Strongly interacting one-dimensional (1D) Bose–Fermi mixtures form a tunable $XXZ$ spin chain. Within the spin-chain model developed here, all properties of these systems can be calculated from states representing the ordering of the bosons and fermions within the atom chain and from the ground-state wave function of spinless noninteracting fermions. We validate the model by means of an exact diagonalization of the full few-body Hamiltonian in the strongly interacting regime. Using the model, we explore the phase diagram of the atom chain as a function of the boson-boson (BB) and boson-fermion (BF) interaction strengths and calculate the densities, momentum distributions, and trap-level occupancies for up to 17 particles. In particular, we find antiferromagnetic (AFM) and ferromagnetic (FM) order and a demixing of the bosons and fermions in certain interaction regimes. We find, however, no demixing for equally strong BB and BF interactions, in agreement with earlier calculations that combined the Bethe ansatz with a local-density approximation.

We investigate the superfluid behavior of a two-dimensional (2D) Bose gas of ${}^{87}\mathrm{Rb}$ atoms using classical field dynamics. In the experiment by R. Desbuquois et al. [Nat. Phys. 8, 645 (2012)], a 2D quasicondensate in a trap is stirred with a blue-detuned laser beam along a circular path around the trap center. Here, we study this experiment from a theoretical perspective. The heating induced by stirring increases rapidly above a velocity $v_c$, which we define as the critical velocity. We identify the superfluid, the crossover, and the thermal regime by a finite, a sharply decreasing, and a vanishing critical velocity, respectively. We demonstrate that the onset of heating occurs due to the creation of vortex-antivortex pairs. A direct comparison of our numerical results to the experimental ones shows a good agreement, if a systematic shift of the critical phase-space density is included. We relate this shift to the absence of thermal equilibrium between the condensate and the thermal wings, which were used in the experiment to extract the temperature. We expand on this observation by studying the full relaxation dynamics between the condensate and the thermal cloud.

A recent experiment reveals the linear scaling property of the superradiant emission rate for a trapped atom cloud that is excited by weak and short light pulses. We adopt a real-space approach to numerically demonstrate that such a linear scaling law can be interpreted as single-photon superradiance, in both the time domain and the frequency domain. The dependence of the single-photon superradiant emission rate on the distance between atoms is investigated. We further study the single-photon superradiance for artificial atoms that have deviations in transition frequencies and decay rates.

We examine theoretically the driven-dissipative Bose-Hubbard dimer in the optical bistable regime. Various approximation schemes based on a Gutzwiller mean-field decoupling are applied and compared. Depending on the system parameters, we show that a decoupling with respect to the real space or to the reciprocal space can be more accurate. The Gutzwiller decoupling is applied both at the level of the density matrix and for the wave function during a quantum trajectory simulation. The latter is shown to be a more accurate approximation. A Gaussian approximation for the nonhomogeneous antibonding mode is also explored. We also show that entanglement in this system is witnessed by squeezing in reciprocal space.

We propose an efficient method to selectively enhance the spontaneous emission rate of a quantum system by changing the polarization of an incident control field, and exploiting the polarization dependence of the system's spontaneous emission rate. This differs from the usual Purcell enhancement of spontaneous emission rates as it can be selectively turned on and off. Using a three-level $\mathrm{\Lambda }$ system in a quantum dot placed in between two silver nanoparticles and a linearly polarized, monochromatic driving field, we present a protocol for rapid quantum state initialization, while maintaining long coherence times for control operations. This process increases the overall amount of time that a quantum system can be effectively utilized for quantum operations, and presents a key advance in quantum computing.

The attainability of the quantum Cramér-Rao bound (QCR), the ultimate limit in the precision of the estimation of a physical parameter, requires the saturation of the quantum information bound (QIB). This occurs when the Fisher information associated to a given measurement on the quantum state of a system which encodes the information about the parameter coincides with the quantum Fisher information associated to that quantum state. Braunstein and Caves [Phys. Rev. Lett. 72, 3439 (1994)] have shown that the QIB can always be achieved via a projective measurement in the eigenvectors basis of an observable called the symmetric logarithmic derivative. However, such projective measurement depends, in general, on the value of the parameter to be estimated, therefore requiring previous knowledge of the quantity one is trying to estimate. For this reason, it is important to investigate under which situation it is possible to saturate the QCR without previous information about the parameter to be estimated. Here, we show the complete solution to the problem of which are the initial pure states and which projective measurements allow the global saturation of the QIB, without the knowledge of the true value of the parameter, when the information about the parameter is encoded in the system by a unitary process.

The definition of accessible coherence is proposed. Through local measurement on the other subsystem and one-way classical communication, a subsystem can access more coherence than the coherence of its density matrix. Based on the local accessible coherence, the part that cannot be locally accessed is also studied, which we call it remaining coherence. We study how the bipartite coherence is distributed by partition for both $l_1$ norm coherence and relative entropy coherence, and the expressions for local accessible coherence and remaining coherence are derived. We also study some examples to illustrate the distribution.

We have performed an experiment demonstrating that loop state-preparation-and-measurement tomography [Jackson and van Enk, Phys. Rev. A 92, 042312 (2015)] is capable of detecting correlated errors between the preparation and the measurement of a quantum system. Specifically, we have prepared pure and mixed states of single qubits encoded in the polarization of heralded individual photons. By performing measurements using multiple state preparations and multiple measurement device settings we are able to detect if there are any correlated errors between them, and we are also able to determine which state preparations are correlated with which measurements. This is accomplished by going around a “loop” in parameter space, which allows us to check for self-consistency. No assumptions are made concerning either the state preparations or the measurements, other than that the dimensions of the states and the positive-operator-valued measures (POVMs) describing the detector are known. In cases where no correlations are found we are able to perform quantum state tomography of the polarization qubits by using knowledge of the detector POVMs, or quantum detector tomography by using knowledge of the state preparations. Note, however, that the detection of correlated errors does not require estimating any state or measurement parameters.

The collision cross sections for alkali-metal–rare-gas spin orbit mixing between the $n^2{P}_{3/2}\to n^2{P}_{1/2}$ levels trend strongly with the Massey parameter, or adiabaticity of the collisions. The strength of the interaction, as characterized by the $C_6$ dispersion coefficient, is a secondary influence on the rates. An analytic expression for the probability of energy transfer in alkali-metal–rare-gas collisions is derived using time-dependent perturbation theory. The model agrees well with a broad literature survey of the observed temperature-dependent cross sections. A simple interaction potential successfully organizes the alkali-metal–rare-gas database. The rates become very large for high-lying states, as the collisions are quite sudden and the radius of the valence electron is large. In contrast, the highly adiabatic cesium $6^{2}P$ mixing rates are six to eight orders of magnitude smaller. The mixing rate for the Rb-He diode pumped alkali laser system varies from $0.20–1.53\times {10}^{-11}\mathrm{cm}{}^3/\mathrm{at}.\mathrm{s}$ for $T=279–893\mathrm{K}$.

We analyze photoionization of hydrogen in the presence of a strong static electric field $F\sim 0.1$ a.u. Such a field essentially modifies the spectrum of the unperturbed atom. Even the ground $n=1$ state acquires a non-negligible width, while the higher field-free bound states become overlapping resonances. At the same time, static-field-induced states (SFISs) found recently [A. V. Gets and O. I. Tolstikhin, Phys. Rev. A 87, 013419 (2013)] emerge in the field-free continuum. We formulate the theory of photoionization from a decaying initial state and define appropriate observables—the reduced photoionization rate and transverse momentum distribution of photoelectrons. These observables are calculated for the four initial states with $n=1$ and 2 in the different polarization cases. The SFISs are shown to manifest themselves as distinct peaks in the observables. Remarkably, even broad SFISs can be seen as narrow well-pronounced peaks at fields where their widths are comparable to that of the initial state. Such a resonance enhancement of the manifestations of SFISs is the main finding of this paper. This finding suggests that SFISs should manifest themselves also in photoelectron momentum distributions produced by photoionization in the presence of a quasistatic field of intense low-frequency laser pulses currently used in strong-field physics.

The spectrum of a high-repetition train of laser pulses consists of many equally spaced lines, forming an optical frequency comb which is useful for high-precision spectroscopy. By exposing atoms to a train of strong pulses, frequency combs reaching into the extreme ultraviolet may be produced via high-order harmonic generation. Here, we report a theoretical study of extreme-ultraviolet frequency-comb generation by trains of few-cycle pulses. We analyze the nontrivial comb structure arising from overlapping harmonic orders. The spacings of the comb lines and their dependence on the offset frequency of the incident pulse train are discussed.

We consider a one-dimensional trapped gas of strongly interacting few spin-1 atoms which can be described by an effective spin chain Hamiltonian. Away from the $\text{SU}\left(3\right)$ integrable point, where the energy spectrum is highly degenerate, the rules of ordering and crossing of the energy levels and the symmetry of the eigenstates in the regime of large but finite repulsion have been elucidated. We study the spin-mixing dynamics which is shown to be very sensitive to the ratio between the two channel interactions $g_0/g_2$ and the effective spin chain transfers the quantum states more perfectly than the Heisenberg bilinear-biquadratic spin chain.

We introduce and study a photonic analog of the Kondo model. The model is defined as a far-detuned regime of photonic scattering off a three-level emitter in a $\mathrm{\Lambda }$-type configuration coupled to a one-dimensional waveguide with linear dispersion. We study dynamics of this local emitter driven by a coherent field pulse as well as the first- and second-order correlation functions of a scattered light. Various polarization-dependent correlation effects and an entanglement between the emitter and transmitted photons are quantified by the purity of an emitter's state. We also show that statistical properties of the scattered light are very sensitive to a polarization of the incoming coherent pulse.

The Heisenberg uncertainty principle suggests that it is impossible to determine the trajectory of a quantum particle in the same way as a classical particle. However, we may still yield insight into novel behavior of photons based on the average photon trajectories (APTs). Here we explore the APTs of optical fields carrying spin angular momentum (SAM) and orbital angular momentum (OAM) under the paraxial condition. We define the helicity and differential helicity for unveiling the three-dimensional spiral structures of the APTs of optical fields carrying the SAM and/or the OAM. We clarify the novel behaviors of the APTs caused by the SAM and OAM as well as the SAM-OAM coupling. The APT concept is also very helpful for profoundly understanding trapped particle motion and has the potential to elucidate other physical systems. Due to the presence of the helical path caused by the SAM and/or the OAM, the actual traveling distance of the photons might be much longer than the geometric distance.

Adopting concurrence as entanglement measure, we study entanglement and quantum phase transition of the Ising spin systems on the triangular lattice and Sierpiński fractal lattices by using the quantum renormalization-group method. It is found that the ground-state entanglement between two spins (or spin blocks) depends on the following factors: the size of system, the magnetic field, the exchange coupling, and the structure of lattice. As the size of the system becomes large, (a) the range of the magnetic field, in which the entanglement exists, contracts gradually and focuses on the critical point; and (b) the first derivative of entanglement shows singular behavior, and its maximum or minimum is approaching to the critical point gradually. The scaling behaviors of entanglement on the different lattices are similar but the scaling relations are diverse. For the triangular lattice, the space dimensionality determines the scaling relationship between the critical exponent of the entanglement and the critical exponent correlation length. However, for fractal lattices, it is the fractal dimensionality but not the space one to determine this relationship.

A three-dimensional harmonic oscillator consisting of $N\ge 2$ Coulomb-interacting charged particles, often called a (many-electron) Hooke atom, is a popular model in computational physics for, e.g., semiconductor quantum dots and ultracold ions. Starting from Thomas-Fermi theory, we show that the ground-state energy of such a system satisfies a nontrivial relation: $E_{gs}=\omega N^{4/3}{f}_{gs}\left(\beta N^{1/2}\right)$, where $\omega$ is the oscillator strength, $\beta$ is the ratio between Coulomb and oscillator characteristic energies, and $f_{gs}$ is a universal function. We perform extensive numerical calculations to verify the applicability of the relation. In addition, we show that the chemical potentials and addition energies also satisfy approximate scaling relations. In all cases, analytic expressions for the universal functions are provided. The results have predictive power in estimating the key ground-state properties of the system in the large-$N$ limit, and can be used in the development of approximative methods in electronic structure theory.