Entanglement between quantum and classical objects is of special interest in the context of fundamental studies of quantum mechanics and potential applications to quantum information processing. In quantum optics, single photons are treated as light quanta while coherent states are considered the most classical among all pure states. Recently, entanglement between a single photon and a coherent state in a free-traveling field was identified to be a useful resource for optical quantum information processing. However, it was pointed out to be extremely difficult to generate such states since it requires a clean cross-Kerr nonlinear interaction. Here, we devise and experimentally demonstrate a scheme to generate such hybrid entanglement by implementing a coherent superposition of two distinct quantum operations. The generated states clearly show entanglement between the two different types of states. Our work opens a way to generate hybrid entanglement of a larger size and to develop efficient quantum information processing using such a new type of qubits.Quantum entanglement is of crucial importance for fundamental tests of quantum mechanics and implementations of quantum information processing. The idea of entangling "classical" and "quantum" states is found in Schrödinger's famous cat paradox [1], where a microscopic atom -as a quantum particle-and a cat -as a classical object-were assumed to be entangled to each other. In quantum optics, coherent states are considered the most classical among all pure states [2]. In many situations they can be treated semi-classically, i.e. as a classical light field with the addition of stochastic noise, and are typically most robust against decoherence [3]. On the other hand, single photons are normally treated as discrete light quanta containing the minimum quantized amount of energy available at a given frequency and any attempt to describe them with an effective noise theory leads to negative probabilities.Recently, entanglement between a single photon and a coherent state was identified to be a very useful resource for optical quantum information processing, enabling one to perform nearly deterministic quantum teleportation and universal gate operations for quantum computation using linear optics [4]. This type of hybrid entanglement, however, is difficult to generate in spite of its conceptual interest and potential usefulness. It is well known that a clean cross-Kerr type interaction between a single photon and a coherent state may generate such a state [5,6]. However, many fundamental problems lie in the way of realizing a suitable interaction of this kind [7][8][9]. Therefore, an experimentally accessible scheme to replace the cross-Kerr nonlinearity and create entanglement between a single photon and a coherent state would be highly desirable. In this article, we introduce such a scheme and use it to experimentally generate small-scale hybrid entanglement. We also outline extensions whereby our methods can be generalized to produce larger scale hybrid entanglement.Th...

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We propose a definition of nonclassicality for a single-mode quantum-optical process based on its action on coherent states. If a quantum process transforms a coherent state to a nonclassical state, it is verified to be nonclassical. To identify nonclassical processes, we introduce a representation for quantum processes, called the process-nonclassicality quasiprobability distribution, whose negativities indicate nonclassicality of the process. Using this distribution, we derive a relation for predicting nonclassicality of the output states for a given input state. We experimentally demonstrate our method by considering the single-photon addition as a nonclassical process and predicting nonclassicality of the output state for an input thermal state.

Single dibenzoterrylene (DBT) molecules offer great promise as bright, reliable sources of single photons on demand, capable of integration into solid-state devices. It has been proposed that DBT in anthracene might be placed close to an optical waveguide for this purpose, but so far there have been no demonstrations of sufficiently thin crystals, with a controlled concentration of the dopant molecules. Here we present a method for growing very thin anthracene crystals from super-saturated vapour, which produces crystals of extreme flatness and controlled thickness. We show how this crystal can be doped with an adjustable concentration of dibenzoterrylene (DBT) molecules and we examine the optical properties of these molecules to demonstrate their suitability as quantum emitters in nanophotonic devices. Our measurements show that the molecules are available in the crystal as single quantum emitters, with a well-defined polarisation relative to the crystal axes, making them amenable to alignment with optical nanostructures. We find that the radiative lifetime and saturation intensity vary little within the crystal and are not in any way compromised by the unusual matrix environment. We show that a large fraction of these emitters can be excited more than 10 times without photo-bleaching, making them suitable for real applications.