We demonstrate the analog of electromagnetically induced transparency in a room temperature cavity optomechanics setup formed by a thin semitransparent membrane within a Fabry-Pérot cavity. Due to destructive interference, a weak probe field is completely reflected by the cavity when the pump beam is resonant with the motional red sideband of the cavity. Under this condition we infer a significant slowing down of light of hundreds of microseconds, which is easily tuned by shifting the membrane along the cavity axis. We also observe the associated phenomenon of electromagnetically induced amplification which occurs due to constructive interference when the pump is resonant with the blue sideband.

Stimulated optical signals obtained by subjecting the system to a narrow band and a broadband pulse show both gain and loss Raman features at the red and blue side of the narrow beam, respectively. Recently observed temperature-dependent asymmetry in these features [Mallick et al., J. Raman Spectrosc. 42, 1883 (2011); Dang et al., Phys. Rev. Lett. 107, 043001 (2011)] has been attributed to the Stokes and anti-Stokes components of the third-order susceptibility, ${\chi }^{\left(3\right)}$. By treating the setup as a steady state of an open system coupled to four quantum radiation field modes, we show that Stokes and anti-Stokes processes contribute to both the loss and gain resonances. ${\chi }^{\left(3\right)}$ predicts loss and gain signals with equal intensity for electronically off-resonant excitation. Some asymmetry may exist for resonant excitation. However, this is unrelated to the Stokes vs anti-Stokes processes. Any observed temperature-dependent asymmetry must thus originate from effects lying outside the ${\chi }^{\left(3\right)}$ regime.

Fully relativistic calculations on the level energies and radiative lifetimes for all the low-lying levels of He-like Ni have been systematically carried out and attention has been paid to provide a complete tabulation of data for all the conceivable one- and two-electron one-photon transitions from levels of $2p^2$ configuration decaying sequentially to the ${}^1{S}_0$ ground state. Large-scale multiconfiguration Dirac-Fock wave functions have been applied to study the level features. The accuracy of our computed data has been highlighted in terms of the uncertainty estimates on the fine structure energy levels and also on the line strengths. The branching ratios of the emitted radiations from various transitions are evaluated. The sensitivity of transition rates to Breit interaction and quantum elctrodynamics effects are analyzed in detail. A comparison of our results with available experimental and previous computations is also made.

We derive the van der Waals energy and pressure in a planar multilayer system with arbitrary number of dissipative films between two half-spaces. A unique feature of this work is that the entire analysis is performed on the real frequency axis instead of summation over Matsubara frequencies on the imaginary frequency axis. The expression we obtain for van der Waals energy is a generalization of van Kampen and Schram's result for dissipationless media. By considering a specific case of a vacuum gap between multilayer objects with dissipative films, we show that the van der Waals energy due to the vacuum gap can not be interpreted simply as a sum of free energy of normal modes.

Simultaneous ionization and excitation processes are studied for initially He-like uranium ions in collisions with xenon gaseous targets at relativistic energy, $220\mathrm{MeV}/u$. The virtue of investigating the process of simultaneous excitation and ionization is that one electron ends up in the continuum, while the other electron ends up in a hydrogen-like final state. Experimentally, this process can be identified by observing the radiative decay of the excited levels in coincidence with ions that lost one electron (U${}^{91+}$). In particular, owing to the large fine-structure splitting of H-like U, the angular distribution of photons for the simultaneous ionization and excitation into the different total angular momentum $j=1/2$ and $j=3/2$ states of the L shell is determined directly from the obser-ved yields of Ly${}_{\alpha 1}$ and Ly${}_{\alpha 2}$ radiation at various observation angles. The experimental data show a progress for the dependence of the alignment on the collision impact parameter. It is shown that the current results confirm the theoretical predictions based on the independent-particle approximation and first-order perturbation, for which the simultaneous ionization and excitation processes occur at small impact parameter.

Interferometric photon-correlation measurements, which correspond to the second-order intensity cross correlations between the two output ports of an unbalanced Michelson (or Mach-Zehnder) interferometer, are sensitive to both amplitude and phase fluctuations of an incoming beam of light. Here, we present the theoretical framework behind these measurements and show that they can be used to unambiguously differentiate a coherent wave undergoing dynamical amplitude and phase fluctuations from a chaotic state of light. This technique may thus be used to characterize the output of nanolasers and monitor the onset of coherent emission.

We report on the origin of an acousto-optic Raman-Nath self-modulation found in a broad-area Nd:YAG single-shot laser. Operating the laser device under vacuum conditions suppresses the spectral splitting associated with acousto-optic modulation by the shock waves produced by the discharge of the pumping flash lamps. This splitting is reproduced by a general class B laser model that takes into account the dynamical density grating generated by a stationary acoustic radial wave.

We investigate the backflow of information in a system with a second-order structural phase transition, namely, a quasi-one-dimensional Coulomb crystal. Using standard Ramsey interferometry which couples a target ion (the system) to the rest of the chain (a phononic environment), we study the non-Markovian character of the resulting open system dynamics. We study two different time scales and show that the backflow of information pinpoints both the phase transition and different dynamical features of the chain as it approaches criticality. We also establish an exact link between the backflow of information and the Ramsey fringe visibility.

In this work we examine the effect of phase-space noncommutativity on some typically quantum properties such as quantum beating, quantum information, and decoherence. To exemplify these issues we consider the two-dimensional noncommutative quantum harmonic oscillator whose component behavior we monitor in time. This procedure allows us to determine how the noncommutative parameters are related to the missing information quantified by the linear quantum entropy and by the mutual information between the relevant Hilbert space coordinates. Particular questions concerning the thermodynamic limit of some relevant properties are also discussed in order to evidence the effects of noncommutativity. Finally, through an analogy with the Zeeman effect, we identify how some aspects of the axial symmetry of the problem suggest the possibility of decoupling the noncommutative quantum perturbations from unperturbed commutative well-known solutions.

We present a number of observables-based proofs of the Kochen-Specker (KS) theorem based on the $N$-qubit Pauli group for $N\ge 4$, thus adding to the proofs that have been presented earlier for the 2- and 3-qubit groups. These proofs have the attractive feature that they can be presented in the form of diagrams from which they are obvious by inspection. They are also irreducible in the sense that they cannot be reduced to smaller proofs by ignoring some subset of qubits and/or observables in them. A simple algorithm is given for transforming any observables-based KS proof into a large number of projectors-based KS proofs; if the observables-based proof has $O$ observables, with each observable occurring in exactly two commuting sets and any two commuting sets having at most one observable in common, the number of associated projectors-based parity proofs is 2${}^O$. We introduce symbols for the observables- and projectors-based KS proofs that capture their important features and also convey a feeling for the enormous variety of both these types of proofs within the $N$-qubit Pauli group. We discuss an infinite family of observables-based proofs, whose members include all numbers of qubits from 2 up, and show how it can be used to generate projectors-based KS proofs involving only 9 bases (or experimental contexts) in any dimension of the form $2^N$ for $N\ge 2$. Some implications of our results are discussed.

We show that ionization of noble gas atoms by a strong infrared circularly polarized laser field under standard experimental conditions can yield electrons with up to 100$\%$ spin polarization in energy-resolved measurements. Spin polarization arises due to the interplay of the electron-core entanglement and the sensitivity of ionization in circularly polarized fields to the sense of electron rotation in the initial state.

We consider the interaction of a two-state quantum system with a class of pulses of finite temporal duration. The pulse shape function $f\left(t\right)$ of such a pulse is a nonanalytic function of time, with discontinuous derivatives at the turn-on and turn-off times. The excitation line width—the excited-state population versus the detuning—is determined primarily by the magnitude of the jumps of the derivative $f^{\left(n\right)}\left(t\right)$ at the points of nonanalyticity, where $n$ is the order of the first discontinuous derivative; this nonanalyticity shows up in the $n$th superadiabatic basis. The excitation line width for such pulses exhibits weak power broadening—it scales up as ${\Omega }_0^{1/(n+1)}$, where ${\Omega }_0$ is the peak Rabi frequency of the transition: $\Omega \left(t\right)={\Omega }_{0}f\left(t\right)$. As a specific example, we consider the power-of-sine class $f\left(t\right)={\sin }^n(\pi t/T)$ ($0\leqq t\leqq T$) and a truncated Gaussian pulse, and we compare their excitation line widths with the well-known excitation profile of the rectangular pulse (the Rabi formula). We find that, because of the reduced power broadening, the ${\sin }^n$ and truncated Gaussian pulses may accelerate manipulation of qubits compared to rectangular pulses. The reason is that the lower power broadening allows one to use higher Rabi frequency, and hence shorter pulse duration, without affecting significantly other closely lying states.

Fermionic atoms trapped in a double-well potential are an ideal setting to study fundamental exchange mechanisms. We use exact diagonalization and complementary analytic calculations to demonstrate that two trapped fermions deliver a minimal model of the direct-exchange mechanism. This is an ideal quantum simulator of the Heisenberg antiferromagnet, exposes the competition between covalent and ionic bonding, and can create, manipulate, and detect quantum entanglement. Three trapped atoms form a faithful simulator of the double-exchange mechanism that is the fundamental building block behind many Heisenberg ferromagnets.

We adopt a three-level bosonic model to investigate the quantum phase transition in an ultracold atom-molecule conversion system which includes one atomic mode and two molecular modes. Through thoroughly exploring the properties of energy-level structure, fidelity, and adiabatic geometric phase, we confirm that the system exhibits a second-order phase transition from an atom-molecule mixture phase to a pure molecule phase. We give the explicit expression of the critical point and obtain two scaling laws to characterize this transition. In particular, we find that both the critical exponents and the behavior of the ground-state geometric phase have obviously changed in contrast to a similar two-level model. Our analytical calculations show that the ground-state geometric phase jumps from zero to $\pi /3$ at the critical point. This discontinuous behavior has been checked by numerical simulations and it can be used to identify the phase transition in the system.

We study the spin correlations of a few fermions in a quasi-one-dimensional trap. Exact diagonalization calculations demonstrate that repulsive interactions between the two species drives ferromagnetic correlations. The ejection probability of an atom provides an experimental probe of the spin correlations. With more than five atoms trapped, the system approaches the itinerant Stoner limit. Losses to Feshbach molecules are suppressed by the discretization of energy levels when fewer than seven atoms are trapped.

We use the equivalence of positron scattering on atomic hydrogen, and positronium-proton scattering, to provide benchmark results for (anti)hydrogen formation in positronium (Ps) scattering on (anti)protons in a low-energy region. Using the two-center $e^{+}$-H convergent close-coupling approach convergence to a few percent accuracy is demonstrated, in the only required zeroth partial wave, by substantial variation in the treatment of the two centers. A simple relation for the cross section at low Ps energies ($k^2$) is given, with the $1/k$ behavior demonstrated to strictly hold only at threshold.

We report an experimental realization of an adaptive quantum state tomography protocol. Our method takes advantage of a Bayesian approach to statistical inference and is naturally tailored for adaptive strategies. For pure states, we observe close to $N^{-1}$ scaling of infidelity with overall number of registered events, while the best nonadaptive protocols allow for $N^{-1/2}$ scaling only. Experiments are performed for polarization qubits, but the approach is readily adapted to any dimension.

Using Tsallis-$q$ entropy, we introduce the generalized concept of global quantum discord, namely the $q$-global quantum discord, and provide its analytic evaluation for two classes of multiqubit states. We also provide a sufficient condition, for which the pairwise quantum correlations in terms of $q$-global quantum discord is monogamous in multiparty quantum systems.

The time-dependent close-coupling method is used to calculate fully differential cross sections for the two-photon double ionization of the He($1s^2{}^1{S}^e$) ground state at a photon energy of 44 eV and the He($1s2s{}^{1,3}{S}^e$) excited states at a photon energy of 34 eV. The total and triple-differential cross sections for the ground state are in good agreement with available calculations. We also used the time-dependent close-coupling method to calculate fully differential nuclear-recoil cross sections of He${}^{2+}$ for the two-photon double ionization of He in the ground and excited states at the same photon energies. The nuclear-recoil differential cross sections of He${}^{2+}$ for the ground state are in good agreement with the measurements recorded with a reaction microscope at the free-electron laser facility in Hamburg (FLASH).

We demonstrate the simultaneous magneto-optical trapping (MOT) of Rb and Sr and examine the characteristic loss of Rb in the MOT due to photoionization by the cooling laser for Sr. The photoionization cross section of Rb in the $5P_{3/2}$ state at 461 nm is determined to be $1.4\left(1\right)\times {10}^{-17}$ cm${}^2$. It is important to consider this loss rate to realize a sufficiently large number of trapped Rb atoms to achieve a quantum degenerate mixture of Rb and Sr.