We present a mixed-dimensional atomic gas system to unambiguously detect and systematically probe mediated interactions. In our scheme, fermionic atoms are confined in two parallel planes and interact via exchange of elementary excitations in a three-dimensional background gas. This interaction gives rise to a frequency shift of the out-of-phase dipole oscillations of the two clouds, which we calculate using a strong-coupling theory taking the two-body mixed-dimensional scattering into account exactly. The shift is shown to be easily measurable for strong interactions and can be used as a probe for mediated interactions.

We are interested in dynamics of quantum many-body systems under continuous observation, and its physical realizations involving cold atoms in lattices. In the present work we focus on continuous measurement of atomic currents in lattice models, including the Hubbard model. We describe a Cavity QED setup, where measurement of a homodyne current provides a faithful representation of the atomic current as a function of time. We employ the quantum optical description in terms of a diffusive stochastic Schrödinger equation to follow the time evolution of the atomic system conditional to observing a given homodyne current trajectory, thus accounting for the competition between the Hamiltonian evolution and measurement back action. As an illustration, we discuss minimal models of atomic dynamics and continuous current measurement on rings with synthetic gauge fields, involving both real space and synthetic dimension lattices (represented by internal atomic states). Finally, by “not reading” the current measurements the time evolution of the atomic system is governed by a master equation, where—depending on the microscopic details of our CQED setups—we effectively engineer a current coupling of our system to a quantum reservoir. This provides interesting scenarios of dissipative dynamics generating “dark” pure quantum many-body states.

We provide a description of spontaneous emission in a dispersive and dissipative linear inhomogeneous medium based on the generalized Huttner-Barnett model [Phys. Rev. A 46, 4306 (1992)]. Our discussion considers on an equal footing both the photonic and material fluctuations which are necessary to preserve unitarity of the quantum evolution. Within this approach we justify the results obtained in the past using the Langevin noise method that neglects the removal of photonic fluctuations. We finally discuss the concept of local density of states in a lossy and dispersive inhomogeneous environment that provides a basis for theoretical studies of fluorescent emitters near plasmonic and polaritonic antennas.

We discuss the collective modes in an alkaline-earth-metal Fermi gas close to an orbital Feshbach resonance. Unlike the usual Feshbach resonance, the orbital Feshbach resonance in alkaline-earth-metal atoms realizes a two-band superfluid system where the fermionic nature of both the open and the closed channel has to be taken into account. We show that, apart from the usual Anderson-Bogoliubov mode which corresponds to the oscillation of total density, there also appears the long-sought Leggett mode corresponding to the oscillation of relative density between the two channels. The existence of the phonon and the Leggett modes and their evolution are discussed in detail. We show how these collective modes are reflected in the density response of the system.

We study the phase diagram of a Rashba spin-orbit-coupled Bose-Einstein condensate confined in a two-dimensional toroidal trap. In the immiscible regime we find an azimuthally periodic density distribution, with the periodicity highly tunable as a function of the spin-orbit-coupling strength and which favors an odd number of petals in each component. This allows for a wide range of states that can be created. We further show that in the miscible regime, both components possess states with persistent flows with a unit winding number difference between them and with the absolute values of these winding numbers depending on the spin-orbit-coupling strength. All features of the odd-petal and the persistent flow states can be explained using a simple but effective model.

In practical applications, quantum systems work in a finite-temperature environment. Quantum entanglement is inevitably destroyed by the thermal noise. Here we experimentally demonstrate the protection of entanglement on the platform of linear optics. The finite-temperature thermal noise is described by the generalized amplitude-damping channel. By performing optimal pre- and postchannel weak measurements, we successfully protect the entanglement from the generalized amplitude-damping channel. Compared with the unprotected case, the quantum coherence is recovered, and the entanglement is enhanced. The demonstrated protocol may have further applications in superconducting quantum qubits.

In this article, we investigate the role of the self-interaction error in the simulation of collisions using time-dependent density functional theory (TDDFT) and Ehrenfest dynamics. We compare many different approximations of the exchange and correlation potential, using as a test system the collision of ${\mathrm{H}}^{+}+{\mathrm{CH}}_4$ at $30\mathrm{eV}$. We find that semilocal approximations, like the Perdew-Burke- Ernzerhof (PBE), and even hybrid functionals, such as the Becke, 3-parameter, Lee-Yang-Parr (B3LYP), produce qualitatively incorrect predictions for the scattering of the proton. This discrepancy appears because the self-interaction error allows the electrons to jump too easily to the proton, leading to radically different forces with respect to the non-self-interacting case. From our results, we conclude that using a functional that is self-interaction free is essential to properly describing charge-transfer collisions between ions and molecules in TDDFT.

Semiclassical open-orbit theory (OOT), previously developed to study photoionization of hydrogenic atoms in a uniform electric field [L. B. Zhao and J. B. Delos, Phys. Rev. A 81, 053417 (2010)], has been generalized to describe the propagation of outgoing electron waves to macroscopic distances from a nonhydrogenic atomic source. The generalized OOT has been applied to calculate spatial distributions of electron probability densities and current densities, produced due to photoionization for lithium in a uniform electric field. The obtained results are compared with those from the fully quantum-mechanical coupled-channel theory (CCT). The excellent agreement between the CCT and OOT confirms the reliability of the generalized OOT. Comparison is also made with theoretical calculations from the wave-packet propagation technique and the recent photoionization microscopy experiment. The existing difference between theory and experiment is discussed.

In the present work, we explore the existence, stability, and dynamics of single- and multiple-vortex-ring states that can arise in Bose-Einstein condensates. Earlier works have illustrated the bifurcation of such states in the vicinity of the linear limit for isotropic or anisotropic three-dimensional harmonic traps. Here, we extend these states to the regime of large chemical potentials, the so-called Thomas-Fermi limit, and explore their properties such as equilibrium radii and inter-ring distance for multi-ring states, as well as their vibrational spectra and possible instabilities. In this limit, both the existence and stability characteristics can be partially traced to a particle picture that considers the rings as individual particles oscillating within the trap and interacting pairwise with one another. Finally, we examine some representative instability scenarios of the multi-ring dynamics, including breakup and reconnections, as well as the transient formation of vortex lines.

We demonstrate experimentally that an electric field can be used to control quantum reflection of matter waves off periodically microstructured surfaces. Applying a voltage of alternating polarity between neighboring grating bars induces an electric field which modifies the interaction between the surface and the impinging matter wave so that quantum reflection is gradually reduced. We find that the measured reflectivities are in good agreement with our numerical simulations and that their suppression is well captured by a simple analytic model. This experiment constitutes a step towards electrically tuned reflective diffraction elements for atomic and molecular matter waves.

We present a general model, based on a Hamiltonian approach, for the joint quantum state of photon pairs generated through pulsed spontaneous four-wave mixing, including nonlinear phase modulation and a finite material response time. For the case of a silica fiber, it is found that the pair-production rate depends weakly on the waveguide temperature, due to higher-order Raman scattering events, and more strongly on pump-pair frequency detuning. From the analytical model, a numerical scheme is derived, based on the well-known split-step method. This scheme allows computation of joint states where nontrivial effects are included, such as group-velocity dispersion and Raman scattering. In this work, the numerical model is used to study the impact of the noninstantaneous response on the prefiltering purity of heralded single photons. We find that for pump pulses shorter than 1 ps, a significant detuning-dependent change in quantum-mechanical purity may be observed in silica. This shows that Raman scattering not only introduces noise, but can also drastically change the spectral correlations in photon pairs when pumped with short pulses.

We analyze theoretically the many-body dynamics of a dissipative Ising model in a transverse field using a variational approach. We find that the steady-state phase diagram is substantially modified compared to its equilibrium counterpart, including the appearance of a multicritical point belonging to a different universality class. Building on our variational analysis, we establish a field-theoretical treatment corresponding to a dissipative variant of a Ginzburg-Landau theory, which allows us to compute the upper critical dimension of the system. Finally, we present a possible experimental realization of the dissipative Ising model using ultracold Rydberg gases.

We study the dependence of the fidelity of the surface code in the presence of a single finite-temperature massless bosonic environment after a quantum error correction cycle. The three standard types of environment are considered: super-Ohmic, Ohmic, and sub-Ohmic. Our results show that, for regimes relevant to current experiments, quantum error correction works well even in the presence of environment-induced, long-range interqubit interactions. A threshold always exists at finite temperatures, although its temperature dependence is very sensitive to the type of environment. For the super-Ohmic case, the critical coupling constant separating high from low fidelity decreases with increasing temperature. For both Ohmic and super-Ohmic cases, the dependence of the critical coupling on temperature is weak. In all cases, the critical coupling is determined by microscopic parameters of the environment. For the sub-Ohmic case, it also depends strongly on the duration of the QEC cycle.

Detection of entanglement in quantum networks consisting of many parties is one of the important steps toward building quantum communication and computation networks. We consider a scenario where the measurement devices used for this certification are uncharacterized. In this case, it is well known that by using quantum states as inputs for the measurement devices it is possible to detect any entangled state (a situation known as measurement-device-independent entanglement witnessing). Here we go beyond entanglement detection and provide methods to estimate the amount of entanglement in a quantum network. We also consider the task of randomness certification and show that randomness can be certified in a variety of cases, including single-partite experiments or setups using only separable states.

Quantum search on the two-dimensional lattice with one marked vertex and cyclic boundary conditions is an important problem in the context of quantum algorithms with an interesting unfolding. It avails to test the ability of quantum walk models to provide efficient algorithms from the theoretical side and means to implement quantum walks in laboratories from the practical side. In this paper, we rigorously prove that the recent-proposed staggered quantum walk model provides an efficient quantum search on the two-dimensional lattice, if the reflection operators associated with the graph tessellations are used as Hamiltonians, which is an important theoretical result for validating the staggered model with Hamiltonians. Numerical results show that on the two-dimensional lattice staggered models without Hamiltonians are not as efficient as the one described in this paper and are, in fact, as slow as classical random-walk-based algorithms.

Rydberg molecules, often exemplified by long-range “trilobite” molecules, are a subject of much recent interest at high principal quantum number $n$. States that use the same bonding mechanism can exist at much lower $n$ and less-extreme internuclear separations that are still quite long range. We use a high-resolution pulsed amplifier to study previously detected transitions to a low-$n$ Rydberg molecular state near the $5s+7p$ asymptote in ${}^{85}\mathrm{Rb}_2$. The observed line structure is modeled using precise ground-state positions and a lower bound of $\tau >1.1\times {10}^{-9}$ s is set on the excited-state autoionization lifetime.

We study the isotope effect on high-order-harmonic generation (HHG) of vibrating ${{\mathrm{H}}_2}^{+}$ and ${{\mathrm{D}}_2}^{+}$ molecular ions aligned parallel to the polarization of the laser field. The time-dependent Schrödinger equation is solved accurately and efficiently by the time-dependent generalized pseudospectral and Fourier grid methods in three spatial coordinates, one of them being the internuclear separation and the other two describing the electronic motion. The laser pulses have a carrier wavelength of 800 nm and duration of 10 or 16 optical cycles. The peak intensities used in the calculations are $2\times {10}^{14}$ and $3\times {10}^{14}\text{W}/{\mathrm{cm}}^2$. The effect of nuclear vibration is visible in both ${{\mathrm{H}}_2}^{+}$ and ${{\mathrm{D}}_2}^{+}$ but more pronounced in the lighter ${{\mathrm{H}}_2}^{+}$ molecule. Striking differences from the fixed nuclei case are a total disappearance of the traditional plateau in the HHG spectrum at the higher intensity and significant redshift of the harmonic peaks in the central part of the spectrum. These phenomena are explained based on the analysis of the dynamics of the nuclear vibrational wave packet.

The photoelectron emission time delay $\tau$ associated with one-photon absorption, which coincides with half the Wigner delay ${\tau }_{\mathrm{W}}$ experienced by an electron scattered off the ionic potential, is a fundamental descriptor of the photoelectric effect. Although it is hard to access directly from experiment, it is possible to infer it from the time delay of two-photon transitions, ${\tau }^{\left(2\right)}$, measured with attosecond pump-probe schemes, provided that the contribution of the probe stage can be factored out. In the absence of resonances, $\tau$ can be expressed as the energy derivative of the one-photon ionization amplitude phase, $\tau ={\partial }_{E}arg{D}_{Eg}$, and, to a good approximation, $\tau ={\tau }^{\left(2\right)}-{\tau }_{\mathrm{cc}}$, where ${\tau }_{\mathrm{cc}}$ is associated with the dipole transition between Coulomb functions. Here we show that, in the presence of a resonance, the correspondence between $\tau$ and ${\partial }_{E}arg{D}_{Eg}$ is lost. Furthermore, while ${\tau }^{\left(2\right)}$ can still be written as the energy derivative of the two-photon ionization amplitude phase, ${\partial }_{E}arg{D}_{Eg}^{\left(2\right)}$, it does not have any scattering counterpart. Indeed, ${\tau }^{\left(2\right)}$ can be much larger than the lifetime of an intermediate resonance in the two-photon process or more negative than the lower bound imposed on scattering delays by causality. Finally, we show that ${\tau }^{\left(2\right)}$ is controlled by the frequency of the probe pulse, ${\omega }_{\mathrm{IR}}$, so that by varying ${\omega }_{\mathrm{IR}}$, it is possible to radically alter the photoelectron group delay.

We used extreme ultraviolet (XUV) transient absorption spectroscopy to study the autoionizing Rydberg states of oxygen in an electronically- and vibrationally-resolved fashion. XUV pulse initiates molecular polarization and near-infrared pulse perturbs its evolution. Transient absorption spectra show positive optical-density (OD) change in the case of $ns{\sigma }_g$ and $nd{\pi }_g$ autoionizing states of oxygen and negative OD change for $nd{\sigma }_g$ states. Multiconfiguration time-dependent Hartree-Fock (MCTDHF) calculations are used to simulate the transient absorption and the resulting spectra and temporal evolution agree with experimental observations. We model the effect of near-infrared perturbation on molecular polarization and find that the laser-induced phase-shift model agrees with the experimental and MCTDHF results, while the laser-induced attenuation model does not. We relate the electronic-state-symmetry-dependent sign of the OD change to the Fano parameters of the static absorption line shapes.

The phenomenon of interference plays a crucial role in the field of precision measurement science. Wave-particle duality has expanded the well-known interference effects of electromagnetic waves to massive particles. The majority of the wave-particle interference experiments require a near monochromatic beam which limits its applications due to the resulting low intensity. Here we demonstrate white beam interference in the far-field regime using a two-phase-grating neutron interferometer and its application to phase-contrast imaging. The functionality of this interferometer is based on the universal moiré effect that allows us to improve upon the standard Lau setup. Interference fringes were observed with monochromatic and polychromatic neutron beams for both continuous and pulsed beams. Far-field neutron interferometry allows for the full utilization of intense neutron sources for precision measurements of gradient fields. It also overcomes the alignment, stability, and fabrication challenges associated with the more familiar perfect-crystal neutron interferometer, as well as avoids the loss of intensity due to the absorption analyzer grating requirement in Talbot-Lau interferometer.