There are a number of tasks in quantum information science that exploit nontransitional adiabatic dynamics. Such a dynamics is bounded by the adiabatic theorem, which naturally imposes a speed limit in the evolution of quantum systems. Here, we investigate an approach for quantum state engineering exploiting a shortcut to the adiabatic evolution, which is based on rapid quenches in a continuous-time Hamiltonian evolution. In particular, this procedure is able to provide state preparation faster than the adiabatic brachistochrone. Remarkably, the evolution time in this approach is shown to be ultimately limited by its “thermodynamical cost,” provided in terms of the average work rate (average power) of the quench process. We illustrate this result in a scenario that can be experimentally implemented in a nuclear magnetic resonance setup.

In addition to their central role in quantum information processing, qubits have proven to be useful tools in a range of other applications such as enhanced quantum sensing and as spectrometers of quantum noise. Here we show that a superconducting qubit strongly coupled to a nonlinear resonator can act as a probe of quantum fluctuations of the intraresonator field. Building on previous work [M. Boissoneault et al., Phys. Rev. A 85, 022305 (2012)], we derive an effective master equation for the qubit which takes into account squeezing of the resonator field. We show how sidebands in the qubit excitation spectrum that are predicted by this model can reveal information about the squeezing factor $r$. The main results of this paper have already been successfully compared to experimental data [F. R. Ong et al., Phys. Rev. Lett. 110, 047001 (2013)], and we present here the details of the derivations.

We present separability criteria for both bipartite and multipartite quantum states. These criteria include the criteria based on the correlation matrix and its generalized form as special cases. We show by detailed examples that our criteria are more powerful than the positive partial transposition criterion, the realignment criterion, and the criteria based on the correlation matrices.

Minimum-weight perfect matching (MWPM) has been the primary classical algorithm for error correction in the surface code, since it is of low runtime complexity and achieves relatively low logical error rates [Phys. Rev. Lett. 108, 180501 (2012)]. A Markov chain Monte Carlo (MCMC) algorithm [Phys. Rev. Lett. 109, 160503 (2012)] is able to achieve lower logical error rates and higher thresholds than MWPM, but requires a classical runtime complexity, which is super-polynomial in $L$, the linear size of the code. In this work we present an MCMC algorithm that achieves significantly lower logical error rates than MWPM at the cost of a runtime complexity increased by a factor $O\left(L^2\right)$. This advantage is due to taking correlations between bit- and phase-flip errors (as they appear, for example, in depolarizing noise) as well as entropic factors (i.e., the numbers of likely error paths in different equivalence classes) into account. For depolarizing noise with error rate $p$, we present an efficient algorithm for which the logical error rate is suppressed as $O\left({(p/3)}^{L/2}\right)$ for $p\to 0$—an exponential improvement over all previously existing efficient algorithms. Our algorithm allows for tradeoffs between runtime and achieved logical error rates as well as for parallelization, and can be also used for correction in the case of imperfect stabilizer measurements.

We investigate to what extent a suitably chosen system Hamiltonian can counteract local dissipative processes and preserve entanglement in the stationary state. The results determine prospects and limitations of dissipative state preparation schemes based on natural dissipative processes—in contrast to engineered, typically nonlocal dissipative schemes. As an exemplary case, we determine the stationary state of two spontaneously decaying two-level systems with optimal entanglement properties. The corresponding system Hamiltonian is derived, and its possible experimental implementation is discussed in detail. Finally, we discuss generalizations for $N$ qubits.

Aaronson and Arkhipov recently used computational complexity theory to argue that classical computers very likely cannot efficiently simulate linear, multimode, quantum-optical interferometers with arbitrary Fock-state inputs [Aaronson and Arkhipov, Theory Comput. 9, 143 (2013)]. Here we present an elementary argument that utilizes only techniques from quantum optics. We explicitly construct the Hilbert space for such an interferometer and show that its dimension scales exponentially with all the physical resources. We also show in a simple example just how the Schrödinger and Heisenberg pictures of quantum theory, while mathematically equivalent, are not in general computationally equivalent. Finally, we conclude our argument by comparing the symmetry requirements of multiparticle bosonic to fermionic interferometers and, using simple physical reasoning, connect the nonsimulatability of the bosonic device to the complexity of computing the permanent of a large matrix.

The orientation and the heat capacity of horizontally adsorbed molecules are investigated in static electric fields. We evaluate the energy spectrum and the wave function to probe the rotational characteristics of the molecule. Numerical results indicate that the electric field and the effect of quantum confinement lead to anticrossing behaviors in the energy levels. The orientation reveals a stepped feature due to the anticrossing in the ground state. Moreover, the heat capacity displays two peaks near the anticrossing. By means of comparison, each peak of the heat capacity corresponds to a particular degree of orientation.

The $C_3$ coefficients and their ratios for the alkali-metal atoms are determined very accurately by taking into account the optical properties of the atoms and four distinct types of trapping materials such as Au (metal), Si (semiconductor), vitreous ${\text{SiO}}_2$ (dielectric), and ${\text{SiN}}_x$ (dielectric). Dynamic dipole polarizabilities are calculated precisely for the alkali-metal atoms that produce $C_3$ coefficients in a perfectly conducting medium within 0.2% accuracy. Thus, uncertainties from the atomic polarizabilities in the evaluation of the material-dependent $C_3$ coefficients are small and accuracies of these $C_3$ values mainly rely on the used optical data which are not taken into account in the present work. Our findings are in very good agreement with the available measurements, but they differ by around 10% from the previously reported high-precision calculations in heavy atoms for the perfect conductors. We also revisit the dispersion coefficients for the alkali-metal dimers and evaluate them using the above dynamic dipole polarizabilities which provide slightly different error bars than the other reported precise results. These coefficients are fitted into a ready-to-use functional form to aid the experimentalists in finding out the interaction potential only with the knowledge of atom-wall distance.

Near-field effects in fluctuational electrodynamics leads to enhancement of radiative energy transfer as well as the emergence of van der Waals and/or Casimir pressure. While much has been learned from the analysis of near-field interactions between two half-spaces separated by a vacuum gap, we shed new light on the problem by finding how much of a surface patch on one of the half-spaces contributes to the energy transfer or van der Waals pressure at any location within the vacuum gap. We show that energy transfer and fluctuation-induced van der Waals pressure at any point on the surface of one half-space are qualitatively and quantitatively different due to the dissimilar zones of influence of interactions. We also show that the contributions from different surface patches are qualitatively similar for half-spaces with dielectric materials (silica, silicon carbide) and half-spaces with metals (gold).

Large-scale $R$-matrix-with-pseudostates calculations for electron collisions with argon atoms, using a recently developed parallel version of our B-spline $R$-matrix code, are reported. The calculations were carried out in the semirelativistic $\mathit{jK}$-coupling scheme. They are intended to provide converged (with respect to the number of coupled states) results for electron-impact excitation of individual target states with dominant configurations $3p^{5}4s$, $3p^{5}4p$, $3p^{5}3d$, and $3p^{5}5s$ for incident electron energies from threshold to 300 eV. The close-coupling expansion includes 500 target states, with the lowest 78 states representing the bound spectrum and the remaining 422 the ionization continuum. The results reveal dramatic reductions of the predicted excitation cross sections at intermediate energies due to a strong influence of coupling to the target continuum and the higher-lying Rydberg states. Comparison with available experimental data for excitation raises questions about the absolute normalization in the measurements.

We propose a scheme to achieve extremely asymmetric electron localization during molecular dissociations. Hydrogen molecular ion (H${}_2$${}^{+}$) dissociation is investigated theoretically. A THz pulse is used to steer the electron motion after the molecular ion is excited by an ultrashort ultraviolet laser pulse. A high probability, as high as 99.3%, to localize electrons on one of the two nuclei is demonstrated, with a dissociation probability of 6.14% and almost no ionization, by optimizing the peak intensities and time delay of the two pulses. Even when the total dissociation probability is increased to 25.6%, more than 96.3% electrons can be localized in all dissociation events. These results represent a significant advancement in the electronic dynamics control in molecules.

Through frequency up-conversion of polarization-shaped, femtosecond laser pulses nonlinearly interacting with xenon atoms, energetic, broadband, coherent, XUV continuum radiation is generated. By exploiting the thus-formed short-duration XUV pulses, all the optically allowed excited states of H${}_2$ are coherently populated. Nuclear and electronic 1-fs-scale dynamics are subsequently investigated by means of XUV-pump–XUV-probe measurements, which are compared to the results of ab initio calculations. The revealed dynamics reflects the intrinsic molecular behavior, as the XUV probe pulse hardly distorts the molecular potential.

We studied the elementary processes of excitation and ionization of atomic hydrogen in an intense 800-nm pulse with intensity in the 1.0 to $2.5\times {10}^{14}$ ${\mathrm{W}/\mathrm{cm}}^2$ range. By analyzing excitation as a continuation of above-threshold ionization (ATI) into the below-threshold negative energy region, we show that modulation of excitation probability and the well-known shift of low-energy ATI peaks vs laser intensity share the same origin. Modulation of excitation probability is a general strong field phenomenon and is shown to be a consequence of channel closing in multiphoton ionization processes. Furthermore, the excited states populated in general have large orbital angular momentum and they are stable against ionization by the intense 800-nm laser—they are the underlying reason for population trapping of atoms and molecules in intense laser fields.

We show that an unconventional superfluid triggered by spin-orbit coupling is realized for repulsively interacting quasi-one-dimensional fermions. A competition between spin-singlet and -triplet pairings occurs due to the breaking of inversion symmetry. We show that both superfluid correlations decay algebraically with the same exponent except for special coupling constants for which a dominant superfluid is controlled by the spin-orbit coupling. We also discuss a possible experiment to observe such phases with cold atoms.

We calculate the ground-state energy and the collective excitation frequency of trapped bosons at large scattering length interacting via the realistic two-body van der Waals potential. Our many-body method keeps two-body correlations produced by all interacting pairs. When the scattering length is small compared to the trap size and the number of bosons in the trap is of the order of a few thousands, the mean-field results are in good agreement with the many-body results. However for large particle numbers, even when the condensate is sufficiently dilute, the interatomic correlation comes into the picture. When the scattering length is quite large near the Feshbach resonance, the Bose gas becomes highly correlated. The many-body results are close to the Gross-Pitaevskii results for a small number of bosons, however, large deviations are noted in the large particle limit. We also calculate the lowest collective excitation and the interaction energy for large scattering lengths. The monopole excitation frequency exhibits a pronounced dependence on the scattering length. We also observe a universal behavior for the interaction energy at the limit of large scattering length.

We examine quantum phases emerging from the double degeneracy of $p$-orbital bands in attractive atomic Fermi gases loaded on a one-dimensional (1D) optical lattice. Our numerical simulations via the density-matrix renormalization group predict the emergence of a state with a charge excitation gap, namely the Haldane insulator phase. A mapping onto an effective spin-1 model reveals its physical origin. Moreover, we show that population imbalance leads to richer diversity of the quantum phases, including a phase-separated polarized state. Finally, we study the effects of the harmonic trap potential in this 1D chain.

Semiconductor lasers with delayed feedback exhibit two fundamentally different dynamical states: weak and strong chaos. We characterize experimentally the mechanism for the emergence of strong chaos. Based on these insights, we demonstrate similarity properties for long delays, i.e., similar dynamics for different pump currents when adjusting the feedback strength. For different delay times, even the same time- and amplitude-rescaled version of the dynamics can be generated. Using a simple rate-equation model, these properties can be corroborated. The results have major consequences for the characterization and tailoring of the dynamics for applications.

We show how two-dimensional waveguide arrays can be used to probe the effect of on-site interaction on Anderson localization of two interacting bosons in one dimension. It is shown that classical light and linear elements are sufficient to experimentally probe the interplay between interaction and disorder in this setting. For experimental relevance, we evaluate the participation ratio and the intensity correlation function as measures of localization for two types of disorder (diagonal and off-diagonal), for two types of interaction (repulsive and attractive), and for a variety of initial input states. Employing a commonly used set of initial states, we show that the effect of interaction on Anderson localization is strongly dependent on the type of disorder and initial conditions, but is independent of whether the interaction is repulsive or attractive. We then analyze a certain type of entangled input state where the type of interaction is relevant and discuss how it can be naturally implemented in waveguide arrays. We conclude by laying out the details of the two-dimensional photonic lattice implementation including the required parameter regime.

We measured the second-order coherence at zero time delay, $g^{\left(2\right)}\left(0\right)$, of a high-$Q$ photonic crystal nanocavity quantum dot laser below and above the lasing threshold, via detecting visible emission originated from the second harmonic generation (SHG) occurring within the laser cavity. The efficiency of the nonlinear frequency conversion process, defined by SHG intensity divided by the square of laser output intensity, is shown to be proportional to $g^{\left(2\right)}\left(0\right)$, and hence is used for the measurement. The obtained $g^{\left(2\right)}\left(0\right)$ values show good agreement with those measured by a Hanbury Brown–Twiss interferometer equipped with ultrafast superconducting single-photon detectors.

We theoretically investigate harmonic generation driven by intense midinfrared lasers. A valleylike structure is observed at very low-order harmonics because of a low-energy photoelectron suppression effect. Further, at the beginning of a broad supercontinuum, a convex structure appears that is distributed from the tail in a very narrow energy band in time-frequency maps. Surprisingly, our quantum dynamics calculations demonstrate the beneficial wavelength scaling of the harmonic yield to be ${\lambda }^{4.6}$ for He and ${\lambda }^{5.1}$ for Ne over selected energy windows. The bandwidth of the harmonic plateau with only a single quantum trajectory contribution can be further extended by adding a controlling laser field, and the harmonic efficiency is found to be significantly enhanced after macroscopic propagation. In addition, ultrashort isolated attosecond pulses can be obtained by properly superposing the harmonics in the plateaus of both the He and Ne systems.