We study Doppler cooling of atoms using a frequency comb and develop a theory to quantitatively describe the process by investigating the interaction of two-level atoms with two counterpropagating pulse trains. We derive the radiative force on atoms and show that each comb mode effectively acts as a cooling laser, thereby indicating that simultaneous laser cooling in multiple cooling channels can be achieved using a single-frequency comb source. Possible experimental applications involving alkali-metal atoms are illustrated.

Several mechanisms by which an external electromagnetic field influences the temperature of a plasma are studied analytically and specialized to the system of an ultracold plasma (UCP) driven by a uniform radio-frequency (rf) field. Heating through collisional absorption is reviewed and applied to UCPs. Furthermore, it is shown that the rf field modifies the three-body recombination process by ionizing electrons from intermediate high-lying Rydberg states and upshifting the continuum threshold, resulting in a suppression of three-body recombination. Heating through collisionless absorption associated with the finite plasma size is calculated in detail, revealing a temperature threshold below which collisionless absorption is ineffective.

Rotational excitation of polar molecules trapped in an optical lattice gives rise to rotational excitons. Here we show that nonlinear interactions of such excitons can be controlled by an electric field. The exciton-exciton interactions can be tuned to induce exciton pairing, leading to the formation of biexcitons. Tunable nonlinear interactions between excitons can be used for many applications ranging from the controlled preparation of entangled quasiparticles to the study of polaron interactions and the effects of nonlinear interactions on quantum energy transport in molecular aggregates.

We compute the integrated doublon production rate in response to a lattice modulation for two-component fermions in an optical lattice. We derive a general formula for the integrated intensity, valid in the presence of inhomogeneous potentials such as the trap, which gives the integrated intensity in terms of equal time correlation functions only. Such a formula is thus well suited for direct numerical calculations. We show that, in the limit of large repulsion for commensurate fillings, or for temperature ranges for which the hopping is incoherent, the integrated doublon spectrum is directly related to the nearest-neighbor spin-spin correlation function. We compute its temperature dependence in this regime using finite temperature quantum Monte Carlo calculation.

We consider a Bell-like inequality performed using various instances of multiphoton entangled states to demonstrate that losses occurring after the unitary transformations used in the nonlocality test can be counteracted by enhancing the size of such entangled states. In turn, this feature can be used to overcome detection inefficiencies affecting the test itself: a slight increase in the size of such states, pushing them towards a more macroscopic form of entanglement, significantly improves the state robustness against detection inefficiency, thus easing the closing of the detection loophole. Differently, losses before the unitary transformations cause decoherence effects that cannot be compensated using macroscopic entanglement.

We propose a measurement scheme for observing quantum correlations and entanglement in the spatial properties of two macroscopic mirrors. Two spatial versions of compass states are generated by using a single Gaussian mode of single photons in a simple interferometer. The chessboard pattern of spatial compass states reflects the spatial properties of a mirror composed of $N$ quantum mirrors. The displacement and tilt correlations of the two mirrors are manifested by single photons and projection measurements through a measuring device which measures the propensities of the compass states. The technique can extract mechanical correlations of the two mirrors and lock them into the Einstein-Podolsky-Rosen (EPR) correlation. The criteria for EPR entanglement of these mirrors are then verified by sub-Planck structures in the propensity. We formulate the discrete-like property of the propensity, a demonstration of quantum jumps of EPR correlations in phase space, which, hence, enables discrete phase-space quantum computing and information processing.

A fundamental problem in quantum information is to describe efficiently multipartite quantum states. An efficient representation in terms of graphs exists for several families of quantum states (graph, cluster, and stabilizer states), motivating us to extend this construction to other classes. We introduce an axiomatic framework for mapping graphs to quantum states of a suitable physical system. Starting from three general axioms we derived a rich structure which includes and generalizes several classes of multipartite entangled state, like graph or stabilizer states, Gaussian cluster states, quantum random networks, and projected entangled pair states. Due to its flexibility we can extend the present formalism to include directed and weighted graphs.

A wide variety of communication channels employ the quantized electromagnetic field to convey information. Their communication capacity crucially depends on losses associated to spatial characteristics of the channel such as diffraction and antenna design. Here we focus on the communication via a finite pupil, showing that diffraction is formally described as a memory channel. By exploiting this equivalence we then compute the communication capacity of an optical refocusing system, modeled as a converging lens. Even though loss of information originates from the finite pupil of the lens, we show that the presence of the refocusing system can substantially enhance the communication capacity. We mainly concentrate on communication of classical information, the extension to quantum information being straightforward.

We present a package of mathematical theorems that allows us to construct multipartite-entanglement criteria. Importantly, establishing bounds for certain classes of entanglement does not take an optimization over continuous sets of states. These bonds are found from the properties of commutativity graphs of operators used in the criterion. We present two examples of criteria constructed according to our method. One of them detects genuine five-qubit entanglement without ever referring to correlations between all five qubits.

In this article, we extend recent results concerning random-pair Einstein-Podolsky-Rosen distillation and the operational gap between separable operations (SEPs) and local operations with classical communication (LOCC). In particular, we consider the problem of obtaining bipartite maximal entanglement from an $N$-qubit $W$-class state (i.e., that of the form $\sqrt{x_0}|00\cdots 0\rangle +\sqrt{x_1}|10\cdots 0\rangle +\cdots +\sqrt{x_n}|00\cdots 1\rangle$) when the target pairs are a priori unspecified. We show that when $x_0=0$, the optimal probabilities for SEPs can be computed using semidefinite programming. On the other hand, to bound the optimal probabilities achievable by LOCC, we introduce entanglement monotones defined on the $N$-qubit $W$ class of states. The LOCC monotones we construct can be increased by SEPs, and in terms of transformation success probability, we are able to quantify a gap as large as 37$\%$ between the two classes. Additionally, we demonstrate transformations ${\rho }^{\otimes n}\to {\sigma }^{\otimes n}$ that are feasible by SEP for any $n$ but impossible by LOCC.

We investigated the light-induced desorption (LID) of Rb atoms from quartz substrates to understand better the LID observed in alkali-metal vapor cells used in laser cooling experiments. The substrate was first exposed to Rb under ultrahigh vacuum conditions. The Rb was found to diffuse into the substrate by x-ray photoelectron spectroscopy. Rb atoms were then deposited with a density of $\sim {10}^{15}$ atoms/cm${}^2$ on the surface, from which LID was observed upon irradiation by ultraviolet light. The LID rate depended linearly on the light intensity in the investigated range up to 15 mW/cm${}^2$. A typical value was ${10}^{10}$ atoms/(s cm${}^2$) per mW/cm${}^2$ at a photon energy of 4.3 eV (290 nm in wavelength). We found that the LID rate increased with photon energy in the range 1.8–4.3 eV (700–290 nm in wavelength).

Fluorescence, resonance, and off-resonance Raman spectroscopy are all precise and versatile techniques for indentifying small quantities of chemical and biological substances. One way to improve the sensitivity and specificity of these measurement techniques is to use enhancement of optical fields in the vicinity of metal nanoparticles. The degree of enhancement, however, is drastically different as Raman enhancement of 10 orders of magnitude or more has been consistently measured in experiment, while the enhancement of the seemingly similar process of fluorescence is typically far more modest. While resonance Raman scattering has the advantages of higher sensitivity and specificity when compared with the ordinary, nonresonant Raman process, its plasmon enhancement is far less spectacular. In fact, both fluorescence and resonance Raman measurements are subject to quenching when the molecule is placed too close to the metal surface. Such an effect, however, is completely absent from the normal nonresonant Raman process. In this work, we present an analytical model that reveals the physics behind the strikingly different orders of magnitude in enhancement that have been observed, provide a fundamental explanation for the quenching effect observed in fluorescence and resonance Raman but not in normal Raman, establish limits for attainable enhancement, and outline the path to optimization of all three processes.

We report on the experimental realization of a microwave decelerator for neutral polar molecules, suitable for decelerating and focusing molecules in high-field-seeking states. The multistage decelerator consists of a cylindrical microwave cavity oscillating in the TE${}_{\mathit{11}\mathit{n}}$ mode, with $n=12$ electric field maxima along the symmetry axis. By switching the microwave field on and off at the appropriate times, a beam of state-selected ammonia molecules with an incident mean velocity of 25 m/s is guided while being spatially focused in the transverse direction and bunched in the forward direction. Deceleration from 20.0 to 16.9 m/s and acceleration from 20.0 to 22.7 m/s are demonstrated.

The quantum dynamics of optomechanical systems has mostly been studied to determine the fluctuations around classical steady states. We present a theoretical approach to determining the system observables of optomechanical systems as genuine quantum objects, for example, a quantum-mechanical oscillator coupled to a cavity single photon. In this approach we study the dynamics of such systems in the strong-coupling regime. We find that, under strong optomechanical coupling, steady quantum states of optomechanical systems driven by continuous-wave single photons exhibit periodic oscillation, and cavity noise considerably affects the system observables.

In this paper we consider a quantum harmonic oscillator interacting with the electromagnetic radiation field in the presence of a boundary condition preserving the continuous spectrum of the field, such as an infinite perfectly conducting plate. Using an appropriate Bogoliubov-type transformation we can diagonalize exactly the Hamiltonian of our system in the continuum limit and obtain nonperturbative expressions for its ground-state energy. From the expressions found, the atom-wall Casimir-Polder interaction energy can be obtained, and well-known lowest-order results are recovered as a limiting case. Use and advantage of this method for dealing with other systems where perturbation theory cannot be used is also discussed.

A field-theoretical approach to the scattering off an oscillating quantum system is developed. As a key ingredient it employs the adiabatic eigenstate basis and consists of a perturbative scheme for the calculation of the geometric phases influencing the transmission through the time-dependent potential landscape. The main advantage is the identification of basic diagrams which allow for an immediate interpretation of the underlying elementary physical processes contributing to the scattering and transmission behavior. We apply our method to the simple, but prototypical, problem of transmission through an one-dimensional oscillating $\delta$ potential and demonstrate how it enables a deeper understanding of the relevant physical processes.

By stating the adiabatic theorem of quantum mechanics in a clear and rigorous way, we establish a necessary condition and a sufficient condition for its validity, where the latter is obtained employing our recently developed adiabatic perturbation theory. Also, we simplify further the sufficient condition into a useful and simple practical test at the expense of its mathematical rigor. We present results for the most general case of quantum systems, i.e., those with degenerate energy spectra. These conditions are of upmost importance for assessing the validity of practical implementations of non-Abelian braiding and adiabatic quantum computation. To illustrate the degenerate adiabatic approximation, and the necessary and sufficient conditions for its validity, we analyze in depth an exactly solvable time-dependent degenerate problem.

Integral cross sections for electron-impact excitation out of the ground-state level $X{}^1{\Sigma }_g^{+}(v^{\prime \prime }=0)$ to the $b{}^1{\Pi }_u$, $c_3{}^1{\Pi }_u$, $o_3{}^1{\Pi }_u$, $b^{\prime }{}^1{\Sigma }_u^{+}$, $c_4^{\prime }{}^1{\Sigma }_u^{+}$, $G{}^3{\Pi }_u$, and $F{}^3{\Pi }_u$ electronic states in N${}_2$ are reported at incident energies ranging between 17.5 and 100 eV. We also provide excitation cross sections using emission-based excitation shape functions and optical oscillator strengths. These cross-section results are of great importance to planetary atmospheric modeling of the emissions observed in Earth's atmosphere as well as those of Titan and Triton, the largest moons of Saturn and Neptune, respectively. Critical comparisons of the present cross sections with previous values are presented in an effort to provide improved cross sections for electron excitation of this fundamental molecule's important transitions.