We propose a scheme to steer the electron localization in dissociating ${{\mathrm{H}}_2}^{+}$ by using the combination of a phase-stabilized few-cycle mid-infrared laser pulse and a low-frequency field. Via manipulating the population dynamics in the $1s{\sigma }_g$ and $2p{\sigma }_u$ states by the weak low-frequency field, the dissociative wave packets on these two degenerate states of opposite parities are managed to share the same kinetic energy and overlap in space. By adjusting the carrier-envelope phase of the mid-infrared pulse or simply changing the intensity of the low-frequency field, the electron localization induced by the spatial interference of the two dissociation pathways can be efficiently controlled.

We theoretically investigate the equation of state and Tan's contact of a nondegenerate three-dimensional Bose gas near a broad Feshbach resonance, within the framework of large-$N$ expansion. Our results agree with the path-integral Monte Carlo simulations in the weak-coupling limit and recover the second-order virial expansion predictions at strong interactions and high temperatures. At resonance, we find that the chemical potential and energy are significantly enhanced by the strong repulsion, while the entropy does not change significantly. With increasing temperature, the two-body contact initially increases and then decreases like $T^{-1}$ at large temperature, and therefore exhibits a peak structure at about $4T_{c0}$, where $T_{c0}$ is the Bose-Einstein condensation temperature of an ideal, noninteracting Bose gas. These results may be experimentally examined with a nondegenerate unitary Bose gas, where the three-body recombination rate is substantially reduced. In particular, the nonmonotonic temperature dependence of the two-body contact could be inferred from the momentum distribution measurement.

Optical limiters transmit low-level radiation while blocking electromagnetic pulses with excessively high energy (energy limiters) or with excessively high peak intensity (power limiters). A typical optical limiter absorbs most of the high-level radiation, which can cause its overheating and destruction. Here we introduce the concept of a reflective energy limiter which blocks electromagnetic pulses with excessively high total energy by reflecting them back to space, rather than absorbing them. The idea is to use a defect layer with temperature-dependent loss tangent embedded in a low-loss photonic structure. The low-energy pulses with central frequency close to that of the localized defect mode will pass through. But if the cumulative energy carried by the pulse exceeds certain level, the entire photonic structure becomes highly reflective (not absorptive) within a broad frequency range. The underlying physical mechanism is based on self-regulated impedance mismatch which increases dramatically with the cumulative energy carried by the pulse.

We study the dynamics of a mobile impurity in a quantum fluid at zero temperature. Two related settings are considered. In the first setting, the impurity is injected in the fluid with some initial velocity ${\mathbf{v}}_0$, and we are interested in its velocity at infinite time, ${\mathbf{v}}_{\infty }$. We derive a rigorous upper bound on $|{\mathbf{v}}_0-{\mathbf{v}}_{\infty }|$ for initial velocities smaller than the generalized critical velocity. In the limit of vanishing impurity-fluid coupling, this bound amounts to ${\mathbf{v}}_{\infty }={\mathbf{v}}_0$, which can be regarded as a rigorous proof of the Landau criterion of superfluidity. In the case of a finite coupling, the velocity of the impurity can drop, but not to zero; the bound quantifies the maximal possible drop. In the second setting, a small constant force is exerted upon the impurity. We argue that two distinct dynamical regimes exist—backscattering oscillations of the impurity velocity and saturation of the velocity without oscillations. For fluids with $v_{c\mathrm{L}}=v_s$ (where $v_{c\mathrm{L}}$ and $v_s$ are the Landau critical velocity and sound velocity, respectively), the latter regime is realized. For fluids with $v_{c\mathrm{L}}<v_s$, both regimes are possible. Which regime is realized in this case depends on the mass of the impurity, a nonequilibrium quantum phase transition occurring at some critical mass. Our results are equally valid in one, two, and three dimensions.

Current approaches for building quantum computing devices focus on two-level quantum systems which nicely mimic the concept of a classical bit, albeit enhanced with additional quantum properties. However, rather than artificially limiting the number of states to two, the use of $d$-level quantum systems (qudits) could provide advantages for quantum information processing. Among other merits, it has recently been shown that multilevel quantum systems can offer increased stability to external disturbances. In this study we demonstrate that topological quantum memories built from qudits, also known as Abelian quantum double models, exhibit a substantially increased resilience to noise. That is, even when taking into account the multitude of errors possible for multilevel quantum systems, topological quantum error-correction codes employing qudits can sustain a larger error rate than their two-level counterparts. In particular, we find strong numerical evidence that the thresholds of these error-correction codes are given by the hashing bound. Considering the significantly increased error thresholds attained, this might well outweigh the added complexity of engineering and controlling higher-dimensional quantum systems.

A detailed experimental and theoretical investigation of the dynamics leading to fragmentation of doubly ionized molecular thiophene is presented. Dissociation of double-ionized molecules was induced by S $2p$ core photoionization and the ionic fragments were detected in coincidence with Auger electrons from the core-hole decay. Rich molecular dynamics was observed in electron-ion-ion coincidence maps exhibiting ring breaks accompanied by hydrogen losses and/or migration. The probabilities of various dissociation channels were seen to be very sensitive to the internal energy of the molecule. Theoretical simulations were performed by using the semiempirical self-consistent charge-density-functional tight-binding method. By running thousands of these simulations, the initial conditions encountered in the experiment were properly taken into account, including the systematic dependencies on the internal (thermal) energy. This systematic approach, not affordable with first-principle methods, provides a good overall description of the complex molecular dynamics observed in the experiment and shows good promise for applicability to larger molecules or clusters, thus opening the door to systematic investigations of complex dynamical processes occurring in radiation damage.

High-order-harmonic generation (HHG) from small linear molecules driven by a circularly polarized laser pulse (CPLP) is investigated. It is found that the obtained high-order harmonics are more pronounced than those from reference atoms with equal ionization potential driven by the same CPLP. By analyzing the dependence of the cutoff position on laser parameters and calculating the recollision trajectories, it is shown that this molecular HHG originates from the recollision mechanism, instead of the bound-bound transition mechanism found to be responsible for molecular HHG by CPLP in earlier works. A semiclassical model is used to analyze the HHG process and discuss the origin of the higher efficiency of molecular HHG. It is found that the higher HHG efficiency for molecules is mainly contributed in the recombination step and at least partly due to the higher recollision probability of continuum electrons.

The Higgs amplitude mode of the order parameter of an ultracold confined Fermi gas in the BCS regime after a quench of the coupling constant is analyzed theoretically. A characteristic feature is a damped oscillation which at a certain transition time changes into a rather irregular dynamics. We compare the numerical solution of the full set of nonlinear equations of motion for the normal and anomalous Bogoliubov quasiparticle excitations with a linearized approximation. In doing so the transition time as well as the difference between resonant systems, i.e., systems where the Fermi energy is close to a sub-band minimum, and off-resonant systems can be well understood and traced back to the system and geometry parameters.

We study Raman scattering of single nanoparticles coupled to a high-$Q$ whispering-gallery microresonator. It is found that cavity resonances greatly enhance the Raman signal, and the enhancement factor is as high as ${10}^8$. Unlike the noncavity case, the signal power exhibits a nonmonotonic dependence on particle size, and it reaches the maximum when the Rayleigh scattering loss and the cavity intrinsic loss are comparable. We further analyze how the Raman signal intensity is influenced by different parameters including cavity quality factors and taper-cavity coupling strength. The detection limit of observing single-nanoparticle Raman signal is discussed finally. As a potential application, this mechanism may provide an alternative way to detect specific biological targets without the need of precovered biorecognitions.

We study the scattering of infrared light by small dielectric core-shell particles taking an ${\mathrm{Al}}_2{\mathrm{O}}_3$ sphere with a CaO core as an example. The extinction efficiency of such a particle shows two intense series of resonances attached, respectively, to in-phase and out-of-phase multipolar polarization-induced surface charges buildup, respectively, at the core-shell and the shell-vacuum interface. Both series, the character of the former may be labeled bonding and the character of the latter antibonding, give rise to anomalous scattering. For a given particle radius and filling factor, the Poynting vector field shows therefore around two wave numbers the complex topology of this type of light scattering. Inside the particle, the topology depends on the character of the resonance. The dissipation of energy inside the particle also reflects the core-shell structure. It depends on the resonance and shows strong spatial variations.

In quantum optimal control theory, kinematic bounds are the minimum and maximum values of the control objective achievable for any physically realizable system dynamics. For a given initial state of the system, these bounds depend on the nature and state of the controller. We consider a general situation where the controlled quantum system is coupled to both an external classical field (referred to as a classical controller) and an auxiliary quantum system (referred to as a quantum controller). In this general situation, the kinematic bound is between the classical kinematic bound (CKB), corresponding to the case where only the classical controller is available, and the quantum kinematic bound (QKB), corresponding to the ultimate physical limit of the objective's value. Specifically, when the control objective is the expectation value of a quantum observable (a Hermitian operator on the system's Hilbert space), the QKBs are the minimum and maximum eigenvalues of this operator. We present, both qualitatively and quantitatively, the necessary and sufficient conditions for surpassing the CKB and reaching the QKB, through the use of a quantum controller. The general conditions are illustrated by examples in which the system and controller are initially in thermal states. The obtained results provide a basis for the design of quantum controllers capable of maximizing the control yield and reaching the ultimate physical limit.

We study an asymmetric form of the two-mode entangled coherent state (ECS), where the two local amplitudes have different values, as a tool for testing the Bell-Clauser-Horne-Shimony-Holt (Bell-CHSH) inequality. We find that the asymmetric ECSs have obvious advantages over the symmetric form of ECSs in testing the Bell-CHSH inequality. We further study an asymmetric strategy in distributing an ECS over a lossy environment and find that such a scheme can significantly increase violation of the inequality.

One of the promising systems to realize quantum computation is a hybrid system where a superconducting flux qubit plays the role of a quantum processor and the nitrogen-vacancy- $\left({\mathrm{NV}}^{-}\right)$ center ensemble is used as a quantum memory. We have theoretically and experimentally studied the effect of magnetic fields on this hybrid system, and found that the lifetime of the vacuum Rabi oscillation is improved by applying a few mT magnetic field to the ${\mathrm{NV}}^{-}$ center ensemble. Here, we construct a theoretical model to reproduce the vacuum Rabi oscillations with and without magnetic fields applied to the ${\mathrm{NV}}^{-}$ centers, and we determine the reason why magnetic fields can affect the coherent properties of the ${\mathrm{NV}}^{-}$ center ensemble. From our theoretical analysis, we quantitatively show that the magnetic fields actually suppress the inhomogeneous broadening from the strain in the ${\mathrm{NV}}^{-}$ centers.

Nowadays, geometric tools are being used to treat a huge class of problems of quantum information science. By understanding the interplay between the geometry of the state space and information-theoretic quantities, it is possible to obtain less trivial and more robust physical constraints on quantum systems. Here we establish a geometric lower bound for the Wigner-Yanase skew information (WYSI), a well-known information-theoretic quantity recently recognized as a proper quantum coherence measure. In the case of a mixed state evolving under unitary dynamics generated by a given observable, the WYSI between the state and the observable is bounded from below by the rate of change of the state's statistical distinguishability from its initial value. Our result shows that, since WYSI fits in the class of Petz's metrics, this lower bound is the change rate of its respective geodesic distance on quantum state space. The geometric approach is advantageous because it raises several physical interpretations of this inequality under the same theoretical umbrella.

We present a simple theoretical model for describing ultracold-atomic collision dynamics and Feshbach resonances (FRs) without onerous numerical computation. An effective quantum-defect phase shift ${\delta }_{\mathrm{Y}}^{\mathrm{eff}}$ is derived and the reference potentials are shown acting as an assistant tool in the calculation. With the help of singlet and triplet scattering lengths $a^{\mathrm{S}}$ and $a^{\mathrm{T}}$, as well as an adjustable parameter ${\gamma }_{\mathrm{sr}}$, this model can be used to reliably predict FRs. An application example of ${}^6\mathrm{Li}-{}^{40}\mathrm{K}$ collision shows that the results given by the simple model agree quite well with both the relevant experimental data and the coupled-channel calculations.

By numerically solving the time-dependent Schrödinger equation in full dimensionality, we discuss the dependance of joint photoelectron angular distributions on the energy sharing of the emitted electrons for the double ionization of helium atoms by ultrashort pulses of extreme ultraviolet (XUV) radiation in coplanar emission geometry with and without the presence of a comparatively weak infrared (IR) laser pulse. For IR-laser-assisted single-XUV-photon double ionization, our joint angular distributions show that the IR-laser field enhances back-to-back electron emission and induces a characteristic splitting in the angular distribution for electrons that are emitted symmetrically relative to the identical linear polarization directions of the XUV and IR pulse. These IR-pulse-induced changes in photoelectron angular distributions are (i) imposed by different symmetry constraints for XUV-pulse-only and laser-assisted XUV double ionization, (ii) robust over a large range of energy sharings between the emitted electrons, and (iii) consistent with the transfer of discrete IR-photon momenta to both photoelectrons from the assisting IR-laser field. While selection-rule forbidden at equal energy sharing, for increasingly unequal energy sharing we find back-to-back emission to become more likely and to compete with symmetric emission.

We demonstrate that a site-dependent driving of a periodic potential allows for the controlled manipulation of a quantum particle on length scales of the lattice spacing. Specifically we observe for distinct driving frequencies a near depletion of certain sites which is explained by a resonant mixing of the involved Floquet-Bloch modes occurring at these frequencies. Our results could be exploited as a scheme for a site-selective loading of, e.g., ultracold atoms into an optical lattice.

While different ways to realize spin-orbit coupling in Bose-Einstein condensates exist, not all are currently experimentally implementable. Here we present a detailed study of gap solitons in a Bose-Einstein condensate with experimentally realizable spin-orbit coupling and discuss two cases relating to a spin-dependent parity symmetry. In the parity symmetric case, two families of fundamental gap solitons in the second linear energy gap are demonstrated with the opposite sign of the parity, with one family having single-humped densities and the other double-humped ones. In the case of broken parity symmetry, the fundamental modes manifest spin polarization. Both families possess an opposite sign of the spin polarization.

We present a detailed, realistic analysis of the implementation of a proposal for dipole-induced electromagnetic transparency (DIET) [R. Puthumpally-Joseph, M. Sukharev, O. Atabek, and E. Charron, Phys. Rev. Lett. 113, 163603 (2014)] using an ensemble of cold atoms at high density. Using both direct numerical simulations and simple analytical models, we show how, in a realistic $N$-level quantum system, narrow transparency windows can appear at large densities. The existence of such windows is attributed to quantum interference effects in overlapping resonances. Our analysis is applied to the $D_1$ transition of Rb atoms, and we show that, at high densities, Rb can behave like a simple three-level emitter exhibiting all the properties of DIET. Some interesting effects such as slow light are also presented, and their limits in the context of DIET are discussed