The atomic properties of Pm-like ions were comprehensively studied using relativistic atomic codes. Excitation energies of the $4f^{14}nl$ (with $nl=5s$, $6s$, $5p$, $6p$, $5d$, $6d$, and $5f$) states in Pm-like ions with nuclear charge $Z$ ranging from 74 to 100 are evaluated within the framework of relativistic many-body theory (RMBPT). First- and second-order Coulomb energies and first- and second-order Breit corrections to the energies are calculated. Two alternative treatments of the Breit interaction are investigated. In the first approach we omit Breit contributions to the Dirac-Fock potential and evaluate Coulomb and Breit-Coulomb corrections through second order perturbatively. In the second approach were included both Coulomb and Breit contributions on the same footing via the Breit-Dirac-Fock potential and then treat the residual Breit and Coulomb interactions perturbatively. The results obtained from the two approaches are compared and discussed. The important question of what is the ground state in Pm-like ions was answered. Properties of the $4f$-core-excited states are evaluated using the multiconfiguration relativistic Hebrew University Lawrence Livermore atomic code (hullac code) and the Hartree-Fock-relativistic method (cowan code). We evaluate excitation energies and transition rates in Pm-like ions with nuclear charge $Z$ ranging from 74 to 92. Our large scale calculations include the following set of configurations: $4f^{14}5s$, $4f^{14}5p$, $4f^{13}5s^2$, $4f^{13}5p^2$, $4f^{13}5s5p$, $4f^{12}5s^{2}5p$, $4f^{12}5s5p^2$, and $4f^{12}5p^3$. Trends of excitation energies as function of $Z$ are shown graphically for selected states. Excitation energies, transition rates, and lifetimes in Pm-like tungsten are evaluated with additional inclusion of the $4f^{11}5s^{2}5p^2$, $4f^{11}5s5p^3$, $4f^{10}5s^{2}5p^3$, and $4f^{10}5s5p^4$ configurations. This represents an unusual example of an atomic system where the even-parity complex [$4f^{14}5s+4f^{13}5s5p+4f^{12}5s5p^2+4f^{11}5s5p^3+4f^{10}5s5p^4$] and the odd-parity complex [$4f^{14}5p+4f^{13}5s^2+4f^{12}5s^{2}5p+4f^{11}5s^{2}5p^2+4f^{10}5s^{2}5p^3$] include so different configurations. Wavelengths of the $4f^{14}5s{}^2{S}_{1/2}$-$4f^{14}5p{}^2{P}_J$ transition obtained by the cowan, hullac, and RMBPT codes are compared with other theoretical results and available measurements.

The existence of resonances in a double differential cross section of anomalous inelastic scattering of an x-ray photon by an excited atom at the attosecond time scale of the photon-electron contact interaction is theoretically predicted. These resonances realize unusual types of the excited atom vacancy decay channels. The subject of investigation is the atom of Be, excited to a discrete spectrum state via the $1s\to np$, $n\ge 2$ channel. Experimental realization of the theoretically predicted quantum effect may provide a new method for increasing the energy of photons generated by an x-ray free electron laser.

The determination of the electronic stopping power for low-energy ions is an experimentally demanding task. In this paper we elaborate on the different effects of nuclear stopping and multiple scattering on the energy spectra for different experimental geometries, i.e., transmission through thin foils and backscattering from thin films. By calculating distributions of path lengths and scattering angles we demonstrate how electronic stopping, nuclear stopping, and multiple scattering add up to the total energy loss. We show that at low energies it is important to properly disentangle these effects to extract electronic stopping from the measured energy loss spectra.

Light propagation through an ensemble of ultracold Rydberg atoms in an electromagnetically-induced-transparency (EIT) configuration is studied. In strongly interacting Rydberg EIT media, nonlinear optical effects lead to a nontrivial dependence of the degree of probe-beam attenuation on the medium density and on its initial intensity. We develop a Monte Carlo rate equation model that self-consistently includes the effect of the probe-beam attenuation to investigate the steady state of the Rydberg medium driven by two laser fields. We compare our results to recent experimental data and to results of other state-of-the-art models for light propagation in Rydberg EIT media. We find that for low probe field intensities, our results match the experimental data best if a density-dependent dephasing rate is included in the model. At higher probe intensities, our model deviates from other theoretical approaches, because it predicts a spectral asymmetry together with line broadening. These are likely due to off-resonant excitation channels, which, however, have not been observed in recent experiments. Atomic motion and coupling to additional Rydberg levels are discussed as possible origins for these deviations.

We consider Bose-Einstein condensation of photons in an optical cavity filled with dye molecules that are excited by laser light. By using the Schwinger-Keldysh formalism we derive a Langevin field equation that describes the dynamics of the photon gas and, in particular, its equilibrium properties and relaxation towards equilibrium. Furthermore we show that the finite lifetime effects of the photons are captured in a single dimensionless damping parameter that depends on the power of the external laser pumping the dye. Finally, as applications of our theory we determine spectral functions and collective modes of the photon gas in both the normal and the Bose-Einstein condensed phases.

When atoms scatter photons from a transverse laser into a high-finesse optical cavity, they form crystalline structures which maximize the intracavity light field and trap the atoms in the ordered array. Stable organization occurs when the laser field amplitude exceeds a certain threshold. For planar single-mode cavities there exist two equivalent possible atomic patterns, which determine the phase of the intracavity light field. Under these premises, we show that the effect of an additional laser pumping the cavity makes one pattern more favorable than the other and that it can dynamically force the system into a predetermined configuration. This is an instance of pattern formation and seeding in a nonlinear quantum-optical system.

We present a theoretical study of the rotational dynamics of asymmetric-top molecules in an electric field and a parallel nonresonant linearly polarized laser pulse. The time-dependent Schrödinger equation is solved within the rigid rotor approximation. Using the benzonitrile molecule as a prototype, we investigate the field-dressed dynamics for experimentally accessible field configurations and compare these results to the adiabatic predictions. We show that for an asymmetric-top molecule in parallel fields, the formation of the pendular doublets and the avoided crossings between neighboring levels are the two main sources of nonadiabatic effects. We also provide the field parameters under which the adiabatic dynamics would be achieved.

We investigate the role of momentum-dependent interactions to determine the ground-state properties in the Bose-Einstein condensate for large scattering lengths ($a$) such that $ka$ $\ge 1$, even for small momentum ($p$) where $p=\hslash k$ and $k$ is the wave number. The results for momentum-dependent and momentum-independent interactions differ significantly, even for moderate values of $a$, and the effect of higher partial-wave interaction increases with the increase in $a$. We have made an attempt to compare the theoretical column density with experimental results at different magnetic fields [Cornish et al., Phys. Rev. Lett. 85, 1795 (2000)]. Since the initial number of atoms changes while swapping the magnetic field and the actual value of the number of atoms at different magnetic fields is not known, we studied the dependence of column densities on the value of $N$ at different magnetic fields. We present here the results for those values of $N$ for which the theoretical column densities are comparable with the experimental results. It is shown that the column density for ${}^{85}$Rb atoms at 100 $\text{nK}$ with momentum-dependent interaction is in fairly good agreement with the experimental values for different values of $N$ at two different values of magnetic fields ($B=157.2$ and 156.4 G).

We present a simple and optimal experimental scheme for an all-optical production of a sodium spinor Bose-Einstein condensate (BEC). With this scheme, we demonstrate that the number of atoms in a pure BEC can be greatly boosted by a factor of 5 over some widely used schemes in a simple single-beam or crossed-beam optical trap. Our scheme avoids technical challenges associated with some all-optical BEC methods and may be applicable to other optically trappable atomic species. In addition, we discuss an upper limit for evaporative cooling efficiency in all-optical BEC approaches and a good agreement between our theoretical model and experimental data.

We study the collective excitation frequencies of a harmonically trapped ${}^{85}$Rb Bose-Einstein condensate (BEC) in the vicinity of a Feshbach resonance. To this end, we solve the underlying Gross-Pitaevskii (GP) equation by using a Gaussian variational approach and obtain the coupled set of ordinary differential equations for the widths and the center of mass of the condensate. A linearization shows that the dipole-mode frequency decreases when the bias magnetic field approaches the Feshbach resonance, so the Kohn theorem is violated.

Starting from a microscopic theory for atomic scatterers, we describe the scattering of light by a single atom and study the coherent propagation of light in a cold atomic cloud in the presence of a magnetic field $\mathbf{B}$ in the mesoscopic regime. Nonpertubative expressions in $B$ are given for the magneto-optical effects and optical anisotropy. We then consider the multiple-scattering regime and address the fate of the coherent-backscattering (CBS) effect. We show that, for atoms with nonzero spin in their ground state, the CBS interference contrast can be increased compared to its value when $B=0$, a result at variance with classical samples. We validate our theoretical results by a quantitative comparison with experimental data.

We provide a theoretical study of frequency-shifted feedback (FSF) lasers, i.e., lasers with an internal frequency shifter, seeded with a monochromatic wave. The resulting spectrum consists in a set of equidistant modes, labeled by $n$, whose phases vary quadratically with $n$. We prove the emergence of a temporal fractional Talbot effect, leading to generation of Fourier-transform-limited pulses at a repetition rate tunable by the parameters of the FSF cavity (cavity length and frequency shift per round trip), and limited by the spectral bandwidth of the laser. We characterize in detail the output field of this so-called “Talbot laser” and emphasize its specific intensity fluctuations. We evidence connections with some aspects of number theory by the appearance of Gauss sums and theta series in the expression of the laser field. Our predictions are in full agreement with the experimental results published in Guillet de Chatellus et al. [Opt. Express 21, 15065 (2013)]. Practical applications and limitations are discussed.

Cold circular Rydberg atoms are produced and magnetically trapped. The trap is characterized by direct spatial imaging of ion distributions, ion counting, and state-selective field ionization. At room temperature, we observe about 70% of the trapped atoms remaining after 6 ms. We measure a trap oscillation frequency increase of the circular Rydberg atom trap relative to the ground-state atom trap due to the larger magnetic moment of the circular Rydberg atoms. Simulations of the center-of-mass and internal-state evolution of circular states in our magnetic trap are performed and results are in good agreement with experimental observations.

We provide a systematic analysis of a prototypical nonlinear oscillator system respecting $\mathcal{P}\mathcal{T}$ symmetry i.e., one of them has gain and the other an equal and opposite amount of loss. Starting from the linear limit of the system, we extend considerations to the nonlinear case for both soft and hard cubic nonlinearities identifying symmetric and antisymmetric breather solutions, as well as symmetry-breaking variants thereof. We propose a reduction of the system to a Schrödinger-type $\mathcal{P}\mathcal{T}$-symmetric dimer, whose detailed earlier understanding can explain many of the phenomena observed herein, including the $\mathcal{P}\mathcal{T}$ phase transition. Nevertheless, there are also significant parametric as well as phenomenological potential differences between the two models and we discuss where these arise and where they are most pronounced. Finally, we also provide examples of the evolution dynamics of the different states in their regimes of instability.

We study how conditional photon operations can affect multipartite quantum correlations, specifically nonlocality and entanglement, of the continuous-variable Greenberger-Horne-Zeilinger (GHZ) states. We find that the violation of the Mermin-Klyshko inequality revealing the multipartite nonlocality can be made stronger with photon subtraction applied on each mode of the original GHZ states, particularly in a weak squeezing regime. Photon addition applied on local modes also turns out to enhance the degree of multipartite nonlocality in a broad range of parameters. We further investigate the effects of the photon operations on the degree of multipartite entanglement by looking into the Gaussian tangle, the fidelity of teleportation network, and the quadrature correlations. We find that photon subtraction applied on two modes enhances those entanglement characteristics in a practical squeezing regime while there is no improvement made by photon addition.

The assumption of free will—the ability of an experimentalist to make random choices—is central to proving the indeterminism of quantum resources, the primary tool in quantum cryptography. Relaxing the assumption in a Bell test allows violation of the usual classical threshold by correlating the random number generators used to select measurements with the devices that perform them. In this paper, we examine not only these correlations, but those across multiple runs of the experiment. This enables an explicit exposition of the optimal cheating strategy and how the correlations manifest themselves within this strategy. Similar to other recent results, we prove that there remain Bell violations for a sufficiently high, yet nonmaximal degree of free will which cannot be simulated by a classical attack, regardless of how many runs of the experiment those choices are correlated over.

Block entanglement entropy in the ground state of a quantum spin chain is investigated. The spins have Kitaev-type nearest-neighbor interaction, of strength $J_x$ or $J_y$, through either $x$ or $y$ components of the spins on alternating bonds, along with a transverse magnetic field $h$. An exact solution is obtained through Jordan-Wigner fermionization, and it exhibits a macroscopically degenerate ground state for $h=0$ and a nondegenerate ground state for $h\ne 0$ for all interaction strengths. For a chain of $N$ spins, we study the block entropy of a partition of $L$ contiguous spins. The block entanglement entropy needs the eigenvalues of the $2^L$-dimensional reduced density matrix. We employ an efficient method that reduces this problem to evaluating eigenvalues of an $L$-dimensional matrix, which enables us to calculate easily the block entanglement for large-$N$ chains numerically. The entanglement entropy grows as $\log L$ at the degeneracy point $h=0$ and only for $J_x=J_y$. For $h\ne 0$, the entropy becomes independent of the size, thus obeying the area law. For $J_x\ne J_y$, the block entropy shows a nonmonotonic behavior for $L<N/2$.

We fully characterize bipartite entanglement-annihilating (EA) channels that destroy entanglement of any state shared by subsystems and, thus, should be avoided in any entanglement-enabled experiment. Our approach relies on extending the problem to EA positive maps, the cone of which remains invariant under concatenation with partially positive maps. Due to this invariancy, positive EA maps adopt a well characterization and their intersection with completely positive trace-preserving maps results in the set of EA channels. In addition to a general description, we also provide sufficient operational criteria revealing EA channels. They have a clear physical meaning since the processes involved contain stages of classical information transfer for subsystems. We demonstrate the applicability of derived criteria for local and global depolarizing noises, and specify corresponding noise levels beyond which any initial state becomes disentangled after passing the channel. The robustness of some entangled states is discussed.

We present a scheme for the dissipative preparation of an entangled steady state of two superconducting qubits in a circuit quantum electrodynamics (QED) setup. Combining resonator photon loss—a dissipative process already present in the setup—with an effective two-photon microwave drive, we engineer an effective decay mechanism which prepares a maximally entangled state of the two qubits. This state is then maintained as the steady state of the driven, dissipative evolution. The performance of the dissipative state preparation protocol is studied analytically and verified numerically. In view of the experimental implementation of the presented scheme we investigate the effects of potential experimental imperfections and show that our scheme is robust to small deviations in the parameters. We find that high fidelities with the target state can be achieved both with state-of-the-art three-dimensional, as well as with the more commonly used two-dimensional transmons. The promising results of our study thus open a route for the demonstration of a highly entangled steady state in circuit QED.

Threshold theorems for fault-tolerant quantum computing assume that errors are of certain types. But how would one detect whether errors of the “wrong” type occur in one's experiment, especially if one does not even know what type of error to look for? The problem is that for many qubits a full state description is almost impossible to analyze due to the exponentially large state space, and a full process description requires even more resources. As a result, one simply cannot detect all types of errors. Here we show through a quantum state estimation example (on up to 25 qubits) how to attack this problem using model selection. We use, in particular, the Akaike information criterion. The example indicates that the number of measurements that one has to perform before noticing errors of the wrong type scales polynomially both with the number of qubits and with the error size.