser field, there is a small portion of electrons staying in the laser field for a long time until the instance when the laser field is strong enough for cascading. This mechanism is relevant for all gases. For high-$Z$ gases there is an additional mechanism associated with the ionization of inner shells at the instance when the laser field is strong enough for cascading. The role of both mechanisms for cascade initiation is revealed.

A collision model (CM) is a framework to describe open quantum dynamics. In its memoryless version, it models the reservoir $\mathcal{R}$ as consisting of a large collection of elementary ancillas: the dynamics of the open system $\mathcal{S}$ results from successive collisions of $\mathcal{S}$ with the ancillas of $\mathcal{R}$. Here, we present a general formulation of memoryless composite CMs, where $\mathcal{S}$ is partitioned into the very open system under study $S$ coupled to one or more auxiliary systems $\left\{S_i\right\}$. Their composite dynamics occurs through internal $S-\left\{S_i\right\}$ collisions interspersed with external ones involving $\left\{S_i\right\}$ and the reservoir $\mathcal{R}$. We show that important known instances of quantum non-Markovian dynamics of $S$—such as the emission of an atom into a reservoir featuring a Lorentzian, or multi-Lorentzian, spectral density or a qubit subject to random telegraph noise—can be mapped on to such memoryless composite CMs.

Based on correlations of coherently displaced photon numbers, we derive entanglement criteria for the purpose of verifying non-Gaussian entanglement. Our construction method enables us to verify bipartite and multipartite entanglement of complex states of light. An important advantage of our technique is that the certified entanglement persists even in the presence of arbitrarily high, constant losses. We exploit experimental correlation schemes for the two-mode and multimode scenarios, which allow us to directly measure the desired observables. To detect entanglement of a given state, a genetic algorithm is applied to optimize over the infinite set of our constructed witnesses. In particular, we provide suitable witnesses for several distinct two-mode states. Moreover, a mixed non-Gaussian four-mode state is shown to be entangled in all possible nontrivial partitions.

Quantum state tomography is an indispensable but costly part of many quantum experiments. Typically, it requires measurements to be carried out in a number of different settings on a fixed experimental setup. The collected data are often informationally overcomplete, with the amount of information redundancy depending on the particular set of measurement settings chosen. This raises a question about how one should optimally take data so that the number of measurement settings necessary can be reduced. Here, we cast this problem in terms of integer programming. For a given experimental setup, standard integer-programming algorithms allow us to find the minimum set of readout operations that can realize a target tomographic task. We apply the method to certain basic and practical state-tomographic problems in nuclear-magnetic-resonance experimental systems. The results show that considerably fewer readout operations can be found using our technique than by using the previous greedy search strategy. Therefore, our method could be helpful for simplifying measurement schemes to minimize the experimental effort.

Based on the algebraic theory of signal processing, we recursively decompose the discrete sine transform of the first kind (DST-I) into small orthogonal block operations. Using a diagrammatic language, we then second-quantize this decomposition to construct a tensor network implementing the DST-I for fermionic modes on a lattice. The complexity of the resulting network is shown to scale as $\frac{5}{4}nlogn$ (not considering swap gates), where $n$ is the number of lattice sites. Our method provides a systematic approach of generalizing Ferris' spectral tensor network for nontrivial boundary conditions.

KCa was produced in a heat-pipe oven and its thermal emission spectrum around $8900{\mathrm{cm}}^{-1}$ was recorded with a high-resolution Fourier-transform spectrometer. In addition, many selected transitions of this spectrum between deeply bound vibrational levels of the $X{\left(1\right)}^2{\mathrm{\Sigma }}^{+}$ and ${\left(2\right)}^2{\mathrm{\Sigma }}^{+}$ states were studied using laser excitation to facilitate the assignment of the lines. The ground state is described for $v^{\prime \prime }=0–5$ and the ${\left(2\right)}^2{\mathrm{\Sigma }}^{+}$ state for $v^{\prime }=0-8$ with rotational levels up to 175. For both states, Dunham coefficients, spin-rotation parameters, and potential energy curves are derived.

The specific energy loss of medium-energy hydrogen, helium, and neon ions in titanium nitride is studied. Electronic stopping cross sections of ions in the energy range of 3–140 keV/amu were measured in backscattering geometry using time-of-flight medium-energy ion scattering, from the energy loss experienced in TiN thin films on Si. For the lowest energies, data for H show a strong deviation from Bragg's rule. For hydrogen and Ne ions, electronic stopping exhibits velocity proportionality at ion velocities below 1 a.u. Comparison to density functional theory calculations of the stopping power yields very good agreement for H, while for He and Ne, the experimentally observed electronic stopping power is considerably higher than predicted. For He the extrapolation of the stopping power at low energies points to a nonvanishing energy loss at vanishing ion velocity. The present data can thus be taken as an indication of additional energy-loss processes different from direct electron-hole pair excitation. Furthermore, the results provide reference values for ion-beam-based analysis of TiN, a material with huge technological relevance.