We propose a scheme for perfect excitation of a single two-level atom by a single photon in free space. The photon state has to match the time reversed photon state originating from spontaneous decay of a twolevel system. We discuss its experimental preparation. The state is characterized by a particular asymmetric exponentially-shaped temporal profile. Any deviations from this ideal state limit the maximum absorption. Although perfect excitation requires an infinite amount of time we demonstrate that there is a class of initial onephoton quantum states which can achieve almost perfect absorption even for a finite interaction time. Our results pave the way for realizing perfect coupling between flying and stationary qubits in free space thus opening a possibility for building scalable quantum networks. PACS numbers:Efficient coupling between light and matter at a single quantum level lies at the heart of scalable quantum information processing, computation and communication [1,2,3]. Information encoded in a flying qubit used for its transfer has to be recorded by a localized stationary qubit (e.g. an atom), i.e. the photon has to excite the atom with unit probability. Experimental realization of these protocols remains challenging due to the weak coupling between a single-photon and a single-atom in free space. Recent approaches investigating this problem focus on the absorption of a single photon by an ensemble of atoms resulting in a distributed single photon excitation entangling the atoms of the ensemble [4,5,6,7,8,9].So far, close-to-perfect interaction has been achieved only in the context of cavity QED in the strong coupling regime where the atom is forced to interact with a single mode of the radiation field only [11,12,13,14,15,16,17]. Scaling up these schemes is difficult because of the requirement for high finesse cavities.Currently, several groups are attempting to quantify [21,22] and to improve light-matter coupling in the absence of any mode-selecting cavity [10,23]. A detailed study of singleatom-single-photon interaction in free space requires the control of all resonant degrees of freedom of the radiation field, i.e. its spatio-temporal vector modes. Van Enk and Kimble showed theoretically that both strong focusing and increased overlap of a light beam with a dipole wave corresponding to the relevant atomic transition improve the coupling [24,25]. Strong focusing of a light beam was demonstrated by Quabis et al. by tailoring the polarization pattern of light in theory [26] and in experiment [27,28].We aim at having full control over the field modes and at exciting and maximizing the coupling to the atom. A first step towards this goal was the demonstration of a significant attenuation of a laser beam by a single trapped ion [29]. Recently several groups succeeded in improving on this result [10,21,30,31,32]. Other groups attempted to control the excitation of a single atom [33,34], the ultimate goal being per- * Electronic address: mstobinska@optik.uni-erlangen.de fect excitation with a single p...

Entanglement between macroscopically populated states can easily be created by combining a single photon and a bright coherent state on a beamsplitter. Motivated by the simplicity of this technique, we report on a method using displacement operations in the phase space and basic photon detections to reveal such an entanglement. We show that this eminently feasible approach provides an attractive way for exploring entanglement at various scales, ranging from 1 to 1000 photons. This offers an instructive viewpoint to gain insight into why it is hard to observe quantum features in our macroscopic world.

We find that Bell's inequality can be significantly violated ͑up to Tsirelson's bound͒ with two-mode entangled coherent states using only homodyne measurements. This requires Kerr nonlinear interactions for local operations on the entangled coherent states. Our example is a demonstration of Bell-inequality violations using classical measurements. We conclude that entangled coherent states with coherent amplitudes as small as 0.842 are sufficient to produce such violations.

We present a technique for measuring the second-order coherence function g(2)(tau) of light using a Hanbury Brown-Twiss intensity interferometer modified for homodyne detection. The experiment was performed entirely in the continuous-variable regime at the sideband frequency of a bright carrier field. We used the setup to characterize g(2)(tau) for thermal and coherent states and investigated its immunity to optical loss. We measured g(2)(tau) of a displaced-squeezed state and found a best antibunching statistic of g(2)(0)=0.11+/-0.18.

A Fokker-Planck equation for the Wigner function evolution in a noisy Kerr medium ͑ ͑3͒ nonlinearity͒ is presented. We numerically solved this equation taking a coherent state as an initial condition. The dissipation effects are discussed. We provide examples of quantum interference, sub-Planck phase space structures, and Gaussian vs non-Gaussian dynamical evolution of the state. The results also apply to the description of a nanomechanical resonator with an intrinsic Duffing nonlinearity.

Almost all Bell inequality experiments to date have used postselection and therefore relied on the fair sampling assumption for their interpretation. The standard form of the fair sampling assumption is that the loss is independent of the measurement settings, so the ensemble of detected systems provides a fair statistical sample of the total ensemble. This is often assumed to be needed to interpret Bell inequality experiments as ruling out hidden-variable theories. Here we show that it is not necessary; the loss can depend on measurement settings, provided the detection efficiency factorizes as a function of the measurement settings and any hidden variable. This condition implies that Tsirelson's bound must be satisfied for entangled states. On the other hand, we show that it is possible for Tsirelson's bound to be violated while the Clauser-Horne-Shimony-Holt (CHSH)-Bell inequality still holds for unentangled states, and present an experimentally feasible example.

Rapid growth of nanoscience and nanotechnology requires new and more powerful modeling tools. Efficient theoretical modeling of large molecular systems at the ab initio and Density Functional Theory (DFT) levels of theory depends critically on the size and completeness of the basis set used. The recently designed variants of STO-3G basis set (STO-3Gel, STO-3Gmag), modified to correctly predict electronic and magnetic properties were tested on simple models of pristine and functionalized carbon nanotube (CNT) systems and fullerenes using the B3LYP and VSXC density functionals. Predicted geometries, vibrational properties, and HOMO/LUMO gaps of the model systems, calculated with typical 6-31G* and modified STO-3G basis sets, were comparable. The (13)C nuclear isotropic shieldings, calculated with STO-3Gmag and Jensen's polarization consistent pcS-2 basis sets, were also identical. The STO-3Gmag basis sets, being half the size of the latter one, are promising alternative for studying (13)C nuclear magnetic shieldings in larger size CNTs and fullerenes.

We study the unconventional topological phases of polaritons inside a cavity waveguide, demonstrating how strong light-matter coupling leads to a breakdown of the bulk-edge correspondence. Namely, we observe an ostensibly topologically nontrivial phase, which unexpectedly does not exhibit edge states. Our findings are in direct contrast to topological tight-binding models with electrons, such as the celebrated Su-Schrieffer-Heeger (SSH) model. We present a theory of collective polaritonic excitations in a dimerized chain of oscillating dipoles embedded inside a photonic cavity. The added degree of freedom from the cavity photons upgrades the system from a typical SSH SU(2) model into a largely unexplored SU(3) model. Tuning the light-matter coupling strength by changing the cavity size reveals three critical points in parameter space: when the polariton band gap closes, when the Zak phase changes from being trivial to nontrivial, and when the edge state is lost. Remarkably, these three critical points do not coincide, and thus the Zak phase is no longer an indicator of the presence of edge states. Our discoveries demonstrate some remarkable properties of topological matter when strongly coupled to light, and could be important for the growing field of topological nanophotonics.