Polariton formalism is applied for studying the propagation of a probe field of light in a cloud of cold atoms influenced by two control laser beams of larger intensity. The laser beams couple resonantly three hyperfine atomic ground states to a common excited state, thus forming a tripod configuration of the atomic energy levels involved. The first control beam can have an optical vortex, with the intensity of the beam going to zero at the vortex core. The second control beam without a vortex ensures the lossless (adiabatic) propagation of the probe beam at a vortex core of the first control laser. We investigate the storage of the probe pulse into atomic coherences by switching off the control beams and its subsequent retrieval by switching the control beams on. The optical vortex is transferred from the control to the probe fields during the storage or retrieval of the probe field. We analyze the conditions needed for the vortex to be transferred efficiently to the regenerated probe beam and discuss the possibilities of experimental implementation of the proposed scheme using atoms such as rubidium or sodium.

Benchmark variational calculations of the four lowest ${}^{2}P$ and ${}^{2}S$ states of the boron atom (including the ground state) have been performed. The wave functions of the states have been expanded in terms of all-particle explicitly correlated Gaussian basis functions and the finite mass of the nucleus has been explicitly accounted for. Variational upper bounds for the nonrelativistic finite- and infinite-nuclear-mass energies of all considered states have been obtained with the relative convergence of the order of 10${}^{-7}$–10${}^{-8}$. Expectation values of the powers of the inter-particle distances and Dirac $\delta$ functions depending on those distances have also been computed. These calculations provide reference values that can be used to test other high-level quantum chemistry methods.

We report photoelectron energy spectra, momentum, and angular distributions for the strong-field single ionization of lithium by 30-fs laser pulses. For peak intensities between ${10}^{11}$ and ${10}^{14}$ W/cm${}^2$ at a central wavelength of 785 nm, the classical over-the-barrier intensity was reached well inside the multiphoton regime. The complete vector momenta of the ionization fragments were recorded by a reaction microscope with a magneto-optically trapped target (MOTREMI). On the theoretical side, the time-dependent Schrödinger equation was solved by two independent methods seeking the solution directly on a radial grid. Distinct differences between the results of both calculations and also in comparison with experiment point to a high sensitivity of this reaction with respect to small details, particularly in the description of the ${\mathrm{Li}}^{+}$ core.

Multichannel molecular high-order harmonic generation (MHOHG) from the asymmetric diatomic molecule HeH${}^{2+}$ in two-color laser fields is investigated from numerical simulation of the corresponding time-dependent Schrödinger equation (TDSE). It is found that the laser-induced electron transfer (LIET) plays a crucial role in MHOHG, which leads to the multichannel harmonic generation from the ground and long-lifetime excited states. LIET is sensitive to the phase differences of the two-color laser pulses, which can be used to control the enhanced excitation (EE) and enhanced ionization (EI) of the system. Both EE and EI have a strong influence on the overall intensity of the MHOHG spectrum, and there may be four orders of magnitude difference in the MHOHG intensity between the enhanced and suppressed cases. In addition, owing to the asymmetry of the two-color laser fields and the recombination of electron with the neighboring ion, multiple cutoff energies are observed. The mechanism of these effects are confirmed by classical simulations.

We perform detailed analytical and numerical studies of a recently proposed method for a spectroscopic measurement of the superfluid fraction of an ultracold atomic gas [N. R. Cooper and Z. Hadzibabic, Phys. Rev. Lett. 104, 030401 (2010)]. Previous theoretical work is extended by explicitly including the effects of nonzero temperature and interactions, and assessing the quantitative accuracy of the proposed measurement for a one-component Bose gas. We show that for suitably chosen experimental parameters the method yields an experimentally detectable signal and a sufficiently accurate measurement. This is illustrated by explicitly considering two key examples: First, for a weakly interacting three-dimensional Bose gas it reproduces the expected result that below the critical temperature the superfluid fraction closely follows the condensate fraction. Second, it allows a clear quantitative differentiation of the superfluid and the condensate density in a strongly interacting Bose gas.

By strongly driving a cyclic-transition three-level artificial atom, demonstrated by such as a flux-based superconducting circuit, we show that coherent microwave signals can be excited along a coupled one-dimensional transmission line. Typically, the intensity of the generated microwave is tunable via properly adjusting the Rabi frequencies of the applied strong-driving fields or introducing a probe field with the same frequency. In practice, the system proposed here could work as an on-chip quantum device with controllable atom-photon interaction to implement a total-reflecting mirror or switch for the propagating probe field.

Using the Hellmann-Feynman theorem, a general comparison theorem is established for an eigenvalue equation of the form $(T+V)|\psi \rangle =E|\psi \rangle$, where $T$ is a kinetic part which depends only on momenta and $V$ is a potential which depends only on positions. We assume that $H^{(1)}=T+V^{(1)}$ and $H^{(2)}=T+V^{(2)}$ ($H^{(1)}=T^{(1)}+V$ and $H^{(2)}=T^{(2)}+V$) support both discrete eigenvalues $E_{\{\alpha \}}^{(1)}$ and $E_{\{\alpha \}}^{(2)}$, where $\{\alpha \}$ represents a set of quantum numbers. We prove that, if $V^{(1)}⩽V^{(2)}$ ($T^{(1)}⩽T^{(2)}$) for all position (momentum) variables, then the corresponding eigenvalues are ordered as $E_{\{\alpha \}}^{(1)}⩽E_{\{\alpha \}}^{(2)}$. Some analytical applications are given.

We study various types of multipartite states lying near the quantum-classical boundary. The so-called classical states are precisely those in which each party can perfectly identify a locally held state without disturbing the global state, a task known as nondisruptive local state identification (NDLID). We show NDLID to be closely related local broadcasting, and we introduce a class of states called generalized classical states which allow for both NDLID and multipartite broadcasting when the most general quantum measurements are permitted. Simple analytical methods and a physical criterion are given for detecting whether a multipartite state is classical or generalized classical. For deciding the latter, a semidefinite programming algorithm is presented which may find use in other fields such as signal processing.

A coherent control scheme for the population distribution in the vibrational states of nonpolar molecules is proposed. Our theoretical analysis and results of numerical simulations for the interaction of the hydrogen molecular ion in its electronic ground state with an infrared laser pulse reveal a selective two-photon transition between the vibrational states via a coupling with the first excited dissociative state. We demonstrate that for a given temporal intensity profile the population transfer between vibrational states, or a superposition of vibrational states, can be made complete for a single chirped pulse or a train of chirped pulses, which accounts for the accumulated phase difference due to the ac Stark effect. Effects of a spatial intensity (or focal) averaging are discussed.

Physical phenomena driven by topological properties, such as the quantum Hall effect, have the appealing feature that they are robust with respect to external perturbations. Lately, a new class of materials has emerged that manifests topological properties at room temperature and without the need of external magnetic fields. These topological insulators are band insulators with large spin-orbit interactions and exhibit the quantum spin-Hall (QSH) effect. Here we investigate the transition between QSH and normal insulating phases under topological deformations of a two-dimensional lattice. We demonstrate that the QSH phase present in the honeycomb lattice loses its robustness as the occupancy of extra lattice sites is allowed. Furthermore, we propose a method for verifying our predictions with fermionic cold atoms in optical lattices. In this context, the spin-orbit interaction is engineered via an appropriate synthetic gauge field.

The challenge of optimizing the emission into single spatial modes of photons from spontaneous parametric down-conversion is addressed from the perspective of spatial entanglement. It is shown that single-mode coupling is most efficient in the absence of entanglement. Evidence of the relationship between spatial entanglement and pump focusing is revealed through experimental results, and numerical simulations show that spatial entanglement and single-mode coupling are optimized under nearly identical pump parameters.

We propose a method to generate analytical quantum Bell inequalities based on the principle of macroscopic locality. By imposing locality over binary processings of virtual macroscopic intensities, we establish a correspondence between Bell inequalities and quantum Bell inequalities in bipartite scenarios with dichotomic observables. We discuss how to improve the latter approximation and how to extend our ideas to scenarios with more than two outcomes per setting.

Steganography is the technique of hiding secret information by embedding it in a seemingly “innocent” message. We present protocols for hiding quantum information by disguising it as noise in a codeword of a quantum error-correcting code. The sender (Alice) swaps quantum information into the codeword and applies a random choice of unitary operation, drawing on a secret random key she shares with the receiver (Bob). Using the key, Bob can retrieve the information, but an eavesdropper (Eve) with the power to monitor the channel, but without the secret key, cannot distinguish the message from channel noise. We consider two types of protocols: one in which the hidden quantum information is stored locally in the codeword, and another in which it is embedded in the space of error syndromes. We analyze how difficult it is for Eve to detect the presence of secret messages, and estimate rates of steganographic communication and secret key consumption for specific protocols and examples of error channels. We consider both the case where there is no actual noise in the channel (so that all errors in the codeword result from the deliberate actions of Alice), and the case where the channel is noisy and not controlled by Alice and Bob.

Quantum systems prepared in pure states evolve into mixtures under environmental action. Continuously realizable ensembles (or physically realizable) are the pure state decompositions of those mixtures that can be generated in time through continuous measurements of the environment. Here, we define continuously realizable entanglement as the average entanglement over realizable ensembles. We search for the measurement strategy to maximize and minimize this quantity through observations on the independent environments that cause two qubits to disentangle in time. We then compare it with the entanglement bounds (entanglement of formation and entanglement of assistance) for the unmonitored system. For some relevant noise sources the maximum realizable entanglement coincides with the upper bound, establishing the scheme as an alternative to protect entanglement. However, for local strategies, the lower bound of the unmonitored system is not reached.

The multimode character of quantum fields imposes constraints on the implementation of high-fidelity quantum gates between individual photons. So far this has only been studied for the longitudinal degree of freedom. Here we show that effects due to the transverse degrees of freedom significantly affect quantum gate performance. We also discuss potential solutions, in particular separating the two photons in the transverse direction.