We study transfer of a quantum state through $XX$ spin chains with static imperfections. We combine the two standard approaches for state transfer based on (i) modulated couplings between neighboring spins throughout the spin chain and (ii) weak coupling of the outermost spins to an unmodulated spin chain. The combined approach allows us to design spin chains with modulated couplings and localized boundary states, permitting high-fidelity state transfer in the presence of random static imperfections of the couplings. The modulated couplings are explicitly obtained from an exact algorithm using the close relation between tridiagonal matrices and orthogonal polynomials [Linear Algebr. Appl. 21, 245 (1978)]. The implemented algorithm and a graphical user interface for constructing spin chains with boundary states (spinGUIn) are provided as Supplemental Material.

An experiment enabling extremely high signal-to-noise ratios in the measurement of spectral line shapes is described. This approach, which combines high-bandwidth locking of a continuous wave probe laser and the frequency-stabilized cavity ring-down spectroscopy technique, enables long-term signal averaging and yields high-resolution spectra with a relatively wide dynamic range and low detection limit. By probing rovibronic transitions of the ${}^{16}$O${}_2$ $B$ band near $\lambda =689$ nm, exceptionally precise measurements of absorption line shape and line position are demonstrated. A signal-to-noise ratio of $220000$ and a minimum detectable absorption coefficient of $2.4\times {10}^{-11}{\mathrm{cm}}^{-1}$ is reported, which corresponds to the lowest line intensity measurable by this setup of approximately $1.3\times {10}^{-30}{\mathrm{cm}}^{-1}/(\mathrm{molecule}$ ${\mathrm{cm}}^{-2})$. Careful analysis of the data revealed a subtle line-shape asymmetry that could be explained by the speed dependence of the collisional shift. The demonstrated measurement precision enables the quantification of systematic line-shape deviations, which were approximately 1 part in 80 000 of the peak absorption. The influence of slowly drifting etaloning effects on the precision of the line-shape analysis is discussed. How this method can enable experiments that address a number of fundamental physical problems including the accurate optical measurement of the Boltzmann constant and tests of the symmetrization postulate is also discussed.

Universality of strongly interacting fermions is a topic of great interest in diverse fields. Here we investigate theoretically the universal dynamic density response of resonantly interacting ultracold Fermi atoms in the limit of either high temperature or large frequency. At high temperature, we use quantum virial expansion to derive universal, nonperturbative expansion functions of the dynamic structure factor; at large momentum, we conjecture that the second-order expansion function gives the Wilson coefficient used in the operator product expansion method. The dynamic structure factor is therefore determined by its second-order expansion function with an overall normalization factor given by Tan's [S. Tan, Ann. Phys. (NY) 323, 2952 (2008); S. Tan, Ann. Phys. (NY) 323, 2971 (2008); S. Tan, Ann. Phys. (NY) 323, 2987 (2008)] contact parameter. We show that the spin-parallel and -antiparallel dynamic structure factors have, respectively, a tail of the form $\sim \pm {\omega }^{-5/2}$ for $\omega \to \infty$, decaying slower than the total dynamic structure factor found previously ($\sim {\omega }^{-7/2}$). Our predictions for the dynamic structure factor at high temperature or large frequency are testable using Bragg spectroscopy for ultracold atomic Fermi gases.

We show by extensive numerical simulations and analytical variational calculations that elongated binary nonmiscible Bose-Einstein condensates subject to periodic modulations of the radial confinement exhibit a Faraday instability similar to that seen in one-component condensates. Considering the hyperfine states of ${}^{87}$Rb condensates, we show that there are two experimentally relevant stationary-state configurations: one in which the components form a dark-bright symbiotic pair (the ground state of the system) and one in which the components are segregated (first excited state). For each of these two configurations, we show numerically that far from resonances the Faraday waves excited in the two components are of similar periods, emerge simultaneously, and do not impact the dynamics of the bulk of the condensate. We derive analytically the period of the Faraday waves using a variational treatment of the coupled Gross-Pitaevskii equations combined with a Mathieu-type analysis for the selection mechanism of the excited waves. Finally, we show that for a modulation frequency close to twice that of the radial trapping, the emergent surface waves fade out in favor of a forceful collective mode that turns the two condensate components miscible.

An intuitive physical mechanism for isolated-attosecond-pulse generation in a multicycle pulse regime is proposed. We suggest that the continuum spectra of the high-order harmonics can be controlled through the modulation of carrier wave, while keeping the multicycle envelope unchanged approximately. By increasing the time interval between the adjacent maximum return kinetic energy peaks, the kinetic energy of the side peaks will be apparently suppressed by the multicycle envelope. The photons with energies near the cutoff burst only at the pulse center with high coherence, which contribute to the generation of isolated-attosecond pulse. We verify this mechanism by using a multicycle tricolor pulse as driver field.

We present a quantum theoretical analysis of triple photons generated by a phase-matched third-order nonlinear process in a KTP crystal in a weak-interaction regime. We show that the quantum properties of the triple photons can be brought to light through optical second- and third-order sum frequency generation processes.

We evaluate the resonance-fluorescence spectrum of a bunch of two-level atoms driven by a gradient coherent laser field. The result shows that we can determine the positions of atoms from the spectrum even when the atoms locate within the subwavelength range and the dipole-dipole interaction is significant. This far-field resonance-fluorescence-localization microscopy method does not require point-by-point scanning, and it may be more time efficient. We also give a possible scheme to extract the position information in an extended region without requiring more peak power of the laser. Finally, we also briefly discuss how to perform two-dimensional imaging based on our scheme.

We theoretically investigate signatures of stimulated emission at the single-photon level for a two-level atom interacting with a one-dimensional light field. We consider the transient regime where the atom is initially excited, and the steady-state regime where the atom is continuously driven with an external pump. The influence of pure dephasing is studied, clearly showing that these effects can be evidenced with state-of-the-art solid-state devices. We finally propose a scheme to demonstrate the stimulation of one optical transition by monitoring another one, in three-level one-dimensional atoms.

We address the problem of optically controlling and quantifying the dissipative dynamics of quantum and classical correlations in a setup of individual quantum emitters under external laser excitation. We show that both types of correlations, the former measured by the quantum discord, are present in the system's evolution even though the emitters may exhibit an early stage disentanglement. In the absence of external laser pumping, we demonstrate analytically, for a set of suitable initial states, that there is an entropy bound for which quantum discord and entanglement of the emitters are always greater than classical correlations, thus disproving an early conjecture that classical correlations are greater than quantum correlations. Furthermore, we show that quantum correlations can also be greater than classical correlations when the system is driven by a laser field. For scenarios where the emitters' quantum correlations are below their classical counterparts, an optimization of the evolution of the quantum correlations can be carried out by appropriately tailoring the amplitude of the laser field and the emitters' dipole-dipole interaction. We stress the importance of using the entanglement of formation, rather than the concurrence, as the entanglement measure, since the latter can grow beyond the total correlations and thus give incorrect results on the actual system's degree of entanglement.

Recently, Sánchez-Moreno et al. [New J. Phys. 8, 330 (2006)] have shown that the three-dimensional Heisenberg uncertainty relation ${\sigma }_r{\sigma }_p\ge 3\hslash /2$ might be significantly sharpened if the relevant quantum state describes the particle in a central potential. I extend that result to the case of states which are not the eigenstates of the square of the angular momentum operator. I derive a lower bound for ${\sigma }_r{\sigma }_p$ which involves the mean value and the variance of the square of the angular momentum operator.

In addition to being a very interesting quantum phenomenon, Schrödinger-cat-state swapping has the potential for application in the preparation of quantum states that could be used in metrology and other quantum processing. We study in detail the effects of field decoherence on a Schrödinger-cat-state-swapping system comprising a set of identical qubits, or spins, all coupled to a field mode. We demonstrate that increasing the number of spins actually mitigates the effects of field decoherence on the collapse and revival of a spin Schrödinger-cat state, which could be of significant utility in quantum metrology and other quantum processing.

We propose an optimal nonlocal entanglement concentration protocol (ECP) for multiphoton systems in a partially entangled pure state, resorting to the projection measurement on an additional photon. One party in quantum communication first performs a parity-check measurement on her photon in an $N$-photon system and an additional photon, and then she projects the additional photon into an orthogonal Hilbert space for dividing the original $N$-photon systems into two groups. In the first group, the $N$ parties will obtain a subset of $N$-photon systems in a maximally entangled state. In the second group, they will obtain some less-entangled $N$-photon systems which are the resource for the entanglement concentration in the next round. By iterating the entanglement concentration process several times, the present ECP has the maximal success probability, which is just equivalent to the entanglement of the partially entangled state. That is, this ECP is an optimal one.

Based on an asymmetric Lanczos-chain subspace algorithm, damped coupled cluster linear response functions have been implemented for the hierarchy of coupled cluster (CC) models including CC with single excitations (CCS), CC2, CC with single and double excitations (CCSD), and CCSD with noniterative triple corrected excitation energies CCSDR(3). This work is a first step toward the extension of these theoretical electronic structure methods of well-established high accuracy in UV-vis absorption spectroscopies to applications concerned with x-ray radiation. From the imaginary part of the linear response function, the near $K$-edge x-ray absorption spectra of neon, water, and carbon monoxide are determined and compared with experiment. Results at the CCSD level show relative peak intensities in good agreement with experiment with discrepancies in transition energies due to incomplete treatment of electronic relaxation and correlation that amount to 1–2 eV. With inclusion of triple excitations, errors in energetics are less than 0.9 eV and thereby capturing 90$\%$, 95$\%$, and 98$\%$ of the relaxation-correlation energies for C, O, and Ne, respectively.

We study the resonant control of two nonreactive polar molecules in an optical lattice site, focusing on the example of RbCs. Collisional control can be achieved by tuning bound states of the intermolecular dipolar potential by varying the applied electric field or trap frequency. We consider a wide range of electric fields and trapping geometries, showing that a three-dimensional optical lattice allows significantly wider avoided crossings than free space or quasi-two dimensional geometries. Furthermore, we find that dipolar confinement-induced resonances can be created with reasonable trapping frequencies and electric fields, and have widths that will enable useful control in forthcoming experiments.

Bound structures among dipolar molecules in multilayers are a topic of great interest in the light of recent experiments that have demonstrated the feasibility of the setup. While it is known that two molecules in two adjacent layers will always bind, larger complexes have only been scarcely addressed thus far. Here we prove rigorously that three- and four-body states will never be bound when the dipoles are oriented perpendicular to the layers. The technique employed is general and can be used for more molecules or layers and other geometries. Our analytical findings are supported by numerical calculations for both fermionic and bosonic molecules. Furthermore, we calculate the reduction in intralayer repulsion necessary to bind large complexes and estimate the influence of bound complexes in systems with many layers.

The momentum-space structure of the Faddeev-Yakubovsky (FY) components of weakly bound tetramers is investigated at the unitary limit using a renormalized zero-range two-body interaction. The results, obtained by considering a given trimer level with binding energy $B_3$, provide further support to a universal scaling function relating the binding energies of two successive tetramer states. The correlated scaling between the tetramer energies comes from the sensitivity of the four-boson system to a short-range four-body scale. Each excited $N$th tetramer energy $B_4^{\left(N\right)}$ moves as the short-range four-body scale changes, while the trimer properties are kept fixed, with the next excited tetramer $B_4^{(N+1)}$ emerging from the atom-trimer threshold for a universal ratio $B_4^{\left(N\right)}/B_3=B_4^{\left(N\right)}/B_4^{(N+1)}\simeq 4.6$, which does not depend on $N$. We show that both channels of the FY decomposition [atom-trimer ($K$ type) and dimer-dimer ($H$ type)] present high-momentum tails that reflect the short-range four-body scale. We also found that the $H$ channel is favored over the $K$ channel at low momentum, when the four-body momentum scale largely overcomes the three-body scale.

We show how to optimally control the creation of quantum superpositions in a bosonic Josephson junction within the two-site Bose-Hubbard-model framework. Both geometric and purely numerical optimal-control approaches are used, the former providing a generalization of the proposal of Micheli et al. [Phys. Rev. A 67, 013607 (2003)]. While this method is shown not to lead to significant improvements in terms of time of formation and fidelity of the superposition, a numerical optimal-control approach appears more promising, as it allows creation of an almost perfect superposition, within a time short compared to other existing protocols. We analyze the robustness of the optimal solution against atom-number variations. Finally, we discuss the extent to which these optimal solutions could be implemented with state-of-the-art technology.

Radiation pressure on a flat mirror submerged in a transparent liquid depends not only on the refractive index $n$ of the liquid but also on the phase angle ${\psi }_0$ of the Fresnel reflection coefficient of the mirror, which could be anywhere between 0${}^{\circ }$ and 180${}^{\circ }$. Depending on the value of ${\psi }_0$, the momentum per incident photon picked up by the mirror covers the range between the Abraham and Minkowski values, i.e., the interval (2$\hslash {\omega }_0/nc$, 2$n\hslash {\omega }_0/c$). Here $\hslash$ is the reduced Planck constant, ${\omega }_0$ is the frequency of the incident photon, and $c$ is the speed of light in vacuum. We argue that a simple experimental setup involving a dielectric slab of refractive index $n$, a vibrating mirror placed a short distance behind the slab, a collimated, monochromatic light beam illuminating the mirror through the slab, and an interferometer to measure the phase of the reflected beam is all that is needed to deduce the precise magnitude of the radiation pressure on a submerged mirror. In the proposed experiment, the transparent slab plays the role of the submerging liquid (even though it remains detached from the mirror at all times), and the adjustable gap between the mirror and the slab simulates the variable phase angle ${\psi }_0$. The phase of the reflected beam, measured as a function of time during one oscillation period of the mirror, then provides the information needed to determine the gap dependence of the reflected beam's Doppler shift and, consequently, the radiation pressure experienced by the mirror.