The ac-Stark shift induced by blackbody radiation is a major source of systematic uncertainty in present-day cesium microwave frequency standards. The shift is parametrized in terms of a third-order electric polarizability ${\alpha }_0^{\left(3\right)}$ that can be inferred from the static electric-field displacement of the clock transition resonance. In this paper, we report on an all-optical coherent-population-trapping pump-probe experiment measuring the differential polarizability $\Delta {\alpha }_0^{\left(3\right)}={\alpha }_0^{\left(3\right)}(F=4)-{\alpha }_0^{\left(3\right)}(F=3)$ on a thermal Cs atomic beam, from which we infer ${\alpha }_0^{\left(3\right)}(F=4)=2.023{\left(6\right)}_{\mathrm{stat}}{\left(9\right)}_{\mathrm{syst}}\mathrm{Hz}/{(\mathrm{kV}/\mathrm{cm})}^2$, which corresponds to a scalar Stark shift parameter $k_s=-2.312{\left(7\right)}_{\mathrm{stat}}{\left(10\right)}_{\mathrm{syst}}\mathrm{Hz}/{(\mathrm{kV}/\mathrm{cm})}^2$. The result agrees within two standard deviations with a recent measurement in an atomic fountain, and rules out another recent result obtained in a Cs vapor cell.

Detailed calculations are carried out for the polarization properties and angular distribution of the x-ray photoemission of highly charged H-like and He-like ions following electron-impact excitation using a fully relativistic distorted-wave method. The contributions of the magnetic quadrupole ($M2$) to the subsequent characteristic x-ray emission are investigated systematically. For the Lyman-${\alpha }_1$ $(2p_{3/2}\to 1s_{1/2}$) radiation of H-like ions, it is shown that the interference between the leading electric dipole ($E1$) decay and the much weaker $M2$ decay may significantly reduce the polarization and slightly increase the angular distribution of the emitted x-rays, these characteristics are very similar to the conclusions regarding the radiative electron capture process [G. Weber et al., Phys. Rev. Lett. 105, 243002 (2010)]. However, for the $2s_{1/2}2p_{3/2}(J=1)\to 1s_{1/2}2s_{1/2}$($J=1$) radiation of He-like ions, the $E1$-$M2$ interference leads to an obvious increase in polarization and a large decrease in the angular distribution of the x-ray photoemission, which is contrary to the Lyman-${\alpha }_1$ line of H-like ions. And all these characters strongly depend on the atomic number and the incident electron energy.

Quantum optimal control has enjoyed wide success for a variety of theoretical and experimental objectives. These favorable results have been attributed to advantageous properties of the corresponding control landscapes, which are free from local optima if three conditions are met: (1) the quantum system is controllable, (2) the Jacobian of the map from the control field to the evolution operator is full rank, and (3) the control field is not constrained. This paper explores how gradient searches for globally optimal control fields are affected by deviations from assumption (2). In some quantum control problems, so-called singular critical points, at which the Jacobian is rank deficient, may exist on the landscape. Using optimal control simulations, we show that search failure is only observed when a singular critical point is also a second-order trap, which occurs if the control problem meets additional conditions involving the system Hamiltonian and/or the control objective. All known second-order traps occur at constant control fields, and we also show that they only affect searches that originate very close to them. As a result, even when such traps exist on the control landscape, they are unlikely to affect well-designed gradient optimizations under realistic searching conditions.

Protocols for probabilistic entanglement-assisted quantum teleportation and for entanglement swapping of material qubits are presented. They are based on a protocol for postselective Bell- state projection which is capable of projecting two material qubits onto a Bell state with the help of ancillary coherent multiphoton states and postselection by balanced homodyne photodetection. Provided this photonic postselection is successful, we explore the theoretical possibilities of realizing unit-fidelity quantum teleportation and entanglement swapping with $25\%$ success probability. This photon-assisted Bell projection is generated by coupling almost resonantly the two material qubits to single modes of the radiation field in two separate cavities in a Ramsey-type interaction sequence and by measuring the emerged field states in a balanced homodyne detection scenario. As these quantum protocols require basic tools of quantum state engineering of coherent multiphoton states and balanced homodyne photodetection, they may offer interesting perspectives in particular for current quantum optical applications in quantum information processing.

The production of long-lived Rydberg positronium (Ps) and correction for Doppler shifts in the excitation laser frequencies are crucial elements of proposed measurements of the gravitational freefall of antimatter and for precision measurements of the optical spectrum of Ps. Using a two-step optical transition via $2P$ levels, we have prepared Ps atoms in Rydberg states up to the term limit. The spectra are corrected for the first-order Doppler shift using measured velocities, and the Balmer transitions are resolved for $15\le n\le 31$. The excitation signal amplitude begins to decrease for $n>50$, consistent with the onset of motional electric field ionization in the 3.5-mT magnetic field at the Ps formation target.

The combination of the configuration interaction method and all-order single-double coupled-cluster technique is used to calculate excitation energies, ionization potentials, and static dipole polarizabilities of superheavy elements nobelium, lawrencium, and rutherfordium. Breit and quantum electrodynamic corrections are also included. The results for the superheavy elements are compared with earlier calculations where available. Similar calculations for lighter analogs, ytterbium, lutetium, and hafnium, are used to study the accuracy of the calculations. The estimated uncertainties of the final results are discussed.

Rate coefficients for photorecombination (PR) and cross sections for electron-impact ionization (EII) of ${\mathrm{Fe}}^{14+}$, forming ${\mathrm{Fe}}^{13+}$ and ${\mathrm{Fe}}^{15+}$, respectively, have been measured by employing the electron-ion merged-beams technique at a heavy-ion storage ring. Rate coefficients for PR and EII of ${\mathrm{Fe}}^{14+}$ ions in a plasma are derived from the experimental measurements. Simple parametrizations of the experimentally derived plasma rate coefficients are provided for use in the modeling of photoionized and collisionally ionized plasmas. In the temperature ranges where ${\mathrm{Fe}}^{14+}$ is expected to form in such plasmas, the latest theoretical rate coefficients of Altun et al. [Astron. Astrophys. 474, 1051 (2007)] for PR and of Dere [Astron. Astrophys. 466, 771 (2007)] for EII agree with the experimental results to within the experimental uncertainties. Common features in the PR and EII resonance structures are identified and discussed.

We present a method for the characterization of bound-electron wave packets generated by a broadband excitation pulse. The technique is based on the photoionization of the electron wave packet by a delayed isolated attosecond pulse and on the measurement of the ionization asymmetry parameter in the direction of the probe pulse polarization, which depends on the pump-probe delay and on the photoelectron energy. By numerically solving the fully three-dimensional time-dependent Schrödinger equation we show that Fourier analysis of the two-dimensional ionization asymmetry parameter, displaying a complex interference pattern, enables a clear observation of quantum beats between pairs of stationary states involved in the generation of the wave packet. An analytical model confirms that the quantum beats’ signal encodes the weight of each stationary state, thus suggesting a feasible approach for the complete characterization of the relative population ratio of the excited-state components of the wave packet. Moreover, an approach based on the further analysis of quantum beats is proposed to retrieve the lifetime added to each excited state.

We present a Green's-function formalism for an interacting Bose-Einstein condensate (BEC) satisfying the two required conditions: (i) the infrared-divergent longitudinal susceptibility with respect to the BEC order parameter, and (ii) the Nepomnyashchii-Nepomnyashchii identity stating the vanishing off-diagonal self-energy in the low-energy and low-momentum limit. These conditions cannot be described by the ordinary mean-field Bogoliubov theory, the many-body $T$-matrix theory, or the random-phase approximation with the vertex correction. In this paper, we show that these required conditions can be satisfied, when we divide many-body corrections into singular and nonsingular parts, and separately treat them as different self-energy corrections. The resulting Green's function may be viewed as an extension of the Popov's hydrodynamic theory to the region at finite temperatures. Our results would be useful in constructing a consistent theory of BECs satisfying various required conditions, beyond the mean-field level.

Hyperbolic metamaterials are materials in which at least one principal dielectric constant is negative. We describe the refractive index surface, and the resulting refraction effects, for a biaxial hyperbolic metamaterial, with principal dielectric constants ${\epsilon }_1<0$, $0<{\epsilon }_2\ne {\epsilon }_3$. In this general case the two sheets of the index surface intersect, forming conical singularities. We derive the ray description of conical refraction in these materials and show that it is topologically and quantitatively distinct from conical refraction in a conventional biaxial material. We also develop a wave optics description, which allows us to obtain the diffraction patterns formed from arbitrary beams incident close to the optic axis. The resulting patterns lack circular symmetry and hence are qualitatively different from those obtained in conventional, positive index materials.

Experimental investigation of the nonlinear dynamics of a quantum oscillator is a long-standing goal of quantum physics. We propose a conditional method for inducing an arbitrary nonlinear potential on a quantum oscillator weakly interacting with light. Such an arbitrary nonlinear potential can be implemented by sequential repetition of an elementary conditional $X$ gate. To implement the $X$ gate, a single photon is linearly coupled to the oscillator and is subsequently detected by optical homodyne detection.

We investigate experimental conditions in a fiber ring laser where noiselike-pulse emission fulfills the rogue wave criteria, highlighting the influence of the chromatic dispersion sign as well as the pivotal role of the pulse measurement design. While pulse-energy rogue waves are exclusively detected in the anomalous dispersion regime, spectral rogue waves are recorded for both positive and negative dispersions. To reveal $\mathsf{L}\text{-shaped}$ statistical series compatible with the generation of spectral rogue wave events, the dispersive Fourier-transform method is implemented to study the shot-to-shot spectral fluctuations of the laser output.

Quantum Zeno and anti-Zeno effects on pure dephasing are studied using exactly solvable microscopic models. The crossover between these two opposite effects is investigated. The case of a single two-level system undergoing dephasing is already different from the previously studied population-decay problem, even without taking into account any back action from the environment. For many two-level systems interacting with a common environment, multiple transitions between Zeno and anti-Zeno regimes are predicted. Finally, if the system-environment coupling strength is not weak, we show that the nontrivial evolution of the environment between measurements can considerably alter the quantum Zeno and anti-Zeno effects. This is qualitatively different from population-decay models where the environment state is reset to equilibrium after each measurement.

In this work we compare two theoretical approaches to modeling the action of the environment on an open quantum system. It is often assumed that as the temperature of the environment surrounding a quantum system increases, so does the speed of environment-induced dephasing, or decoherence (dynamical noise), and so the efficacy of processes such as quantum tunneling drops. An alternative way of viewing the action of the environment is to consider it as carrying out von Neumann–type measurements that, in the limit of continuous observation, lead to the so-called quantum Zeno effect, whereby the system is never allowed to evolve because its wave function is collapsed to its initial state with overwhelming likelihood. However, it has been established in recent years that under certain conditions quantum processes such as tunneling can actually be enhanced (thermally assisted) when the system couples to its environment, as this allows transitions to higher-energy eigenstates closer to the top of the potential barrier. Here we show that, over a specific temperature range, increasing the temperature of the heat bath to encourage such thermally induced tunneling is equivalent to increasing the frequency of a von Neumann–type measurement on the system by the environment (an anti-Zeno effect). However, this correspondence between these two independent pictures of quantum measurement breaks down above a certain limit: Increasing the frequency of measurement above this limit leads to a reversal from an anti-Zeno to a Zeno effect and the tunneling rate decreases again, whereas raising the temperature further leads to a leveling off in the tunneling probability.

Following recent work of Beigi and Shor, we investigate positive partial transpose (PPT) states that are “heavily entangled.” We first exploit volumetric methods to show that in a randomly chosen direction, there are PPT states whose distance in trace norm from separable states is (asymptotically) at least $1/4$. We then provide explicit examples of PPT states which are nearly as far from separable ones as possible. To obtain a distance of $2-\epsilon$ from the separable states, we need a dimension of $2^{\mathrm{poly}[log\left(\frac{1}{\epsilon }\right)]}$, as opposed to $2^{\mathrm{poly}\left(\frac{1}{\epsilon }\right)}$ given by the construction of Beigi and Shor [J. Math. Phys. 51, 042202 (2010)]. We do so by exploiting the so-called private states, introduced earlier in the context of quantum cryptography. We also provide a lower bound for the distance between private states and PPT states and investigate the distance between pure states and the set of PPT states.

The strong-driving regime occurs when a quantum two-level system is driven with an external field whose amplitude is greater than or equal to the energy splitting between the system's states and is typically identified with the breaking of the rotating wave approximation (RWA). We report an experimental study in which the spin of a single nitrogen-vacancy (NV) center in diamond is strongly driven with microwave fields of arbitrary polarization. We measure the NV center spin dynamics beyond the RWA and characterize the limitations of this technique for generating high-fidelity quantum gates. Using circularly polarized microwave fields, the NV spin can be harmonically driven in its rotating frame regardless of the field amplitude, thus allowing rotations around arbitrary axes. Our approach can effectively remove the RWA limit in quantum-sensing schemes and assist in increasing the number of operations in QIP protocols.

We use a specific geometric method to determine speed limits for the implementation of quantum gates in controlled quantum systems that have a specific class of constrained control functions. We achieve this by applying a recent theorem of Shen, which provides a connection between time optimal navigation on Riemannian manifolds and the geodesics of a certain Finsler metric of Randers type. We use the lengths of these geodesics to derive the optimal implementation times (under the assumption of constant control fields) for an arbitrary quantum operation (on a finite dimensional Hilbert space), and explicitly calculate the result for the case of a controlled single spin system in a magnetic field and a swap gate in a Heisenberg spin chain.

The atomic exchange effect has previously been demonstrated to have a great influence at low energy on the ${}^{241}\mathrm{Pu}$ ${\beta }^{-}$ transition. The screening effect has been given as a possible explanation for a remaining discrepancy. Improved calculations have been made to consistently evaluate these two atomic effects, compared here to the recent high-precision measurements of ${}^{241}\mathrm{Pu}$ and ${}^{63}\mathrm{Ni}$ $\beta$ spectra. In this paper a screening correction has been defined to account for the spatial extension of the electron wave functions. Excellent overall agreement of about 1% from 500 eV to the end-point energy has been obtained for both $\beta$ spectra, which demonstrates that a rather simple $\beta$ decay model for allowed transitions, including atomic effects within an independent-particle model, is sufficient to describe well the current most precise measurements.

Universal behavior has been found inside the window of Efimov physics for systems with $N=4,5,6$ particles. Efimov physics refers to the emergence of a number of three-body states in systems of identical bosons interacting via a short-range interaction becoming infinite at the verge of binding two particles. These Efimov states display a discrete scale invariance symmetry, with the scaling factor independent of the microscopic interaction. Their energies in the limit of zero-range interaction can be parametrized, as a function of the scattering length, by a universal function. We have found, using the form of finite-range scaling introduced by A. Kievsky and M. Gattobigio [Phys. Rev A 87, 052719 (2013)], that the same universal function can be used to parametrize the ground and excited energy of $N\le 6$ systems inside the Efimov-physics window. Moreover, we show that the same finite-scale analysis reconciles experimental measurements of three-body binding energies with the universal theory.