We theoretically demonstrate a scheme to produce elliptically polarized isolated attosecond pulses, based on the high-order harmonic generation from the current-carrying state of an atom exposed in linearly polarized laser fields. It is shown that the nonzero ellipticity of the attosecond pulse generated at the single-atom level is attributed to the nonvanishing angular momentum of the state, which is further elucidated by the strong-field approximation model. With specific carrier envelope phases (CEPs), isolated attosecond pulses with large ellipticity can be obtained. Moreover, the ellipticity of high-order harmonics remains almost invariable versus the CEP of the few-cycle driving field. Hence, the temporal profile and ellipticity of the attosecond-pulse radiation can be manipulated separately, which is beneficial to shape the attosecond-pulse radiation without compromising the ellipticity. The obtained elliptical attosecond pulse may serve as a potential tool to explore and manipulate the ultrafast dynamics in magnetic materials and chiral media.

We investigate the persistence, in time and space, of ferromagnetic domains in two dimensions subjected to the influence of both the static disorder of variable strength and weak interactions. The domains are represented by a two-species bosonic mixture of ${}^{87}\mathrm{Rb}$ ultracold atoms in different hyperfine states, such that initially one lies on the left half and the other on the right half of a square lattice. The dynamics of the double domain is followed by describing the two-component superfluid through the time-dependent Gross-Pitaevskii coupled equations, with values of the intra- and interspecies interaction that guarantee miscibility of the components. A robust analysis of the magnetization dynamics for several values of the interspecies interaction, reachable in current experimental setups, and the investigation of the density-weighted magnetization correlator lead us to conclude that the presence of structural disorder yields a slowdown the process of destruction of the initial ferromagnetic order. As shown by our numerical calculations, magnetization is maintained up to 50% of its initial value for the largest disorder amplitude considered.

We study the Fermi polaron problem of one mobile spin-up impurity immersed atop the bath consisting of spin-down fermions in one- and two-dimensional square lattices. We solve this problem by applying a variational approach with non-Gaussian states after separating the impurity and the background via the Lee-Low-Pines transformation. The ground state with a fixed total momentum can be obtained via imaginary-time evolution. For the one-dimensional case, the variational ground-state energy is compared with exact Bethe ansatz solutions and numerical density matrix renormalization-group results with excellent agreement. In two-dimensional lattices, we focus on the dilute limit, and find a polaron-molecule evolution consistent with previous results obtained by variational and quantum Monte Carlo methods for models in continuum space. Compared to previous works, our method provides the lowest ground-state energy in the entire parameter region considered, and has an apparent advantage as it does not need to assume a priori any specific form of the variational wave function.

We investigate the propagation of Gaussian beams through optical waveguide lattices characterized by correlated non-Hermitian disorder. In the framework of coupled mode theory, we demonstrate how the imaginary part of the refractive index needs to be adjusted to achieve perfect beam transmission, despite the presence of disorder. Remarkably, the effects of both diagonal and off-diagonal disorder in the waveguides and their couplings can be efficiently eliminated by our non-Hermitian design. Waveguide arrays thus provide an ideal platform for the experimental realization of non-Hermitian phenomena in the context of discrete photonics.

The dynamics of squeezing across a quantum phase transition in two basic models, viz., the one-axis twisting model in a transverse field and the Dicke model, is investigated using Holstein-Primakoff representation in the large spin limit. Near the phase boundary between the disordered (normal) and the ordered (superradiant) phases, the strength of spin and photon squeezing and the duration of time for which the system stays in the highly squeezed state are found to exhibit strong power-law growth with distance from the quantum critical point. The critical exponent for squeezing time is found to be 1/2 in both of the models, and for squeezing strength, it is shown to be 1/2 in the one-axis twisting model and 1 for the Dicke model, which in the limit of extreme detuning also becomes 1/2.

We address the problem of detecting bipartite Bell nonlocality whenever the only experimental information are the intensities produced in each run of the experiment by an unknown number of particles. We point out that this scenario naturally occurs in Bell experiments with parametric down-conversion when the crystal is pumped by strong pulses, in Bell tests with distant sources and in which particles suffer different delays during their flight, in Bell experiments using living cells as photo detectors, and in Bell experiments where the pairing information is physically removed. We show that, although Bell nonlocality decreases as the number of particles increases, if the parties can distinguish arbitrarily small differences of intensities and the visibility is larger than 0.98, then Bell nonlocality can still be experimentally detected with fluxes of up to 15 particles. We show that this prediction can be tested with current equipment in a Bell experiment where pairing information is physically removed, but requires the assumption of fair sampling.

We suggest a dynamical vector model of entanglement in a three-qubit system based on isomorphism between su(4) and so(6) Lie algebras. This model allows one to write an evolution equation for three-qubit entanglement parameters under an arbitrary pairwise qubit coupling. Generalizing a Plücker-type description of three-qubit local invariants we introduce three pairs of real-valued three-dimensional vectors (denoted here as ${\mathbf{A}}_{R,I}$, ${\mathbf{B}}_{R,I}$, and ${\mathbf{C}}_{R,I}$). Magnitudes of these vectors determine two- and three-qubit entanglement parameters of the system. We show that evolution of vectors $\mathbf{A}$, $\mathbf{B}$, and $\mathbf{C}$ under local SU(2) operations is identical to SO(3) evolution of single-qubit Bloch vectors of qubits $a$, $b$, and $c$ correspondingly. At the same time, general two-qubit su(4) Hamiltonians incorporating $a\text{−}b$, $a-c$, and $b\text{−}c$ two-qubit coupling terms generate SO(6) coupling between vectors $\mathbf{A}$ and $\mathbf{B}$, $\mathbf{A}$ and $\mathbf{C}$, and $\mathbf{B}$ and $\mathbf{C}$ correspondingly. It turns out that dynamics of entanglement induced by different two-qubit coupling terms is entirely determined by mutual orientation of vectors $\mathbf{A}$, $\mathbf{B}$, $\mathbf{C}$. We illustrate the power of this vector description of entanglement by solving quantum control problems involving transformations between $W$, Greenberg-Horne-Zeilinger, and biseparable states.

A parity violation experiment based on stimulated emission in atomic hydrogen is analyzed. The intensity of a laser field within an optical cavity provides the experimental signal. We find that such a measurement would offer several advantages compared to previous experiments based on microwave spectroscopy, including a well-defined interaction region and a short coherent interaction time.

Radiative and nonradiative decay rates of the $C{}^2{\mathrm{\Sigma }}_u^{+}$ electronic state of ${\mathrm{N}}_2^{+}$ are calculated as a function of the temperature. This makes it possible to study the effect of the temperature on the photoemission spectrum and on the ion yields $I$(${\mathrm{N}}^{+}$) and $I$(${\mathrm{N}}_2^{+}$). It is shown that the temperature has different effects on the isotopes ${}^{28}\mathrm{N}_2^{+}$, ${}^{29}\mathrm{N}_2^{+}$, and ${}^{30}\mathrm{N}_2^{+}$. Our results confirm that, regardless of the temperature, predissociation is the dominant relaxation mechanism for vibrational states ${\nu }^{\prime }⩾3$ of the ${}^{28}\mathrm{N}_2^{+}$ and ${}^{29}\mathrm{N}_2^{+}$ molecules.

We numerically investigate the quench expansion dynamics of an initially confined state in a two-dimensional Gross-Pitaevskii lattice in the presence of external disorder. The expansion dynamics is conveniently described in the control parameter space of the energy and norm densities. The expansion can slow down substantially if the expected final state is a nonergodic non-Gibbs one, regardless of the disorder strength. Likewise stronger disorder delays expansion. We compare our results with recent studies for quantum many-body quench experiments.

We demonstrate that an ultracold many-body bosonic ensemble confined in a one-dimensional double-well potential exhibits a population imbalance between the two wells at large timescales, when the depth of the wells is modulated by a time-dependent driving force. The specific form of the driving force is shown to break spatial parity and time-reversal symmetries, which leads to such an asymptotic population imbalance (API). The value of the API can be flexibly controlled by changing the phase of the driving force and the total number of particles. While the API is highly sensitive to the initial state in the few-particle regime, this dependence on the initial state is lost as we approach the classical limit of large particle numbers. We perform a Floquet analysis in the few-particle regime and an analysis based on a driven classical nonrigid pendulum in the many-particle regime. Although the obtained API values in the many-particle regime agree very well with those obtained in the classical limit, we show that there exists a significant disagreement in the corresponding real-time population imbalance due to quantum correlations.

We study the expansion of a cold, non-neutral ion plasma into the vacuum. The plasma is initialized by photoionization of cold rubidium atoms in a magneto-optical trap by an ultraviolet laser pulse. We employ time-delayed plasma extraction and imaging onto a position- and time-sensitive microchannel plate detector to analyze the plasma. We report on the formation and persistence of plasma shock shells, pair correlations in the plasma, and external-field-induced plasma focusing effects. We also develop trajectory and fluid descriptions to model the data and to gain further insight. The simulations verify the formation of shock shells and correlations, and allow us to model time- and position-dependent density, temperature, and Coulomb coupling parameter, $\mathrm{\Gamma }(\mathbf{r},t)$. The analysis also shows that the experimental plasma is strongly coupled and elucidates its expansion dynamics.

Based on the Richards-Wolf theory, it is strictly shown that in the sharp focus of a linearly polarized laser beam, the flux of a spin vector has only transverse components (the effect of photonic wheels or a photonic helicopter). For a linearly polarized optical vortex, the orbit-spin conversion leads to the appearance of both longitudinal and transverse components of the spin angular momentum (SAM) vector in the focus. We show that in the strong focus of a circularly polarized Gaussian beam, the longitudinal component of the SAM is maximal on the optical axis, with the longitudinal component of the orbital angular momentum (OAM) being maximal on a ring. In this way, the effect of SAM and OAM on the motion of a trapped microparticle can be evaluated separately. Spin-orbit conversion is experimentally demonstrated for a circularly polarized Gaussian beam when transverse energy flux (orbital angular momentum) arises in the focus, which is transmitted to a microparticle and causes it to rotate. Switching the handedness of circular polarization (from left to right) switches the direction of microparticle rotation. It is also shown here that an azimuthally polarized vortex beam with an arbitrary integer topological charge generates in the focus a SAM vector with only an axial component (pure magnetization), whereas there is no transverse spin flux.

Disordered media are often susceptible to optical damage even at low irradiation doses, since the corresponding microscopic structure may suffer from processes such as bleaching, chemical reactions, or morphological changes. The disorder often introduces linear and nonlinear (NL) light scattering, and currently it is a challenge to perform single-shot characterization of NL refraction, NL absorption, and NL scattering using the more common NL transmittance techniques. This work discusses how the scattered light imaging method and $\mathrm{D}4\sigma$ techniques can be combined to characterize the NL optical constants of thick turbid media. Therefore, it should help researchers aiming to draw a detailed physical picture of turbid samples at the single laser shot level.

We analyze and compare three different schemes that can be used to generate entanglement between spin qubits in optically active single solid-state quantum systems. Each scheme is based on first generating entanglement between the spin degree of freedom and the photon number, the time bin, or the polarization degree of freedom of photons emitted by the systems. We compute the time evolution of the entanglement generation process by decomposing the dynamics of a Markovian master equation into a set of propagation superoperators conditioned on the cumulative detector photon count. We then use the conditional density operator solutions to compute the efficiency and fidelity of the final spin-spin–entangled state while accounting for spin decoherence, optical pure dephasing, spectral diffusion, photon loss, phase errors, detector dark counts, and detector photon number resolution limitations. We find that the limit to fidelity for each scheme is restricted by the mean wave-packet overlap of photons from each source but that these bounds are different for each scheme. We also compare the performance of each scheme as a function of the distance between spin qubits.

Van der Waals forces between atoms and molecules are universally assumed to act along the line separating them. Inspired by recent works on effects which can propel atoms parallel to a macroscopic surface via the Casimir-Polder force, we predict a lateral van der Waals force between two atoms, one of which is in an excited state with nonzero angular momentum and the other is isotropic and in its ground state. The resulting force acts in the same way as a planetary gear, in contrast to the rack-and-pinion motion predicted in works on the lateral Casimir-Polder force in the analogous case, for which the force predicted here is the microscopic origin. We illustrate the effect by predicting the trajectories of an excited caesium in the vicinity of ground-state rubidium, finding behavior qualitatively different to that if lateral forces are ignored.

In any natural science, measurements are the essential link between theory and observable reality. Is it possible to obtain accurate and relevant information via measurement whose action on the probed system is unknown? In other words, can one be convinced to know something about the nature without knowing in detail how the information was obtained? In this paper, we show that the answer is, surprisingly, yes. We construct and experimentally implement a quantum optical probing measurement where measurements on the probes, i.e., the photons' polarization states, are used to extract information on the systems, i.e., the frequency spectra of the same photons. Unlike the preexisting probing protocols, our measurement does not require any knowledge of the interaction between the probe and the system.

High quality, fully programmable quantum processors are available with small numbers ($<1000$) of qubits, and the scientific potential of these near-term machines is not well understood. If the small number of physical qubits precludes practical quantum error correction, how can these error-susceptible processors be used to perform useful tasks? We present a strategy for developing quantum error detection for certain gate imperfections that utilizes additional internal states and does not require additional physical qubits. Examples for adding error detection are provided for a universal gate set in the trapped-ion platform. Error detection can be used to certify individual gate operations against certain errors, and the irreversible nature of the detection allows a result of a complex computation to be checked at the end for error flags.