We study the dynamics of a two-level quantum system under the influence of sinusoidal driving in the intermediate-frequency regime. Analyzing the Floquet quasienergy spectrum, we find combinations of the field parameters for which population transfer is optimal and takes place through a series of well-defined steps of fixed duration. We also show how the corresponding evolution operator can be approximated at all times by a very simple analytical expression. We propose this model as being specially suitable for treating periodic driving at avoided crossings found in complex multilevel systems, and thus show a relevant application of our results to designing a control protocol in a realistic molecular model.

We investigate the main limitations which prevent the continuous-variable quantum key distribution protocols from achieving long distances in the finite-size setting. We propose a double-modulation protocol which allows using each state for both channel estimation and key distribution. As opposed to the standard method, we optimize the parameters of the protocol and consider squeezed as well as coherent states as a signal. By optimally combining the resources the key rate can approach the theoretical limit for long distances, and one can obtain about ten times higher key rate using ten times shorter block size than in the current state-of-the-art implementation.

The fragmentation of the ${\mathrm{CO}}_2{}^{2+}$ dication following 200 eV electron impact double ionization was studied using a cold target recoil-ion momentum spectroscopy. Both two-body and three-body fragmentation channels were observed with an ion-ion coincidence technique. The slopes of the peaks in the coincidence spectra were extracted and compared with previous results and theoretical predictions. Overall, good agreement between theoretical and experimental results was achieved, while different behaviors were observed for the channel of fragmentation into $\mathrm{C}{}^{+}+\mathrm{O}{}^{+}+\mathrm{O}$ with previous high energy electron impact results. The momentum vectors of ionic fragments were measured, and thus the momentum vector of the neutral particle was deduced and kinetic energy release (KER) distributions for the different fragmentation channels were obtained. KER distribution behavior also shows a different signature from that induced by high energy electron impact. Furthermore, the three-body fragmentation mechanism, i.e., dissociation in one- or two-step processes, was distinguished from their distinct signatures on the Dalitz plots and the slopes of the islands from the covariance mapping spectrum. For example, the fragmentation channel $\mathrm{C}{}^{+}+\mathrm{O}{}^{+}+\mathrm{O}$ was found to be very similar to the 60 eV photon impact result [J. Laksman, E. P. Månsson, C. Grunewald, A. Sankari, M. Gisselbrecht, D. Céolin, and S. L. Sorensen, J. Chem. Phys. 136, 104303 (2012)], which shows a predominant sequential dissociation process.

The capture of polarized electrons by H-like ions provides a method for polarizing the nuclei of He-like heavy ions with zero total electron angular momentum in storage rings for high-energy ions. A detailed analysis for ${}_{63}{}^{151}\mathrm{Eu}$ ions with nuclear spin $I=5/2$ predicts a nuclear polarization degree of about $47\%$ already after one passage through a target containing $100\%$ polarized electrons. Almost $50\%$ of the polarized He-like ions are predicted to be in states with zero total electron angular momentum. Such ions were recently considered as the most promising candidates in experiments at storage rings for the search for violations of the fundamental symmetries and for a nuclear and an electron electric dipole moment.

Without using the relativity principle, we show how the Dirac equation in three space dimensions emerges from the large-scale dynamics of the minimal nontrivial quantum cellular automaton satisfying unitarity, locality, homogeneity, and discrete isotropy. The Dirac equation is recovered for small wave vector and inertial mass, whereas Lorentz covariance is distorted in the ultrarelativistic limit. The automaton can thus be regarded as a theory unifying scales from Planck to Fermi. A simple asymptotic approach leads to a dispersive Schrödinger equation describing the evolution of narrowband states at all scales.

High-fidelity, efficient quantum nondemolition readout of quantum bits is integral to the goal of quantum computation. As superconducting circuits approach the requirements of scalable, universal fault tolerance, qubit readout must also meet the demand of simplicity to scale with growing system size. Here we propose a fast, high-fidelity, scalable measurement scheme based on the state-selective ring-up of a cavity followed by photodetection with the recently introduced Josephson photomultiplier (JPM), a current-biased Josephson junction. This scheme maps qubit state information to the binary digital output of the JPM, circumventing the need for room-temperature heterodyne detection and offering the possibility of a cryogenic interface to superconducting digital control circuitry. Numerics show that measurement contrast in excess of 95% is achievable in a measurement time of 140 ns. We discuss perspectives to scale this scheme to enable readout of multiple qubit channels with a single JPM.

The standard construction of complete sets of mutually unbiased bases (MUBs) in prime power dimensions is based on the quadratic Gauss sums. We introduce complete MUB sets for three, four, and five qubits that are unitarily inequivalent to all existing MUB sets. These sets are constructed by using certain exponential sums, where the degree of the polynomial appearing in the exponent can be higher than 2. Every basis of these MUBs (except the computational) consists of two disjoint blocks of vectors with different factorization structures and associated with a unique hypergraph (or graph) that represents an interaction between the qubits.

This work develops measures for quantifying the effects of field noise upon targeted unitary transformations. Robustness to noise is assessed in the framework of the quantum control landscape, which is the mapping from the control to the unitary transformation performance measure (quantum gate fidelity). Within that framework, a geometric interpretation of stochastic noise effects naturally arises, where more robust optimal controls are associated with regions of small overlap between landscape curvature and the noise correlation function. Numerical simulations of this overlap in the context of quantum information processing reveal distinct noise spectral regimes that better support robust control solutions. This perspective shows the dual importance of both noise statistics and the control form for robustness, thereby opening up new avenues of investigation on how to mitigate noise effects in quantum systems.

We present a consistent analysis of linear spectroscopy for arrays of nearest-neighbor dipole-coupled two-level molecules that reveals distinct signatures of weak- and strong-coupling regimes separated for infinite-size arrays by a quantum critical point. In the weak-coupling regime, the ground state of the molecular array is disordered, but in the strong-coupling regime, it has (anti)ferroelectric ordering. We show that multiple molecular excitations [odd (even) in the weak- (strong-) coupling regime] can be accessed directly from the ground state. We analyze the scaling of absorption and emission with system size and find that the oscillator strengths show enhanced superradiant behavior in both ordered and disordered phases. As the coupling increases, the single-excitation oscillator strength rapidly exceeds the well-known Heitler-London value. In the strong-coupling regime we show the existence of a unique spectral transition with excitation energy that can be tuned by varying the system size and that asymptotically approaches zero for large systems. The oscillator strength for this transition scales quadratically with system size, showing an anomalous one-photon superradiance. For systems of infinite size, we find a novel singular spectroscopic signature of the quantum phase transition between disordered and ordered ground states. We outline how arrays of ultracold dipolar molecules trapped in an optical lattice can be used to access the strong-coupling regime and observe the anomalous superradiant effects associated with this regime.

We report an investigation on electron collisions with two nitrogen-containing compounds, namely ammonia (${\mathrm{NH}}_3$) and formamide (${\mathrm{NH}}_2\mathrm{CHO}$). For ammonia, both theoretical and experimental differential, integral, and momentum-transfer cross sections, as well as calculated grand-total and total absorption cross sections, are reported in the 50–500 eV incident energy range. Calculated results of various cross sections are also reported for energies below 50 eV. Experimentally, angular distributions of the scattered electrons were measured using a crossed electron beam-molecular beam geometry and then converted to absolute differential cross sections using the relative flow technique. Absolute integral and momentum-transfer cross sections for elastic $e^{-}$-ammonia scattering were also derived from the measured differential cross sections. For formamide, only theoretical cross sections are presented in the 1–500 eV incident energy range. A single-center-expansion technique combined with the method of Padé was used in our calculations. For both targets, our calculated cross sections are compared with the present measured data and with the theoretical and experimental data available in the literature and show generally good agreement. Moreover, for formamide, two shape resonances located at 3.5 eV and 15 eV which correspond to the continuum ${}^2{A}^{\prime \prime }$ and ${}^2{A}^{\prime }$ scattering symmetries, respectively, are identified. The former can be associated to the ${}^2{B}_1$ shape resonance in formaldehyde located at around 2.5 eV, whereas the latter can be related to the ${}^{2}E$ resonance in ammonia at about 10 eV. Such correspondence is very interesting and so supports the investigation on electron interaction with small building blocks, instead of with larger biomolecules.

We report theoretical results of the low-energy photodetachment cross sections of the ${\mathrm{C}}_2\mathrm{H}{}^{-},{\mathrm{C}}_4\mathrm{H}{}^{-}$, and ${\mathrm{C}}_6\mathrm{H}{}^{-}$ hydrocarbon-chain anions. The complex Kohn variational technique is used to calculate molecular-frame transition dipole moments from the anion ground state to a photoelectron in the continuum of the neutral radical. We employ the Franck-Condon approximation and include interchannel electronic coupling to determine the low-energy photodetachment cross sections and asymmetry parameters. We discuss the behavior of the cross section, especially near thresholds, and describe the role of electronic resonances and excited channels. The theoretical results reproduce the main characteristics of recent measurements of absolute photodetachment cross sections.

We study the dynamics of bound pairs in the extended Hubbard model driven by a linear external field. It is shown that two interacting bosons or singlet fermions with nonzero on-site and nearest-neighbor interaction strengths can always form bound pairs in the absence of an external field. There are two bands of bound pairs, one of which may have incomplete wave vectors when it has an overlap with the scattering band, referred to as an imperfect band. In the presence of the external field, the dynamics of the bound pair in the perfect band exhibits distinct Bloch-Zener oscillation (BZO), while in the imperfect band the oscillation presents a sudden death. The pair becomes uncorrelated after the sudden death and the BZO never comes back. Such dynamical behaviors are robust even for the weak-coupling regime and thus can be used to characterize the phase diagram of the bound states.

We introduce a scheme of single-photon emission based on four-wave mixing in a three mode system with weak Kerr-type nonlinearity. A highly populated lower energy mode results in strong stimulated scattering of particle pairs out of the central mode, which consequently limits the central mode occupation. Thus, the system can be reduced to a ${\chi }^{\left(2\right)}$ nonlinear medium with greatly enhanced interaction constant. As a model setup we consider dipolaritons in semiconductor microcavities. Using the master equation approach we show strong antibunching under continuous-wave pump, which largely exceeds the conventional blockade mechanism. Finally, using a pulsed excitation we demonstrate theoretically a triggered single-photon emitter in a weakly nonlinear system with $33\%$ emission probability.

We demonstrate all-optical multilogic gate operations and transistor functionalities using a Kerr phase gate method in a room-temperature ${}^{85}\mathrm{Rb}$ vapor. Two symmetric Mach-Zehnder interferometers are constructed in the same vapor cell in which a Raman gain medium is established. We show three basic logic gates (and, or, and not) by controlling the output combinations from the two interferometers. With one weakly driven interferometer acting as the phase control light for a strongly driven interferometer, we further demonstrate optical field-effect transistor functionalities. More complex combinations of this Kerr phase gate method and scheme allow all eight basic logic gate operations including the controlled-not gate to be constructed and implemented.

The nonadiabatic dynamics of a many-body system driven through a quantum critical point can be controlled using counterdiabatic driving, where the formation of excitations is suppressed by assisting the dynamics with auxiliary multiple-body nonlocal interactions. We propose an alternative scheme which circumvents practical challenges to realize shortcuts to adiabaticity in mesoscopic systems by tailoring the functional form of the auxiliary counterdiabatic interactions. A driving scheme resorting in short-range few-body interactions is shown to generate an effectively adiabatic dynamics.

Squeezing is a nonlinear Gaussian operation that is a key component in the construction of other nonlinear Gaussian gates. In our implementation of the squeezing gate, the amount and the orientation of the squeezing can be controlled by an external driving signal with a 1-MHz operational bandwidth. This provides another way to view dynamic Gaussian processing. In particular, the gate can be immediately employed as the feedforward needed for the deterministic implementation of the quantum cubic phase gate, which is a key piece of universal quantum information processing.

We study laser-sub-cycle control over electron trajectories concomitantly in space and time using orthogonally polarized two-color laser fields. We compare experimental photoelectron spectra of neon recorded by coincidence momentum imaging with photoelectron spectra obtained by semiclassical and numerical solutions of the time-dependent Schrödinger equation. We find that a resolution of a quarter optical cycle in the photoelectron trajectories can be achieved. It is shown that depending on their sub-cycle birth time the trajectories of photoelectrons are affected differently by the ion's Coulomb field.

We analyze the ground state of a system of dipolar bosons moving in the $XY$ plane and such that their dipolar moments are all aligned in a fixed direction in space. We focus on the general case where the polarization field forms a generic angle $\alpha$ with respect to the $Z$ axis. We use the path-integral ground-state method to analyze the static properties of the system as both $\alpha$ and the density $n$ vary over a wide range where the system is stable. We use the maximum of the static structure function as an order parameter to characterize the different phases and the transition lines among them. We find that, in addition to a superfluid gas and a solid phase, the system reaches a stripe phase at large tilting angles that is entirely induced by the anisotropic character of the interaction. We also show that the quantum phase transition from the gas to the stripe phase is of second order and report approximate values for the critical exponents.

Multibarrier-tunneling invisible systems may be formed by a finite chain of building blocks that are tunneling invisible. Here we study these systems as a function of the energy and of the distance among building blocks. We find that the transmission coefficient exhibits a very complex behavior both around unity transmission and near the border of energy gaps. Yet, tunneling invisibility remains for a broad range values of these parameters. We investigate on the relationship of the energy spectra in these systems with the corresponding distribution of the complex poles of the transmission amplitude on the energy plane, in particular with the formation of energy dips and gaps in the transmission spectra. We find that these findings hold also in general for multibarrier resonant tunneling systems.

In two-dimensional systems, predictability and visibility satisfy a complementarity relation, which has been generalized to bipartite systems by including the concurrence. We consider systems in which the two alternatives of a which-state experiment are nonorthogonal states and propose generalized measures of predictability and visibility. Criteria like symmetry under exchange of the alternatives, vanishing visibility when only one of the alternatives is present, and clear operational meaning, favor definitions of predictability and visibility which use a three-dimensional basis, previously employed in the unambiguous discrimination of nonorthogonal states. We derive generalized single-partite and bipartite complementarity relations and find that predictability and visibility do not exhaust the single-partite properties for a nonorthogonal basis. A recent experiment on the quantum-classical border is analyzed using generalized complementarity relations.