It is well established that the collective motion of ion crystals can be used as a quantum bus for multiqubit entanglement. However, as the number of ions increases, it becomes difficult to directly entangle ions far apart and resolve all motional modes of the ion crystal. We introduce a scalable and flexible scheme for efficient entanglement between any pair of ions within a large ion chain, using an evenly distributed 50-ion crystal as an example. By performing amplitude and frequency modulation, we find high-fidelity pulse sequences that primarily drive a transverse motional mode with a wavelength of four ion spacings. We present two $500\mu \mathrm{s}$ pulses that can in theory suppress gate errors due to residual motion to below ${10}^{-4}$, and observe a trade-off between gate power and robustness against unwanted frequency offsets.

We study, from a thermodynamic perspective, the equilibrium states of a qubit interacting with an arbitrary environment of dimension $N\gg 2$. We show that even in the presence of memory about the initial state, in some cases the qubit can be considered in a thermal state characterized by an entanglement Hamiltonian, which encodes the effects of the environment, and an initial-state-dependent entanglement temperature that measures the degree of entanglement generated between the system and its environment. Geometrical aspects of the thermal states are studied and the results are confirmed for the concrete case of the quantum walk on the line.

We study the dynamic evolution of the excited dye molecule IR144 in the liquid phase by performing transient-absorption measurement. The time-delay-dependent absorption spectra show different behaviors when the pump pulse either precedes or follows the probe pulse. Assuming the excited molecule as a multilevel system, we reproduce the experimental results based on the density matrix theory. As a system's quantum dynamics is closely related to its absorption spectral shape, the quantum evolution of the excited dye molecule IR144 is analyzed and the system's phase information is extracted from the intensity-dependent absorption spectra. The present work finds us a route for the full understanding and control of the quantum dynamics in a complex system excited by an external pulsed laser field.

We present an in-depth theoretical study of ${\mathrm{N}}_2$ photoionization in the region between the second (${}^2{\mathrm{\Pi }}_u$) and third (${}^2{\mathrm{\Sigma }}_u^{+}$) ionization thresholds. In this region, the electronic continuum includes the Hopfield series of autoionizing states, corresponding to excitations to $ns{\sigma }_d$, $nd{\sigma }_d$, and $nd{\pi }_g$ molecular orbitals. Calculations have been performed by using the xchem code, which makes use of a Gaussian and B-spline hybrid basis in the framework of a close-coupling approach. We provide total and partial photoionization cross sections for all open channels, energy positions, and widths for the five lowest resonances of each series and, when resonances are well isolated from each other, Fano and Starace parameters. We also discuss how the coupling between the two series of overlapping resonances, $ns{\sigma }_d$ and $nd{\sigma }_d$, affects their energies and autoionization widths. These results show the potential of the xchem method to describe resonant photoionization in molecules.

In the field of continuous-variable systems a fundamental role is played by Gaussian unitaries, that is, operators that preserve the Gaussian character of the states. The paper deals with the implementation of an arbitrary multimode Gaussian unitary by means of primitive components, namely, single-mode operators and simple two-mode operators (beam splitters). A partial solution to this problem is provided by the Bloch-Messiah reduction, which gives an architecture consisting of a multimode displacement, a diagonal squeezer, and two rotation operators. In this architecture displacement and squeezing are already formed by primitive components, so that it remains to find the implementation of the rotation operators. To this end, a suitable factorization of unitary matrices allows one to complete the desired implementation with primitive components. The theory is illustrated with an example of application to a four-mode Gaussian unitary.

Quantum coherence and nonlocality capture the nature of quantumness from different aspects. For the two-qubit states with a diagonal correlation matrix, we prove strictly a hierarchy between the nonlocal advantage of quantum coherence (NAQC) and Bell nonlocality by showing geometrically that the NAQC created on one qubit by local measurement on another qubit captures quantum correlation which is stronger than Bell nonlocality. For general states, our numerical results present strong evidence that this hierarchy may still hold. So the NAQC states form a subset of the states that can exhibit Bell nonlocality. We further propose a measure of NAQC that can be used for a quantitative study of it in bipartite states.

We theoretically demonstrate the feasibility of producing an electron spin beam splitter using Kapitza-Dirac diffraction on bichromatic standing waves which are created by the fundamental frequency and its third harmonic. The relativistic electron in the Bragg regime absorbs three photons with a frequency of $\omega$ and emits a photon with a frequency of $3\omega$; we introduce this as the four-photon Kapitza-Dirac effect. In this four-photon Kapitza-Dirac effect distinct spin effects arise in different polarizations of the third harmonic laser beam. It is shown that the shape of the Rabi oscillation between the initial and scattered states is changed and finds two unequal peaks for all polarizations of laser beams. For the case of circular polarization of the fundamental and third harmonic, despite Rabi oscillation, the spin down electron beam in 0.23 fs intervals maintains its momentum and spin. Also we find that if a suitable combination of a linearly polarized fundamental laser beam and a third harmonic with circular polarization in the condition $\vec{p}·\vec{A}$ term is involved, it results in a spin-dependent oscillation that preserves the spin state of the initial electron.

A kinematically complete experiment on two-photon double ionization of Ar by free-electron laser radiation with a photon energy of 27.93 eV was performed. The electron energy spectra show that double ionization is dominated by the sequential process. Comparison of the electron angular distributions to our data for single ionization and to theory confirms that even in the sequential process the electrons from both ionization steps are correlated with each other through polarization of the intermediate $\mathrm{A}{\mathrm{r}}^{+}$ state. Furthermore, a very important role of autoionization in both ionization steps is found.

We simulate the dynamics of varying density quasi-two-dimensional spin ensembles in solid-state systems, focusing on the nitrogen-vacancy centers in diamond. We consider the effects of various control sequences on the averaged dynamics of large ensembles of spins, under a realistic “spin-bath” environment. We reveal that spin locking is efficient for decoupling spins initialized along the driving axis, both from coherent dipolar interactions and from the external spin-bath environment, when the driving is two orders of magnitude stronger than the relevant coupling energies. Since the application of standard pulsed dynamical decoupling sequences leads to strong decoupling from the environment, while other specialized pulse sequences can decouple coherent dipolar interactions, such sequences can be used to identify the dominant interaction type. Moreover, a proper combination of pulsed decoupling sequences could lead to the suppression of both interaction types, allowing additional spin manipulations. Finally, we consider the effect of finite-width pulses on these control protocols and identify improved decoupling efficiency with increased pulse duration, resulting from the interplay of dephasing and coherent dynamics.

Absolute total, partial, and differential cross sections for the photodetachment of the oxygen anion are reported for photon energies ranging from threshold (1.46 eV) to 5.5 eV. The total cross section was measured using the animated-crossed-beam technique while partial and differential cross sections were obtained using velocity map imaging. The measured values are in good agreement with theoretical results obtained from an $R$-matrix calculation using polarized pseudostates.

By confining atoms in a state-insensitive optical lattice, the lifetime of the ground-state–Rydberg coherence is increased to $\ge 20\mu \mathrm{s}$, an order of magnitude improvement over previous experiments using freely diffusing atoms. Using these enhanced lifetimes, we measure the so-called magic lattice wavelengths for Rb and use them to extract the $6p_{3/2}\text{−}n{s}_{1/2}$ reduced electric dipole matrix elements. Good agreement is found with values obtained using an effective one-electron potential for principal quantum numbers $n$ between $n=30$ and $n=70$. We develop a theoretical model based on quantized motion to map out the ground-state–Rydberg coherence as a function of time that is in good agreement with the experimental results. The availability of long coherence times may present new opportunities for high-resolution spectroscopy and quantum information science.

We report an experimental observation of staircase pulses generated from an Er:Yb co-doped fiber laser operating in the anomalous dispersion regime. The staircase pulses consist of two distinct square pulses associated with different wavelengths in a dual peak spectrum and they exhibit particular temporal characteristics. To establish a theory behind the experimental results, we simulate numerically the fiber oscillator by using a master equation and taking into consideration the gain inhomogeneities as well as other nonlinear variables.

In [Phys. Rev. Lett. 102, 010401 (2009)], Spekkens et al. have shown that quantum preparation contextuality can power the parity-oblivious multiplexing (POM) task. The bound on the optimal success probability of an $n$-bit POM task performed with the classical resources was shown to be the same as in a preparation noncontextual theory. This noncontextual bound is violated if the task is performed with quantum resources. While in the 2-bit POM task the optimal quantum success probability is achieved, in the 3-bit case optimality was left as an open question. In this paper, we show that the quantum success probability of an $n$-bit POM task is solely dependent on a suitable Bell expression and the optimal value of it optimizes the success probability of the said POM task. Further, we discuss how the degree of quantum preparation contextuality restricts the amount of quantum violations of Bell's inequalities, and consequently, the success probability of a POM task.

Implementing quantum algorithms is essential for quantum computation. We study the implementation of three quantum algorithms on a two-dimensional temporal continuous-variable cluster state. We first review the generation of temporal cluster states and the implementation of gates using the measurement-based model. Alongside this we discuss methods to introduce non-Gaussianity into the cluster states. The first algorithm we consider is Gaussian boson sampling in which only Gaussian unitaries need to be implemented. Taking into account the fact that input states are also Gaussian, the errors due to the effect of finite squeezing can be partially corrected and a high fidelity can be achieved, provided a moderate amount of online squeezing is available. This helps to construct a large Gaussian boson sampling machine. The second algorithm is the continuous-variable instantaneous quantum polynomial circuit in which one needs to implement non-Gaussian gates, such as the cubic phase gate. We discuss several methods of implementing the cubic phase gate and fit them into the temporal cluster state architecture. The third algorithm is the continuous-variable version of Grover's search algorithm, the main challenge of which is the implementation of the inversion operator. We propose a method to implement the inversion operator by injecting a resource state into a teleportation circuit. The resource state is simulated using the Strawberry Fields quantum software package.

We investigate magnetically tunable Feshbach resonances between ultracold europium atoms and between europium and alkali-metal atoms using multichannel quantum scattering calculations. For ultracold gases of europium atoms both homonuclear ${}^{153}\mathrm{Eu}+{}^{153}\mathrm{Eu}$ and heteronuclear ${}^{151}\mathrm{Eu}+{}^{153}\mathrm{Eu}$ systems are studied. Calculations for mixtures of europium and alkali-metal atoms are carried out for prototype systems of ${}^{153}\mathrm{Eu}+{}^{87}\mathrm{Rb}$ and ${}^{153}\mathrm{Eu}+{}^7\mathrm{Li}$. We analyze the prospects for the control of scattering properties, observation of quantum chaotic behavior, and magnetoassociation into ultracold polar and paramagnetic molecules. We show that favorable resonances can be expected at experimentally feasible magnetic-field strengths below $1000\mathrm{G}$ for all investigated atomic combinations. For Eu atoms, a rich spectrum of resonances is expected as a result of the competition between relatively weak short-range spin-exchange and strong long-range magnetic dipole-dipole interactions, where the dipolar interaction induces measurable resonances. A high density of resonances is expected at magnetic-field strengths below $200\mathrm{G}$ without pronounced quantum chaos signatures. The present results may be useful for the realization and application of dipolar atomic and molecular quantum gases based on europium atoms in many-body physics.

We consider the transversal modes of ions in a linear radio-frequency trap where we control the time-dependent axial confinement to show that we can excite quanta of motion via a two-mode squeezing process. This effect is analogous to phenomena predicted to occur in the early universe, in general out of reach for experimental investigation. As a substantial advantage of this proposal in comparison to previous ones we propose to exploit the radial and axial modes simultaneously to permit experimental access of these effects based on state-of-the-art technology. In addition, we propose to create and explore entanglement between the two ions.

Recent studies in the realization of Majorana fermion (MF) quasiparticles have focused on engineering topological superconductivity that derives from proximity effects of conventional superconductors and spin textures. We propose an effective model to create unpaired MFs at a honeycomb lattice edge by generalizing a two-dimensional topologically nontrivial Haldane model and introducing textured pairings. The core idea is to add both the spin-singlet and textured spin-triplet pairings to a pseudospin-state-dependent, time-reversal-symmetry (TRS) noninvariant honeycomb lattice, and to satisfy generalized “sweet spot” conditions as in the Kitaev chain model. Our model has a gapped superconducting phase and a gapless phase; either phase may have zero or nonzero topological winding numbers. The discriminant that distinguishes those two phases gives a measure of TRS breaking and may have more general implications. Effective Majorana zero modes arise at edges in distinct phases with different degrees of degeneracy. Our theoretical model motivates concepts, such as “textured pairings” and the “strength” of TRS breaking, that may play important roles in future implementation of MFs with cold atoms in optical lattices.

We investigate the response to superlattice modulation of a bosonic quantum gas confined to arrays of tubes emulating the one-dimensional Bose-Hubbard model. We demonstrate, using both time-dependent density-matrix renormalization-group and linear response theory, that such a superlattice modulation gives access to the excitation spectrum of the Bose-Hubbard model at finite momenta. Deep in the Mott insulator, the response is characterized by a narrow energy-absorption peak at a frequency approximately corresponding to the on-site interaction strength between bosons. This spectroscopic technique thus allows for an accurate measurement of the effective value of the interaction strength. On the superfluid side, we show that the response depends on the lattice filling. The system can either respond at infinitely small values of the modulation frequency or only above a frequency threshold. We discuss our numerical findings in light of analytical results obtained for the Lieb-Liniger model. In particular, for this continuum model, bosonization predicts power-law onsets for both responses.

Loss and decoherence are major problems in the transmission of nonclassical states of light over large distances. It was recently shown that the effects of decoherence can be reduced by applying a probabilistic noiseless attenuator before transmitting a quantum state through a lossy channel, followed by probabilistic noiseless amplification [M. Micuda, I. Straka, M. Mikova, M. Dusek, N. J. Cerf, J. Fiurasek, and M. Jezek, Phys. Rev. Lett. 109, 180503 (2012)]. Here we show that similar results can be obtained for certain kinds of macroscopic quantum states by squeezing the signal before transmission, followed by deterministic amplification and antisqueezing to restore the original amplitude of the state. This approach can greatly reduce the effects of decoherence in the transmission of non-Gaussian states, such as Schrödinger cat states, without any reduction in the data transmission rate.

A systematic elementary linear-algebraic construction of non-Hermitian Hamiltonians $H=H\left(\gamma \right)$ possessing exceptional points $\gamma ={\gamma }^{\left(\mathrm{EP}\right)}$ of higher orders is proposed. The implementation of the method leading to EPs of orders $K=4$ and $K=5$ is described in detail. Two distinct areas of applicability of our user-friendly benchmark models are conjectured: (1) in quantum  mechanics of non-Hermitian systems and (2) in their experimental simulations via classical  systems (e.g., coupled waveguides).