This paper presents high-precision spectroscopic measurements of atomic tin using five different resonance-ionization schemes performed with the collinear resonance-ionization spectroscopy technique. Isotope shifts were measured for the stable tin isotopes from the $5s^{2}5p^2{}^3{P}_{0,1,2}$ and ${}^1{S}_0$ to the $5s^{2}5p6s{}^1{P}_1,{}^3{P}_{1,2}$ and $5s^{2}5p7s{}^1{P}_1$ atomic levels. The magnetic dipole hyperfine constants $A_{\mathrm{hf}}$ have been extracted for six atomic levels with electron angular momentum $J>0$ from the hyperfine structures of nuclear spin $I=1/2$ tin isotopes, ${}^{115,117,119}\mathrm{Sn}$. State-of-the-art atomic calculations using a relativistic Fock-space coupled-cluster method and the configuration interaction approach combined with many-body perturbation theory allow accurate and reliable calculations of both field- and mass-shift factors for the studied transitions, in addition to the hyperfine magnetic fields and electric-field gradients of the atomic levels. The excellent agreement with the experimental results highlights the accuracy of modern atomic theory and establishes an important foundation for precision measurements of nuclear moments and charge radii of the most exotic isotopes of tin.

A near-infrared laser generates gain on transitions between the $B{}^2\mathrm{\Sigma }_{\text{u}}^{+}$ and $X{}^2\mathrm{\Sigma }_{\text{g}}^{+}$ states of the nitrogen molecular cation in part by coupling the $X{}^2\mathrm{\Sigma }_{\text{g}}^{+}$ and $A{}^2\mathrm{\Pi }_{\text{u}}$ states in the $V$ system. Traditional time resolved pump-probe measurements rely on postionization coupling by the pump pulse to initialize dynamics in the $A{}^2\mathrm{\Pi }_{\text{u}}$ state. Here we show that a weak second excitation pulse reduces ambiguity because it acts only on the ion independent of ionization. The additional control pulse can increase gain by moving population to the $A{}^2\mathrm{\Pi }_{\text{u}}$ state, which modifies the lasing emission in two distinct ways. The presence of fast decoherence on $X{}^2\mathrm{\Sigma }_{\text{g}}^{+}$ to $A{}^2\mathrm{\Pi }_{\text{u}}$ transitions may prevent the formation of a coherent rotational wave packet in the ground state in our experiment, but the control pulse can reverse impulsive alignment by the pump pulse to remove rotational wave packets in the $B{}^2\mathrm{\Sigma }_{\text{u}}^{+}$ state.

Motivated by recent experiments, we present the time-optimal variational control of a bright matter-wave soliton trapped in the harmonic trap by manipulating the atomic interaction through Feshbach resonances. More specifically, we first apply the variational technique to derive the motion equation for capturing the soliton's shape and, second, combine an inverse-engineering method with optimal control theory to design the scatter length for implementing time-optimal decompression. Since the minimum-time solution is of the “bang-bang” type, the smooth regularization is further adopted to smooth the on-off controller out, thus avoiding the heating and atom loss induced from the magnetic field ramp across a Feshbach resonance, in practice.

We study saturable absorption in graphene-comprising nanophotonic waveguides taking into account the finite relaxation time as well as the carrier diffusion due to the nonuniform, tightly confined spatial profile of the guided modes. We discuss various details of graphene SA and comment on the necessary conditions that allow for directly comparing our model with available experimental. The mathematical framework is based on the nonlinear Schrödinger equation which provides a strict framework for our analysis and is developed for two optical channels. We explore the propagation of cw, long and short pulsed signals in a silicon slot waveguide and show the importance of our model in order to capture the ultrafast dynamics of graphene and the spatial distribution of guided modes. Finally, we demonstrate how cross absorption modulation can be exploited in order to imprint data from a high power optical channel to a low power channel.

We study the photon antibunching effect in a bimode nano-optical cavity filled with the second-order nonlinear material and embedded with a quantum dot. Analytical results show that both the conventional photon blockade (CPB) induced by the nonlinear interaction between polaritons and the unconventional photon blockade (UCPB) induced by the quantum interference appear in our model. In particular, we find that CPB and UCPB can simultaneously occur in an overlapped parameter regime, which allows one to prepare a single-photon source with high purity and high brightness in the strong-coupling regime. Furthermore, owing to the multipathway interference in this composite model, the photon antibunching effect is significantly enhanced compared to that in the Jaynes-Cummings model and in the degenerate optical parametric amplifier in the weak-coupling regime.

We investigate the role played by quantum operator ordering in the correlations that characterize two-photon polarization Bell measurements. The Clauser-Horne-Shimony-Holt (CHSH) criterion is investigated in the normal ordering imposed by the photodetection theory and in the symmetric ordering that constitutes the standard prescription for building Hermitian operators from products of noncommuting observables. The two approaches are obtained in a single theoretical framework, where operator ordering is directly associated with the representation used for the density matrix. Moreover, this discussion can be recast in terms of the contribution given by the vacuum fluctuations to the detected signals. We also envisage possible detection schemes sensitive to these fluctuations with recent technological developments.

An average gate fidelity is a standard performance metric to quantify deviation between an ideal unitary gate transformation and its realistic experimental implementation. The average is taken with respect to states uniformly distributed over the full Hilbert space. We analytically (single qubit) and numerically (two qubit) show how this average changes if the uniform distribution condition is relaxed, replaced by parametrized distributions—polar cap and von Mises–Fisher distributions—and how the resulting fidelities can differentiate certain noise models. In particular, we demonstrate that Pauli channels with different noise rates along the three axes can be faithfully distinguished using these augmented fidelities.

Molecular hydrogen, despite being the most abundant molecule in the universe, cannot be efficiently formed in the gas phase from two hydrogen atoms without the presence of a third body. In this work we show that the electron-impact production of vacancies in the various valence shells of ethylene results in an abundant ejection of neutral ${\mathrm{H}}_2$ and H. Using the technique of delayed-extraction time-of-flight spectrometry, the cross section of ${\mathrm{H}}_2$ and H production within the 20- to 800-eV impact energy range is obtained on an absolute scale. This is achieved by combining kinematic and dynamic measurements in conjunction with an association between the cross sections for production of fragment ions and the molecular orbitals where the primary vacancy is produced. A comparison with other common small molecules containing hydrogen shows that ethylene has a greater and outstanding efficiency in the production of ${\mathrm{H}}_2$ in gaseous media subjected to ionizing radiation.

We use the two-center wave-packet convergent close-coupling approach to calculate singly differential cross sections for direct scattering, electron capture, and ionization in proton-hydrogen collisions at intermediate energies. The distinct feature of the approach is that it gives a complete differential picture of all the interconnected processes at once, subject to the unitary principle. Results obtained for the angular differential cross sections of elastic scattering, excitation, and electron capture, as well as the ionization cross sections differential in the ejected-electron angle, and in the ejected-electron energy agree well with available experimental data. It is concluded that the two-center wave-packet convergent close-coupling approach is capable of providing a realistic differential picture of all collision processes taking place in proton-hydrogen collisions.

The dissociative ionization of ${\text{H}}_2^{+}$ in a linearly polarized, 400-nm laser pulse is simulated by solving a three-particle time-dependent Schrödinger equation in full dimensionality without using any data from quantum chemistry computation. The joint energy spectrum (JES) is computed using a time-dependent surface-flux method, the details of which are given. The calculated ground energy is $-0.597$ atomic units and the internuclear distance is 1.997 atomic units if the kinetic energy term of protons is excluded, consistent with the reported precise values from quantum chemistry computation. If the kinetic term of the protons is included, the ground energy is $-0.592$ atomic units with an internuclear distance of 2.05 atomic units. Energy sharing is observed in JES and we find the peak of the JES with respect to the nuclear kinetic energy release is within 2–4 eV, which is different from previous two-dimensional computations (over 10 eV), but is close to the reported experimental values. The projected energy distribution on the azimuth angles shows that the electron and the protons tend to dissociate in the direction of polarization of the laser pulse.

We experimentally observe a coexisting pair of topological anomalous Floquet interface states in a ($1+1$)-dimensional discrete photon walk. We explicitly verify the robustness of these states against local static perturbations respecting the chiral symmetry of the system, as well as their vulnerability against nonstationary perturbations. The walk is implemented based on pulses propagating in a pair of coupled fiber loops of dissimilar lengths with dynamically variable mutual coupling. The topological interface is created via phase modulation in one of the loops, which allows for an anomalous Floquet topological transition at the interface.

We suggest a method of achieving an extremely long lifetime of a collective singly excited state in a generic small-size ensemble of $N$ identical atoms. The decay rate of such a “superdark” state can be proportional to ${(r/\lambda )}^{2(N-1)}$, where $r$ and $\lambda$ are the system size and the wavelength of the radiation, respectively, i.e., considerably smaller than in any of the systems suggested up to now. The method is based on a special fine-tuning of the atomic Hamiltonian, namely, on a proper position-dependent adjustment of atomic transition frequencies. The chosen set of the control parameters is sufficient to ensure the minimum of the spontaneous decay rate of the engineered state in a generic ensemble of atoms.

Contrary to what was previously believed, two-loop radiative corrections to the $g$ factor of an electron bound in a hydrogenlike ion at $O({\alpha }^2{\left(Z\alpha \right)}^5)$ exhibit logarithmic enhancement. This previously unknown contribution is due to a long-distance light-by-light scattering amplitude. Taking an effective field theory approach, and using the Euler-Heisenberg Lagrangian, we find $\mathrm{\Delta }g=-{\left(\frac{\alpha }{\pi }\right)}^2{\left(Z\alpha \right)}^5\frac{56\pi }{135}lnZ\alpha$.

An operational probabilistic theory where all systems are classical, and all pure states of composite systems are entangled, is constructed. The theory is endowed with a rule for composing an arbitrary number of systems, and with a nontrivial set of transformations. Hence, we demonstrate that the presence of entanglement is independent of the existence of incompatible measurements. We then study a variety of phenomena occurring in the theory, some of them contradicting both classical and quantum theories, including cloning, entanglement swapping, dense coding, additivity of classical capacities, nonmonogamous entanglement, hypersignaling. We also prove the existence, in the theory, of a universal processor. The theory is causal and satisfies the no-restriction hypothesis. At the same time, it violates a number of information-theoretic principles enjoyed by quantum theory, most notably, local discriminability, purity of parallel composition of states, and purification. Moreover, we introduce an exhaustive procedure to construct generic operational probabilistic theories, and a sufficient set of conditions to verify their consistency. In addition, we prove a characterization theorem for the parallel composition rules of arbitrary theories, and specialize it to the case of bilocal-tomographic theories. We conclude pointing out some open problems. In particular, on the basis of the fact that every separable state of the theory is a statistical mixture of entangled states, we formulate a no-go conjecture for the existence of a local-realistic ontological model.

To compare quantum estimation theory schemes we must acknowledge that, in some cases, the quantitative difference between them might be small and hence sensitive to numerical errors. Here, we are concerned with comparing estimation schemes for the quantum state under continuous measurement (quantum trajectories), namely quantum state filtering and, as introduced by us [Phys. Rev. Lett. 115, 180407 (2015)], quantum state smoothing. Unfortunately, the cumulative errors in the most typical simulations of quantum trajectories with time step $\mathrm{\Delta }t$ and total simulation time $T$ can scale as $T\mathrm{\Delta }t$. Moreover, these errors may correspond to deviations from valid quantum evolution as described by a completely positive map. Here we introduce a higher-order method that reduces the cumulative errors in the complete positivity of the evolution to the order of $T{\left(\mathrm{\Delta }t\right)}^2$, whether for linear (unnormalized) or nonlinear (normalized) quantum trajectories. Our method also guarantees that the discrepancy in the average evolution between different detection methods (different “unravelings,” such as quantum jumps or quantum diffusion) is similarly small. This equivalence is essential for comparing quantum state filtering to quantum state smoothing, as the latter assumes that all irreversible evolution is unraveled, although the estimator only has direct knowledge of some records. In particular, here we compare the average difference between filtering and smoothing conditioned on an event of which the estimator lacks direct knowledge: a photon detection within a certain time window. We find that the smoothed state is actually less pure, both before and after the time of the jump. Similarly, the fidelity of the smoothed state with the “true” (maximal knowledge) state is also lower than that of the filtered state before the jump. However, after the jump, the fidelity of the smoothed state is higher.

One of the challenges in quantum computing is the synthesis of unitary operators into quantum circuits with polylogarithmic gate complexity. Exact synthesis of generic unitaries requires an exponential number of gates in general. We propose a novel approximate quantum circuit synthesis technique by relaxing the unitary constraints and interchanging them for ancilla qubits via block encodings. This approach combines smaller block encodings, which are easier to synthesize, into quantum circuits for larger operators. Due to the use of block encodings, our technique is not limited to unitary operators and can be applied for the synthesis of arbitrary operators. We show that operators which can be approximated by a canonical polyadic expression with a polylogarithmic number of terms can be synthesized with polylogarithmic gate complexity with respect to the matrix dimension.

Spontaneous parametric down-conversion (SPDC) has been a reliable process for the generation of entangled photon pairs. In this process, a nonlinear quadratic crystal is pumped by a laser field in order to convert (high-energy) photons into correlated photon pairs whose efficient control plays an essential role in various applications of quantum information processing. In particular, the amount of entanglement has been successfully controlled by adjusting the spatial structure of the incident pump field. Here, we theoretically analyze how the entanglement of the down-converted two-photon state can be further enhanced by using Ince-Gaussian beams with well-defined ellipticity $\epsilon$, i.e., solutions of the paraxial wave equation in elliptical coordinates. These spatially structured beams are quite universal as they include both the Laguerre-Gaussian beams for $\epsilon \to 0$ as well as the Hermite-Gaussian beams for $\epsilon \to \infty$. We demonstrate that the entanglement of the generated photon pairs in SPDC can be maximized by a proper choice of $\epsilon$ and that such an enhanced entanglement can be observed experimentally in terms of the Schmidt number.

Machine-learning techniques are used to check the theoretical and experimental predictions of positron binding to general molecules. The bound or unbound character of previous calculations for polar molecules are mostly confirmed. Binding for so far unexplored polar molecules is predicted. For apolar molecules, a formula for the binding energy in terms of isotropic polarizability and ionization potential is obtained, leading to unprecedented agreement with experiments as well as prediction of previously unidentified bound systems. The role of the ionization potential is suggested as a consequence of enhanced formation of virtual positronium at short distances.

We demonstrate a method to determine the position of single atoms in a three-dimensional optical lattice. Atoms are sparsely loaded from a far-off-resonant optical tweezer into a few vertical planes of a cubic optical lattice positioned near a high-resolution microscope objective. In a single realization of the experiment, we pin the atoms in deep lattices and then acquire multiple fluorescence images with single-site resolution. The objective is translated between images, bringing different vertical planes of the lattice into focus. The applicability of our method is assessed using simulated fluorescence images where the atomic filling fraction in the lattice is varied. This opens up the possibility of extending the domain of quantum simulation using quantum gas microscopes from two to three dimensions.