We report on the experimental observation of quantum-network-compatible light described by a nonpositive Wigner function. The state is generated by photon subtraction from a squeezed vacuum state produced by a continuous wave optical parametric amplifier. Ideally, the state is a coherent superposition of odd photon number states, closely resembling a superposition of weak coherent states |alpha > - |-alpha >. In the limit of low squeezing the state is basically a single photon state. Light is generated with about 10,000 and more events per second in a nearly perfect spatial mode with a Fourier-limited frequency bandwidth which matches well atomic quantum memory requirements. The generated state of light is an excellent input state for testing quantum memories, quantum repeaters, and linear optics quantum computers.

By performing cryogenic laser spectroscopy under a scanning probe electrode that induces a local electric field, we have resolved two individual fluorescent molecules separated by 12 nanometers in an organic crystal. The two molecules undergo a strong coherent dipole-dipole coupling that produces entangled sub- and superradiant states. Under intense laser illumination, both molecules are excited via a two-photon transition, and the fluorescence from this doubly excited system displays photon bunching. Our experimental scheme can be used to optically resolve molecules at the nanometer scale and to manipulate the degree of entanglement among them.

Rapid progress in science on nanoscopic scales has promoted increasing interest in techniques of ultrahigh-resolution optical microscopy. The diffraction limit can be surpassed by illuminating an object in the near field through a sub-wavelength aperture at the end of a sharp metallic probe. Proposed modifications of this technique involve replacing the physical aperture by a nanoscopic active light source. Advances in the spatial and spectral detection of individual fluorescent molecules, using near-field and far-field methods, suggest the possibility of using a single molecule as the illumination source. Here we present optical images taken with a single molecule as a point-like source of illumination, by combining fluorescence excitation spectroscopy with shear-force microscopy. Our single-molecule probe has potential for achieving molecular resolution in optical microscopy; it should also facilitate controlled studies of nanometre-scale phenomena (such as resonant energy transfer) with improved lateral and axial spatial resolution.

Using scanning probe techniques, we show the controlled manipulation of the radiation from single dipoles. In one experiment we study the modification of the fluorescence lifetime of a single molecular dipole in front of a movable silver mirror. A second experiment demonstrates the changing plasmon spectrum of a gold nanoparticle in front of a dielectric mirror. Comparison of our data with theoretical models allows determination of the quantum efficiency of each radiating dipole.It is a well established matter that the radiation of an oscillating electric dipole can be manipulated if it is placed in front of a planar interface [1]. Experiments investigating this system date back to Drexhage [2] who looked at the influence of a metallic mirror on the fluorescence lifetime of ensembles of Eu 3+ ions. By preparing a large number of samples, each with a different spacing between the mirror and the emitter layer, two major effects were observed. Firstly, it was shown that the decay rate (Γ) oscillates at large distances due to the retarded interaction of the dipoles with their own reflected fields. Secondly, it was shown that Γ is strongly modified very close to the mirror due to the energy transfer to the metal [1,2]. Since that time, numerous works have investigated the modification of spontaneous emission from ensembles in thin dielectric layers [1]. Various key parameters such as the dipole's orientation, its distance to the interface and its quantum efficiency are, however, averaged in ensemble measurements.Due to challenges such as detection sensitivity, photostability and position control, experiments with single emitters have been scarce. Some researchers have nevertheless shown effects of the local dielectric environment by adding an index matching fluid to eliminate an interface [3,4] or by introducing the subwavelength boundary of a sharp tip [5,6]. In this work we study the fluorescence lifetime and intensity of a single molecule at a well-defined orientation and position, while moving an external silver mirror in its vicinity. We also examine the plasmon spectrum of a well-characterized single gold nanoparticle at various locations in front of a dielectric mirror. These experiments allow us to demonstrate, for the first time, both the far-field modulation and the nearfield modification of the total decay rate (Γ) for individual dipoles. Since the far-field modulations are only due to changes in the radiative decay rate (Γ r ) we can determine the quantum efficiency η = Γ r /Γ of each dipole.A theoretical description of dipole decay in multi-layer structures was first developed by Chance et al. [7] and has been expanded by many authors to cover numerous * Present address:FOM-Institute for Atomic and Molecular Physics (AMOLF), 1098 SJ Amsterdam, The Netherlands † Present address:Niels Bohr Institute, 2100 Copenhagen, Denmark ‡ Electronic address: vahid.sandoghdar@ethz.ch situations. In particular, Sullivan and Hall [8] present an elegant plane wave solution that can easily be adapted to our system. For a s...

We present a parametrization of a self-consistent charge density functional-based tight-binding scheme (SCC-DFTB) to describe gold-organic hybrid systems by adding new Au-X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold-thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and Aun SCH3 (n = 3 and 25) molecular-sized models. The geometrical, energetic, and electronic parameters obtained at the SCC-DFTB level for the small Au3 SCH3 gold-thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold-thiolate model, Au25 SCH3 , the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC-DFTB parameters are also in good agreement with DFT results.