We investigate the effects of many-body correlations on the ground-state properties of a single-component ultracold Rydberg-dressed Fermi liquid with purely repulsive interparticle interactions in both three and two spatial dimensions. We employed the Fermi-hypernetted-chain Euler-Lagrange approximation and observed that the contribution of the correlation energy on the ground-state energy becomes significant at intermediate values of the soft-core radius and large coupling strengths. For small and large soft-core radii, the correlation energy is negligible and the ground-state energy approaches the Hartree-Fock value. The positions of the main peaks in static structure factor and pair distribution function in the homogeneous fluid phase signal the formation of quantum droplet crystals with several particles confined inside each droplet.

We theoretically investigate superradiance effects in quantum field theories in curved space-times by proposing an analog model based on Bose-Einstein condensates subject to a synthetic vector potential. The breaking of the irrotationality constraint of superfluids allows us to study superradiance in simple planar geometries and obtain intuitive insight into the amplified scattering processes at ergosurfaces. When boundary conditions are modified to allow for reflections, dynamical instabilities are found, similar to the ones of ergoregions in rotating space-times. Their stabilization by horizons in black hole geometries is discussed. All these phenomena are reinterpreted through an exact mapping with the physics of one-dimensional relativistic charged scalar fields in electrostatic potentials. Our study provides a deeper understanding of the basic mechanisms of superradiance: By disentangling the different ingredients at play, it shines light on some misconceptions on the role of dissipation and horizons and on the competition between superradiant scattering and instabilities.

Recent experiments with ultracold quantum gases have successfully realized integer-quantized topological charge pumping in optical lattices. Motivated by this progress, we study the effects of static disorder on topological Thouless charge pumping. We focus on the half-filled Rice-Mele model of free spinless fermions and consider random diagonal disorder. We use both static and time-dependent simulations to characterize the charge pump. The set of measures that we compute in the instantaneous basis include the polarization, the entanglement spectrum, and the space-integrated local Chern marker. As a first main result, we conclude that the space-integrated local Chern marker is best suited for a quantitative determination of topological transitions in a disordered system. In the time-dependent simulations we use the time-integrated current to obtain the pumped charge in slowly periodically driven systems. As a second main result, we observe and characterize a disorder-driven breakdown of the quantized charge pump. There is an excellent agreement between the static and the time-dependent ways of computing the pumped charge. The topological transition occurs well in the regime where all states are localized on the given system sizes. This observation is consistent with previous studies and the topological transition is therefore not tied to a delocalization-localization transition of Hamiltonian eigenstates. For individual disorder realizations, the breakdown of the quantized pumping occurs for parameters where the spectral bulk gap inherited from the band gap of the clean system closes, leading to a globally gapless spectrum. As a third main result and with respect to the analysis of finite-size systems, we show that the disorder average of the bulk gap severely overestimates the stability of quantized pumping. A much better estimate is the typical value of the distribution of energy gaps, also called mode of the distribution. We discuss our results in the context of recent quantum-gas experiments that realized charge pumps.

For a periodic structure sandwiched between two homogeneous media, a bound state in the continuum (BIC) is a guided Bloch mode with a frequency in the radiation continuum. Optical BICs have found many applications, mainly because they give rise to resonances with ultrahigh-quality factors. If the periodic structure has a relevant symmetry, a BIC may have a symmetry mismatch with incoming and outgoing propagating waves of the same frequency and compatible wave vectors and is considered as protected by symmetry. Propagating BICs with a nonzero Bloch wave vector have been found in many highly symmetric periodic structures. They are not protected by symmetry in the usual sense (i.e., there is no symmetry mismatch), but some of them seem to depend on symmetry for their existence and robustness. In this paper we show that the low-frequency propagating BICs (with only one radiation channel) in two-dimensionally periodic structures with an inversion symmetry in the plane of periodicity and a reflection symmetry in the perpendicular direction are robust to symmetry-preserving structural perturbations. In other words, a propagating BIC continues its existence with a slightly different frequency and a slightly different Bloch wave vector when the periodic structure is perturbed slightly preserving the inversion and reflection symmetries. Our study enhances theoretical understanding for BICs in periodic structures and provides useful guidelines for their applications.

Quantum conversion or interface is one of the most prominent protocols in quantum information processing and quantum state engineering. We propose a photon-phonon conversion protocol in a hybrid magnomechanical system comprising a microwave optical mode, a driven magnon mode, and a mechanical-vibrating mode, which has attracted much interest and is expected to become a building block of the future quantum information network due to its controllability in coupling strengths. The microwave photons in the optical cavity are coupled to the magnons by the Zeeman interaction, and the latter are coupled to the mechanical phonons by the magnetostrictive interaction. With a strong photon-magnon interaction and a strong driving on the magnon, an effective Hamiltonian is constructed to describe the conversion between photons and phonons near their resonant point. The cavity-magnon system can then play the role of quantum memory. Moreover, the faithfulness of the photon-phonon conversion is estimated in terms of fidelities for state evolution and state-independent transfer. The former is discussed in the Lindblad master equation, taking account of the leakages of photons, phonons, and magnons into consideration. The latter is derived by the Heisenberg-Langevin equation considering the non-Markovian noise from the structured environments for both optical and mechanical modes. The state-evolution fidelity is found to be robust to the weak leakage. The transfer fidelity can be maintained by the Ohmic and sub-Ohmic environments of the photons and is insensitive to the $1/f$ noise of the phonons. Our work thus provides an interesting application for the magnon system as a photon-phonon converter in the microwave regime.

We construct inequalities between Rényi $\alpha$-entropy and the indexes of coincidence of probability distributions, based on which we obtain improved state-dependent entropic uncertainty relations for general symmetric informationally complete positive operator-valued measures (SIC-POVMs) and mutually unbiased measurements (MUMs). We show that our uncertainty relations for general SIC-POVMs and MUMs can be tight for sufficiently mixed states, and, moreover, comparisons to the numerically optimal results are made via information diagrams.

We investigate the capability of attosecond transient absorption spectroscopy to characterize the dynamics of inner-shell-excited systems. In the transient absorption spectroscopy setup considered, wave packets are prepared by an attosecond XUV pulse and probed by a femtosecond NIR pulse. By using this, we study coherent electron dynamics in core-excited xenon atoms. In particular, we clarify which aspects of the dynamics can be revealed when the wave packets are probed using an NIR pulse and analyze why the inner-shell hole dynamics is more difficult to probe than the dynamics of the excited electron. We perform a theoretical analysis of the transient absorption signal as a function of the time delay between the XUV pump and NIR probe pulses, treating the excitation pulse perturbatively and the probe pulse nonperturbatively. We also demonstrate that an additional NIR dressing field can dramatically influence the transient absorption spectrum. Our theoretical predictions are compared with experimental results, suggesting that a precise characterization of the NIR pulse is necessary for a qualitative and quantitative comparison.

We investigate local quantum field theories for one-dimensional (1D) Bose and Fermi gases with contact interactions, which are closely connected with each other by Girardeau's Bose-Fermi mapping. While the Lagrangian for bosons includes only a two-body interaction, a marginally relevant three-body interaction term is found to be necessary for fermions. Because of this three-body coupling, the three-body contact characterizing a local triad correlation appears in the energy relation for fermions, which is one of the sum rules for a momentum distribution. In addition, we apply in both systems the operator product expansion to derive large-energy and momentum asymptotics of a dynamic structure factor and a single-particle spectral density. These behaviors are universal in the sense that they hold for any 1D scattering length at any temperature. The asymptotics for the Tonks-Girardeau gas, which is a Bose gas with a hardcore repulsion, as well as the Bose-Fermi correspondence in the presence of three-body attractions are also discussed.

Creation of entangled photon pairs is one of the key topics in contemporary quantum optics. In the present article, we theoretically describe the generation of photon pairs in the process of spontaneous parametric down-conversion of light by a resonant spherical nanoparticle made of a dielectric material with bulk ${\widehat{\chi }}^{\left(2\right)}$ nonlinearity. We consider the nanoparticle size that satisfies the condition of excitation of resonant eigenmodes described by Mie theory. We demonstrate that a highly directional photon pair generation can be observed utilizing the nonlinear Kerker-type effect, and that this regime provides useful polarization correlations between the emitted photons.

Numerical simulations are frequently required for quantum scattering problems and often face difficulties with finite grids and unwanted, unphysical reflections. For decades, improved complex absorbing potentials (CAPs) have been sought. Today, the rise of ultracold physics makes a solution essential as CAP errors increase at lower energies. We present a method that provides a physical, semiclassical picture of how to improve CAPs based on the behavior of classical trajectories. The method does not rely on the mathematical formalism often required by existing methods and reduces the error associated with CAP-based calculation of the low-energy scattering wavefunctions by up to several orders of magnitude relative to the standard Woods-Saxon approach, as demonstrated via the distorted-wave Born approximation. This indicates the method may be applied to the numerical simulation of collisions in the ultracold regime.

We investigate wave-packet dynamics across supercritical barriers for the Klein-Gordon and Dirac equations. Our treatment is based on a multiple scattering expansion (MSE). For spin-0 particles, the MSE diverges, rendering invalid the use of the usual connection formulas for the scattering basis functions. In a time-dependent formulation, the divergent character of the MSE naturally accounts for charge creation at the barrier boundaries. In the Dirac case, the MSE converges and no charge is created. We show that this time-dependent charge behavior dynamics can adequately explain the Klein paradox in a first-quantized setting. We further compare our semianalytical wave-packet approach to exact finite-difference solutions of the relativistic wave equations.

We consider Markovian dynamics of a finitely dimensional open quantum system featuring a weak unitary symmetry, i.e., when the action of a unitary symmetry on the space of density matrices commutes with the master operator governing the dynamics. We show how to encode the weak symmetry in quantum stochastic dynamics of the system by constructing a weakly symmetric representation of the master operator: a symmetric Hamiltonian, and jump operators connecting only the symmetry eigenspaces with a fixed eigenvalue ratio. In turn, this representation simplifies both the construction of the master operator as well as quantum jump Monte Carlo simulations, where, for a symmetric initial state, stochastic trajectories of the system state are supported within a single symmetry eigenspace at a time, which is changed only by the action of an asymmetric jump operator. Our results generalize directly to the case of multiple Abelian weak symmetries.

We develop a protocol for continuous operation of a quantum error correcting code for protection of coherent evolution due to an encoded Hamiltonian against environmental errors, using the three-qubit bit-flip code and bit-flip errors as a canonical example. To detect errors in real time, we filter the output signals from continuous measurement of the error syndrome operators and use a double thresholding protocol for error diagnosis, while correction of errors is done as in the conventional operation. We optimize our continuous operation protocol for evolution under quantum memory and under quantum annealing, by maximizing the fidelity between the target and actual logical states at a specified final time. In the case of quantum memory, we show that our continuous operation protocol yields a logical error rate that is slightly larger than the one obtained from using the optimal Wonham filter for error diagnosis while being simpler to implement. For quantum annealing, we show that our continuous quantum error correction protocol can significantly reduce the final logical state infidelity when the continuous measurements are sufficiently strong relative to the strength of the time-dependent Hamiltonian. We also show that this continuous quantum error correction protocol can reduce the time-to-solution relative to the value obtained from a classical parallelization scheme. These results suggest that a continuous implementation is suitable for quantum error correction in the presence of encoded time-dependent Hamiltonians, opening the possibility of many applications in quantum simulation and quantum annealing.

We study single ionization of a heteroatomic system by charged projectiles whose velocity $v$ approaches the speed of light $c$. The system is formed by two loosely bound atomic species, $A$ and $B$, with the ionization potential of $A$ being smaller than excitation energy for a dipole-allowed transition in $B$. In such a case, three single ionization channels occur: (i) single-center ionization of atom $A$, (ii) single-center ionization of atom $B$, and (iii) two-center ionization of $A$. While (i) and (ii) are the well known mechanism of direct impact ionization of a single atom, in channel (iii) ionization of $A$ proceeds via impact excitation of $B$ with consequent radiationless transfer of excitation energy—via (long-range) two-center electron-electron correlations—to $A$, leading to its ionization. We show that, close to the resonance energy, the two-center channel (iii) is so enormously strong that its contribution remains dominant even if the range of emission energies $\sim 1$ eV, which is orders of magnitude broader than its width, is considered. The influence of relativistic effects, caused by a high collision velocity, on the angular distribution of emitted electrons may be quite strong even at $\gamma =1/\sqrt{1-v^2/c^2}\approx 2$. However, in the energy distribution and the total cross section, these effects become substantial only at $\gamma \gg 1$. Relativistic effects arising due to a large size of the two-atomic system are shown to be very weak even for a ${}^7\mathrm{Li}\text{−}\mathrm{He}$ dimer whose mean size is about 28 Å.

We report new measurements of the positronium (Ps) $2{}^{3}S_1\to 2{}^{3}P_J$ fine-structure intervals, ${\nu }_J$ ($J=0,1,2$). In the experiments, Ps atoms, optically excited to the radiatively metastable $2{}^{3}S_1$ level, flew through microwave radiation fields tuned to drive transitions to the short-lived $2{}^{3}P_J$ levels, which were detected via the time spectrum of subsequent ground-state Ps annihilation radiation. Both the ${\nu }_1$ and ${\nu }_2$ line shapes were found to be asymmetric, which, in the absence of a complete line-shape model, prevents accurate determination of these fine-structure intervals. Conversely, the ${\nu }_0$ line shape did not exhibit any significant asymmetry; the observed interval, however, was found to disagree with QED theory by 4.2 standard deviations.

Precision measurements of quantum systems often seek to probe or must account for the interaction with blackbody radiation. Over the past several decades, much attention has been given to ac Stark shifts and stimulated state transfer. For a blackbody in thermodynamic equilibrium, these two effects are determined by the expectation value of photon number in each mode of the Planck spectrum. Here we explore how the photon number variance of an equilibrium blackbody generally leads to a parametric broadening of the energy levels of quantum systems that is inversely proportional to the square root of the blackbody volume. We consider the effect in two cases which are potentially highly sensitive to this broadening: Rydberg atoms and atomic clocks. We find that even in blackbody volumes as small as 1 ${\mathrm{cm}}^3$, this effect is unlikely to contribute meaningfully to transition linewidths.

We propose a self-consistent model which utilizes the polarization vector to theoretically describe the evolution of spin polarization of relativistic electrons in an intense electromagnetic field. The variation of radiative polarization due to instantaneous no photon emission is introduced into our model, which extends the applicability of the polarization vector model derived from the nonlinear Compton scattering under local constant crossed-field approximation to the complex electromagnetic environment in laser plasma interaction. According to this model, we develop a Monte Carlo method to simulate the electron spin under the influence of radiation and precession simultaneously. Our model is consistent with the quantum physical picture that spin can only be described by a probability distribution before measurement, and it contains the entire information on the spin. The correctness of our model is confirmed by the successful reproduction of the Sokolov-Ternov effect and the comparison of the simulation results with other models in the literature. The results show the superiority in accuracy, applicability, and computational efficiency of our model, and we believe that our model is a better choice to deal with the electron spin in particle-in-cell simulation for laser plasma interaction.

We study the reconstruction of a wave packet and the corresponding electron dynamics in an atom via photoelectron angular distributions (PADs) in a pump-probe scheme as a function of time delay. The method is applied to the superposition of ground and one or two excited states in helium atom representing field-free charge migrations on the attosecond timescale in form of ring currents around the core. It is based on the interference between one- and two-photon transitions from ground and excited states into the continuum. In the reconstruction predictions of first- and second-order perturbation theory are used to determine the unknown phases and amplitudes from the PADs, which we simulate via solutions of the time-dependent Schrödinger equation in single-active-electron approximation. Results of calculations show that the reconstruction technique works well for peak laser intensities less than ${10}^{13}$ $\mathrm{W}/{\mathrm{cm}}^2$. Knowledge of the electric field of the probe pulse is required with shot-to-shot variations of carrier-to-envelope phase and peak intensity of up to 10% and 20%, respectively. The relevance of different one- and two-photon pathways for the reconstruction as a function of peak intensity and pulse duration is analyzed—specifically their role for ultrashort probe pulses with broad bandwidths.

We have derived the complete formula for the ${\alpha }^{7}m$ contribution to energy levels of an arbitrary triplet state of the helium atom, performed numerical calculations for the $2{}^{3}S$ and $2{}^{3}P$ states, and thus improved the theoretical accuracy of ionization energies of these states by more than an order of magnitude. Using the nuclear charge radius extracted from the muonic helium Lamb shift, we obtain the theoretical prediction in excellent agreement with the measured $2{}^{3}S-2{}^{3}P$ transition energy [X. Zheng et al., Phys. Rev. Lett. 119, 263002 (2017)]. At the same time, we observe significant discrepancies with experiments for the $2{}^{3}S-3{}^{3}D$ and $2{}^{3}P-3{}^{3}D$ transitions.