Quantum mechanics dictates the band structure of materials that is essential for functional electronic components. With increased miniaturization of devices, it becomes possible to exploit the full potential of quantum mechanics through the principles of superposition and entanglement. We propose a class of quantum rectifiers that can leverage entanglement to dramatically increase performance by coupling two small spin chains through an effective double-slit interface. Simulations show that rectification is enhanced by several orders of magnitude even in small systems, and that the effect survives in a noisy environment. Realizable using several of the quantum technology platforms currently available, our findings reveal the importance of quantum entanglement in seemingly contradictory applications such as heat and noise control.

Optical two-dimensional coherent spectroscopy (2DCS) has become a powerful tool for studying energy-level structure, dynamics, and coupling in many systems including atomic ensembles. Various types of two-dimensional (2D) spectra, including the so-called single-quantum, zero-quantum, and double-quantum 2D spectra, of both $D$ lines ($D_1$ and $D_2$ transitions) of potassium (K) atoms have been reported previously. For rubidium (Rb), a major difference is that the $D$ lines are about 15 nm apart as opposed to only about 3 nm for K. Simultaneously exciting both $D$ lines of Rb atoms requires a broader laser bandwidth for the experiment. Here, we report a broadband optical 2DCS experiment in a Rb atomic vapor. A complete set of single-quantum, zero-quantum, and double-quantum 2D spectra including both $D$ lines of Rb atoms were obtained. The experimental spectra were reproduced by simulated 2D spectra based on the perturbation solutions to the optical Bloch equations. This work in Rb atoms complements previous 2DCS studies of K and Rb with a narrower bandwidth that covers two $D$ lines of K or only a single $D$ line of Rb. The broadband excitation enables the capability to perform double-quantum and multiquantum 2DCS of both $D$ lines of Rb to study many-body interactions and correlations in comparison with K atoms.

A laser-atom interaction simulator derived from quantum electrodynamics (LASED) is presented, which has been developed in the python programming language. LASED allows a user to calculate the time evolution of a laser-excited atomic system. The model allows for any laser polarization, a Gaussian laser beam profile, a rotation of the reference frame chosen to define the states, and an averaging over the Doppler profile of an atomic beam. Examples of simulations using LASED are presented for excitation of calcium from the $4{}^{1}S_0$ state to the $4{}^{1}P_1$ state, for excitation from the helium $3{}^{1}D_2$ state excited by electron impact to the $10{}^{1}P_1$ state, and for laser excitation of caesium via the $D_2$ line.

Using the Hubbard model we study how the process of high-order harmonic generation (HHG) is modified by electron-electron correlation for both finite and bulk systems. A finite-size enhancement of the HHG signal is found and attributed to electrons backscattering off the lattice edges. Additionally, with the increasing strength of the electron-electron correlation an enhancement of the high-frequency regime of the HHG spectrum is found. This is attributed to the on-site Coulomb repulsion between electrons giving rise to a localized correlation-induced quiver motion of the electrons. The finite-lattice enhancement dominates the HHG spectra from a few harmonic orders until a threshold from which the correlational enhancement dominates. This threshold is determined by the degree of correlation and decreases into the low-frequency regime for increasing electron-electron correlation. This infers that as the electron-electron correlation increases, the finite-size effect on the electron dynamics decreases.

Strong-field triple ionization of atoms is studied by performing classical-trajectory Monte Carlo simulations. The momentum distributions of three electrons are visualized via Dalitz plots, exhibiting clear electronic correlations. Separated traces of various nonsequential paths are identified in these ternary spectra, including channels with multiple recollisions. It is proposed that Dalitz plots can be used as a valuable tool to study correlated electron dynamics in strong fields, in particular, as a method to retrieve from experimental data relative roles of various channels of nonsequential ionization such as direct ionization, recollision-induced excitation with subsequent ionization, or a multirecollison.

Within Gross-Pitaevskii (GP) theory we derive the interface potential $V\left(\ell \right)$ which describes the interaction between the interface separating two demixed Bose-condensed gases and an optical hard wall at a distance $\ell$. Previous work revealed that this interaction gives rise to extraordinary wetting and prewetting phenomena. Calculations that explore nonequilibrium properties by using $\ell$ as a constraint provide a thorough explanation for this behavior. We find that at bulk two-phase coexistence, $V\left(\ell \right)$ for both complete wetting and partial wetting is monotonic with exponential decay. Remarkably, at the first-order wetting phase transition, $V\left(\ell \right)$ is independent of $\ell$. This anomaly explains the infinite continuous degeneracy of the grand potential reported earlier. As a physical application, using $V\left(\ell \right)$ we study the three-phase contact line where the interface meets the wall under a contact angle $\theta$. Employing an interface displacement model we calculate the structure of this inhomogeneity and its line tension $\tau$. Contrary to what happens at a usual first-order wetting transition in systems with short-range forces, $\tau$ does not approach a nonzero positive constant for $\theta \to 0$, but instead approaches zero (from below) in the manner $\tau \propto -\theta$ as would be expected for a critical wetting transition. This hybrid character of $\tau$ is a consequence of the absence of a barrier in $V\left(\ell \right)$ at wetting. For a typical $V\left(\ell \right)=\mathcal{S}exp(-\ell /\xi )$, with $\mathcal{S}$ the spreading coefficient and $\xi$ a decay length, we conjecture that $\tau =-2(1-ln2)\gamma \xi sin\theta$ is exact within GP theory, with $\gamma$ the interfacial tension and $0\le \theta \le \pi$.

We propose a general and experimentally feasible approach to realize simultaneous ground-state cooling of an arbitrary number of near-degenerate or even fully degenerate mechanical modes, overcoming the limit imposed by the formation of mechanical dark modes. Multiple optical modes are employed to provide different dissipation channels that prevent complete destructive interference of the cooling pathway, and thus eliminating the dark modes. The cooling rate and limit are explicitly specified, in which the distinguishability of the optical modes from the mechanical modes is found to be critical for an efficient cooling process. In a realistic multimode optomechanical system, ground-state cooling of all mechanical modes is demonstrated by sequentially introducing optical drives, proving the feasibility and scalability of the proposed scheme. The work may provide new insights in preparing and manipulating multiple quantum states in macroscopic systems.

A laser with superb frequency stability plays an important role in fundamental physics research and advanced technologies. Frequency references are usually utilized to stabilize the laser, such as high-finesse cavities and atomic ensembles, of which thermal atomic beams are attractive because of their balanced performance between simplicity and stability. However, due to the transverse-velocity distribution in the thermal atomic beams, most atoms (generally $>99\%$) are wasted when interacting with the laser. Here, we propose a laser referenced on a version of velocity-grating Ramsey-Bordé atom interferometry with greatly improved atom utilization and thus superior laser frequency stability. Compared with a conventional atomic interferometry applied to stabilize the laser, the proposed system in principle generates optical Ramsey fringes with the amplitude enhanced by 1000-fold or more. Such a configuration has promising applications in diverse areas, including precision measurements and quantum metrology.

Electronic friction is an important energy loss channel for atoms and molecules scattering off, reacting, or simply vibrating at metallic surfaces. It is usually well described by mixed classical-quantum approaches where the nuclei evolve classically according to Langevin-type equations of motion, and Born-Oppenheimer forces and friction kernels are obtained from first-principles electronic structure calculations. However, classical dynamics falls short when light atoms are involved, which is also the situation where electronic friction becomes the dominant dissipation channel and its role in the dynamics can be unambiguously assessed. Furthermore, the interplay between electronic friction and nuclear quantum effects in molecular processes at surfaces is largely unknown; in fact, it is not even clear how to include electronic friction in a quantum setting. Here we fill this gap by developing a fully quantum theory of electronic friction at $T=0\mathrm{K}$. The electronic bath is considered to be entirely general and can be made of interacting electrons, potentially in a strongly correlated state. The derived friction kernel agrees with a recently obtained mixed quantum-classical result [Dou, Miao, and Subotnik, Phys. Rev. Lett. 119, 046001 (2017)], except for a pseudomagnetic contribution in the latter that is removed here. The ensuing equation of motion for the nuclear wave function is a nonlinear Schrödinger equation with a frictional vector potential that depends on the past wave function behavior. The equation becomes local-in-time in the typical situation where the electrons respond rapidly on the slow timescale of the nuclear dynamics (Markov limit) and generalizes previously known Schrödinger-Langevin equations to coordinate-dependent, tensorial friction kernels.

In this paper, we explore the Wigner entropy production rate in the stationary state of a two-mode Gaussian system. The interacting modes dissipate into different local thermal baths. Also, one of the bosonic modes evolves into the squeezed-thermal state because of the parametric amplification process. Using the Heisenberg-Langevin approach combined with the quantum phase-space formulation, we get an analytical expression for the steady-state Wigner entropy production rate. It contains two key terms. The first one is an Onsager-like expression that describes heat flow within the system. The second term results from vacuum fluctuations of the baths. Analyses show that self-correlation between the quadratures of the parametrically amplified mode pushes the mode toward the thermal squeezed state. It increases vacuum entropy production of the total system and reduces the heat current between the modes. The results imply that, unlike in previous proposals, squeezing can constrain the efficiency of actual nonequilibrium heat engines by irreversible flows.

Using the confined variational method, the low-energy $S$-wave elastic scattering of positronium from molecular hydrogen is studied nonadiabatically at incident energy up to 0.13 eV. Accurate $S$-wave phase shifts, scattering lengths, and cross sections are calculated and compared with other theoretical and experimental values. Using the modified effective-range formula, the $S$-wave scattering length is determined to be 1.97 Bohr radii, which is about $2.5\%$ smaller than that of the confined variational calculation under the fixed-nucleus approximation. The consistency of the present cross sections with those from the angular correlation of annihilation radiation experiment indicates that the results of Doppler broadening spectroscopy might be problematic. We also discuss the distortion effects of positronium and molecular hydrogen.

Magnetic monopoles and kinks are topological excitations that have been extensively investigated in quantum spin systems, but usually, they are studied in different setups. We explore the conditions for the coexistence and interaction effects of these quasiparticles in the pseudospin chain of an atomic dipolar superlattice gas. In this chain, the magnetic kink is the intrinsic quasiparticle, and the particle (hole) defect takes over the role of the north (south) magnetic monopole, exerting monopolar magnetic fields on neighboring spins. A binding effect between the monopole and kink is revealed, which renormalizes the dispersion of the kink. The corresponding dynamical antibinding process is observed and arises due to the kink-antikink annihilation. The rich interaction effects of the two quasiparticles could stimulate corresponding investigations in bulk spin systems.

Considering the paradigmatic case of a cloud of two-level atoms interacting through common vacuum modes, we show how cooperative spontaneous emission, which is at the origin of superradiance, leads the system to long-lived entangled states at late times. These subradiant modes are characterized by an entanglement between all particles, independently of their geometrical configuration. While there is no threshold on the interaction strength necessary to entangle all particles, stronger interactions lead to longer-lived entanglement.

In this paper, we investigate the influence of the symmetry of the biphoton wave function on the coincidence measurement of the generalized Mach-Zehnder (MZ) interferometer, where there are a temporal and frequency shift operations between the two beam splitters. We show that the generalized MZ interferometer allows the measurement of the short-time Fourier transform of the function modeling the energy conservation of a spontaneous parametric down-conversion process if the full biphoton state is symmetric, and of the symmetric characteristic distribution of the phase-matching function if the state is antisymmetric. Thus, this technique is phase sensitive to the spectral distribution of the photon pairs. Finally, we investigate in detail the signature of a pair of anyons whose peculiar statistics can be simulated by engineering the spectrum of photon pairs.

The extended quantum Rabi models make a significant contribution to understanding of the quantum nature of the atom-light interaction. We transform two kinds of extended quantum Rabi model, the anisotropic Rabi model and the asymmetric Rabi model, into the rotating frame and regard them as periodically driven quantum systems. The analytical solutions of the quasienergy spectrum as well as the Floquet modes for both models are constructed by applying the Floquet theory and the high-frequency expansion, which is applied to the nonstroboscopic dynamics of physical observables such as atomic inversion, transverse magnetization, and atom-field correlation. For the anisotropic Rabi model, the quasienergy fits well with the numerical results even when the rotating-wave coupling is in the deep strong-coupling regime $g\simeq \hslash \omega$ if the counter-rotating term is small enough compared to the driving frequency. Avoided level crossing may occur for quasienergy with the same parity when the positive branch spectrum lines for the total excitation number $N$ cross the negative branch lines for $N+2$, while the high-frequency expansion fails to predict this due to the conservation of the total excitation number. Furthermore, we present analytical and numerical studies of the long-time evolution of population and conclude that the analytical method is credible for the population dynamics. For the asymmetric Rabi model, we find that the external bias field which breaks the parity symmetry of the total excitation number tends to cluster the upper and lower branches into two bundles, and the detuning induced gap in the first temporal Brillouin zone shows a quadratic dependence on the bias. Fourier analysis is applied to extract the frequency composition and two-frequency driving behavior is revealed. Varying the bias strength will change the time-averaged value of the oscillation, which shows how the bias competes with the detuning and atom-field coupling in the driving dynamics. Both models prove that treating the Hamiltonian in the rotating frame by Floquet theory gives an alternative tool in the study of interaction between atom and light.

The effective Lagrangian for plasmons in an electron liquid is given and is further generalized to a mixture of electron and hole liquids, which we take to model the carriers in a semiconductor. The coupling between plasmon and external electromagnetic field is also discussed. These results show the main conclusions in the paper by Jung and Keller [Phys. Rev. A 103, 063501 (2021)] are uncorrect.

We reformulate the Floquet theory for periodically driven quantum systems following a perfect analogy with the proof of the Bloch theorem. We observe that the current standard method for calculating the Floquet eigenstates using the quasienergy alone is incomplete and unstable and pinpoint an overlooked quantum number, the average energy. This new quantum number resolves many shortcomings of the Floquet method stemming from the quasienergy degeneracy issues, particularly in the continuum limit. Using the average-energy quantum number, we get properties similar to those of the static energy, including a unique lower-bounded ordering of the Floquet states, from which we define a ground state, and a variational method for calculating the Floquet states. This is a first step towards reformulating Floquet first-principles methods, which have long been thought to be incompatible due to the limitations of the quasienergy.

The standard topological approach to indistinguishable particles formulates exchange statistics by using the fundamental group to analyze the connectedness of the configuration space. Although successful in two and more dimensions, this approach gives only trivial or near-trivial exchange statistics in one dimension because two-body coincidences are excluded from configuration space. Instead, we include these path-ambiguous singular points and consider configuration space as an orbifold. This orbifold topological approach allows unified analysis of exchange statistics in any dimension and predicts possibilities for anyons in one-dimensional systems, including non-Abelian anyons obeying alternate strand groups. These results clarify the nontopological origin of fractional statistics in one-dimensional anyon models.

The quantum communication process usually consists of three stages: the sender who prepares encoded carriers, the transmission in noisy channels, and the quantum receivers. The transmitted quantum information can be inevitably affected by kinds of quantum noise in the environment. Thus, quantum protocols are extensively studied to improve communication efficiency and accuracy under the influence of quantum noise. The optimization strategies usually occur in these three stages. In this paper, we focus on the optimization strategy of quantum receivers in the third stage. In quantum receiver algorithms, the key to distinguish received non-orthogonal coherent states in free-space optical quantum communication is to construct an optimum displacement operator for transforming the current coherent state into a state that is easier to distinguish than before. To improve the antinoise ability and accuracy of quantum communication, this paper proposes a universal optimization strategy of quantum receivers called learnable antinoise receiver (LAN receiver). In this strategy, a parametrized quantum circuit is constructed as a quantum feedforward neural network as the displacement operator to improve the antinoise ability. The parameters used in the quantum circuit are updated by gradient descent continuously to find the best parameter combination of the quantum circuit that minimizes the error rate and the qubits affected by quantum noise are used as training and testing data. The simulation of the proposed algorithm shows that the LAN receiver can resist different kinds of strong quantum noise. The average error rate of the proposed algorithm LAN receiver under the strong noise channel is 0.18, which has better performance than other type of receivers under the influence of strong quantum noise.