Nonadiabatic coupling terms between the four lowest singlet states of ${\mathrm{H}}_3{}^{+}$ were calculated ab initio. The analysis according to the criteria suggested by Baer and Alijah [Baer and Alijah, Chem. Phys. Lett. 319, 489 (2000)] shows that as many as four electronic states may be required for an accurate description of the reactions $\mathrm{H}+{\mathrm{H}}_2{}^{+}(v,j)\rightleftarrows {\mathrm{H}}^{+}+{\mathrm{H}}_2(v^{\prime },j^{\prime })$, depending on the collision energy.

Electric-field noise in proximity to metallic surfaces is a poorly understood phenomenon that appears in different areas of physics. Trapped ion quantum information processors are particularly susceptible to this noise, leading to motional decoherence, which ultimately limits the fidelity of quantum operations. On the other hand, trapped ions present an ideal tool to study this effect, opening new possibilities in surface science. In this work we analyze and measure the polarization of the noise field in a microfabricated ion trap for various noise sources. We find that technical noise sources and noise emanating directly from the surface give rise to different degrees of polarization, which allows us to differentiate between the two noise sources. Based on this we demonstrate a method to infer the magnitude of surface noise in the presence of technical noise.

A model for studying atomtronic devices and circuits based on finite-temperature Bose-condensed gases is presented. The approach involves numerically solving equations of motion for atomic populations and coherences, derived using the Bose-Hubbard Hamiltonian and the Heisenberg picture. The resulting cluster expansion is truncated at a level giving balance between physics rigor and numerical demand mitigation. This approach allows parametric studies involving time scales that cover both the rapid population dynamics relevant to nonequilibrium state evolution, as well as the much longer time durations typical for reaching steady-state device operation. The model is demonstrated by studying the evolution of a Bose-condensed gas in the presence of atom injection and extraction in a double-well potential. In this configuration phase locking between condensates in each well of the potential is readily observed, and its influence on the evolution of the system is studied.

Describing local system-reservoir interaction in a quantum kinetic approach allows a first-principles investigation of basic nonequilibirium effects in ultracold atomic vapors. We derive such a quantum kinetic theory and apply it to heat transport in a Bose-condensed gas coupled collisionally to two spatially separated reservoir gases at different temperatures. We show that in the collisionless regime, where a Bogoliubov expansion is applicable, heat is transported by counterpropagating steady currents of condensate and noncondensate fractions, with the condensate flowing towards the source of greater heat. This phenomenon has hitherto only be seen in more strongly interacting systems as superfluid helium and is known as superfluid internal convection. We derive explicit formulas for the steady-state energy current and differential particle currents (with zero net particle flux) in the collisionless regime. As a significant technical detail in this derivation, we show that a correct treatment of this nonequilibrium scenario requires the inclusion of post-Bogoliubov and nonresonant interaction effects.

We investigate the excitation of quantum plasmonic states of light in graphene using end-fire and prism coupling. In order to model the excitation process quantum mechanically, we quantize the transverse-electric and transverse-magnetic surface plasmon polariton (SPP) modes in graphene. A selection of regimes are then studied that enable the excitation of SPPs by photons and we show that efficient coupling of photons to graphene SPPs is possible at the quantum level. Furthermore, we study the excitation of quantum states and their propagation under the effects of loss induced from the electronic degrees of freedom in the graphene. Here we investigate whether it is possible to protect quantum information using quantum-error correction techniques. We find that these techniques provide a robust-to-loss method for transferring quantum states of light in graphene over large distances.

In Jaouadi et al. [Phys. Rev. A 83, 023616 (2011)] the authors derive an analytical finite-size expansion for the Bose-Einstein condensation critical temperature of an ideal Bose gas in a generic power-law trap. In the case of a harmonic trap, this expansion adds higher-order terms to the well-known first-order correction. We point out a delicate point in connection to these results, showing that the claims of Jaouadi et al. should be treated with caution. In particular, for a harmonic trap, the given expansion yields results that, depending on what is considered to be the critical temperature of the finite system, do not generally improve on the established first-order correction. For some nonharmonic traps, the results differ at first order from other results in the literature.

In this Reply to Comment we show that the criticisms raised by J. Noronha are based on a misapplication of the model we have proposed in A. Jaouadi, M. Telmini, and E. Charron [Phys. Rev. A 83, 023616 (2011)]. Here we explicitly discuss the range of validity of the approximations underlying our analytical model. We also show that the discrepancies pointed out for very small atom numbers and for very anisotropic traps are not surprising since these conditions exceed the range of validity of the model.

Hardy's is one of the simplest arguments concerning nonlocality. Recently, Chen et al. [Phys. Rev. A 88, 062116 (2013)] have proposed a more generalized Hardy-like argument and have shown that the probability of success increases with the local system's dimension. Here we study the same in a minimally constrained theory, namely, the generalized no-signaling theory(GNST). We find that not only does the probability of success of this argument increase with the local system dimension in GNST, but it also takes a very simple functional form.

We address quantum communication channels based on phase modulation of coherent states and analyze in detail the effects of static and dynamical (stochastic) phase diffusion. We evaluate mutual information for an ideal phase receiver and for a covariant phase-space-based receiver, and compare their performances by varying the number of symbols in the alphabet and/or the overall energy of the channel. Our results show that phase communication channels are generally robust against phase noise, especially for large alphabets in the low-energy regime. In the presence of dynamical (non-Markovian) noise the mutual information is preserved by the time correlation of the environment, and when the noise spectra are detuned with respect to the information carrier, revivals of mutual information appear.

Relative cross sections for photoionization of endohedral fullerene ions ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{q+}$ $\left(q=1,2,3\right)$ have been measured employing the photon-ion merged-beam technique. The investigations include various ionization channels ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+q\to +p}$ $(q=1,2,3$ and $p=2,3,4,5,6)$, in some cases accompanied by fragmentation of the carbon cage. Prominent structures related to the carbon $K$-shell ionization threshold were observed in the energy range 280–330 eV. These resonance structures have been analyzed in seven product channels and are compared with previously known absorption spectra of several fullerene species. Within the statistical significance of the data, we do not observe any signatures of the N $1s$ and Lu $3d$ thresholds in the energy ranges 390–435 eV and 1500–1700 eV, respectively. By comparing the energies for double ionization of ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{+}$ and ${\mathrm{Lu}}_3\mathrm{N}@{\mathrm{C}}_{80}{}^{2+}$ at the carbon $1s$ $K$-shell threshold, we derive a value of $5.0\pm 0.4\text{Å}$ for the radius of the carbon cage.

We study a type of Fermi polaron induced by an impurity interacting with an ultracold Fermi superfluid. Due to the three-component nature of the system, the polaron can become trimerlike with a nonuniversal energy spectrum. We identify multiple avoided crossings between impurity- and trimerlike solutions in both the attractive and the repulsive polaron spectra. In particular, the widths of avoided crossings gradually increase as the Fermi superfluid undergoes a crossover from the BCS side towards the Bose-Einstein condensate side, which suggests instabilities towards three-body losses. Such losses can be reduced for interaction potentials with small effective ranges. We also demonstrate, using the second-order perturbation theory, that the mean-field evaluation of the fermion-impurity interaction energy is inadequate even for small fermion-impurity scattering lengths, due to the essential effects of Fermi superfluid and short-range physics in such a system. Our results are practically useful for cold-atom experiments on mixtures.

We study the dynamics of a general quartic interaction Hamiltonian under the influence of dissipation and nonclassical driving. We show that this scenario could be realized with a cascaded superconducting cavity-qubit system in the strong dispersive regime in a setup similar to recent experiments. In the presence of dissipation, we find that an effective Hartree-type decoupling with a Fokker-Planck equation yields a good approximation. We find that the stationary state is approximately a squeezed vacuum, which is enhanced by the $Q$ factor of the cavity but conserved by the interaction. The qubit nonlinearity, therefore, does not significantly influence the highly squeezed intracavity microwave field but, for a range of realistic parameters, enables characterization of itinerant squeezed fields.

A novel strategy is introduced in order to include variations of the nonlinearity in the nonlinear Schrödinger equation. This technique, which relies on renormalization, is in particular well adapted to nanostructured optical systems where the nonlinearity exhibits large variations up to two orders of magnitude larger than in bulk material. We show that it takes into account in a simple and efficient way the specificity of the nonlinearity in nanostructures that is determined by geometrical parameters like the effective mode area and the group index. The renormalization of the nonlinear Schrödinger equation is an occasion for physics-oriented considerations and unveils the potential of photonic crystal waveguides for the study of new nonlinear propagation phenomena.

We study the entanglement generated by a weak cross-Kerr nonlinearity between two initial coherent states, one of which has an amplitude close to the single-photon level, while the other one is macroscopic. We show that strong micro-macro entanglement is possible for weak phase shifts by choosing the amplitude of the macroscopic beam sufficiently large. We analyze the effects of loss and discuss possible experimental demonstrations of the micro-macro entanglement based on homodyne tomography and an entanglement witness. We also discuss the macroscopicity of the generated entanglement from the point of view of a recently proposed measure.

The stability of Bose gases near resonance has been a puzzling problem in recent years. In this article, we demonstrate that in addition to generating thermal pressure, thermal atoms enhance the repulsiveness of the scale-dependent interactions between condensed atoms due to a renormalization effect and further stabilize the Bose gases. Consequently, we find that, as a precursor of instability, the compressibility develops an anomalous structure as a function of scattering length and is drastically reduced compared with the mean-field value. Furthermore, the density profile of a Bose gas in a harmonic trap is found to develop a flat top near the center. This is due to the anomalous behavior of compressibility and can be a potential smoking gun for probing such an effect.

Random laser action is observed in a holey photonic-crystal fiber (PCF) whose holes are filled with a solution of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran laser dye. Although the holes of the PCF are periodically ordered, the laser action does not stem from the Bragg reflection due to the periodic order. A detailed investigation demonstrates that random laser action is the only possible mechanism for the laser oscillation. The PCF is a promising quasi-two-dimensional random laser platform that is convenient for investigating the mechanism of random laser oscillation.

A scheme to achieve spin squeezing using a geometric phase induced by a single mechanical mode is proposed. The analytical and numerical results show that the ultimate degree of spin squeezing depends on the parameter $\frac{n_{th}+1/2}{Q\sqrt{N}}$, which is the ratio between the thermal excitation, the quality factor, and square root of ensemble size. The undesired coupling between the spin ensemble and the bath can be efficiently suppressed by bang-bang control pulses. With high quality factor, the ultimate limit of the ideal one-axis twisting spin squeezing can be obtained for a nitrogen-vacancy ensemble in diamond.

A quantum system can be prepared in an entangled state that is very sensitive to changes introduced to its dynamics and thus potentially can be utilized as a precise sensor of dynamics parameters. $N$ entangled two-level atoms evolving unitarily can be used for frequency estimation with Heisenberg-limited precision which scales as $N^{-2}$, whereas uncorrelated states give only a standard-limited precision which scales as $N^{-1}$. As entangled states are also sensitive to disturbances introduced by interactions with external environments, quantum enhancement of the precision may be significantly limited for open quantum systems. This is the case when an interaction with an environment leads to local Markovian dephasing and the precision scales as $N^{-1}$ for all atom preparations. For local non-Markovian dephasing, however, Matsuzaki et al. [Phys. Rev. A 84, 012103 (2011)] and Chin et al. [Phys. Rev. Lett. 109, 233601 (2012)] showed that the atoms prepared in the maximally entangled state achieve Zeno scaling $\propto N^{-3/2}$ in precision and argued that this limit should hold for any entangled state. Here we prove that Zeno scaling is indeed the best possible scaling for all local non-Markovian dephasing models which feature initial Zeno dynamics. Moreover, Zeno scaling can hold only for experiments in the Zeno-dynamics regime, otherwise scaling of the precision is standard.

We introduce an operational and statistically meaningful measure, the quantum tomographic transfer function, that possesses important physical invariance properties for judging whether a given informationally complete quantum measurement performs better tomographically in quantum-state estimation relative to other informationally complete measurements. This function is independent of the unknown true state of the quantum source and is directly related to the average optimal tomographic accuracy of an unbiased state estimator for the measurement in the limit of many sampling events. For the experimentally appealing minimally complete measurements, the transfer function is an extremely simple formula. We also give an explicit expression for this transfer function in terms of an ordered expansion that is readily computable and illustrate its usage with numerical simulations and its consistency with some known results.