We report on a method to generate extreme ultraviolet vortices from high-order harmonic generation with two-color counter-rotating Laguerre-Gaussian ($\text{LG}$) beams that carry a well-defined orbital angular momentum (OAM). Our calculations show that the OAM of each harmonic can be directly controlled by the OAM of the incident $\text{LG}$ modes. Furthermore, we show how the incoming $\text{LG}$ modes have to be tailored, in order to generate every possible value of OAM in the emitted harmonics. In addition, we analyze the emitted harmonics with respect to their divergence and find that it decreases with the harmonic order and increases with the OAM of the emitted harmonic.

We rigorously investigate the quantum non-Markovian dissipative dynamics of a system coupled to a harmonic-oscillator bath by deriving hierarchical Schrödinger equations of motion (HSEOM) and studying their dynamics. The HSEOM are the equations for wave functions derived on the basis of the Feynman-Vernon influence functional formalism for the density operator, $\langle q\left|\rho \left(t\right)\right|q^{\prime }\rangle$, where $\langle q|$ and $|q^{\prime }\rangle$ are the left- and right-hand elements. The time evolution of $\langle q|$ is computed from time 0 to $t$, and, subsequently, the time evolution of $|q^{\prime }\rangle$ is computed from time $t$ to 0 along a contour in the complex time plane. By appropriately choosing functions for the bath correlation function and the spectral density, we can take advantage of an HSEOM method to carry out simulations without the need for a great amount of computational memory. As a demonstration, quantum annealing simulation for a ferromagnetic $p$-spin model is studied.

We numerically explore molecular orientation dynamics with moderately intense nanosecond two-color laser pulses. It is believed that the nanosecond two-color pulse can adiabatically control the molecular orientation. However, in our simulation based on the time-dependent Schrödinger equation, which naturally includes nonadiabatic effects, the orientation dynamics shows clear deviations from the adiabatic approximation (AA) results, while the molecular alignment dynamics is in good agreement with the AA results. The nonadiabaticity is significantly influenced by three parameters, the intensities, and the relative delay of the two wavelengths. In this work, we clarify the reason behind the nonadiabaticity and provide the solutions for achieving higher degrees of orientation.

We study the anisotropic, elliptic expansion of a thermal atomic Bose gas released from an anisotropic trapping potential, for a wide range of interaction strengths across a Feshbach resonance. We show that this hydrodynamic phenomenon is for all interaction strengths fully described by a microscopic kinetic model with no free parameters. The success of this description crucially relies on taking into account the reduced thermalizing power of elastic collisions in a strongly interacting gas, for which we derive an analytical theory. We also perform time-resolved measurements that directly reveal the dynamics of the energy transfer between the different expansion axes.

An essential ingredient in many examples of the conflict between quantum theory and noncontextual hidden variables (e.g., the proof of the Kochen-Specker theorem and Hardy's proof of Bell's theorem) is a set of atomic propositions about the outcomes of ideal measurements such that, when outcome noncontextuality is assumed, if proposition $A$ is true, then, due to exclusiveness and completeness, a nonexclusive proposition $B$ ($C$) must be false (true). We call such a set a true-implies-false set (TIFS) [true-implies-true set (TITS)]. Here we identify all the minimal TIFSs and TITSs in every dimension $d\ge 3$, i.e., the sets of each type having the smallest number of propositions. These sets are important because each of them leads to a proof of impossibility of noncontextual hidden variables and corresponds to a simple situation with quantum vs classical advantage. Moreover, the methods developed to identify them may be helpful to solve some open problems regarding minimal Kochen-Specker sets.

While entanglement plays an important role in characterizing quantum many-body systems, it is hardly possible to directly access many-body entanglement in real experiments. In this paper, we study how bipartite entanglement of many-body states is manifested in the correlation of local measurement outcomes. In particular, we consider a measure of correlation defined as the statistical distance between the joint probability distribution of local measurement outcomes and the product of its marginal distributions. Various bounds of this measure are obtained and several examples of many-body states are considered as a testbed for the measure. We also generalize the framework to the case of imprecise measurement and argue that the considered measure is related to the concept of quantum macroscopicity.

The dynamics of an open quantum system can be fully described and tomographically reconstructed if the experimenter has complete control over the system of interest. Most real-world experiments do not fulfill this assumption, and the amount of control is restricted by the respective experimental setup. That is, the set of performable manipulations of the system is limited. In this paper, we provide a general reconstruction scheme that yields an operationally well-defined description of non-Markovian quantum dynamics for a large class of realistic experimental situations. The resultant “restricted” descriptor for the process, surprisingly, does not have the property of complete positivity. Based on these restricted dynamics, we construct witnesses for correlations and the presence of memory effects. We demonstrate the applicability of our framework for the two important cases where the set of performable operations comprises only unitary operations or projective measurements, respectively, and show that it provides a powerful tool for the description of quantum control experiments.

We propose a protocol for state transfer and entanglement generation between two distant spin qubits (sender and receiver) that have different energies. The two qubits are permanently coupled to a far-off-resonant spin chain, and the qubit of the sender is driven by an external field, which provides the energy required to bridge the energy gap between the sender and the receiver. State transfer and entanglement generation are achieved via virtual single-photon and multiphoton transitions to the eigenmodes of the channel.

Quantum mechanics puts a restriction on the number of observers who can simultaneously steer another observer's system, known as the monogamy of steering. In this work, we find the limit of the number of observers (Bobs) who can steer another party's (Alice's) system, invoking a scenario where half of an entangled pair is shared between a single Alice in one wing and several Bobs on the other wing, who act sequentially and independently of each other. When all the observers measure two dichotomic observables, we find that two Bobs can steer Alice's system, going beyond the monogamy restriction. We further show that three Bobs can steer Alice's system, considering a three-settings linear steering inequality, and then conjecture that, at most, $n$ Bobs can demonstrate steering of Alice's system when steering is probed through an $n$-settings linear steering inequality.

A joint experimental and theoretical investigation of the valence-shell excitations of hydrogen deuteride has been performed by the fast electron impact method at an incident electron energy of 1500 eV and the multireference single- and double-excitation configuration-interaction method. Momentum-transfer-dependent inelastic squared form factors for the vibronic states belonging to the $B{}^1{{\mathrm{\Sigma }}_u}^{+}$, $C{}^1{\mathrm{\Pi }}_u$, and $EF{}^1{{\mathrm{\Sigma }}_g}^{+}$ of molecular hydrogen deuteride have been derived with a high-energy resolution of 70 meV. Similar to the results of molecular hydrogen [L. Q. Xu, K. Xu, Y. G. Peng, X. Xu, Y. W. Liu, Y. Wu, K. Yang, N. Hiraoka, K. D. Tsuei, J. G. Wang, and L. F. Zhu, Phys. Rev. A 97, 032503 (2018)], the present calculations do not satisfactorily reproduce the profiles of the inelastic squared form factors for the higher vibronic states of the $B{}^1{{\mathrm{\Sigma }}_u}^{+}$ of hydrogen deuteride. However, the discrepancies between the experimental and calculated results of the $B{}^1{{\mathrm{\Sigma }}_u}^{+}$ of hydrogen deuteride show a weaker dependence on the vibronic number than those of molecular hydrogen, i.e., the present experimental results for the $B{}^1{{\mathrm{\Sigma }}_u}^{+}$ of hydrogen deuteride are in better agreement with the calculated ones than those of the molecular hydrogen. It is very likely that the electronic-vibrational coupling effect in hydrogen deuteride is less important than that in molecular hydrogen. For the $C{}^1{\mathrm{\Pi }}_u$ state and some vibronic excitations of the $EF{}^1{{\mathrm{\Sigma }}_g}^{+}$ state, the present experimental results are lower than the present calculations, which may be due to the fact that the first Born approximation does not hold at an incident electron energy of 1500 eV.

Ion-beam cancer therapy has become increasingly favored worldwide in treatment of certain types of cancer over the last decade. Whereas the clinical effects of the therapy are well documented, the understanding of the underlying physical mechanisms is somewhat incomplete. The problem arises due to the multiscale nature of the effects involved in ion-beam cancer therapy, as the effects range from quantum-mechanical to macroscopic scales. The present study investigates the production of free electrons in the vicinity of the Bragg peak through quantum-mechanical simulations of the collision between a ${\mathrm{C}}^{4+}$ ion with a cytosine-guanine nucleotide pair taken from a DNA double helix. Time-dependent density-functional theory was employed using the octopus 6.0 software. The results show that such a collision triggers the release of a large amount of electrons into the cellular environment, as only a fraction is captured by the ${\mathrm{C}}^{4+}$ ion. Furthermore, it is demonstrated that the impact angle and projectile kinetic energy have much more influence on the number of ejected electrons than the impact parameter.

Fully differential cross sections for electron impact of gaseous ${\mathrm{H}}_2\mathrm{O}$ at an impact energy of 81 eV are calculated within the framework of the continuum-distorted-wave eikonal-initial-state model. Present results are benchmarked against recent experimental data obtained in MPIK-Heidelberg by means of a reaction microscope. Cross sections calculated for different molecular orientations are properly averaged to mimic the experimental conditions. Present results are in very good agreement with the experimental data for coplanar geometries. For the denominated perpendicular emission planes the agreement tends to deteriorate as the projectile momentum transfer increases.

We analyze the energy spectrum and eigenstates of cold atoms in a tilted brick-wall optical lattice. When the tilt is applied, the system exhibits a sequence of topological phase transitions reflected in an abrupt change of the eigenstates. It is demonstrated that these topological phase transitions can be easily detected in a laboratory experiment by observing Bloch oscillations of cold atoms.

Rapidly scanning magnetic and optical dipole traps have been widely utilized to form time-averaged potentials for ultracold quantum gas experiments. Here we theoretically and experimentally characterize the dynamic properties of Bose-Einstein condensates in ring-shaped potentials that are formed by scanning an optical dipole beam in a circular trajectory. We find that unidirectional scanning leads to a nontrivial phase profile of the condensate that can be approximated analytically using the concept of phase imprinting. While the phase profile is not accessible through in-trap imaging, time-of-flight expansion manifests clear density signatures of an in-trap phase step in the condensate, coincident with the instantaneous position of the scanning beam. The phase step remains significant even when scanning the beam at frequencies 2 orders of magnitude larger than the characteristic frequency of the trap. We map out the phase and density properties of the condensate in the scanning trap, both experimentally and using numerical simulations, and find excellent agreement. Furthermore, we demonstrate that bidirectional scanning flattens the phase profile, rendering the system more suitable for coherent matter-wave interferometry.

We study the physics of $\nu =1/2$ bosonic fractional quantum Hall droplets confined in a ring-shaped region delimited by two concentric cylindrically symmetric hard-wall potentials. Trial wave functions based on an extension of the Jack polynomial formalism including two different chiral edges are proposed and validated for a wide range of confinement potentials in terms of their excellent overlap with the eigenstates numerically found by exact diagonalization. In the presence of a single repulsive potential centered in the origin, a recursive structure in the many-body spectra and a massively degenerate ground-state manifold are found. The addition of a second hard-wall potential confining the fractional quantum Hall droplet from the outside leads to a nondegenerate ground state containing a well-defined number of quasiholes at the center and, for suitable potential parameters, to a clear organization of the excitations on the two edges. The utility of this ring-shaped configuration in view of theoretical and experimental studies of subtle aspects of fractional quantum Hall physics is outlined.

We propose a scheme that can generate a microwave-controlled optical double optomechanical induced transparency (OMIT) in a hybrid piezo-optomechanical cavity system in which a piezoelectric optomechanical crystal AlN-nanobeam resonator is placed in a superconducting coplanar microwave cavity. We show that when an intense microwave field is applied to the superconducting microwave cavity, with a strong pump optical field and a weak probe optical field applied to the optomechanical crystal cavity through the optical waveguide, a double transmission window can be obtained in the weak output probe field. This phenomenon arises because an $N$-type four-level system can be formed in our scheme, because under the effects of the radiation pressure and the piezoelectric interaction, the quantum interference between different energy-level pathways induces the occurrence of the double-OMIT. Our scheme can be applied in the fields of optical switches, high-resolution spectroscopy, coherent population trapping, and quantum information processing in solid-state quantum systems.

We consider the $F_g=1\to F_e=1$ transition between the ground and excited hyperfine levels in alkali-metal vapor interacting with $\sigma$ linearly polarized control and probe fields whose polarizations can be either parallel or perpendicular to each other. We develop a matrix formulation that allows a solution of the Bloch equations to all orders in the pump and probe Rabi frequencies. Using this formalism, we calculate the steady-state probe absorption spectrum, the coherent population oscillations (CPOs), and two-photon coherence, in the absence (degenerate case) and presence (nondegenerate case) of a longitudinal magnetic field. We then calculate the probe storage when the pump is switched off and on again. We are particularly interested in whether the probe regains its original temporal shape when the pump is switched on again for the case of identical pump and probe frequencies. We show that, in the nondegenerate case, the restored probe does not regain its original shape whereas, in the degenerate case, the original shape is restored. This can be explained by considering the relative magnitudes of the CPOs which do not remember the temporal shape of the probe and the two-photon coherence oscillations which store the probe shape when the pump is switched off, as in electromagnetically induced transparency memories. In the nondegenerate case, the CPOs are much stronger than the two-photon coherence oscillations whereas, in the degenerate case, they are of similar magnitudes. Thus it is the two-photon coherence oscillations that are responsible for the restoration of the temporal shape of the probe in the degenerate case.

The achievement of Bose-Einstein condensation of photons (phBEC) in a dye-filled microcavity has led to a renewed interest in the density distribution of the ideal Bose gas in a two-dimensional harmonic oscillator. We present measurements of the radial profile of photons inside the microcavity below and above the critical point for phBEC with a good signal-to-noise ratio. We obtain a good agreement with theoretical profiles obtained using exact summation of eigenstates.

Single-photon detectors are widely used in modern quantum optics experiments and applications. Like all detectors, it is important for these devices to be accurately calibrated. A single-photon detector is calibrated by determining its detection efficiency; the common method to measure this quantity requires comparison to another detector. Here, we suggest a method to measure the detection efficiency of a single-photon detector without requiring an external reference detector. Our method is valid for individual single-photon detectors as well as multiplexed detectors, which are known to be photon number resolving. The method exploits the even-number photon-statistics of a nonlinear source, as well as the nonlinear loss of a single-photon detector that occurs when multiple photons are incident simultaneously. We have analytically modeled multiplexed detectors and used the results to experimentally demonstrate the calibration of a single-photon detector without the need for an external reference detector.