Due to atomic interactions and dispersion in the total atom number, the order parameter of a pair-condensed Fermi gas experiences a collapse in a time that we derive microscopically. As in the bosonic case, this blurring time depends on the derivative of the gas chemical potential with respect to the atom number and on the variance of that atom number. The result is obtained first using linearized time-dependent Bogoliubov–de Gennes equations, then in the random-phase approximation, and then it is generalized to beyond the mean field. In this framework, we construct and compare two phase operators for the paired fermionic field: The first one, issuing from our study of the dynamics, is the infinitesimal generator of adiabatic translations in the total number of pairs. The second one is the phase operator of the amplitude of the field of pairs on the condensate mode. We explain that these two operators differ due to the dependence of the condensate wave function on the atom number.

Recent studies have shown that topological models with interacting anyonic quasiparticles can be used as self-correcting quantum memories. Here we study the behavior of these models at thermal equilibrium. It is found that the interactions allow topological order to exist at finite temperature, not only in an extension of the ground-state phase, but also in a novel form of topologically ordered phase. Both phases are found to support self-correction in all models considered, and the transition between them corresponds to a change in the scaling of memory lifetime with system size.

Dielectronic and higher-order resonant electron recombination processes including a $K$-shell excitation were systematically measured at high electron energy resolution in electron beam ion traps. After storing highly charged Ar, Fe, and Kr ions, the dependence on atomic number $Z$ of the contribution of these processes to the total recombination cross section was studied and compared with theoretical calculations. Large higher-order resonant recombination contributions were found, especially for systems with $10<Z<36$. In some cases, they even surpass the strength of the dielectronic channel, which was hitherto presumed to be always the dominant one. These findings have consequences for modeling high-temperature plasmas. Features attributed to intershell quadruelectronic recombination were also observed. The experimental data obtained for the He-like to O-like isoelectronic sequences compare well with the results of advanced relativistic distorted-wave calculations employing multiconfiguration Dirac-Fock bound-state wave functions that include threefold and fourfold excitations.

Tapered optical fibers with a nanofiber waist are widely used tools for efficient coupling of light to photonic devices or quantum emitters via the nanofiber's evanescent field. In order to ensure well-controlled coupling, the phase and polarization of the nanofiber guided light field have to be stable. Here, we show that in typical tapered optical fibers these quantities exhibit high-frequency thermal fluctuations. They originate from high-$Q$ torsional oscillations that optomechanically couple to the nanofiber-guided light. We present a simple ab initio theoretical model that quantitatively explains the torsional mode spectrum and that can be used to design tapered optical fibers with tailored mechanical properties.

We observed two-photon phase superresolution in an unbalanced Michelson interferometer with classical Gaussian laser pulses. Our work is a time-reversed version of a two-photon interference experiment using an unbalanced Michelson interferometer. A measured interferogram exhibits two-photon phase superresolution with a high visibility of $97.9\%\pm 0.4\%$. Its coherence length is about 22 times longer than that of the input laser pulses. It is a classical analog to the large difference between the one- and two-photon coherence lengths of entangled photon pairs.

We analyze the influence of dipole-dipole interactions between Rydberg atoms on the generation of Abelian artificial gauge potentials and fields. When two Rydberg atoms are driven by a uniform laser field, we show that the combined atom-atom and atom-field interactions give rise to nonuniform, artificial gauge potentials. We identify the mechanism responsible for the emergence of these gauge potentials. Analytical expressions for the latter indicate that the strongest artificial magnetic fields are reached in the regime intermediate between the dipole blockade regime and the regime in which the atoms are sufficiently far apart such that atom-light interaction dominates over atom-atom interactions. We discuss the differences and similarities of artificial gauge fields originating from resonant dipole-dipole and van der Waals interactions. We also give an estimation of experimentally attainable artificial magnetic fields resulting from this mechanism and we discuss their detection through the deflection of the atomic motion.

We report on measurements of the differential polarizability between the nearly degenerate, opposite parity states in atomic dysprosium at 19 797.96 cm${}^{-1}$, and the differential blackbody radiation induced Stark shift of these states. The differential scalar and tensor polarizabilities due to additional states were measured for the $\left|M\right|=7,\cdots ,10$ sublevels in ${}^{164}$Dy and ${}^{162}$Dy and determined to be ${\overline{\alpha }}_{BA}^{\left(0\right)}=180{\left(45\right)}_{\mathrm{stat}}{\left(8\right)}_{\mathrm{sys}}$ $\text{mHz}/{(\mathrm{V}/\mathrm{cm})}^2$ and ${\overline{\alpha }}_{BA}^{\left(2\right)}=-163{\left(65\right)}_{\mathrm{stat}}{\left(5\right)}_{\mathrm{sys}}$ $\text{mHz}/{(\mathrm{V}/\mathrm{cm})}^2$, respectively. The average blackbody radiation induced Stark shift of the Zeeman spectrum was measured around 300 K and found to be $-34\left(4\right)$ mHz/K and $+29\left(4\right)$ mHz/K for ${}^{164}$Dy and ${}^{162}$Dy, respectively. We conclude that ac Stark related systematics will not limit a search for variation of the fine-structure constant, using dysprosium, down to the level of $|\dot{\alpha }/\alpha |=2.6\times {10}^{-17}$ yr${}^{-1}$, for two measurements of the transition frequency one year apart.

We have developed an approach based on first principles to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest in collisions between the molecules and electrons at energies below 1 eV. The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. The electronic continuum states are obtained with the complex-Kohn variational method. The benchmark calculation for the REA to the simplest negative ion CN${}^{-}$, which was recently observed in the interstellar medium, has produced a relatively low rate coefficient $\alpha \left(T\right)=7.3\times {10}^{-16}$ cm${}^3/$s at $T=30$ K. Moreover, our results are shown to agree well with microscopic reversibility applied on a recent photodetachment experiment on CN${}^{-}$. Finally, the study confirms a previous assessment that the CN${}^{-}$ ion is unlikely be formed by REA in the interstellar medium.

We investigate time-dependent stochastic disorder in the one-dimensional Jaynes-Cummings-Hubbard model and show that it gives rise to diffusive behavior. We find that disorder correlation frequency is effective in controlling the level of diffusivity. In the defectless system the mean squared displacement (MSD), which is a measure of the diffusivity, increases with increasing disorder frequency. Contrastingly, when static defects are present, the MSD increases with disorder frequency only at lower frequencies; at higher frequencies, increasing disorder frequency actually reduces the MSD.

Epitaxially grown quantum dots (QDs) are promising sources of nonclassical states of light such as single photons and entangled photons. However, in order for them to be used as a resource for long-distance quantum communication, distributed quantum computation, or linear optics quantum computing, these photons must be coupled efficiently to long-lived quantum memories as part of a quantum repeater network. Here, we theoretically examine the prospects for efficient storage and retrieval of a QD-generated single photon with a 1-ns lifetime in a multilevel atomic system. We calculate using an experimentally demonstrated optical depth of 150 that the storage (total) efficiency can exceed 46$\%$ (28$\%$) in a dense, ultracold ensemble of ${}^{87}$Rb atoms. Furthermore, we find that the optimal control pulse required for storage and retrieval can be obtained using a diode laser and an electro-optic modulator rather than a mode-locked, pulsed laser source. Increasing the optical depth, for example, by using Bose-condensed ensembles or an optical cavity, can increase the efficiencies to near unity. Aside from enabling a high-speed quantum network based on QDs, such an efficient optical interface between an atomic ensemble and a QD can also lead to entanglement between collective spin-wave excitations of atoms and the spin of an electron or hole confined in the QD.

We revisit the problem of quantum state reconstruction of light beams from the photocurrent quantum noise. As is well known, but often overlooked, two longitudinal field modes contribute to each spectral component of the photocurrent (sideband modes). We show that spectral homodyne detection is intrinsically incapable of providing all the information needed for the full reconstruction of the two-mode spectral quantum state. Such a limitation is overcome by the technique of resonator detection. A detailed theoretical description and comparison of both methods is presented, as well as an experiment to measure the six-mode quantum state of pump-signal-idler beams of an optical parametric oscillator above the oscillation threshold.

The use of retroreflection in light-pulse atom interferometry under microgravity conditions naturally leads to a double-diffraction scheme. The two pairs of counterpropagating beams induce simultaneously transitions with opposite momentum transfer that, when acting on atoms initially at rest, give rise to symmetric interferometer configurations where the total momentum transfer is automatically doubled and where a number of noise sources and systematic effects cancel out. Here we extend earlier implementations for Raman transitions to the case of Bragg diffraction. In contrast with the single-diffraction case, the existence of additional off-resonant transitions between resonantly connected states precludes the use of the adiabatic elimination technique. Nevertheless, we have been able to obtain analytic results even beyond the deep Bragg regime by employing the so-called “method of averaging,” which can be applied to more general situations of this kind. Our results have been validated by comparison to numerical solutions of the basic equations describing the double-diffraction process.

We investigate the onset of parity-time ($\mathcal{PT}$) symmetry breaking in non-Hermitian tight-binding lattices with spatially-extended loss-gain regions in the presence of an advective term. Similarly to the instability properties of hydrodynamic open flows, it is shown that $\mathcal{PT}$ symmetry breaking can be either absolute or convective. In the former case, an initially localized wave packet shows a secular growth with time at any given spatial position, whereas in the latter case the growth is observed in a reference frame moving at some drift velocity while decay occurs at any fixed spatial position. In the convective unstable regime, $\mathcal{PT}$ symmetry is restored when the spatial region of gain or loss in the lattice is limited (rather than extended). We consider specifically a non-Hermitian extension of the Rice-Mele tight-binding lattice model, and show the existence of a transition from absolute to convective symmetry breaking when the advective term is large enough. An extension of the analysis to ac-dc driven lattices is also presented, and an optical implementation of the non-Hermitian Rice-Mele model is suggested, which is based on light transport in an array of evanescently coupled optical waveguides with a periodically bent axis and alternating regions of optical gain and loss.

We demonstrate nanowatt-level saturated absorption using a subwavelength diameter tapered optical fiber (TOF) suspended in a gas of metastable xenon atoms. This ultralow-power nonlinearity is enabled by a small optical mode area propagating over a relatively long distance through the Xe gas. The use of inert noble gases in these kinds of TOF experiments may offer practical advantages over the use of reactive alkali-metal vapors, such as rubidium.

We present numerical calculations of the snake instability in a Fermi superfluid within the Bogoliubov-de Gennes theory of the Bose-Einstein condensate (BEC) to BCS crossover using the random-phase approximation complemented by time-dependent simulations. We examine the snaking behavior across the crossover and quantify the time scale and length scale of the instability. While the dynamics shows extensive snaking before eventually producing vortices and sound on the BEC side of the crossover, the snaking dynamics is preempted by decay into sound due to pair breaking in the deep BCS regime. At the unitarity limit, hydrodynamic arguments allow us to link the rate of snaking to the experimentally observable ratio of inertial to physical mass of the soliton. In this limit we witness an unresolved discrepancy between our numerical estimates for the critical wave number of suppression of the snake instability and recent experimental observations with an ultracold Fermi gas.

Beams of helium atoms in selected Rydberg-Stark states with principal quantum number $n=52$ and electric dipole moments as large as 9910 D have been guided and deflected in the electrostatic fields surrounding surface-based electrical transmission lines. The guided atoms were detected by pulsed-electric-field ionization. Information on their position at the time of ionization was obtained from the flight time of the resulting ions to a microchannel plate detector. Comparison of the experimentally recorded data with the results of numerical calculations of particle trajectories through the guide highlight the sensitivity of the guided signal to the interaction of the atoms with each other and the surrounding 300 K blackbody radiation field.

We present a number of arguments to demonstrate that a quantum analog of the Cherenkov effect occurs when two nondispersive half spaces are in relative motion. We show that they experience friction beyond a threshold velocity which, in their center-of-mass frame, is the phase speed of light within their medium, and the loss in mechanical energy is radiated through the medium before getting fully absorbed in the form of heat. By deriving various correlation functions inside and outside the two half spaces, we explicitly compute this radiation and discuss its dependence on the reference frame.

Stimulated optical forces offer a simple and efficient method for providing optical forces far in excess of the saturated radiative force. The bichromatic force, using a counterpropagating pair of two-color beams, has so far been the most effective of these stimulated forces for deflecting and slowing atomic beams. We have numerically studied the evolution of a two-level system under several different bichromatic and polychromatic light fields, while retaining the overall geometry of the bichromatic force. New insights are gained by studying the time-dependent trajectory of the Bloch vector, including a better understanding of the remarkable robustness of bi- and polychromatic forces with imbalanced beam intensities. We show that a four-color polychromatic force exhibits great promise. By adding new frequency components at the third harmonic of the original bichromatic detuning, the force is increased by nearly 50$\%$ and its velocity range is extended by a factor of 3, while the required laser power is increased by only 33$\%$. The excited-state fraction, crucial to possible application to molecules, is reduced from 41$\%$ to 24$\%$. We also discuss some important differences between polychromatic forces and pulse trains from a high-repetition-rate laser.

Because the commonly adopted viewpoint that the Keldysh parameter $\gamma$ determines the dynamical regime in strong field physics has long been demonstrated to be misleading, one can ask what happens as relevant physical parameters, such as laser intensity and frequency, are varied while $\gamma$ is kept fixed. We present results from our one- and fully three-dimensional quantum simulations of high-order-harmonic generation (HHG) from various bound states of hydrogen with $n$ up to 40, where the laser intensities and the frequencies are scaled from those for $n=1$ in order to maintain a fixed Keldysh parameter $\gamma <1$ for all $n$. We find that as we increase $n$ while keeping $\gamma$ fixed, the position of the cutoff scales in a well-defined manner. Moreover, a secondary plateau forms with a new cutoff, splitting the HHG plateau into two regions. The first of these subplateaus is composed of lower harmonics, and has a higher yield than the second one. The latter extends up to the semiclassical $I_p+3.17U_p$ cutoff. We find that this structure is universal, and the HHG spectra look the same for all $n\gtrsim 10$ when plotted as a function of the scaled harmonic order. We investigate the $n$, $l$, and momentum distributions to elucidate the physical mechanism leading to this universal structure.

We demonstrate theoretically that by employing an intense few-cycle, circularly polarized laser pulse, two-dimensional localization of the electronic wave function in laser-driven H${}_3$${}^{2+}$ can be controlled by varying the carrier-envelope phase (CEP). In the reaction ${{\mathrm{H}}_3}^{2+}\to \mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{H}}^{+}$, it is possible to select which one of the protons the electron finally binds to by tuning the value of the CEP. A complex asymmetry measure is introduced to quantify the degree of asymmetry induced by the laser field. In contrast to previous investigations of electron localization where only diatomic molecules were considered, our results extend CEP control of intramolecular electron motion to triatomic systems. This provides a step toward strong-field spatial and temporal control of electron dynamics in polyatomic molecules.