The efficiency of an atomic interferometer in proximity of a surface is discussed. We first study which is the best choice of frequency for a pulse acting on internal atomic transitions in the same well. Then, considering the modification of atomic energy levels in vicinity of the surface, we propose the application of two simultaneous Raman lasers and numerically study the associated interference fringes. We show that the efficiency of the interferometric scheme is limited by the existence of a residual phase depending on the atomic path. We propose a symmetric scheme in order to avoid these contributions. We finally show that the suggested modifications make the contrast of the interference fringes close to 1 in any configuration, both close and far from the surface, and with one or more initially populated wells.

The quantum evolution of a cloud of bosons initially localized on part of a one-dimensional optical lattice and suddenly subjected to a linear ramp is studied, realizing a quantum analog of the “Galileo ramp” experiment. The main remarkable effects of this realistic setup are revealed using analytical and numerical methods. Only part of the particles are ejected for a high enough ramp, while the others remain self-trapped. Then, the trapped density profile displays rich dynamics with Josephson-type oscillations around a plateau. This setup, by coupling bound states to propagative modes, creates two diverging condensates for which the entanglement is computed and related to the equilibrium one. Further, we address the role of integrability on the entanglement and on the damping and thermalization of simple observables.

The thermodynamic and transport properties of the unitary Fermi gas at finite temperature $T$ are governed by a quantum critical point at $T=0$ and zero density. We compute the universal shear viscosity to entropy ratio $\eta /s$ in the high-temperature quantum critical regime $T\gg \left|\mu \right|$ and find that this strongly coupled quantum fluid comes close to perfect fluidity $\eta /s=\hslash /\left(4\pi k_B\right)$. Using a controlled large-$N$ expansion, we show that already at the first nontrivial order the equation of state and the Tan contact density $C$ agree well with the most recent experimental measurements and theoretical Luttinger-Ward and bold diagrammatic Monte Carlo calculations.

We investigate the temperature dependence of the shear viscosity and spin diffusion in a two-dimensional Fermi gas with contact interactions, as realized in ultracold atomic gases. We describe the transport coefficients in terms of a Boltzmann equation and present a full numerical solution for the degenerate gas. In contrast to previous works we take the medium effects due to finite density fully into account. This effect reduces the viscosity-to-entropy ratio $\eta /s$ by a factor of 3, and similarly for spin diffusion. The trap-averaged viscosity agrees well with recent measurements by Vogt et al. [Phys. Rev. Lett. 108, 070404 (2012)].

The coherent combining of supercontinuum radiation from multiple filaments induced by an ultrashort laser pulse is demonstrated by supercomputer simulations. Four-wave mixing of supercontinuum field waveforms with an ultrashort reference pulse is shown to retrieve the coherent components from multiple supercontinua and combine them into a beam whose spatial and temporal coherence is much higher than those of supercontinua in multiple filaments.

We consider the effect of the quadratic term ${\vec{A}}^2$ on quantum state transfer, nonclassical state preparation, and collapse-revival dynamics in the strong and ultrastrong coupling regime. Using the Bogoliubov transformation, we define a field mode which allows us to reduce the Hamiltonian to the original Rabi form without the ${\vec{A}}^2$ term. We calculate the error introduced by the ${\vec{A}}^2$ term in quantum state transfer and find it cannot be neglected in the strong coupling regime. We also study how it impacts the characteristics of the prepared nonclassical states and collapse-revival dynamics. The double-peak characteristic of the revival dynamics can be experimentally observed as manifestation of the effect of the ${\vec{A}}^2$ term.

In this Brief Report, we present a perturbation theory that omits the weak multiphoton process to obtain analytical solutions for the Rabi model. Under this approximation, the Rabi model can be mapped into the solvable Jaynes-Cummings-like model by choosing a proper variation parameter. As a result, both the ground- and excited-state-energy spectrums and wave functions can be derived easily. Moreover, these explicit results agree well with the direct numerical simulation in a wide range of the experimental parameters. In addition, based on our obtained energy spectrums, the recent experimental observation of the Bloch-Siegert shift in the circuit quantum electrodynamics with ultrastrong coupling can be explained perfectly. We believe that our results have potential applications in solid-state quantum-information processing.

The nonlocal properties of arbitrary dimensional bipartite quantum systems are investigated. A complete set of invariants under local unitary transformations is presented. These invariants give rise to both sufficient and necessary conditions for the equivalence of quantum states under local unitary transformations: two density matrices are locally equivalent if and only if all these invariants have equal values.

The local (nonlocal) content in the sense of Elitzur, Popescu, and Rohrlich [Phys. Lett. A 162, 25 (1992)] is a natural measure for the locality (nonlocality) of quantum states. Its computation is in general difficult, even in low dimensions, and is one of the few open questions about pure two-qubit states. We present a complete solution to this long-lasting problem.

We study the decoherence of spin squeezing under non-Markovian channels and consider an ensemble of $N$ independent spin-$1/2$ particles with exchange symmetry. Each spin interacts with its own bath, and the baths are independent and identical. For this kind of open system, the spin squeezing under decoherence can be investigated from the dynamics of the local expectations. The reduced dynamics is obtained by the exact hierarchy equation method. The numerical results show that the spin squeezing may display both sudden and asymptotic vanishing; however, the revival phenomenon does not happen. In contrast, the concurrence shows multiple sudden vanishing and revival phenomena.

We investigate evolution of quantum correlations in ensembles of two-qubit nuclear spin systems via nuclear magnetic resonance techniques. We use discord as a measure of quantum correlations and the Werner state as an explicit example. We, first, introduce different ways of measuring discord and geometric discord in two-qubit systems and then describe the following experimental studies: (a) We quantitatively measure discord for Werner-like states prepared using an entangling pulse sequence. An initial thermal state with zero discord is gradually and periodically transformed into a mixed state with maximum discord. The experimental and simulated behavior of rise and fall of discord agree fairly well. (b) We examine the efficiency of dynamical decoupling sequences in preserving quantum correlations. In our experimental setup, the dynamical decoupling sequences preserved the traceless parts of the density matrices at high fidelity. But they could not maintain the purity of the quantum states and so were unable to keep the discord from decaying. (c) We observe the evolution of discord for a singlet-triplet mixed state during a radio-frequency spin-lock. A simple relaxation model describes the evolution of discord, and the accompanying evolution of fidelity of the long-lived singlet state, reasonably well.

We discuss the physical properties of a noninteracting electron gas constrained to a spherical surface. In particular we consider its chemical potentials, its ionization potential, and its electric static polarizability. All these properties are discussed analytically as functions of the number $N$ of electrons. The trends obtained with increasing $N$ are compared with those of the corresponding properties experimentally measured or theoretically evaluated for quasispherical hollow atomic and molecular clusters. Most of the properties investigated display similar trends, characterized by a prominence of shell effects. This leads to the definition of a scale-invariant distribution of magic numbers which follows a power law with critical exponent $-0.5$. We conclude that our completely mechanistic and analytically tractable model can be useful for the analysis of self-assembling complex systems.

Polarization-dependent resonant inelastic x-ray scattering (RIXS) has been shown to be a probe of molecular-field effects on the electronic structure of isolated molecules. In this experimental analysis we explain the linear dichroism observed in Cl 2$p$ polarized RIXS following Cl 1$s$ excitation of a series of chlorofluoromethanes (CF${}_3$Cl, CF${}_2$Cl${}_2$, CFCl${}_3$, and CCl${}_4$) as due to molecular-field effects, including singlet-triplet exchange. We present an approach to extract directly the $2p$ inner-shell electronic state populations from the experimental measurements. Using the angular properties of the measured $KV$ emission we also are able to determine the value of the polarization anisotropy parameter ${\beta }_p$ for each resolved component of the $KV$ emission spectra.

Laser-driven Rydberg gases are many-body quantum systems that exhibit a pronounced collective excitation behavior due to the strong interaction between atoms in high-lying electronic states. Atoms located within a so-called blockade volume share a single Rydberg excitation, which is dynamically created and annihilated. For sufficiently long times, this driven system approaches a steady state, which lends its properties from a maximum entropy state of a Tonks gas. Using this connection, we show that spatial correlations between Rydberg atoms are controlled by the number of atoms contained within a blockade volume. For a small number, the system favors a disordered arrangement of Rydberg atoms, whereas in the opposite limit, Rydberg atoms tend to arrange in an increasingly ordered configuration. We argue that this is an entropic effect which is observable in current experiments. Our work demonstrates how ordered structures can spontaneously emerge in a strongly interacting, closed many-body system out of equilibrium.

We provide a general method to find the Hamiltonian of a linear circuit in the presence of a nonlinearity. Focusing on the case of a Josephson junction embedded in a transmission-line resonator, we solve for the normal modes of the system by taking into account exactly the effect of the quadratic (i.e., inductive) part of the Josephson potential. The nonlinearity is then found to lead to self and cross-Kerr effects, as well as beam-splitter-type interactions between modes. By adjusting the parameters of the circuit, the Kerr coefficient $K$ can be made to reach values that are weak ($K<\kappa$), strong ($K>\kappa$), or even very strong ($K\gg \kappa$) with respect to the photon-loss rate $\kappa$. In the latter case, the $\mathrm{resonator}+\mathrm{junction}$ circuit corresponds to an in-line version of the transmon. By replacing the single junction by a SQUID, the Kerr coefficient can be tuned in situ, allowing, for example, the fast generation of Schrödinger cat states of microwave light. Finally, we explore the maximal strength of qubit-resonator coupling that can be reached in this setting.

Higher-order sidebands in optomechanically induced transparency are discussed in a generic optomechanical system. We take account of nonlinear terms and give an effective method to deal with such problems. It is shown that, if a strong control field with frequency ${\omega }_1$ and a weak probe field with frequency ${\omega }_p$ are incident upon the optomechanical system, then there are output fields with frequencies ${\omega }_1\pm 2\Omega$ generated, where $\Omega ={\omega }_p-{\omega }_1$. We analyze the amplitude of the output field ${\omega }_1+2\Omega$ and look at how it varies with the control field and show that the amplitude of the second-order sideband can be controlled by the strong control field.

How would the world appear to us if its ontology was that of classical mechanics but every agent faced a restriction on how much they could come to know about the classical state? We show that in most respects it would appear to us as quantum. The statistical theory of classical mechanics, which specifies how probability distributions over phase space evolve under Hamiltonian evolution and under measurements, is typically called Liouville mechanics, so the theory we explore here is Liouville mechanics with an epistemic restriction. The particular epistemic restriction we posit as our foundational postulate specifies two constraints. The first constraint is a classical analog of Heisenberg's uncertainty principle; the second-order moments of position and momentum defined by the phase-space distribution that characterizes an agent's knowledge are required to satisfy the same constraints as are satisfied by the moments of position and momentum observables for a quantum state. The second constraint is that the distribution should have maximal entropy for the given moments. Starting from this postulate, we derive the allowed preparations, measurements, and transformations and demonstrate that they are isomorphic to those allowed in Gaussian quantum mechanics and generate the same experimental statistics. We argue that this reconstruction of Gaussian quantum mechanics constitutes additional evidence in favor of a research program wherein quantum states are interpreted as states of incomplete knowledge and that the phenomena that do not arise in Gaussian quantum mechanics provide the best clues for how one might reconstruct the full quantum theory.

Collisions of protons $p$ with antiprotonic helium atoms ${\overline{p}\mathrm{He}}^{+}$ (bound orbital states of an antiproton $\overline{p}$ and a helium ion He${}^{+}$) are investigated from the viewpoint of chemical reaction. The $\overline{p}{\mathrm{He}}^{+}$ atoms with high orbital angular momentum quantum numbers $L>40$ can be abundantly produced in the capture of $\overline{p}$ by metastable helium atoms He($2^{1,3}S$). Since such orbital states are considered to be practically stable despite having Auger decay channels ($\overline{p}{\mathrm{He}}^{+}\to \overline{p}{\mathrm{He}}^{2+}+e$), atomic and molecular collision processes involving $\overline{p}{\mathrm{He}}^{+}(L>40)$ are experimentally measurable. In this study, adiabatic electron energies in the Born-Oppenheimer approximation are calculated for the $p+\overline{p}{\mathrm{He}}^{+}$ system. The $p+\overline{p}{\mathrm{He}}^{+}$ dynamical calculations of $\overline{p}$ exchange ($\to \overline{p}p+{\mathrm{He}}^{+}$) and dissociation ($\to p+\overline{p}+{\mathrm{He}}^{+}$) reactions on the ground-state adiabatic potential energy surface are carried out for various high orbital states of $\overline{p}\mathrm{He}$ by using a classical trajectory Monte Carlo method. The reaction cross sections and the state distributions of antiprotonic hydrogen atoms (protonium) $\overline{p}p$ produced in the exchange reaction are presented. If the orbital shape of $\overline{p}{\mathrm{He}}^{+}$ is near circular, the exchange reaction becomes inactive at low energies because the repulsive part of the interaction plays a critical role. In the $p+\overline{p}p$ system, however, the low-energy $\overline{p}$ exchange reaction remains active for any type of the initial $\overline{p}p$ orbital motion.

Many-body effects may influence properties, such as scattering parameters, nature of pairing, etc., close to a Feshbach resonance in the fermion BEC-BCS crossover problem. We study effects such as these using a tractable crossing-symmetric approach. This method allow us to include quantum fluctuations, such as density, current, spin, spin-current, and the higher-order fluctuations in a self-consistent fashion. The underlying fermion interaction is reflected in the “driving” term. We perform calculations here on both Bose-Einstein condensate (BEC) and BCS sides and taking the driving term to be finite range and of arbitrary strength. These are related to two-body singlet and triplet scattering parameters and can be connected with experimental $s$- and $p$-wave Feshbach resonances. We include the $\ell =0$ density and spin fluctuations as well as $\ell =1$ current and spin-current fluctuations. We calculate renormalized scattering amplitudes, pairing amplitudes, nature of pairing, etc., on both the BEC and BCS sides. We then compare our results qualitatively with experiments.