We investigate strong-coupling effects on normal-state properties of an ultracold Fermi gas. Within the framework of $T$-matrix approximation in terms of pairing fluctuations, we calculate the single-particle density of states (DOS), as well as the spectral weight, over the entire BCS–Bose-Einstein condensate (BEC) crossover region above the superfluid phase-transition temperature $T_{\text{c}}$. Starting from the weak-coupling BCS regime, we show that the so-called pseudogap develops in DOS above $T_{\text{c}}$, which becomes remarkable in the crossover region. The pseudogap structure continuously changes into a fully gapped one in the strong-coupling BEC regime, where the gap energy is directly related to the binding energy of tightly bound molecules. We determine the pseudogap temperature $T^{\ast }$ where the dip structure in DOS vanishes. The value of $T^{\ast }$ is shown to be very different from another characteristic temperature $T^{\ast \ast }$ where a BCS-type double-peak structure disappears in the spectral weight. While one finds $T^{\ast }>T^{\ast \ast }$ in the BCS regime, $T^{\ast \ast }$ becomes higher than $T^{\ast }$ in the crossover region and BEC regime. Including this, we determine the pseudogap region in the phase diagram of ultracold Fermi gases. Our results would be useful in the search for the pseudogap region in ultracold ${}^6\text{L}\text{i}$ and ${}^{40}\text{K}$ Fermi gases.

Temporal coherence is a fundamental property of macroscopic quantum systems, such as lasers in optics and Bose-Einstein condensates in atomic gases and it is a crucial issue for interferometry applications with light or matter waves. Whereas the laser is an “open” quantum system, ultracold atomic gases are weakly coupled to the environment and may be considered as isolated. The coherence time of a condensate is then intrinsic to the system and its derivation is out of the frame of laser theory. Using quantum kinetic theory, we predict that the interaction with noncondensed modes gradually smears out the condensate phase, with a variance growing as $\mathcal{A}{t}^2+\mathcal{B}t+\mathcal{C}$ at long times $t$, and we give a quantitative prediction for $\mathcal{A}$, $\mathcal{B}$, and $\mathcal{C}$. Whereas the coefficient $\mathcal{A}$ vanishes for vanishing energy fluctuations in the initial state, the coefficients $\mathcal{B}$ and $\mathcal{C}$ are remarkably insensitive to these fluctuations. The coefficient $\mathcal{B}$ describes a diffusive motion of the condensate phase that sets the ultimate limit to the condensate coherence time. We briefly discuss the possibility to observe the predicted phase spreading, also including the effect of particle losses.

We calculate the pump and probe absorption spectra for the cycling $F_g=4\to F_e=5$ transition $D_2$ line of ${}^{133}\text{C}\text{s}$ in an atomic beam, interacting with a strong resonant ${\sigma }_{+}$-polarized pump and a probe of comparable intensity and either ${\sigma }_{-}$ or $\pi$ polarization. The aim is to reproduce and analyze the experiments of Dahl et al. [Opt. Lett. 33, 983 (2008)] who showed for a ${\sigma }_{+}$-polarized pump and ${\sigma }_{-}$-polarized probe that the pump absorption spectrum switches from an “absorption within transparency” (AWT) structure, when the probe is weaker than the pump, to a “transparency within transparency” (TWT) structure, when the probe is stronger than the pump. For all other polarization combinations, the pump spectrum displays AWT behavior at all probe intensities. We analyze our results by considering the contributions that derive from the individual $m_g\to m_e$ transitions. When the ${\sigma }_{+}$-polarized pump is stronger than the ${\sigma }_{-}$-polarized probe, the population is swept toward the $m_g\to m_e=m_g+1$ transitions with the highest values of $m_g$, and the pump absorption spectrum has an AWT structure and resembles that of an $N$ system. However, when the probe is stronger than the pump, the population is swept toward the $m_g=-F_g\to m_e=m_g-1$ transition when the probe is near resonance, and to the $m_g=F_g\to m_e=m_g+1$ transition when the probe is detuned from resonance. The pump and probe spectra are mirror images of each other and resemble those of a $V$ system where the probe has a peak at line center and the pump spectrum has a TWT structure. For a strong ${\sigma }_{+}$ pump and an even stronger $\pi$ probe, the population concentrates in the intermediate transitions, and the AWT to TWT changeover does not occur. We also show that the narrow features in the spectra at line center derive from transfer of coherence from the excited to the ground hyperfine levels.

We present a model for the scaling laws of the electric field noise spectral density as a function of the distance $d$ above a conducting surface. Our analytical approach models the patch potentials by introducing a correlation length $\zeta$ of the electric potential on the surface. The predicted scaling laws are in excellent agreement with two different classes of experiments (cold trapped ions and cantilevers), for which the $d$ parameter differs by four orders of magnitude. According to this model, heating rate in miniature ion traps could be greatly reduced by proper material engineering.

We theoretically study a circuit QED architecture based on a superconducting flux qubit directly coupled to the center conductor of a coplanar waveguide transmission-line resonator. As already shown experimentally [A. A. Abdumalikov, Jr. et al., Phys. Rev. B 78, 180502(R) (2008)], the strong coupling regime of cavity QED can readily be achieved by optimizing the local inductance of the resonator in the vicinity of the qubit. In addition to yielding stronger coupling with respect to other proposals for flux qubit based circuit QED, this approach leads to a qubit-resonator coupling strength $g$ which does not scale as the area of the qubit but is proportional to the total inductance shared between the resonator and the qubit. Strong coupling can thus be attained while still minimizing sensitivity to flux noise. Finally, we show that by taking advantage of the large kinetic inductance of a Josephson junction in the center conductor of the resonator can lead to coupling energies of several tens of percent of the resonator frequency, reaching the ultrastrong coupling regime of cavity QED where the rotating-wave approximation breaks down. This should allow an on-chip implementation of the $E\otimes \beta$ Jahn-Teller model.

We use the system-plus-reservoir approach to study the quantum dynamics of a bipartite continuous variable system (two generic particles). We present an extension of the traditional model of a bath of oscillators which is capable of inducing an effective coupling between the two parts of the system depending on the choice made for the spectral density of the bath. The coupling is nonlinear in the system variables and an exponential dependence on these variables is imposed in order to guarantee the translational invariance of the model if the two particles are not subject to any external potential. The reduced density operator is obtained by the functional integral method. The dynamical susceptibility of the reservoir is modeled in order to introduce, besides a characteristic frequency, a characteristic length that determines if the effective interaction potential is strong enough to induce entanglement between the particles. Our model provides a criterion of distance for identifying in which cases a common environment can induce entanglement. Three regimes are found: the short distance regime, equivalent to a bilinear system-reservoir coupling, the long distance regime in which the particles act like coupled to independent reservoirs, and the intermediate regime suitable for the competition between decoherence and induced entanglement.

We use the Unruh effect to investigate how the teleportation of quantum states is affected when one of the entangled qubits used in the process is under the influence of some external force. In order to reach a comprehensive understanding, a detailed analysis of the acceleration effect on such entangled qubit system is performed. In particular, we calculate the mutual information and concurrence between the two qubits and show that the latter has a “sudden death” at a finite acceleration, whose value will depend on the time interval along which the detector is accelerated.

A time-dependent close-coupling method is developed to treat the double ionization of ${\text{H}}_2$ by fast bare-ion collisions. At high incident energies, for which charge transfer to the projectile may be ignored, multipole expansions are made for the electron-electron and electron-projectile interactions in a fixed target nuclei coordinate system. The time-dependent Schrödinger equation for the six dimensional target electron wave function is reduced to a set of close-coupled equations on a four dimensional numerical lattice in $\left(r_1,{\theta }_1,r_2,{\theta }_2\right)$ center-of-mass spherical polar coordinates. Time-dependent close-coupling calculations are carried out for $p+{\text{H}}_2$ collisions at an incident energy of 1.0 MeV. The ratio of double to single ionization is found to be 0.3%, which is in reasonable agreement with experimental measurements.

We present a systematic study of the stability of highly charged cationic and anionic fullerenes whose most stable neutral counterparts follow the isolated pentagon rule (IPR). In agreement with recent studies, we have found that, for many highly charged fullerenes, non-IPR isomers are significantly more stable than the IPR ones. To understand this behavior, we compare the results of elaborate density-functional theory (DFT) calculations to those of a simple Hückel molecular-orbital theory in which the DFT energies of the corresponding neutral systems are used as a reference. The model leads to a reasonable estimate of the relative stability of the IPR and non-IPR isomers as a function of charge, which can be used to identify, among the thousands of possible isomers and charge states, the non-IPR species that are likely more stable than the IPR isomers.

The dynamics of two-component Bose-Einstein condensates in rotating traps is investigated. In the Thomas-Fermi limit, equations of motion are derived showing multiple static solutions for a vortex-free condensate. Dynamic stability analysis of these solutions and comparison to truncated Wigner simulations enable us to identify the regimes for which vortex states will occur. In addition, our analysis predicts center-of-mass oscillations that are induced by interspecies interactions and affect each component separately. For attractive interspecies interactions, these oscillations lead to a stable symmetry-broken state.

We predict the existence of rogue waves in Bose-Einstein condensates either loaded into a parabolic trap or embedded in an optical lattice. In the latter case, rogue waves can be observed in condensates with positive scattering length. They are immensely enhanced by the lattice. Local atomic density may increase up to tens times. We provide the initial conditions necessary for the experimental observation of the phenomenon. Numerical simulations illustrate the process of creation of rogue waves.

We calculate the spectrum for three atoms in a one-dimensional harmonic well near an $s$-wave Feshbach resonance using a two-channel model to describe the Feshbach physics. This is relevant to the study of cold ${}^{40}\text{K}$ atoms trapped in a one-dimensional optical lattice. Our results indicate that, for this one-dimensional system, one- and two-channel models will differ close to the Feshbach resonance, although the two theories would converge in the limit of strong Feshbach coupling. We also find level crossings in the low-energy spectrum of a single well with three atoms that may lead to quantum phase transition for an optical lattice of many wells. We discuss the stability of the system to a phase with nonuniform density.

A long wavelength optical lattice is generated in a two-level medium by low-frequency contrapropagating beams. Then a short wavelength gap soliton generated by evanescent boundary instability (supratransmission) undergoes a dynamics shown to obey the Newton equation of the parametrically driven pendulum, hence presenting extremely rich, possibly chaotic, dynamical behavior. The theory is sustained by numerical simulations and provides an efficient tool to study soliton trajectories.

We investigate the quantum scattering involving a single photon and a chain of $N$ polarized $\Lambda$-type three-level atoms inside a one-dimensional waveguide. By using a transfer matrix method, we obtain analytic expressions of eigenvectors of the Hamiltonian, which determine the polarization dependent transmission and reflection spectra for a single photon. We show that the scattering is most significant when the interatomic distance equals an integer multiple plus a quarter of the resonance wavelength. In such a configuration, an incident photon with an unknown polarization can be converted into a specified polarization with a transmission probability higher than that in polarizers obeying Malus’s law.

The conventional method for calculating the dielectric tensor for metamaterial relies on finding a transformation from the physical space containing the medium to a virtual electromagnetic space. Such a transformation exists if, and only if, the space is flat, which greatly restricts the way in which we are able to manipulate light. Furthermore, the transformation medium must not alter the temporal coordinate. Here, we show an alternate method for calculating the dielectric tensor that stems from the light ray trajectory equation derived from the metric. Thus, the process of defining an explicit transformation is circumvented entirely. We show that this method reproduces the correct results for the cylindrical cloaking device, and we use it to derive the material parameters of a device that gives rise to a curved space in which light rays follow closed Keplerian orbits.

We show that nonlocal Popescu-Rohrlich correlations can be observed in the postselected results of binned position measurements on a two-party Gaussian state. Our experiment is based on the spatial correlations of entangled photons and lens systems. We obtain a maximum violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality of 3.42, which corresponds to the implementation of a nonlocal AND gate with success probability of 0.93. These results do not conflict with quantum mechanics due to the postselection required and open up the possibility of experimental investigation of fundamental aspects of Popescu-Rohrlich nonlocality with a reliable and simple experimental setup.

Dynamically corrected gates were recently introduced [K. Khodjasteh and L. Viola, Phys. Rev. Lett. 102, 080501 (2009)] as a tool to achieve decoherence-protected quantum gates based on open-loop Hamiltonian engineering. Here, we further expand the framework of dynamical quantum error correction, with emphasis on elucidating under what conditions decoherence suppression can be ensured while performing a generic target quantum gate, using only available bounded-strength control resources. Explicit constructions for physically relevant error models are detailed, including arbitrary linear decoherence and pure dephasing on qubits. The effectiveness of dynamically corrected gates in an illustrative non-Markovian spin-bath setting is investigated numerically, confirming the expected fidelity performance in a wide parameter range. Robustness against a class of systematic control errors is automatically incorporated in the perturbative error regime.

In a recent series of papers, Higuchi and Higuchi defined an extended constrained-search procedure by extending the Levy constrained search by adding additional constraints. As shown here, this procedure can be equivalently formulated in terms of Lieb’s Legendre transformation functional. The Legendre transform approach has advantages in cases where the additional constraints are restrictive enough to cause problems with $N$-representability.

In the polar RbCs molecule, the strong spin-orbit coupling between the $A{}^1\Sigma ^{+}$ and $b{}^3\Pi$ diabatic electronic states correlated to the $\mathrm{Rb}\left(5s\right)\mathrm{Cs}\left(6p\right)$ dissociation limit is at the origin of a global mixing of the two vibrational $0^{+}\left(P_{1∕2,3∕2}\right)$ series coupled radiatively to both $X{}^1\Sigma ^{+}$ and $a{}^3\Sigma ^{+}$ states. This so-called “resonant coupling” plays a crucial role in the formation of ultracold stable RbCs molecules through photoassociation into $0^{+}$ levels followed by stabilization through spontaneous emission. We analyze quantitatively the mechanisms of photoassociation and stabilization through $0^{+}$ levels, starting from and leading to either the singlet or the triplet states and we compare the efficiency of the four paths leading to the formation of stable RbCs molecules. Comparison between the two isotopomers ${}^{87}\mathrm{Rb}{}^{133}\mathrm{Cs}$ and ${}^{85}\mathrm{Rb}{}^{133}\mathrm{Cs}$ is also reported: the overall process of formation of stable molecules is one order of magnitude larger for ${}^{87}\mathrm{Rb}\mathrm{Cs}$. The simple two-channel model presented here yields the general rules for a standard analysis of the effects of resonant coupling between electronic states. To underline the specificity of heteronuclear molecules, the $0^{+}$ coupled system of the RbCs molecule is compared to its analog, the $0_u^{+}$ system of the ${\mathrm{Rb}}_2$ molecule.

Transition rates for electric quadrupole $\left(E2\right)$ and magnetic dipole $\left(M1\right)$ transitions among the ${\text{Fe}}^{13+}3s^{2}3p$ and ${\text{Fe}}^{11+}3s^{2}3p^3$ ground configuration levels and for $E2$, $M1$, and magnetic quadrupole $\left(M2\right)$ transitions from the singly excited ${\text{Fe}}^{11+}3s^{2}3p^{2}3d{{}^{4}F}_{9/2}$, ${{}^{2}G}_{9/2}$, and ${\text{Fe}}^{13+}3s3p3d{{}^{4}F}_{9/2}^o$ long-lived levels have been calculated using nonorthogonal orbitals in the multiconfiguration Hartree-Fock approach. The calculated excitation energies for ground configuration levels deviate from the experimental values by $12–100{\text{cm}}^{-1}$. The rates for $M1$ transitions dominate by far over the $E1$ transition rates for the forbidden transitions among the ground configuration levels. The present results for transition probabilities and lifetimes have been compared with other available theoretical calculations and with recent experiments. Our lifetime results compare very well with some theoretical calculations and experiments.