State mapping between atoms and photons, and photon-photon interactions play an important role in scalable quantum information processing. We consider the interaction of a two-level atom with a quantized propagating pulse in free space and study the probability $P_e(t)$ of finding the atom in the excited state at any time $t$. This probability is expected to depend on (i) the quantum state of the pulse field and (ii) the overlap between the pulse and the dipole pattern of the atomic spontaneous emission. We show that the second effect is captured by a single parameter $\Lambda \in [0,8\pi /3]$, obtained by weighting the dipole pattern with the numerical aperture. Then, $P_e(t)$ can be obtained by solving time-dependent Heisenberg-Langevin equations. We provide detailed solutions for both single-photon Fock state and coherent states and for various temporal shapes of the pulses.

We analyze the three-mode correlation properties of the electromagnetic field in a optical parametric oscillator below-threshold. We employ a perturbative expansion of the Itô equations derived from the positive-P representation of the density matrix. Using the generalized Cauchy-Schwarz inequality, we investigate the genuine quantum nature of the triple correlations between the interacting fields, since in this case continuous-variable entanglement is not detected by the van Loock-Furusawa criterion [Phys. Rev. A 67, 052315 (2003)]. Although not being a necessary condition, these triple correlations are sufficient evidence of tripartite entanglement. Of course, our characterization of the quantum correlations is applicable to non-Gaussian states, which we show to be the case of the optical parametric oscillator below-threshold, provided nonlinear quantum fluctuations are properly taken into account.

We study theoretically the dynamics of two-component Bose-Einstein condensates in two dimensions, which admit topological excitations related to the skyrmions of nuclear physics. We investigate a branch of uniformly propagating solitary waves, which, at high momentum, can be viewed as skyrmion-antiskyrmion pairs. We study these solitary waves for a range of interaction regimes and show that, for experimentally relevant cases, there is a transition to spatially extended spin-wave states at low momentum. We explain how this can be understood by analogy to the two-dimensional ferromagnet. Finally, we discuss how such solitary waves can be generated and studied in experiment.

We show that a parametrically coupled qubit can be used to fully reconstruct the quantum state of a harmonic oscillator even when both systems are subject to decoherence. By controlling the coupling strength of the qubit over time, the characteristic function of the oscillator at any phase-space point can be directly measured by combining the expectation values of two Pauli operators. The effect of decoherence can be filtered out from the measured data, provided a sufficient number of experimental runs are performed. In situations where full state reconstruction is not practical or not necessary, the method can still be used to estimate low-order moments of the mechanical quadratures. We also show that in the same framework it is possible to prepare superposition states of the oscillator. The model is very general but particularly appropriate for nanomechanical systems.

We study the evolution of superposition states under the influence of a reservoir at zero and finite temperatures in cavity quantum electrodynamics aiming to know how their purity is lost over time. The superpositions studied here are composed of coherent states, orthogonal coherent states, squeezed coherent states, and orthogonal squeezed coherent states, which we introduce to generalize the orthogonal coherent states. For comparison, we also show how the robustness of the superpositions studied here differs from that of a qubit given by a superposition of zero- and one-photon states.

We extend the program initiated by T. Werlang et al. [Phys. Rev. Lett. 105, 095702 (2010)] in several directions. Firstly, we investigate how useful quantum correlations, such as entanglement and quantum discord, are in the detection of critical points of quantum phase transitions when the system is at finite temperatures. For that purpose we study several thermalized spin models in the thermodynamic limit, namely, the $\mathit{XXZ}$ model, the $\mathit{XY}$ model, and the Ising model, all of which with an external magnetic field. We compare the ability of quantum discord, entanglement, and some thermodynamic quantities to spotlight the quantum critical points for several different temperatures. Secondly, for some models we go beyond nearest neighbors and also study the behavior of entanglement and quantum discord for second nearest neighbors around the critical point at finite temperature. Finally, we furnish a more quantitative description of how good all these quantities are in spotlighting critical points of quantum phase transitions at finite $T$, bridging the gap between experimental data and those theoretical descriptions solely based on the unattainable absolute zero assumption.

Laser excitation of the $4s4p\hspace{0.5em}{}^3{P}_2\hspace{0.5em}–\hspace{0.5em}4s3d\hspace{0.5em}{}^1{D}_2$ transition in atomic calcium has been observed and the wavelength determined to 1530.5298(6) nm. The metastable $4s4p\hspace{0.5em}{}^3{P}_2$ atoms were magnetically trapped in the quadrupole magnetic field of a magneto-optical trap. This state represents the only “loss” channel for the calcium atoms when laser cooled on the $4s^2\hspace{0.5em}{}^1{S}_0\hspace{0.5em}–\hspace{0.5em}4s4p\hspace{0.5em}{}^1{P}_1$ transition. A rate equation model shows that an order of magnitude more atoms are trapped in this state compared with those taking part in the main cooling cycle. Excitation of the ${}^3{P}_2$ atoms back up to the $4s3d\hspace{0.5em}{}^1{D}_2$ state provides a means of accessing these atoms. Efficient repumping is achieved if the 1530-nm laser is used in conjunction with a 672-nm laser driving the $4s3d\hspace{0.5em}{}^1{D}_2\hspace{0.5em}–\hspace{0.5em}4s5p\hspace{0.5em}{}^1{P}_1$ transition. In the present experiment, we detected about $4.5\times {10}^4$ trapped ${}^3{P}_2$ atoms, a relatively low atom density, and measured a lifetime of approximately 1 s, which is limited by background collisions.

In this work, we present integral, differential, and momentum transfer cross sections for elastic scattering of low-energy electrons by nitromethane, for energies up to 10 eV. We calculated the cross sections using the Schwinger multichannel method with pseudopotentials, in the static-exchange and in the static-exchange plus polarization approximations. The computed integral cross sections show a $\pi {}^{*}$ shape resonance at 0.70 eV in the static-exchange-polarization approximation, which is in reasonable agreement with experimental data. We also found a $\sigma {}^{*}$ shape resonance at 4.8 eV in the static-exchange-polarization approximation, which has not been previously characterized by the experiment. We also discuss how these resonances may play a role in the dissociation process of this molecule.

We examine the effect of cluster size on the interaction of Ar${}_{55}$-Ar${}_{2057}$ with intense extreme ultraviolet (XUV) pulses, using a model we developed earlier that includes ionization via collisional excitation as an intermediate step. We find that the dynamics of these irradiated clusters is dominated by collisions. Larger clusters are more highly collisional, produce higher charge states, and do so more rapidly than smaller clusters. Higher charge states produced via collisions are found to reduce the overall photon absorption, since charge states of Ar${}^{2+}$ and higher are no longer photoaccessible. We call this mechanism collisionally reduced photoabsorption, and it decreases the effective cluster photoabsorption cross section by more than 30% for Ar${}_{55}$ and 45% for Ar${}_{2057}$. The time evolution of the electron kinetic energy distribution begins as a (mostly) Maxwellian distribution. Further, the electron velocity distribution of large clusters quickly become isotropic while smaller clusters retain the inherent anisotropy created by photoionization. Last, the total electron kinetic-energy distribution is integrated over the spatial profile of the laser and the log-normal distribution of cluster size for comparison with a recent experiment [C. Bostedt et al., Phys. Rev. Lett. 100, 133401 (2008)], and good agreement is found.

The free-free transition is studied for an electron-hydrogen atom system in the ground state at very low incident energies in the presence of an external homogeneous, monochromatic, and linearly polarized laser field. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing the Volkov solutions in both the initial and final channels. The space part of the scattering wave function for the electron is solved numerically by taking into account the effect of electron-exchange interactions, short-range interactions, as well as of long-range interactions. The laser-assisted differential as well as total elastic cross sections are calculated for single-photon absorption or emission in the soft photon limit, the laser intensity being much less than the atomic field intensity. A strong suppression is noted in the laser-assisted cross sections as compared to the field-free situations. A significant difference is noted in the singlet and the triplet cross sections.

The strong field ionization dynamics of an atom in a strong extreme-ultraviolet radiation field is investigated using numerical solutions of the time-dependent Schrödinger equation. The analysis of the electron quantum distribution in phase space after the interaction provides detailed information on the ionization time evolution. It evidences the interrelation among the periodic slow drift of the bound electron wave packet in the highly oscillating field, the modulation of the ionization wave packet, and the previously unknown fine structure of above-threshold ionization spectra in the stabilization regime.

We report the experimental observation of electromagnetically induced transparency (EIT) in an inhomogeneously broadened V-type Na${}_2$ molecular system. The experiment is performed with both co- and counterpropagating arrangements for the propagation directions of the coupling and probe laser beams. In our theoretical model we employ the density matrix formalism, as well as perturbative methods for obtaining the probe field absorption profile for both open and closed systems. Simulations of the experimental data show excellent agreement with the predictions derived from the basic theory. Our fluorescent intensity measurements show that, in the copropagating configuration, the EIT plus saturation window depth is about $95\%$, while under similar conditions in the counterpropagating geometry we observed $40\%$–$45\%$ reduction in the fluorescence signal around the line center. To separate the two simultaneously occurring mechanisms in a V-type system (i.e., EIT and saturation) that are induced by the coupling field, we have carried out theoretical calculations which show that, in the copropagating case, a significant fraction of the depth of the dip is due to the coherent effect of EIT. When the coupling and probe beams are in the counterpropagating configuration, the dip is mostly due to saturation effects alone.

Recent experiments revealed the importance of higher-band effects for the Mott-insulator(MI)–superfluid transition(SF) of ultracold bosonic atoms or mixtures of bosons and fermions in deep optical lattices [Best et al., Phys. Rev. Lett. 102, 030408 (2009); Will et al., Nature (London) 465, 197 (2010)]. In the present work we derive an effective lowest-band Hamiltonian in three dimensions that generalizes the standard Bose-Fermi-Hubbard model taking these effects as well as nonlinear corrections of the tunneling amplitudes mediated by interspecies interactions into account. It is shown that a correct description of the lattice states in terms of the bare-lattice Wannier functions, rather than approximations such as harmonic-oscillator states, is essential. In contrast to self-consistent approaches based on effective Wannier functions, our approach captures the observed reduction of the superfluid phase for repulsive interspecies interactions.

We have observed the signatures of quantum trajectories while generating harmonics in metal plasmas by using a high pulse repetition rate (1-kHz) Ti:sapphire laser source. We present how focusing conditions, laser intensity, plasma concentration, and the chirp of laser radiation influence the observation of separated quantum paths. Our studies demonstrate the clear fingerprints of the separated quantum paths in the case of an ionized medium.

We investigate theoretically the influence of laser phase noise on the cooling and heating of a generic cavity optomechanical system. We derive the back-action damping and heating rates and the mechanical frequency shift of the radiation-pressure-driven oscillating mirror, and derive the minimum phonon occupation number for small laser linewidths. We find that, in practice, laser phase noise does not pose serious limitations to ground-state cooling. Additionally, we explore the regime of parametric amplification where coherent oscillations of the mirror are realizable. It is found that heating from laser phase noise is of significance and can cause the onset of instabilities. We then consider the effects of laser phase noise in a parametric cavity driving scheme that minimizes the back-action heating of one of the quadratures of the mechanical oscillator motion. Laser linewidths, narrow compared to the decay rate of the cavity field, do not pose any significant problems in an experimental setting, but broader linewidths limit the practicality of this back-action evasion method.

We present measurements of the optical absorption of K vapor at 795 nm due to the presence of high-pressure He gas. The results set a limit on the polarization attainable in hybrid spin-exchange optical pumping of ${}^3\mathrm{He}$.

We report a proof-of-principle experiment demonstrating that appropriately chosen set of Hermite-Gaussian modes constitutes a Schmidt decomposition for transverse momentum states of biphotons generated in the process of spontaneous parametric down-conversion. We experimentally realize projective measurements in the Schmidt basis and observe correlations between appropriate pairs of modes. We perform tomographical state reconstruction in the Schmidt basis, by direct measurement of single-photon density matrix eigenvalues.

We develop the partitioning technique for quantum discrete systems. The graph consists of several subgraphs: a central graph and several branch graphs, with each branch graph being rooted by an individual node on the central one. We show that the effective Hamiltonian on the central graph can be constructed by adding additional potentials on the branch-root nodes, which generates the same result as does the the original Hamiltonian on the entire graph. Exactly solvable models are presented to demonstrate the main points of this paper.

A constraint on a spin-dependent interaction, induced by a pseudovector light boson, is presented. The interaction includes a Yukawa-type contribution $\alpha {}^{″}({\mathbf{s}}_1·{\mathbf{s}}_2)e^{-\lambda r}/r$ and a contact spin-spin term. To disentangle the long-range and contact terms we utilize experimental data on the $1s$ and $2s$ hyperfine intervals for light two-body atoms and construct a specific difference $8\times E_{\mathrm{hfs}}(2s)-E_{\mathrm{hfs}}(1s)$. That allows one to constrain the spin-dependent coupling constant $\alpha {}^{″}$ of an electron-nucleus Yukawa-type interaction in hydrogen, deuterium, and the helium-3 ion at the level below a part in ${10}^{16}$. The derived constraint is related to the range of masses below $4\hspace{0.5em}\mathrm{keV}/c^2$. The combined constraint including the contact terms is also presented.

We investigate the error due to the dc Stark effect for quantum information processing for trapped ion quantum computers using the scalable architecture proposed in D. Wineland et al. [J. Res. Natl. Inst. Stand. Technol. 103, 259 (1998)] and D. Kielpinski et al. [Nature (London) 417, 709 (2002)]. As the operational speed increases, dephasing and decoherence due to the dc Stark effect become prominent as a large electric field is applied for rapidly transporting ions. We estimate the relative significance of the decoherence and dephasing effects and find that the latter is dominant. We find that the minimum possible dephasing is quadratic in the length of a trap and an inverse cubic in the operational time scale. From these relations, we obtain the operational speed range at which the shifts, caused by the dc Stark effect, are no longer negligible, no matter on which trajectory the ion is transported. Without phase correction, the shortest time a qubit can be transferred across a 100-micrometer-long trap, without excessive error, is about 10 ns for a ${}^{40}{\mathrm{Ca}}^{+}$ ion and 50 ps for a ${}^9{\mathrm{Be}}^{+}$ ion. In practice, the accumulated error is difficult to track and to calculate; thus, our paper gives an estimate for the range of the speed limit imposed by the dc Stark effect.