We describe the effect of thermal motion and buffer-gas collisions on a four-level closed $N$ system interacting with strong pump(s) and a weak probe. This is the simplest system that experiences electromagnetically induced absorption (EIA) due to transfer of coherence via spontaneous emission from the excited state to the ground state. We investigate the influence of Doppler broadening, velocity-changing collisions (VCC), and phase-changing collisions (PCC) with a buffer gas on the EIA spectrum of optically active atoms. In addition to exact expressions, we present an approximate solution for the probe absorption spectrum, which provides physical insight into the behavior of the EIA peak due to VCC, PCC, and the wave-vector difference between the pump and probe beams. VCC are shown to produce a wide pedestal at the base of the EIA peak, which is scarcely affected by the pump-probe angular deviation, whereas the sharp central EIA peak becomes weaker and broader due to the residual Doppler-Dicke effect. Using diffusionlike equations for the atomic coherences and populations, we construct a spatial-frequency filter for a spatially structured probe beam and show that Ramsey narrowing of the EIA peak is obtained for beams of finite width.

The efficiency of extracting single atoms or molecules from an ultracold bosonic reservoir is theoretically investigated for a protocol based on lasers, coupling the hyperfine state in which the atoms form a condensate to another stable state, in which the atom experiences a tight potential in the regime of collisional blockade, the quantum tweezers. The transfer efficiency into the single-atom ground state of the tight trap is fundamentally limited by the collective modes of the condensate, which are thermally and dynamically excited. The noise due to these excitations can be quenched for sufficiently long laser pulses, thereby achieving high efficiencies. These results show that this protocol can be applied to initializing a quantum register based on tweezer traps for neutral atoms.

The position-momentum uncertainty-like inequality based on moments of arbitrary order for $d$-dimensional quantum systems, which is a generalization of the celebrated Heisenberg formulation of the uncertainty principle, is improved here by use of the Rényi-entropy-based uncertainty relation. The accuracy of the resulting lower bound is physico-computationally analyzed for the two main prototypes in $d$-dimensional physics: the hydrogenic and oscillator-like systems.

Source noise affects the security of continuous-variable quantum key distribution (CV QKD) and is difficult to analyze. We propose a model to characterize Gaussian source noise through introducing a neutral party (Fred) who induces the noise with a general unitary transformation. Without knowing Fred’s exact state, we derive the security bounds for both reverse and direct reconciliations and show that the bound for reverse reconciliation is tight.

Absolute vibrational excitation cross sections were measured for diacetylene (1,3-butadiyne). The selectivity of vibrational excitation reveals detailed information about the shape resonances. Excitation of the $\mathrm{C}\equiv$C stretch and of double quanta of the $\mathrm{C}-$H bend vibrations reveals a ${}^2{\Pi }_u$ resonance at 1 eV (autodetachment width $~$30 meV) and a ${}^2{\Pi }_g$ resonance at 6.2 eV (autodetachment width 1–2 eV). There is a strong preference for excitation of even quanta of the bending vibration. Excitation of the $\mathrm{C}-$H stretch vibration reveals ${\sigma }^{*}$ resonances at 4.3, 6.8, and 9.8 eV, with autodetachment widths of $~$2 eV. Detailed information about resonances permits conclusions about the mechanism of the dissociative electron attachment.

Sympathetic cooling of two-ion system, in which one is laser-cooled and the other is sympathetically cooled and their masses are mismatched, in a linear rf trap with a double-well potential is proposed. The double-well potential consists of two wells, and there is one ion in each well. The axial frequencies of the two wells and the spatial interval between them are experimentally controllable. By theoretical analysis, the normal modes of the small oscillations around the equilibrium are derived, and a measure of the sympathetic cooling rate is obtained. As a result, it is found that the sympathetic cooling rate is fast when the frequency of the axial motion of the sympathetically cooled ion is close to that of the laser-cooled ion. In the double-well potential, the sympathetic cooling rate of the ion species whose mass is much heavier or lighter than that of the laser-cooled ion can be fast. The sympathetic cooling rate of ${\mathrm{C}}_{60}$${}^{+}$ by the laser-cooled Ba${}^{+}$ in the double-well potential is estimated to be about 80 times faster than in the conventional setup. The double-well potential may be made by the microfabricated electrode configuration or by the optical dipole force trap.

High-resolution $4f$ photoelectron and subsequent Auger-electron spectra have been measured from free Pb atoms using synchrotron radiation. The fine structure of the spectra has been investigated theoretically by calculating the energies and intensities for $4f$ photoionization and Auger decay processes using the multiconfigurational Dirac-Fock approach. The role of the relativistic effects in the ground and singly and doubly ionized states has been studied on the basis of computed results and their comparison with experiment.

We present a multisite formulation of mean-field theory applied to the disordered Bose-Hubbard model. In this approach the lattice is partitioned into clusters, each isolated cluster being treated exactly, with intercluster hopping being treated approximately. The theory allows for the possibility of a different superfluid order parameter at every site in the lattice, such as what has been used in previously published site-decoupled mean-field theories, but a multisite formulation also allows for the inclusion of spatial correlations allowing us, e.g., to calculate the correlation length (over the length scale of each cluster). We present our numerical results for a two-dimensional system. This theory is shown to produce a phase diagram in which the stability of the Mott-insulator phase is larger than that predicted by site-decoupled single-site mean-field theory. Two different methods are given for the identification of the Bose-glass–to–superfluid transition, one an approximation based on the behavior of the condensate fraction, and one that relies on obtaining the spatial variation of the order parameter correlation. The relation of our results to a recent proposal that both transitions are non-self-averaging is discussed.

We consider a trapped atomic Bose-Einstein condensate penetrated by a repulsive Gaussian potential and theoretically investigate the dynamics induced by oscillating the Gaussian potential. Our study is based on the numerical calculation of the two-dimensional Gross-Pitaevskii equation. Our calculation reveals the dependence of the characteristic behavior of the condensate on the amplitude and frequency of the oscillating potential. These dynamics are deeply related to the nucleation and dynamics of quantized vortices and solitons. When the potential oscillates with a large amplitude, it nucleates many vortex pairs that move away from the potential. When the amplitude of the oscillation is small, it nucleates solitons through an annihilation of vortex pairs. We discuss three issues concerning the nucleation of vortices. The first is the phase diagram for the nucleation of vortices and solitons near the oscillating potential. The second is the mechanism and critical velocity of the nucleation. The critical velocity of the nucleation is an important issue in quantum fluids, and we propose an expression for the velocity containing both the coherence length and the size of the potential. The third is the divergence of the nucleation time, which is the time it takes for the potential to nucleate vortices, near the critical parameters for vortex nucleation.

We propose to construct a family of states of one-dimensional spin chains by replacing the finite-dimensional matrices in matrix product states by infinite-dimensional operators constructed from bosonic annihilation and creation operators. The resulting states are demonstrated to violate the area law and to exhibit long-range correlations. In addition, we propose an efficient way to prepare the states experimentally, in which the spins interact sequentially with an ancilla system.

In this paper, we propose a theoretical scheme of many-ghost imaging in terms of $N$th-order correlated thermal light. We obtain the Gaussian thin lens equations in the many-ghost imaging protocol. We show that it is possible to produce $N-1$ ghost images of an object at different places in a nonlocal fashion by means of a higher order correlated imaging process with an $N$th-order correlated thermal source and correlation measurements. We investigate the visibility of the ghost images in the scheme and obtain the upper bounds of the visibility for the $N$th-order correlated thermal-light ghost imaging. It is found that the visibility of the ghost images can be dramatically enhanced when the order of correlation becomes larger. It is pointed out that the many-ghost imaging phenomenon is an observable physical effect induced by higher order coherence or higher order correlations of optical fields.

Conformal invisibility devices are only supposed to work within the valid range of geometrical optics. Here, we show by numerical simulations and analytical arguments that for certain quantized frequencies, they are nearly perfect even in a regime that clearly violates geometrical optics. The quantization condition follows from the analogy between the Helmholtz equation and the stationary Schrödinger equation.

We extend the idea of weak measurements to the general case, provide a complete treatment, and obtain results for both the regime when the preselected and postselected states (PPS) are almost orthogonal and the regime when they are exactly orthogonal. We surprisingly find that for a fixed interaction strength, there may exist a maximum signal amplification and a corresponding optimum overlap of PPS to achieve it. For weak measurements in the orthogonal regime, we find interesting quantities that play the same role that weak values play in the nonorthogonal regime.

Entanglement represents an important resource for quantum information processing, but its generation itself requires physical resources that are limited. We propose a scheme for generating a wide class of entangled qudit-type states of optical field modes at sites separated by noisy medium when only weak optical nonlinearities are available at both sites. The protocol is also based on exploiting a weak probe field, transmitted between the sites and used for generation of quantum correlations between two spatially separated field modes. The idea of probabilistic entanglement enhancement by measurement is discussed and a corresponding scheme for measuring the probe field state with linear optics and photodetectors not resolving photon numbers is proposed. It is shown that the protocol is applicable in the case when limited efficiency and dark counts of photodetectors, decoherence, and uncertainty of nonlinear coupling constants are present.

Laser-induced dissociation of ${\mathrm{O}}_2^{+}$ is studied in the strong-field limit using two independent methods, namely a crossed laser–ion-beam coincidence 3D momentum imaging method and a supersonic gas jet velocity map imaging technique (790 and 395 nm, 8–40 fs, $~$10${}^{15}$ W/cm${}^2$). The measured kinetic energy release spectra from dissociation of ${\mathrm{O}}_2^{+}$ and dissociative ionization of ${\mathrm{O}}_2$ reveal vibrational structure which persists over a wide range of laser intensities. The vibrational structure is similar for ${\mathrm{O}}_2^{+}$ produced incoherently in an ion source and coherently by laser pulses. By evaluation of the potential energy curves, we assign the spectral energy peaks to dissociation of the $v=10$–15 vibrational states of the metastable $a\hspace{0.5em}{}^4{\Pi }_u$ state via the dissociation pathway $|a\hspace{0.5em}{}^4{\Pi }_u$$\rangle \to |f\hspace{0.5em}{}^4{\Pi }_g-1\omega$$\rangle$—a mechanism equivalent to bond softening in ${\mathrm{H}}_2^{+}$.

The cross sections for Compton scattering from positronium are calculated in the range from 1 to 100 keV incident photon energy. The calculations are based on the $A^2$ term of the photon-electron or photon-positron interaction. Unlike in hydrogen, the scattering occurs from two centers and the interference effect plays an important role for energies below 8 keV. Because of the interference, the criterion for validity of the impulse approximation for positronium is more restrictive compared to that for hydrogen.