Quantum entanglement is among the most fascinating aspects of quantum theory. Entangled optical photons are now widely used for fundamental tests of quantum mechanics and applications such as quantum cryptography. Several recent experiments demonstrated entanglement of optical photons with trapped ions, atoms and atomic ensembles, which are then used to connect remote long-term memory nodes in distributed quantum networks. Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique, and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks.

We present a procedure that makes use of group theory to analyze and predict the main properties of the negatively charged nitrogen-vacancy (NV) center in diamond. We focus on the relatively low temperatures limit where both the spin-spin and spin-orbit effects are important to consider. We demonstrate that group theory may be used to clarify several aspects of the NV structure, such as ordering of the singlets in the (e 2 ) electronic configuration, the spin-spin and the spin-orbit interactions in the (ae) electronic configuration. We also discuss how the optical selection rules and the response of the center to electric field can be used for spin-photon entanglement schemes. Our general formalism is applicable to a broad class of local defects in solids. The present results have important implications for applications in quantum information science and nanomagnetometry. I. INTRODUCTIONDuring the past few years nitrogen-vacancy (NV) centers have emerged as promising candidates for a number of applications [1-4] ranging from high spatial resolution imaging [5] to quantum computation [6]. At low temperatures, the optical transitions of the NV center become very narrow and can be coherently manipulated, allowing for spin-photon entanglement generation [7] for quantum communication and all optical control [8]. A detailed understanding of the properties of this defect is critical for many of these applications. Several studies have addressed this issue both experimentally [9, 10] and theoretically [11,12]. Furthermore, other atom-like defects can potentially be engineered in diamond [13] and other materials with similar or perhaps better * Electronic address: jmaze@puc.cl † Electronic address: agali@eik.bme.hu

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We report an experimental study of group-velocity dispersion effect on an entangled two-photon wavepacket, generated via spontaneous parametric downconversion and propagating through a dispersive medium. Even in the case of using CW laser beam for pump, the biphoton wavepacket and the secondorder correlation function spread significantly. The study and understanding of this phenomenon is of great importance for quantum information applications, such as quantum communication and distant clock synchronization.

We formulate a general complementarity relation starting from any Hermitian operator with discrete non-degenerate eigenvalues. We then elucidate the relationship between quantum complementarity and the Heisenberg-Robertson's uncertainty relation. We show that they are intimately connected. Finally we exemplify the general theory with some specific suggested experiments.Comment: 9 pages, 4 figures, REVTeX, uses epsf.sty and multicol.st

Correlative light and electron microscopy promises to combine molecular specificity with nanoscale imaging resolution. However, there are substantial technical challenges including reliable co-registration of optical and electron images, and rapid optical signal degradation under electron beam irradiation. Here, we introduce a new approach to solve these problems: imaging of stable optical cathodoluminescence emitted in a scanning electron microscope by nanoparticles with controllable surface chemistry. We demonstrate well-correlated cathodoluminescence and secondary electron images using three species of semiconductor nanoparticles that contain defects providing stable, spectrally-distinguishable cathodoluminescence. We also demonstrate reliable surface functionalization of the particles. The results pave the way for the use of such nanoparticles for targeted labeling of surfaces to provide nanoscale mapping of molecular composition, indicated by cathodoluminescence colour, simultaneously acquired with structural electron images in a single instrument.

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We characterized backscattering effects in optical fiber using a photon counting technique and considered its implications for quantum key distribution (QKD). We found that Rayleigh (elastic) backscattering can put strong limitations on a two-way QKD system’s performance. Raman (inelastic) scattering can restrict the ability of wavelength multiplexing of a quantum channel with strong classical data channel(s).