We report molecular-frame electron-scattering cross sections of ${\mathrm{N}}_2$, obtained using an electron-ion coincidence technique. Vector correlation between the scattered electron and the fragment ion has been measured at incident electron energy of 1.4 keV for the dipole-forbidden $2{\sigma }_g\to 1{\pi }_g$ transition of ${\mathrm{N}}_2$ as well as for the inner-valence ionization. The generalized oscillator strength thus obtained exhibits characteristic molecular-orientation dependence due to anisotropic shapes of the molecular orbitals participating in the transition. The present method enables one to explore stereodynamics of dipole-forbidden transitions to autoionization states, which are inaccessible by photoabsorption experiments.

Einstein-Podolsky-Rosen (EPR) steering is an intermediate type of quantum nonlocality which sits between entanglement and Bell nonlocality. A set of correlations is Bell nonlocal if it does not admit a local hidden variable (LHV) model, while it is EPR nonlocal if it does not admit a local hidden variable–local hidden state (LHV-LHS) model. It is interesting to know what states can generate EPR-nonlocal correlations in the simplest nontrivial scenario, that is, two projective measurements for each party sharing a two-qubit state. Here we show that a two-qubit state can generate EPR-nonlocal full correlations (excluding marginal statistics) in this scenario if and only if it can generate Bell-nonlocal correlations. If full statistics (including marginal statistics) is taken into account, surprisingly, the same scenario can manifest the simplest one-way steering and the strongest hierarchy between steering and Bell nonlocality. To illustrate these intriguing phenomena in simple setups, several concrete examples are discussed in detail, which facilitates experimental demonstration. In the course of study, we introduce the concept of restricted LHS models and thereby derive a necessary and sufficient semidefinite-programming criterion to determine the steerability of any bipartite state under given measurements. Analytical criteria are further derived in several scenarios of strong theoretical and experimental interest.

We consider the problem of certifying binary observables based on a Bell inequality violation alone, a task known as self-testing of measurements. We introduce a family of commutation-based measures, which encode all the distinct arrangements of two projective observables on a qubit. These quantities by construction take into account the usual limitations of self-testing and since they are “weighted” by the (reduced) state, they automatically deal with rank-deficient reduced density matrices. We show that these measures can be estimated from the observed Bell violation in several scenarios and the proofs rely only on standard linear algebra. The trade-offs turn out to be tight, and in particular, they give nontrivial statements for arbitrarily small violations. On the other extreme, observing the maximal violation allows us to deduce precisely the form of the observables, which immediately leads to a complete rigidity statement. In particular, we show that for all $n\ge 3$ the $n$-partite Mermin-Ardehali-Belinskii-Klyshko inequality self-tests the $n$-partite Greenberger-Horne-Zeilinger state and maximally incompatible qubit measurements on every party. Our results imply that any pair of projective observables on a qubit can be certified in a truly robust manner. Finally, we show that commutation-based measures give a convenient way of expressing relations among more than two observables.

The rotational and hyperfine spectrum of the $X{}^1{\mathrm{\Sigma }}^{+}\to B{}^3{\mathrm{\Pi }}_1$ transition in TlF molecules was measured using laser-induced fluorescence from both a thermal and a cryogenic molecular beam. Rotational and hyperfine constants for the $B$ state are obtained. The large magnetic hyperfine interaction of the Tl nuclear spin leads to significant mixing of the lowest $B$ state rotational levels. Updated, more precise measurements of the $B\to X$ vibrational branching fractions are also presented. The combined rovibrational branching fractions allow for the prediction of the number of photons that can be scattered in a given TlF optical cycling scheme.

We investigate the prospects of using magnetic Feshbach resonance to control charge exchange in ultracold collisions of heteroisotopic combinations of atoms and ions of the same element. The proposed treatment, readily applicable to alkali or alkaline-earth metals, is illustrated on cold collisions of ${}^9\mathrm{Be}^{+}$ and ${}^{10}\mathrm{Be}$. Feshbach resonances are characterized by quantum scattering calculations in a coupled-channel formalism that includes non-Born-Oppenheimer terms originating from the nuclear kinetic operator. Near a resonance predicted at 322 G, we find the charge exchange rate coefficient to rise from practically zero to values greater than ${10}^{-12}{\mathrm{cm}}^3$/s. Our results suggest controllable charge exchange processes between different isotopes of suitable atom-ion pairs, with potential applications to quantum systems engineered to study charge diffusion in trapped cold atom-ion mixtures and emulate many-body physics.

We perform time-resolved UV pump (4.77 eV) and VUV probe (7.94 eV) measurements of internal conversion of 1,3-cyclohexadiene (CHD). Our measurements reveal a substantial ionization of the “hot” ground state following internal conversion despite the fact that our probe photon energy is below the ionization potential (8.21 eV). With the aid of electronic structure calculations, we interpret our results in terms of vibrationally assisted below-threshold ionization, where vibrational energy is converted to electronic energy. The effect relies on both having vibrational modes which allow for this vibrational-electronic coupling and exciting these modes during the internal conversion. We contrast our measurements in CHD with another similar molecule, cis,cis-1,3-cyclooctadiene (cis,cis-COD), for which we do not see the effect.

We study theoretically the interaction-induced generation of mesoscopic coherent spin state superpositions (small-particle-number cat states) from an initial coherent spin state in bimodal Bose-Einstein condensates and the subsequent phase revival, including decoherence due to particle losses and fluctuations of the total particle number. In a full multimode description, we propose a preparation procedure of the initial coherent spin state and we study the effect of preexisting thermal fluctuations on the phase revival, and on the spin and orbito-spinorial cat-state fidelities.

The fluctuations or disordered motion of the electromagnetic fields are described by statistical properties rather than instantaneous values. This statistical description of the optical fields is underlying in the Stokes-Mueller formalism that applies to measurable intensities. However, the fundamental concept of optical coherence, which is assessed by the ability of waves to interfere, is not treatable by this formalism because it omits the global phase. In this work we show that using an analogy between deterministic matrix states associated with optical media and quantum mechanical wave functions, it is possible to construct a general formalism that accounts for the additional terms resulting from the coherency effects that average out for incoherent treatments. This method generalizes further the concept of coherent superposition to describe how deterministic states of optical media can superpose to generate another deterministic media state. Our formalism is used to study the combined polarimetric response of interfering plasmonic nanoantennas.

We investigate the peculiarities of light scattering from subwavelength particles made of high-refractive-index materials caused by the coexistence of particular anapole modes of both electric and magnetic character. The similarities and differences of such anapole modes are discussed in detail. We also show that these two types of anapole modes can be supported simultaneously by subwavelength high-index spherical dielectric particles.

When measuring a time-varying phase, the standard quantum limit and Heisenberg limit as usually defined, for a constant phase, do not apply. If the phase has Gaussian statistics and a power-law spectrum $1/{\left|\omega \right|}^p$ with $p>1$, then the generalized standard quantum limit and Heisenberg limit have recently been found to have scalings of $1/{\mathcal{N}}^{(p-1)/p}$ and $1/{\mathcal{N}}^{2(p-1)/(p+1)}$, respectively, where $\mathcal{N}$ is the mean photon flux. We show that this Heisenberg scaling can be achieved via adaptive measurements on squeezed states. We predict the experimental parameters analytically, and test them with numerical simulations. Previous work had considered the special case of $p=2$.

We previously demonstrated near-field speckle-scanning-based x-ray imaging to be an easy-to-implement phase-sensing method capable of providing both high sensitivity and high resolution. Yet this performance combination could only be achieved at the cost of a significant number of sample exposures and of extensive data acquisition time, thus tempering its implementation for tomography applications. Here we show ways of drastically lowering the number of exposures for the speckle scanning method to become attractive for computed tomography imaging. As the method presented can cope with a high divergence beam, it is also expected to attract the attention of the laboratory sources community.

In this work we study the impact of chromatic focusing of few-cycle laser pulses on high-order-harmonic generation (HHG) through analysis of the emitted extreme ultraviolet (XUV) radiation. Chromatic focusing is usually avoided in the few-cycle regime, as the pulse spatiotemporal structure may be highly distorted by the spatiotemporal aberrations. Here, however, we demonstrate it as an additional control parameter to modify the generated XUV radiation. We present experiments where few-cycle pulses are focused by a singlet lens in a Kr gas jet. The chromatic distribution of focal lengths allows us to tune HHG spectra by changing the relative singlet-target distance. Interestingly, we also show that the degree of chromatic aberration needed for this control does not degrade substantially the harmonic conversion efficiency, still allowing for the generation of supercontinua with the chirped-pulse scheme, demonstrated previously for achromatic focusing. We back up our experiments with theoretical simulations reproducing the experimental HHG results depending on diverse parameters (input pulse spectral phase, pulse duration, and focus position) and proving that, under the considered parameters, the attosecond pulse train remains very similar to the achromatic case, even showing cases of isolated attosecond pulse generation for near-single-cycle driving pulses.

Rains' bound is arguably the best known upper bound of the distillable entanglement by operations completely preserving positivity of partial transpose (PPT) and was conjectured to be additive and coincide with the asymptotic relative entropy of entanglement. We disprove both conjectures by explicitly constructing a special class of mixed two-qubit states. We then introduce an additive semidefinite programming lower bound ($E_M$) for the asymptotic Rains' bound, and it immediately becomes a computable lower bound for entanglement cost of bipartite states. Furthermore, $E_M$ is also proved to be the best known upper bound of the PPT-assisted deterministic distillable entanglement and gives the asymptotic rates for all pure states and some class of genuinely mixed states.

The Hylleraas-$B$-spline basis set is introduced for solving the Hamiltonian eigenvalue problem of a two-electron atomic system. We demonstrate this method by calculating the static dipole polarizabilities of the $1{}^{1}S–5{}^{1}S,2{}^{3}S–6{}^{3}S$ states of helium in both the length and velocity gauge, which shows a good agreement. Comparing to the traditional $B$-spline basis, the present approach can significantly improve the rate of convergence of energy eigenvalues. The final extrapolated values of the polarizabilities for these states are accurate to at least eight significant digits, indicating that this method can handle intermediate states, including the continuum, of the Hamiltonian sufficiently well.

By establishing coincidences between target ions and scattered projectiles, and coincidences between target ions, scattered projectiles, and ejected electrons, triply differential cross-section (TDCS) information was generated in terms of projectile energy loss and scattering angles for interactions between 1-keV positrons and electrons and Ar atoms. The conversion of the raw experimental information to the TDCS is discussed. The single-ionization TDCS exhibits two distinguishable regions (lobes) where binary and recoil interactions can be described by two peaks. A comparison of the positron and electron impact data shows that the relative intensity of both binary and recoil interactions decreases exponentially as a function of the momentum transfer and is larger when ionization is induced by positron impact, when compared with electron impact.

We study high-order harmonic generation (HHG) from the oriented asymmetric molecule ${\mathrm{HeH}}^{2+}$ and its isotopic variant ${\mathrm{HeT}}^{2+}$ in strong few-cycle laser pulses, by numerically solving the non-Born-Oppenheimer time-dependent Schrödinger equation. Our simulations show that the influence of nuclear motion on HHG of the asymmetric system is dependent on the molecular orientation and the laser parameters. At relatively weak laser intensities, the HHG yields of ${\mathrm{HeH}}^{2+}$ are higher than those of ${\mathrm{HeT}}^{2+}$ for the parallel orientation and the situation reverses for the perpendicular orientation. However, at high laser intensities, they become comparable for the parallel case. With a developed simple HHG model that considers the influences of nuclear motion on all of the ionization, propagation, and recombination processes of HHG, we show the permanent dipole of the system plays an important role in these phenomena. Our results shed light on the complex dynamics of the asymmetric system in strong laser fields.

We propose and evaluate a method to construct a quantum-correlated twin-atom laser using a pumped and damped Bose-Hubbard in-line trimer which can operate in a stationary regime. With pumping via a source condensate filling the middle well and damping using either an electron beam or optical means at the two end wells, we show that bipartite quantum correlations build up between the ends of the chain and that these can be measured either in situ or in the outcoupled beams. While nothing similar to our system has yet been achieved experimentally, recent advances mean that it should be practically realizable in the near future.

Our spectroscopic method using laser trapping and heating has demonstrated that thermal emission from a metal oxide microsphere is enhanced at frequencies resonant with the whispering gallery modes of the spherical resonator. Only a mode series of a specific order effectively emits thermal photons, and spectral peaks shift from higher-order whispering gallery modes to fundamental whispering gallery modes as the size parameter decreases. These spectral profiles are analyzed with the Mie scattering theory and a semiclassical rate-equation model. The observed mode selectivity in thermal radiation is attributed to a matching between the rates of cavity damping and internal absorption. Excellent reproducibility of the observed spectral profiles leads to a precise determination of optical constants of extremely hot materials.

In this paper we study the full quantum approach to the interaction of $N$ trapped two-level ions with $N$ quantized fields in such a way that each of the trapped ions separately interacts with one of the quantized fields. A semiclassical model of this type of interaction has been already studied, using classical laser fields instead of quantized fields, by Cirac and Zoller [Phys. Rev. Lett. 74, 4091 (1995)]. During each interaction, the excitation of a trapped ion occurs due to the occurrence of resonance between the trapped ion and one of the modes of multimode quantized field. In this way, we are able to find the quantized radiation field, which propagates from each excited trapped ion after its deexcitation. It is demonstrated that such a propagated field is a function of the position of ion within the trap, ion number, trapping and ionic transition frequencies, spontaneous emission rate, and time evolution of trapped ionic operators. In the continuation, to achieve the explicit form of such a field we propose a method to evaluate the temporal evolution of operators of the system by utilizing the Heisenberg operator approach. Via the propagated field of each trapped ion, the spectral properties such as intensities as well as power spectra of their radiations are evaluated. Furthermore, the quantum statistics of field and in particular, its photon antibunching and sub-Poissonian statistical properties are investigated. Finally, resonance spectroscopy of the trapped ions is obtained by which one can allocate a characteristic spectrum to each of them.