We give the complete list of 175 facets of the local polytope for the case where Alice and Bob each choose their measurements from a set of four binary outcome measurements. For each inequality we compute the maximum quantum violation for qubits, the resistance to noise, and the minimal detection efficiency required for closing the detection loophole with maximally entangled qubit states, in the case where both detectors have the same efficiency (symmetric case).

Time-dependent Lindblad master equations have important applications in areas ranging from quantum thermodynamics to dissipative quantum computing. In this paper we outline a general method for writing down exact solutions of time-dependent Lindblad equations whose superoperators form closed algebras. We focus on the particular case of a single qubit and study the exact solution generated by both coherent and incoherent mechanisms. We also show that if the time dependence is periodic, the problem may be recast in terms of Floquet theory. As an application, we give an exact solution for a two-level quantum heat engine operating in a finite time.

One out of many emerging implications from solutions of Einstein's general relativity equations are closed timelike curves (CTCs), which are trajectories through space-time that allow for time travel to the past without exceeding the speed of light. Two main quantum models of computation with the use of CTCs were introduced by Deutsch (D-CTC) and by Bennett and Schumacher (P-CTC). Unlike the classical theory in which CTCs lead to logical paradoxes, the quantum D-CTC model provides a solution that is logically consistent due to the self-consistency condition imposed on the evolving system, whereas the quantum P-CTC model chooses such a solution through postselection. Both models are nonequivalent and imply nonstandard phenomena in the field of quantum computation and quantum mechanics. In this paper, we study the implications of these two models on the second law of thermodynamics—the fundamental principle which states that in an isolated system the entropy never decreases. In particular, we construct CTC-based quantum circuits which lead to a decrease in entropy.

We theoretically investigate the carrier-envelope-phase (CEP) effect of laser pulses in the generation of solid high-order harmonics under nonadiabatic conditions using the time-dependent density functional theory. It is found that the CEP dependence of solid harmonics is still complex even if the Keldysh parameter $\gamma$ remains unchanged. Further analysis shows that the multiphoton ionization still contributes to the generation of high-order harmonics for solids with inversion symmetry when the minimum band gap between the highest valence band and the lowest conduction band is equal to even multiples of the laser frequency. The relative contribution of tunneling ionization and multiphoton ionization to the harmonic generation is responsible for this complex CEP dependence.

We develop a time-dependent perturbation theory (PT) beyond the dipole approximation to analyze the two-photon ionization dynamics of atoms exposed to short laser pulses with an arbitrary polarization. In a wide range of laser parameters, the good performance of the PT method is validated by comparing the results of a number of physical quantities with those calculated by numerically solving the time-dependent Schrödinger equation beyond the dipole approximation. Subsequent applications of the PT method in the nonresonant regime allow us to unveil the important role of the interferences between the dipole and nondipole transition pathways to the photoelectron momentum shift along the laser-propagation direction. In particular, we find that the ratio of the probability of each dipole transition channel to the total ionization probability oscillates with the increase of the photon energy. The interferences among different pathways and the oscillatory behavior of the ratios jointly lead to a series of minima in the linear momentum transfer for the case of the linear polarization.

The emergence of Weyl semimetals provides an important opportunity to reexamine the Goos-Hänchen (GH) and Imbert-Fedorov (IF) effects at the unique optical interface. In this paper, we establish a general model to describe the GH and IF shifts at the interface of Weyl semimetals. Based on this model, both the GH and IF shifts are obtained when a light beam impinges on the interface of Weyl semimetals. It is demonstrated that the GH and IF shifts are sensitive to the separation of two Weyl points in momentum space. We believe this result to be of fundamental significance and may provide a possible scheme for the direct optical measurement of the parameters of Weyl semimetals. Our model can also be applied to describe the GH and IF shifts in other topological materials due to their similar topological origins.

A scheme is proposed for fast conversion of two-atom Dicke states to three-atom Dickes state and three-atom Dicke states to four-atom Dicke states based on transitionless quantum driving. This scheme can be achieved within one step through appropriately regulating the Rabi frequencies of classical fields. The effects of decoherence induced by atomic spontaneous emission and leakage of the cavity are also taken into account. The numerical simulation result shows that this scheme is robust against both atomic spontaneous emission and cavity leakage. Furthermore, when multiple atoms are trapped in a cavity, the ($n+1$)-atom Dicke states can be derived directly from the $n$-atom Dicke states with the same principle, which provides a scalable way for the preparation of Dicke states in theory.

In this study, the transmission characteristics of one-dimensional (1D) periodic optical waveguide networks (POWNs) composed of simple or complex unit cells are systematically studied with the transfer matrix method. Using this method, the general transmission formula is analytically derived for any 1D POWNs. Due to the periodicity, three types of transmission resonance peaks can be produced, one of which shows an interesting missing order effect. The conditions for producing these phenomena are analytically obtained from the transmission formula. Moreover, it is found that transmission valleys in the passbands can be described by a simple envelope function. A greater number of waveguides in a simple unit cell or more nodes in a complex unit cell result in smaller minima of the passband transmission valley. Finally, the formula for the depth of photonic band gaps (PBGs) is analytically determined. We find that the depth of PBGs exponentially increases with the cell number and the width of PBGs becomes wider with the increase of the node in unit cells. Our work enhances people's understanding of the optical characteristics of periodic waveguide networks, and may be useful for designing all-optical devices, such as dense wavelength division multiplexers, high-efficiency optical switches, and wideband and/or narrowband optical filters.

We investigate the relationship between no-backwards-in-time signaling and classically causal correlations. We discover that, unlike the case for two parties, the no-backwards-in-time signaling paradigm for three parties is not enough to ensure that correlations can be reproduced with a classical causal ordering. We demonstrate this with an explicit example. We also generalize some existing results for linear two-time states to multiparty scenarios.

Quantum metrology exploits quantum correlations in specially prepared entangled or other nonclassical states to perform measurements that exceed the standard quantum limit. Typically, though, such states are hard to engineer, particularly when larger numbers of resources are desired. As an alternative, this paper aims to establish quantum jump metrology, which is based on generalized sequential measurements as a general design principle for quantum metrology and discusses how to exploit open quantum systems to obtain a quantum enhancement. By analyzing a simple toy model, we illustrate that parameter-dependent quantum feedback can indeed be used to exceed the standard quantum limit without the need for complex state preparation.

Charged wave packets with non-Gaussian spatial profiles are shown to possess intrinsic multipole moments. The magnetic dipole moment and the electric quadrupole moment are found for a wide class of packets, including the vortex electrons with orbital angular momentum $\ell$, the Airy beams, the so-called Schrödinger-cat states, and their generalizations. For the packets with no phase vortices, the electric quadrupole moment is shown to grow quadratically with the packet's width $|Q_{\alpha \beta }|\sim e{\sigma }_{\perp }^2$, while it is also $\left|\ell \right|$ times enhanced for the vortex beams. For available beams of electron microscopes, these multipole moments are relatively easily adjusted and can be quite large, which affects the packets' electromagnetic fields and also allows one to develop new diagnostic tools for materials science, atomic and molecular physics, nuclear physics, and so forth.

We have designed and realized magnetic trapping geometries for ultracold atoms based on permanent magnetic films. Magnetic chip based experiments give a high level of control over trap barriers and geometric boundaries in a compact experimental setup. These structures can be used to study quantum spin physics in a wide range of energies and length scales. By introducing defects into a triangular lattice, kagome and hexagonal lattice structures can be created. Rectangular lattices and (quasi-)one-dimensional structures such as ladders and diamond chain trapping potentials have also been created. Quantum spin models can be studied in all these geometries with Rydberg atoms, which allow for controlled interactions over several micrometers. We also present some nonperiodic geometries where the length scales of the traps are varied over a wide range. These tapered structures offer another way to transport large numbers of atoms adiabatically into subwavelength traps and back.

In this work we consider a numerically solvable model of a two-electron diatomic molecule to study a recently proposed approximation based on the density functional theory of so-called strictly correlated electrons (SCE). We map out the full two-particle wave function for a wide range of bond distances and interaction strengths and obtain analytic results for the two-particle states and eigenenergies in various limits of strong and weak interactions, and in the limit of large bond distance. We then study the so-called Hartree-exchange-correlation (Hxc) kernel of time-dependent density functional theory which is a key ingredient in calculating excitation energies. We study an approximation based on adiabatic SCE (ASCE) theory which was shown to display a particular feature of the exact Hxc kernel, namely, a spatial divergence as function of the bond distance. This makes the ASCE kernel a candidate for correcting a notorious failure of the commonly used adiabatic local density approximation (ALDA) in the calculation of excitation energies of dissociating molecules. Unlike the ALDA, we obtain nonzero excitation energies from the ASCE kernel in the dissociation regime but they do not correspond to those of the true spectrum unless the interaction strength is taken to be very large such that the SCE theory has the right regime of validity, in which case the excitation energies become exact and represent the so-called zero-point oscillations of the strictly correlated electrons. The commonly studied physical dissociation regime, namely, large molecular separation at intermediate interaction strength, therefore remains a challenge for density functional approximations based on SCE theory.

Jahn and Teller used degenerate perturbation theory to prove that open shell molecules with symmetry Jahn-Teller distort. In that vein, we have developed a perturbative approach to computing Jahn-Teller distortions in Kohn-Sham density-functional theory, starting from a symmetrized molecule with an electronic shell closed by fractional occupation numbers. The resulting highly symmetric state is an energy extremum but not necessarily a minimum. Using second-order perturbation theory, we find the changes in geometry that occur when the symmetry of the electron density is broken to form a state with integer occupation numbers. This methodology allows us to retain many of the computational benefits of working in higher symmetry. As a demonstration, we solve the resulting equations for ten electrons in a superatomlike harmonic-oscillator potential.

We report on the atom loss spectroscopy of strontium Rydberg atoms in a magnetooptical trap, using a two-photon excitation scheme through the intermediate state $5s5p{}^3{P}_1$. Energies of the $5sns{}^3{S}_1$ and $5snd{}^3{D}_{1,2}$ Rydberg series of ${}^{88}\mathrm{Sr}$ in the range $13\le n\le 50$ are determined with an absolute accuracy of 10 MHz, including the perturbed region where the $5snd{}^3{D}_2$ series couples to the $5snd{}^1{D}_2$ series. This represents an improvement by more than two orders of magnitude compared to previously published data. The quantum defects for each series are determined using the extended Rydberg-Ritz formula in the range where there is no strong perturbation. A value of $1377012721\left(10\right)\text{MHz}$ for the first ionization limit of ${}^{88}\mathrm{Sr}$ is extracted.

We theoretically investigate the two-color above-threshold ionization of atoms and ions by twisted XUV Bessel and Laguerre-Gaussian (LG) beams in the presence of a strong circularly polarized near-infrared (NIR) laser field. The presence of the NIR field modifies the continuum states accessible to the photoelectron. Based on the strong-field approximation, we explore the resulting energy and angular distributions of photoelectron as a function of the beam parameters. In particular, we analyze dichroism signals that arise due to the twisted nature of the XUV beam and the helicity of the NIR field. We focus on the comparison between LG beams and Bessel beams in the paraxial approximation. Here, we find that both beams yield similar results when the paraxial regime is valid. For localized targets, the dichroism signals strongly depend on the size and position of the atoms relative to the beam axis. Moreover, the dichroism signal tends to zero when the XUV LG beam is linear polarized. Detailed computations of the dichroism are performed and discussed for the $4s$ valence-shell photoionization of ${\mathrm{Ca}}^{+}$ ions.

We propose a scheme to produce spatially and temporally controlled spin-polarized photoelectrons by strong-field ionization of atoms. We calculate the momentum-resolved electron spin polarization from two spin-orbit coupled ionic states ($J=1/2$ and $J=3/2$) of krypton atoms in orthogonal two-color laser fields using the strong-field approximation, and obtain the instantaneous spin-polarization degree. With this scheme, the spin-up and -down photoelectrons can be well separated in the space, and the spin polarization is controlled on the sub-laser-cycle timescale. This study illustrates and reveals the mechanism of the generation of spin-polarized photoelectrons with an arbitrary spatially and temporally tailored laser field. We expect this work has implications for controlling the spatiotemporal spin polarization in strong-field physics.

Measuring and understanding electric-field noise from bulk material and surfaces is important for many areas of physics. In this work, we demonstrate the probing of electric-field noise from different sources with an ion, $225\mu \mathrm{m}$ above the trap surface. We detect noise levels as small as $S_E=5.2\left(11\right)\times {10}^{-16}{\mathrm{V}}^2{\mathrm{m}}^{-2}{\mathrm{Hz}}^{-1}$ at ${\omega }_{\text{z}}=2\pi \times 1.51\mathrm{MHz}$ and $T=12\mathrm{K}$. Our setup incorporates a controllable noise source utilizing a high-temperature superconductor. This element allows us, first, to benchmark and validate the sensitivity of our probe. Second, it allows us to probe noninvasively the bulk properties of the superconductor, observing a superconducting transition with an ion. For temperatures below the transition, we use our setup to assess different surface noise processes. The measured surface noise shows a deviation from a power law in the frequency domain. However, the temperature scaling of the data is not in a good agreement with existing surface noise models. Our results open perspectives for models in surface science and pave the way to test them experimentally.

We investigate the stability of a hard-axis polar state in a spin-1 antiferromagnetic Bose-Einstein condensate under a magnetic field gradient, where the easy-plane spin anisotropy is controlled by a negative quadratic Zeeman energy $q<0$. In a uniform magnetic field, the axial polar state is dynamically unstable and relaxes into the planar polar ground state. However, under a field gradient $B^{\prime }$, the excited spin state becomes metastable down to a certain threshold $q_{\text{th}}$, and as $q$ decreases below $q_{\text{th}}$ its intrinsic dynamical instability is rapidly recalled. The incipient spin excitations in the relaxation dynamics appear with stripe structures, indicating the rotational symmetry breaking by the field gradient. We measure the dependences of $q_{\text{th}}$ on $B^{\prime }$ and the sample size, and we find that $q_{\text{th}}$ is highly sensitive to the field gradient in the vicinity of $B^{\prime }=0$, exhibiting power-law behavior of $|q_{\text{th}}|\propto B^{\prime \alpha }$ with $\alpha \sim 0.5$. Our results demonstrate the significance of the field gradient effect in the quantum critical dynamics of spinor condensates.