We extend the Su-Schrieffer-Heeger model to include both an additional real intercell coupling and a complex intracell coupling whose phase can be interpreted as the Peierls phase associated with a synthetic gauge field. Using different Peierls phases, we can realize a heterostructure supporting a topologically protected interface state, depending on the existence of one trivial and two nontrivial phases of distinct winding numbers. The spatial adiabatic passage of this localized state from the inner interface to either open boundary can be attained simply by modulating the corresponding Peierls phase, while its decay or growth rate is controlled by the on-site parity-time symmetric loss and gain terms. Our results provide a scheme to achieve dynamical control of topologically protected states while being of potential interest to topological lasers with an adjustable spatial profile.

A free electron can form a bound state with an atomic center $A$ upon photoemission (radiative recombination). In the presence of a neighboring atom $B$, such a bound state can, under certain conditions, be also formed via resonant transfer of energy to $B$, with its subsequent relaxation through radiative decay (two-center dielectronic recombination). This two-center process is very efficient in the “static” case where $A$ and $B$ form a weakly bound system, dominating over single-center radiative recombination up to internuclear distances as large as several nanometers. Here we study its dynamic variant in which recombination occurs when a beam of species $A$ collides with a gas of atoms $B$ and show that, even though the average distance between $A$ and $B$ in collisions is orders of magnitude larger than the typical size of a bound system, the two-center recombination can still outperform the single-center radiative recombination.

Air lasing in the nitrogen molecular ion is not well understood because the complex physics responsible for gain is interwoven with pulse propagation in an extreme environment. Here we use a short gas jet to limit the interaction length, thereby removing the propagation effects. We report on several mechanisms that contribute to the decay of gain in different conditions, and experimentally isolate two decay timescales: the decay of long-term gain due to collisional state mixing, and short-term gain that cannot be explained by population inversion. To test the former, we control the inelastic electron scattering rate by varying the gas concentration while keeping the propagation length fixed, and predict the change of the decay using a model of collisional state mixing. We show that the same mechanism causes the decay of rotational wave packets in the states of the ion. Finally, we simulate the complex modulations of gain due to rotational wave packets and the propagation of the probe pulse through the evolving rotationally excited and inverted medium.

A photoelectron forced to pass through two atomic energy levels before receding from the residual ion shows interference fringes in its angular distribution as a manifestation of a two-slit-type interference experiment in wave-vector space. This scenario was experimentally realized by irradiating a rubidium atom by two low-intensity continuous-wave lasers [Pursehouse et al., Phys. Rev. Lett. 122, 053204 (2019)]. In a one-photon process, the first laser excites the $5p$ level whereas the second uncorrelated photon elevates the excited population to the continuum. This same continuum state can also be reached when the second laser excites the $6p$ state, and the first photon then triggers the ionization. As the two lasers are weak and their relative phases uncorrelated, the coherence needed for generating the interference stems from the atom itself. Increasing the intensity or shortening the laser pulses enhances the probability that two photons from both lasers act at the same time, and hence, the coherence properties of the applied lasers are expected to affect the interference fringes. Here, this aspect is investigated in detail, and it is shown how tuning the temporal shapes of the laser pulses allows for tracing the time dependence of the interference fringes. We also study the influence of applying a third laser field with a random amplitude, resulting in a random fluctuation of one of the ionization amplitudes and discuss how the interference fringes are affected.

We discuss the nonequilibrium time evolution of the phase field in the sine-Gordon model using two very different approaches: the truncated Wigner approximation and the truncated conformal space approach. We demonstrate that the two approaches agree for a period covering the first few oscillations, thereby giving a solid theoretical prediction in the framework of the sine-Gordon model, which is thought to describe the dynamics of two bosonic condensates in quasi-one-dimensional traps coupled via a Josephson tunneling term. We conclude, however, that the recently observed phase-locking behavior cannot be explained in terms of homogeneous sine-Gordon dynamics, which hints at the role of other degrees of freedom or inhomogeneity in the experimental system.

I consider nonrelativistic bosons interacting via pairwise potentials with infinite scattering length and supporting no two-body bound states. To lowest order in effective field theory, these conditions lead to noninteracting bosons, since the coupling constant of the Lieb-Liniger model vanishes identically in this limit. Since any realistic pairwise interaction is not a mere delta function, the noninteracting picture is an idealization indicating that the effect of interactions is weaker than in the case of off-resonant potentials. I show that the leading-order correction to the ground-state energy for more than two bosons is accurately described by the lowest-order three-body force in effective field theory that arises due to the off-shell structure of the two-body interaction. For natural two-body interactions with a short-distance repulsive core and an attractive tail, the emergent three-body interaction is repulsive and, therefore, three bosons do not form any bound states. This situation is analogous to the two-dimensional repulsive Bose gas, when treated using the lowest-order contact interaction, where the scattering amplitude exhibits an unphysical Landau pole. The avoidance of this state in the three-boson problem proceeds in a way that parallels the two-dimensional case. These results pave the way for the experimental realization of one-dimensional Bose gases with pure three-body interactions using ultracold atoms.

We consider the formation of temporal localized structures or Kerr-comb generation in a microresonator with inhomogeneities. We show that the introduction of even a small inhomogeneity in the injected beam widens the stability region of localized solutions. The homoclinic snaking bifurcation associated with the formation of localized structures and clusters of them with decaying oscillatory tails is constructed. Furthermore, the inhomogeneity allows us to not only control the position of localized solutions, but strongly affects their stability domains. In particular, a new stability domain of a single peak localized structure appears outside of the region of multistability between multiple peaks of localized states. We identify a regime of larger detuning, where localized structures do not exhibit a snaking behavior. In this regime, the effect of inhomogeneities on localized solutions is far more complex: They can act either attracting or repelling. We identify the pitchfork bifurcation responsible for this transition. Finally, we use a potential well approach to determine the force exerted by the inhomogeneity and summarize with a full analysis of the parameter regime, where localized structures and therefore Kerr-comb generation exist, and analyze how this regime changes in the presence of an inhomogeneity.

The influence of quantum fluctuations of the electromagnetic field on the propagation of a polarized light wave in a nonlinear dielectric is investigated. It is shown that in some cases the fluctuations couple to the optical nonlinearities of the medium and make its refractive index dependent on the polarization of the propagating wave. As a consequence, two light waves propagating in the same direction but with different polarizations will have different speeds, so there will be a Kerr-effect birefringence induced by quantum fluctuations. We consider the case of the electromagnetic field in a squeezed vacuum state, where there are regions where the mean squared electric field can be negative and the birefringence effect has the opposite sign to the Kerr effect in classical physics. We give some estimates for the magnitude of these effects, and discuss their possible observability.

We study the propagation and storage of a quantum field using ultranarrow coherent population oscillations (CPOs) in a $\mathrm{\Lambda }$-type atomic medium. The predictions for classical fields are checked experimentally in a vapor at room temperature. We derive the evolution of its squeezing spectrum in the presence of a large classical pump field which enables CPOs to exist. We show that the spontaneous emission of the residual population pumped into the excited state progressively destroys the quantum noise properties of the quantum field along propagation. The output quantum field therefore tends to be a coherent state, discarding the possibility to store quantum states of light with CPO.

We demonstrate that the transport of coherent quasiresonant light through a dense cloud of immobile two-level atoms subjected to a static external electric field can be described by a simple diffusion process up to atomic number densities of the order of at least ${10}^2$ atoms per wavelength cubed. Transport mean-free paths well below the wavelength of light in the free space can be reached without inducing any sign of Anderson localization of light or of any other mechanism of breakdown of diffusion.

We investigate detailed balance for a quantum system interacting with thermal radiation within mixed quantum-classical theory. For a two-level system coupled to classical radiation fields, three semiclassical methods are benchmarked: (1) Ehrenfest dynamics overestimate the excited-state population at equilibrium due to the failure of capturing vacuum fluctuations. (2) The coupled Maxwell-Bloch equations, which supplement Ehrenfest dynamics by damping at the full golden rule rate, underestimate the excited state population due to double-counting of the self-interaction effect. (3) $\mathrm{Ehrenfest}+\mathrm{R}$ dynamics recover detailed balance and the correct thermal equilibrium by enforcing the correct balance between the optical excitation and spontaneous emission of the quantum system. These results highlight the fact that, when properly designed, mixed quantum-classical electrodynamics can simulate thermal equilibrium in the field of nanoplasmonics.

We study nuclear spin frequency shifts in a ${}^3\mathrm{He}\text{−}{}^{129}\mathrm{Xe}$ comagnetometer caused by spin polarization of ${}^3\mathrm{He}$. We use stemless cylindrical cells to systematically vary the cell geometry and separately measure the cell shape-dependent and shape-independent frequency shifts. We find that a certain aspect ratio for a cylindrical cell cancels the dipolar effects of ${}^3\mathrm{He}$ magnetization in the regime of fast spin diffusion. Using this control, we observe a scalar ${}^3\mathrm{He}\text{−}{}^{129}\mathrm{Xe}$ collisional frequency shift characterized by an enhancement factor ${\kappa }_{\text{HeXe}}=-0.011\pm 0.001$, which is in excellent agreement with theoretical calculations.

Achieving higher sensitivity is an earnest purpose for precision metrology. As a response to this goal, the weak-value amplification approach has been developed for measuring ultrasmall physical effects, realizing sensitivity that had never been reached before. Encouraged by the successes, many efforts have been devoted to obtain ultimate sensitivity of weak-value amplification. However, the benefit would be easily compromised in practice, because the cost of significant reduction on signal intensity leads to an ultralow signal-to-noise ratio. In this work, we bridge this gap by proposing an alternative weak-value amplification approach, which provides sensitivity several orders of magnitude higher than the standard approach while being compatible with practical imperfections. In the proof-of-principle experiment of measuring longitudinal phase change in time domain, sensitivity up to $5\times {10}^{-4}$ attosecond is exemplified. Our approach can be applied to measure other small parameters with extremely high sensitivity, providing a method for future precision metrology.

The nitrogen-vacancy (NV) center spin represents an appealing candidate for quantum information processing. Besides the widely used microwave control, its coherent manipulation may also be achieved using a laser as mediated by the excited energy levels. Nevertheless, the multiple levels of the excited state of the NV center spin make the coherent transition process become complex and may affect the fidelity of coherent manipulation. Here, we adopt the strategy of optimal quantum control to accelerate coherent-state transfer in the ground-state manifold of the NV center spin using a laser. The results demonstrate improved performance in both the speed and the fidelity of coherent-state transfer which will be useful for optical control of the NV center spin in diamond.

Electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS) are similar, but different quantum optical phenomena: EIT results from a Fano interference, whereas ATS is described by the ac Stark effect. Likewise, despite their close resemblance, light-storage techniques based on the EIT memory protocol and the recently proposed ATS memory protocol [Saglamyurek et al., Nat. Photon. 12, 774 (2018)] are distinct: The EIT protocol relies on adiabatic elimination of absorption, whereas the ATS protocol is based on absorption. In this article, we elaborate on the distinction between EIT and ATS memory protocols through numerical analysis and experimental demonstrations in a cold rubidium ensemble. We find that their storage characteristics manifest opposite limits of the light-matter interaction due to their inherent adiabatic versus nonadiabatic nature. Furthermore, we determine optimal memory conditions for each protocol and analyze ambiguous regimes in the case of broadband storage, where nonoptimal memory implementations can possess characteristics of both EIT and ATS protocols. We anticipate that this investigation will lead to deeper understanding and improved technical development of quantum memories, while clarifying distinctions between the EIT and ATS protocols.

The existence and stability of the linear hydrogenic chain $\mathrm{H}{}_3$ and $\mathrm{H}{{}_2}^{-}$ in a strong magnetic field is established. Variational calculations for $\mathrm{H}{}_3$ and $\mathrm{H}{{}_2}^{-}$ are carried out in magnetic fields in the range ${10}^{11}\le B\le {10}^{13}$ G with a 17-parametric (13-parametric for $\mathrm{H}{{}_2}^{-}$), physically adequate trial function. Protons are assumed to be infinitely massive, fixed along the magnetic line. States with total spin projection $S_z=-3/2$ and magnetic quantum numbers $M=-3,-4,-5$ are studied. It is shown that for both $\mathrm{H}{}_3$ and $\mathrm{H}{{}_2}^{-}$, the lowest-energy state corresponds to $M=-3$ in the whole range of magnetic fields studied. As for a magnetic field $B\gtrsim {10}^{11}$ G, both $\mathrm{H}{}_3$ and $\mathrm{H}{{}_2}^{-}$ exist as metastable states, becoming stable for $B\ge 1.9\times {10}^{11}$ G and for $B\ge 2.7\times {10}^{11}$ G, respectively. The excited states ${}^4{(-4)}^{+}$, ${}^4{(-5)}^{+}$ of ${\mathrm{H}}_3$ and $\mathrm{H}{{}_2}^{-}$ appear at magnetic fields $B>7\times {10}^{11}$ and ${10}^{12}$ G, respectively.

We investigate the mechanism of formation of supermixed solitonlike states in bosonic binary mixtures loaded in ring lattices. We evidence the presence of a common pathway which, irrespective of the number of lattice sites and upon variation of the interspecies attraction, leads the system from a mixed and delocalized phase to a supermixed and localized one, passing through an intermediate phase where the supermixed soliton progressively emerges. The degrees of mixing, localization, and quantum correlation of the two condensed species, quantified by means of suitable indicators commonly used in statistical thermodynamics and quantum information theory, allow one to reconstruct a bidimensional mixing-supermixing phase diagram featuring two characteristic critical lines. Our analysis is developed both within a semiclassical approach capable of capturing the essential features of the two-step mixing-demixing transition and with a fully quantum approach.

We theoretically analyze the depletion dynamics of an ensemble of cold atoms in a quasi-one-dimensional optical lattice where atoms in one of the lattice sites are subject to decay. Unlike the previous studies of this problem in Labouvie et al., Phys. Rev. Lett. 116, 235302 (2016), we focus on the case where the system is brought to the chaotic regime, which crucially modifies the depletion dynamics as compared to the regular case. It is shown that depletion of the affected site results in gradual depletion of the neighboring sites according to the $t^{1/3}$ scaling law. We also show that by measuring occupations of the lattice sites one can extract important information on chaotic dynamics of the original conservative system.

We investigate harmonically trapped, laser-pumped bosons with infinite-range interactions induced by a dissipative high-finesse red-detuned optical cavity with numerical and analytical methods. We obtain multiple cavity and atomic observables as well as the full phase diagram of the system using the multiconfigurational time-dependent Hartree method for indistinguishable particles (MCTDH-X) approach. Besides the transition from an unorganized normal phase to a superradiant phase where the atoms self-organize, we focus on an in-depth investigation of the self-organized superfluid to self-organized Mott-insulator phase transition in the superradiant phase as a function of the cavity-atom coupling. The numerical results are substantiated by an analytical study of an effective Bose-Hubbard model. We numerically analyze cavity fluctuations and emergent strong correlations between atoms in the many-body state across the Mott transition via the atomic density distributions and Glauber correlation functions. Unexpectedly, the weak harmonic trap leads to features like a lattice switching between the two symmetry-broken ${\mathbb{Z}}_2$ configurations of the untrapped system and a reentrance of superfluidity in the Mott-insulating phase. Our analytical considerations quantitatively explain the numerically observed correlation features.