We derive and evaluate equations of motion for the mean values and variances of the components of spins collectively coupled to a broadband squeezed radiation reservoir. Our formalism bridges between a single two-level emitter, represented by spin components that relax at different rates, depending on the degree of squeezing, to an ensemble of emitters represented by a large collective spin, whose components relax independently of the squeezing. For a single spin, the steady-state fluctuations in the transverse components are independent of the squeezing, while the steady state of a large spin ensemble reflects the statistics of the squeezed reservoir. This follows from an analysis of the Langevin noise contributions to the equations of motion and their consequences for the first and second moments of the spin operators. We argue that the difference between a single and many spins is related to whether vacuum fluctuations or radiation reaction dominate the coupling of the spin system to the radiation environment.

For a digital quantum simulator (DQS) imitating a target system, we ask the following question: Under what conditions is the simulator dynamics similar to that of the target in the presence of coupling to a bath? In this paper, we derive conditions for close simulation for three different physical regimes, replacing previous heuristic arguments on the subject with rigorous statements. In fact, we find that the conventional wisdom that the simulation cycle time should always be short for good simulation need not always hold up. Numerical simulations of two specific examples strengthen the evidence for our analysis and we go beyond to explore broader regimes.

Symmetric top molecules of methyl iodide are irradiated with a terahertz pulse generated by a two-color plasma and shaped by a short propagation in air. Free-induction decay is emitted by the excited molecular sample and then propagates in air before detection. The experimental data show that the input terahertz (THz) pulse undergoes strong reshaping through absorption and dispersion. This leads to narrow wave packets at revival times due to the excitation of high rotational energy levels. Typically, a THz burst of duration $\simeq 15-20$ ps is produced periodically, with a central frequency of $\simeq 1$ THz and a width that can be as narrow as 60–80 GHz. Pulse shaping based on propagation can be useful for quantum control in molecules. We provide a theoretical description of this wave propagation based on the Maxwell-Bloch equation. The observed experimental signal is in good agreement with the numerical simulations.

We investigate theoretically the photoassociation process of ${}^{85}\mathrm{Rb}$ atoms driven by a short laser pulse at several millikelvins. At this temperature, the actions of the rotational barrier, weights of initial scattering states, and shape resonance emerge and have an influence on the thermally averaged photoassociation probability. The calculated thermal weighted population distribution of rotational states agrees well with photoassociation spectrum experiment. The thermally averaged photoassociation probability decreases with an increase in temperature.

We study the fidelity susceptibility of a quantum antiferromagnetic Ising chain with a long-range power-law interaction $1/r^{\alpha }$ using the large-scale density matrix renormalization group method. We find that the critical adiabatic dimension $\mu =2$ and the critical exponent of the correlation length $\nu =1$ for arbitrary $\alpha >0$, indicating all quantum phase transitions are second-order Ising transitions. In addition, we numerically determine the complete phase diagram for $0<\alpha \le 3$ from the data collapse of the fidelity susceptibility and show that the critical point $h_c$ changes monotonously with respect to $\alpha$. This work will shed light on the nature of phase transitions in the quantum long-range antiferromagnetic Ising chain from a quantum information perspective.

The introduction of metamaterials and transformation optics has brought the possibilities for manipulating electromagnetic waves to an unprecedented level, suggesting applications like super-resolution imaging, cloaking, subwavelength focusing, and field localization. The refractive index of metamaterial structures in transformation optics typically has to be spatially graded. This paper presents a full analytical method for description of the field propagation through composites with gradient refractive index. The remarkable property of this approach is that it gives explicit general expressions for the field intensity and transmission and reflection coefficients, without reference to any boundary conditions. This opens a possibility for a novel fundamental theory of a number of important electromagnetic phenomena. The method enables calculation of wave propagation parameters within structures with arbitrary losses, arbitrary spectral dispersions, and arbitrary slopes of permittivity and permeability gradients, from mild to abrupt.

We propose a theoretical approach to manipulate the counter-rotating (CR) interactions in the quantum Rabi model by introducing a sinusoidal modulation to the transition frequency of the quantum two-level system. By choosing appropriate modulation frequency and amplitude, enhancement and suppression of the CR interactions can be achieved in the Jaynes-Cummings regime (including both weak- and strong-coupling cases) as well as the ultrastrong-coupling regime. In particular, we calculate the output excitation emission of the bosonic vacuum state under enhanced CR terms. Our results show that continuous and steady bosonic excitation emission from the bosonic vacuum can be observed in the Jaynes-Cummings regime as a consequence of the enhancement. Our approach is general and system independent, and hence it works for various physical systems described by the quantum Rabi model. As an example, we discuss the implementation of this scheme with superconducting quantum circuits.

We align nonadiabatically an ensemble of ${\mathrm{N}}_2$ in air, and irradiate the ensemble with linearly polarized intense few-cycle laser pulses to generate population-inverted ${\mathrm{N}}_2{}^{+}$ ions. By probing the self-seeded lasing signals of ${\mathrm{N}}_2{}^{+}$ at 391 nm, we show that the ultrafast population inversion in ${\mathrm{N}}_2{}^{+}$ between the electronic ground $X{}^2\mathrm{\Sigma }_{\mathrm{g}}$ state and the electronically excited $B{}^2\mathrm{\Sigma }_{\mathrm{u}}$ state is sensitively influenced by the extent of the alignment of ${\mathrm{N}}_2$ with respect to the polarization direction of the few-cycle laser field. The observed alignment dependence of the air lasing is reproduced well by numerical calculations performed based on a theoretical model in which the population transfer to the $B{}^2\mathrm{\Sigma }_{\mathrm{u}}$ state from the $X{}^2\mathrm{\Sigma }_{\mathrm{g}}$ state of ${\mathrm{N}}_2{}^{+}$ in the ultrashort laser field is mediated by the $A{}^2\mathrm{\Pi }_{\mathrm{u}}$ state. Our findings show that the intensity of the air lasing can be manipulated precisely by the timing between the laser pulses inducing the alignment of ${\mathrm{N}}_2$ and the laser pulse ionizing ${\mathrm{N}}_2$ into ${\mathrm{N}}_2{}^{+}$ as well as by the difference in the laser polarization directions of these laser pulses.

Important properties of a quantum system are not directly measurable, but they can be disclosed by how fast the system changes under controlled perturbations. In particular, asymmetry and entanglement can be verified by reconstructing the state of a quantum system. Yet, this usually requires experimental and computational resources which increase exponentially with the system size. Here we show how to detect metrologically useful asymmetry and entanglement by a limited number of measurements. This is achieved by studying how they affect the speed of evolution of a system under a unitary transformation. We show that the speed of multiqubit systems can be evaluated by measuring a set of local observables, providing exponential advantage with respect to state tomography. Indeed, the presented method requires neither the knowledge of the state and the parameter-encoding Hamiltonian nor global measurements performed on all the constituent subsystems. We implement the detection scheme in an all-optical experiment.

We extend the recent bounds of Sason and Verdú relating Rényi entropy and Bayesian hypothesis testing (arXiv:1701.01974.) to the quantum domain and show that they have a number of different applications. First, we obtain a sharper bound relating the optimal probability of correctly distinguishing elements of an ensemble of states to that of the pretty good measurement, and an analogous bound for optimal and pretty good entanglement recovery. Second, we obtain bounds relating optimal guessing and entanglement recovery to the fidelity of the state with a product state, which then leads to tight tripartite uncertainty and monogamy relations.

The multiple Auger decays of the $\mathrm{Ne}1s^{-1}$ state including double and triple Auger processes are investigated within the framework of perturbation theory. The contributions of the cascade and direct processes are determined for the double Auger decay. In the cascade processes, the choice of the orbital sets and configuration interaction can strongly affect the partial probabilities for the specific configurations of $\mathrm{N}{\mathrm{e}}^{3+}$. The multistep approaches, i.e., the knockout and shakeoff mechanisms, are implemented to deal with the direct double Auger processes for which the total and partial probabilities corresponding to specific configurations of $\mathrm{N}{\mathrm{e}}^{3+}$ are calculated and reveal that the knockout is dominant. Finally, the probabilities of the triple Auger decays that are decomposed into a double Auger process and a subsequent emission of a single electron are obtained using the cascade and knockout mechanisms. The calculated probabilities agree reasonably well with the available experimental data.

We analyze the possibility of macroscopic quantum effects in the form of coupled structural oscillations and shuttle motion of bright two-component spin-orbit-coupled striped (one-dimensional, 1D) and semivortex (two-dimensional, 2D) matter-wave solitons, under the action of linear mixing (Rabi coupling) between the components. In 1D, the intrinsic oscillations manifest themselves as flippings between spatially even and odd components of striped solitons, while in 2D the system features periodic transitions between zero-vorticity and vortical components of semivortex solitons. The consideration is performed by means of a combination of analytical and numerical methods.

van Dam [Phys. Rev. A 93, 052512 (2016)] claims that the one-particle reduced density matrix (1RDM) of an interacting system can be represented by means of a single-determinant wave function of fictitious noninteracting particles. van Dam [Phys. Rev. A 93, 052512 (2016)] introduced orbitals within a mean-field framework that produce energy levels similar to Hartree-Fock orbital energies, therefore he also claims that conventional analyses based on Koopmans' theorem are possible in 1RDM functional theory. In this Comment, we demonstrate that both claims are unfounded.

We consider the problem of optimally approximating an unavailable quantum state $\rho$ by the convex mixing of states drawn from a set of available states $\left\{{\nu }_i\right\}$. The problem is recast to look for the least distinguishable state from $\rho$ among the convex set ${\sum }_i{p}_i{\nu }_i$, and the corresponding optimal weights $\left\{p_i\right\}$ provide the optimal convex mixing. We present the complete solution for the optimal convex approximation of a qubit mixed state when the set of available states comprises the three bases of the Pauli matrices.

Einstein-Podolsky-Rosen (EPR) steering is an asymmetric form of correlations which is intermediate between quantum entanglement and Bell nonlocality, and can be exploited as a resource for quantum communication with one untrusted party. In particular, steering of continuous-variable Gaussian states has been extensively studied theoretically and experimentally, as a fundamental manifestation of the EPR paradox. While most of these studies focused on quadrature measurements for steering detection, two recent works revealed that there exist Gaussian states which are only steerable by suitable non-Gaussian measurements. In this paper we perform a systematic investigation of EPR steering of bipartite Gaussian states by pseudospin measurements, complementing and extending previous findings. We first derive the density-matrix elements of two-mode squeezed thermal Gaussian states in the Fock basis, which may be of independent interest. We then use such a representation to investigate steering of these states as detected by a simple nonlinear criterion, based on second moments of the correlation matrix constructed from pseudospin operators. This analysis reveals previously unexplored regimes where non-Gaussian measurements are shown to be more effective than Gaussian ones to witness steering of Gaussian states in the presence of local noise. We further consider an alternative set of pseudospin observables, whose expectation value can be expressed more compactly in terms of Wigner functions for all two-mode Gaussian states. However, according to the adopted criterion, these observables are found to be always less sensitive than conventional Gaussian observables for steering detection. Finally, we investigate continuous-variable Werner states, which are non-Gaussian mixtures of Gaussian states, and find that pseudospin measurements are always more effective than Gaussian ones to reveal their steerability. Our results provide useful insights on the role of non-Gaussian measurements in characterizing quantum correlations of Gaussian and non-Gaussian states of continuous-variable quantum systems.

Cold collisions of ${}^{14}\mathrm{N}^{16}\mathrm{O}$ molecules in the ${}^2\mathrm{\Pi }_{1/2}$ ground state, subject to electric and magnetic fields, are investigated. It is found that elastic collision rates significantly exceed state-changing inelastic rates only at temperatures above 0.5 K at laboratory-strength fields. It is found, however, that in very large fields, $>{10}^4$ V/cm, inelastic rates can be somewhat suppressed. Magnetic fields have a negligible influence on scattering for this nearly nonmagnetic state.

A recent PISCES-B Mod experiment [Nishijima et al., J. Phys. B 43, 225701 (2010)] has revealed up to a factor of 5 discrepancy between measurement and the two existing theoretical models [Badnell et al., J. Phys. B 29, 3683 (1996); Bartschat et al., J. Phys. B 35, 2899 (2002)], providing important diagnostics for Mo i. In the following paper we address this issue by employing a relativistic atomic structure and $R$-matrix scattering calculations to improve upon the available models for future applications and benchmark results against a recent Compact Toroidal Hybrid experiment [Hartwell et al., Fusion Sci. Technol. 72, 76 (2017)]. We determine the atomic structure of Mo i using grasp${}^0$, which implements the multiconfigurational Dirac-Fock method. Fine structure energies and radiative transition rates are presented and compared to existing experimental and theoretical values. The electron-impact excitation of Mo i is investigated using the relativistic $R$-matrix method and the parallel versions of the Dirac atomic $R$-matrix codes. Electron-impact excitation cross sections are presented and compared to the few available theoretical cross sections. Throughout, our emphasis is on improving the results for the $z{}^{5}P_{1,2,3}^o\to a{}^{5}S_2,z{}^{7}P_{2,3,4}^o\to a{}^{7}S_3$ and $y{}^{7}P_{2,3,4}^o\to a{}^{7}S_3$ electric dipole transitions of particular relevance for diagnostic work.

Ultracold Bose gases can be used to experimentally test many-body theory predictions. Here we point out that both exact conservation laws and dynamical invariants exist in the topical case of the one-dimensional Bose gas, and these provide an important validation of methods. We show that the first four quantum conservation laws are exactly conserved in the approximate truncated Wigner approach to many-body quantum dynamics. Center-of-mass position variance is also exactly calculable. This is nearly exact in the truncated Wigner approximation, apart from small terms that vanish as $N^{-3/2}$ as $N\to \infty$ with fixed momentum cutoff. Examples of this are calculated in experimentally relevant, mesoscopic cases.

We study the Casimir potential between two impurities immersed in a spin-orbit-coupled Bose-Einstein condensate (BEC) with plane-wave order. We find that by exchanging the virtual excitations, a remarkable anisotropic oscillating potential with both positive and negative parts can be induced between the impurities, with the period of the oscillation depending on the spin-orbit-coupling strength. As a consequence, this would inevitably lead to a noncentral Casimir force, which can be tuned by varying the strength of the spin-orbit coupling (SOC). These results are elucidated for BECs with one-dimensional Raman-induced and two-dimensional Rashba-type SOCs.