We experimentally demonstrate a high-fidelity visible-to-telecommunication wavelength conversion of a photon by using a solid-state-based difference frequency generation. In the experiment, one half of a picosecond visible entangled photon pair at 780 nm is converted to a 1522-nm photon, resulting in the entangled photon pair between 780 and 1522 nm. Using superconducting single-photon detectors with low dark count rates and small timing jitters, we selectively observed well-defined temporal modes containing the two photons. We achieved a fidelity of $0.93\pm 0.04$ after the wavelength conversion, indicating that our solid-state-based scheme can be used for faithful frequency down conversion of visible photons emitted from quantum memories composed of various media.

We use frequency-chirped light on the nanosecond time scale to produce ultracold ${}^{87}$Rb${}_2$ molecules in the lowest triplet state via the process of photoassociation. Comparing to quantum simulations of the molecular formation, we conclude that coherent stimulated emission plays an important role and is primarily responsible for the significant difference observed between positive and negative chirps.

We study the decay behaviors of ultracold atoms in metastable states with spin-orbit coupling (SOC), and demonstrate that there are two SOC-induced decay mechanisms. One arises from the trapping potential and the other is due to interatomic collision. We present general schemes for calculating decay rates from these two mechanisms, and illustrate how the decay rates can be controlled by experimental parameters. We experimentally measure the decay rates over a broad parameter region, and the results agree well with theoretical calculations. This work provides an insight for both quantum simulation involving metastable dressed states and studies on few-body problems with SO coupling.

We discuss how the thermalization of an elementary quantum system is modified when the system is placed in an environment out of thermal equilibrium. To this aim we provide a detailed investigation of the dynamics of an atomic system placed close to a body of arbitrary geometry and dielectric permittivity, whose temperature $T_{\mathrm{M}}$ is different from that of the surrounding walls $T_{\mathrm{W}}$. A suitable master equation for the general case of an $N$-level atom is first derived and then specialized to the cases of a two- and three-level atom. Transition rates and steady states are explicitly expressed as a function of the scattering matrices of the body and become both qualitatively and quantitatively different from the case of radiation at thermal equilibrium. Out of equilibrium, the system steady state depends on the system-body distance, on the geometry of the body, and on the interplay of all such parameters with the body optical resonances. While a two-level atom tends toward a thermal state, this is not the case already in the presence of three atomic levels. This peculiar behavior can be exploited, for example, to invert the populations ordering and to provide an efficient cooling mechanism for the internal state of the quantum system. We finally provide numerical studies and asymptotic expressions when the body is a slab of finite thickness. Our predictions can be relevant for a wide class of experimental configurations out of thermal equilibrium involving different physical realizations of two- or three-level systems.

Quantum memories provide intermediate storage of quantum information until it is needed for the next step of a quantum algorithm or a quantum communication process. Relevant figures of merit are therefore the fidelity with which the information can be written and retrieved, the storage time, and also the speed of the read-write process. Here, we present experimental data on a quantum memory consisting of a single ${}^{13}$C nuclear spin that is strongly coupled to the electron spin of a nitrogen-vacancy (NV) center in diamond. The strong hyperfine interaction of the nearest-neighbor carbon results in transfer times of 300 ns between the register qubit and the memory qubit, with an overall fidelity of 88$\%$ for the write-storage-read cycle. The observed storage times of 3.3 ms appear to be limited by the $T_1$ relaxation of the electron spin. We discuss a possible scheme that may extend the storage time beyond this limit.

In this paper, the $E1$, $E2$, and $M1$ transition amplitudes are calculated along with the hyperfine $A$ and $B$ constants of doubly ionized gallium using the relativistic coupled-cluster approach. Electron correlations and the Gaunt interactions are considered to all orders using the coupled-cluster theory in a relativistic framework and their contributions are discussed explicitly in the calculations of all these amplitudes. Some interesting features of the correlation effects on the Gaunt interactions are noticed in the calculations of the hyperfine constants. The lifetimes of some low-lying states are also calculated using the transition amplitudes obtained by the present theory and the experimental transition energies. The calculated $E1$ transition amplitudes and lifetimes are in good agreement with the results obtained by the other theories and experiment. The hyperfine splitting of the ground states of ${}^{71}$Ga iii and ${}^{69}$Ga iii are found to be about 35 and 27.5 GHz, respectively, which shows the importance of these isotopes for the possible use of microwave frequency standards. The calculated hyperfine constants are associated with linewidth estimations of some transition lines in the visible and ultraviolet regions of electromagnetic spectrum.

Electron emission occurring in transfer ionization for He${}^{2+}$ collisions with argon has been investigated using cold target recoil ion momentum spectroscopy. The double differential cross sections for electron capture to the continuum of the projectile (cusp-shaped electrons) are presented for collision energies from 17.5 to 75 keV/u. For an energy of 30 keV/u, we find a maximum in the experimental ratio of the cusp-shaped electron yield to the total electron yield. This result is explained in terms of the velocity matching between the projectile ion and the electron initially bound to the target. One of the important issues for double electron transitions is the role of electron-electron correlation. If this correlation is weak, then the transfer-ionization process can be viewed as two separate sequential processes. If this correlation is strong, then the transfer-ionization process would happen simultaneously and not sequentially. Our experimental and theoretical results indicate that correlation is weak and that the first step is target ionization followed by charge capture.

We study quantum reflection of antihydrogen atoms from matter slabs due to the van der Waals–Casimir-Polder potential. By taking into account the specificities of antihydrogen and the optical properties and width of the slabs, we calculate realistic estimates for the potential and quantum reflection amplitudes. Next we discuss the paradoxical result of larger reflection coefficients estimated for weaker potentials in terms of the Schwarzian derivative. We analyze the limiting case of reflections at small energies, which are characterized by a scattering length and have interesting applications for trapping and guiding antihydrogen using material walls.

A quasiresonant laser induces a long-range attractive force within a cloud of cold atoms. We take advantage of this force to build in the laboratory a system of particles with a one-dimensional gravitational-like interaction, at a fluid level of modeling. We give experimental evidences of such an interaction in a cold Strontium gas, studying the density profile of the cloud, its size as a function of the number of atoms, and its breathing oscillations.

The photofragmentation of the water cation H${}_2$O${}^{+}$ through dicationic states has been studied at $35.0\pm 0.2$ nm ($35.4\pm 0.3$ eV) and $21.8\pm 0.2$ nm ($56.8\pm 0.5$ eV) with a crossed ion-photon beams experiment at the free electron laser FLASH. The dissociation of the dications is found to be similar at the two wavelengths and to proceed into ${\mathrm{O}}^0+2{\mathrm{H}}^{+}$, ${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$, and ${\mathrm{O}}^{+}+{\mathrm{H}}_2{}^{+}$, with determined ratios ${\sigma }_{{\mathrm{OH}}^{+}+{\mathrm{H}}^{+}}/{\sigma }_{{\mathrm{O}}^{+}+{\mathrm{H}}_2{}^{+}}=4.2\pm 0.3$ and ${\sigma }_{{\mathrm{OH}}^{+}+{\mathrm{H}}^{+}}/{\sigma }_{{\mathrm{O}}^0+2{\mathrm{H}}^{+}}>0.7$. The measured kinetic-energy releases for these processes are consistent with three-body breakup (${\mathrm{O}}^0+2{\mathrm{H}}^{+}$) occurring mainly through the $2{}^3{A}^{\prime \prime }$ and $2{}^1{A}^{\prime \prime }$ states of ${\mathrm{H}}_2{\mathrm{O}}^{2+}$ and two-body breakup (${\mathrm{OH}}^{+}+{\mathrm{H}}^{+}$) occurring through $X{}^3{A}^{\prime \prime }$, $1{}^3{A}^{\prime }$, and $1{}^1{A}^{\prime \prime }$ states of H${}_2$O${}^{2+}$, as predicted in a recent theoretical study [Gervais et al., J. Chem. Phys. 131, 024302 (2009)]. In addition to the kinetic-energy releases, we also report on fragment correlation in the three-body channel where the two protons carry the major part of the released momentum.

We present a fully ab initio model and calculations of the spectral shapes of absorption lines in a pure molecular gas under conditions where the influences of collisions and of the Doppler effect are significant. Predictions of the time dependence of dipole autocorrelation functions (DACFs) are made for pure CO${}_2$ at room temperature using requantized classical molecular dynamics simulations. These are carried, free of any adjusted parameter, on the basis of an accurate anisotropic intermolecular potential. The Fourier-Laplace transforms of these DACFs then yield calculated spectra which are analyzed, as some measured ones, through fits using Voigt line profiles. Comparisons between theory and various experiments not only show that the main line-shape parameters (Lorentz pressure-broadening coefficients) are accurately predicted, but that subtle observed non-Voigt features are also quantitatively reproduced by the model. These successes open renewed perspectives for the understanding of the mechanisms involved (translational-velocity and rotational-state changes and their dependences on the molecular speed) and the quantification of their respective contributions. The proposed model should also be of great help for the test of widely used empirical line-shape models and, if needed, the construction of more physically based ones.

By using a two-fluid model the effect of impurities on the transition temperature of a dipolar trapped Bose gas is investigated. By treating Gaussian spatial correlation for impurities from the interaction modified spectra of the system, the formula for the shift of the transition temperature is derived. The shift of the transition temperature contains essentially three contributions due to contact, dipole-dipole, and impurity interactions. Applying our results to dipolar Bose gases shows that the shift of the transition temperature due to impurities could be measured for an isotropic trap (dipole-dipole contribution is zero) and the Feshbach resonance technique (contact potential contribution is negligible).

We develop a simple, mean-field-like theory for the normal phase of a unitary Fermi gas by deriving a self-consistent equation for its self-energy via a momentum-dependent coupling constant for both attractive and repulsive universal fermions. For attractive universal fermions in the lower branch of a Feshbach resonance, we use zero-temperature Monte Carlo results as a starting point for one-step iteration in order to derive an analytical expression for the momentum-dependent self-energy. For repulsive universal fermions in the upper branch of a Feshbach resonance, we iteratively calculate the momentum-dependent self-energy via our self-consistent equation. Lastly, for the case of population imbalance, we propose an ansatz for higher-order virial expansion coefficents. Overall, we find that our theory is in good agreement with currently available, high-temperature experimental data.

We use an alternative approach to study the quantum phase transition in a coupled cavity lattice at finite temperature. As an illustrative example, we investigate the behaviors of the trace distance and quantum phase transition in a Jaynes-Cummings lattice at finite temperature. It is found that the trace distance can be used to describe the critical point of the quantum phase transition at finite low temperatures and the critical points are sensitive to the atom-field interaction strength and the detuning factor. For nonequilibrium states, we demonstrate that the time evolution of the trace distance's maximum value is also a good indicator of the critical points. Moreover, we show that the scaling behavior of the derivative of the trace distance at the critical points and the scaling rule are dependent on the external parameters of the Hamiltonian.

We consider the behavior of fluctuations near the sonic horizon and the role of the nonlocality of interaction (nonlinearity) on their regularization. The nonlocality dominates if its characteristic length scale is larger than the regularization length. The influence of nonlocality may be important in the current experiments on the transonic flow in Kerr nonlinear media. Experimental conditions, under which the observation of straddled fluctuations can be observed, are discussed.

The requirement for symmetry of the biphoton wave function results in a double-peak structure of the coincidence spectra of spontaneous parametric down-conversion (SPDC) [Fedorov, Mikhailova, and Volkov, Phys. Scr. 85, 058105 (2012)]. In this work we report the experimental observation of this effect by carefully tailoring parameters of the SPDC crystal, the ultrafast pump laser, and the measurement setup. The results are shown to be relevant to quantum state engineering of ultrafast polarization or frequency-entangled mixed states.

Quantum correlations have fundamental and technological interest, and hence many measures have been introduced to quantify them. Some hierarchical orderings of these measures have been established, for example, discord is bigger than entanglement, and we present a class of bipartite states, called premeasurement states, for which several of these hierarchies collapse to a single value. Because premeasurement states are the kind of states produced when a system interacts with a measurement device, the hierarchy collapse implies that the uncertainty of an observable is quantitatively connected to the quantum correlations (entanglement, discord, etc.) produced when that observable is measured. This fascinating connection between uncertainty and quantum correlations leads to a reinterpretation of entropic formulations of the uncertainty principle, so-called entropic uncertainty relations, including ones that allow for quantum memory. These relations can be thought of as lower bounds on the entanglement created when incompatible observables are measured. Hence, we find that entanglement creation exhibits complementarity, a concept that should encourage exploration into “entanglement complementarity relations.”

A recent scheme for perfect transmission of quantum states through quasi-one-dimensional chains requires application of global control at regular intervals of time. We study the effect of stochastic noise in this control and find that the scheme is robust for reasonable values of disorder. Both uncorrelated and correlated noise in the external control are studied, and it is remarkably found that the efficiency of the protocol is much higher in the presence of correlated noise.

The aim of this paper is to show that within the open-system framework the sum-over-modes approach of Casimir leads to the Lifshitz formula for the Casimir free energy. A general result applicable to arbitrary geometries is obtained through the use of Ford, Lewis, and O’Connell's remarkable formula. Additionally, we address the possibility of obtaining the Casimir energy as a sum over complex “modes.” We show in this case that the standard sum-over-modes formula must be suitably generalized to avert unphysical complex energies. Finally, we apply our results to several standard examples.

Motivated by recently developed techniques making it possible to compute Casimir energies for any object whose scattering $S$ matrix (or, equivalently, $T$ matrix) is available, we develop a variable phase method to compute the $S$ matrix for localized but asymmetric sources. Starting from the case of scalar potential scattering, we develop a combined inward-outward integration algorithm that is numerically efficient and extends robustly to imaginary wave number. We then extend these results to electromagnetic scattering from a position-dependent dielectric. This case requires additional modifications to disentangle the transverse and longitudinal modes.