In our recent paper [1], we reported observations of photon blockade by one atom strongly coupled to an optical cavity. In support of these measurements, here we provide an expanded discussion of the general phenomenology of photon blockade as well as of the theoretical model and results that were presented in Ref. [1]. We describe the general condition for photon blockade in terms of the transmission coefficients for photon number states. For the atom-cavity system of Ref.[1], we present the model Hamiltonian and examine the relationship of the eigenvalues to the predicted intensity correlation function. We explore the effect of different driving mechanisms on the photon statistics. We also present additional corrections to the model to describe cavity birefringence and ac-Stark shifts.

On the occasion of the hundredth anniversary of Albert Einstein's annus mirabilis, we reflect on the development and current state of research in cavity quantum electrodynamics in the optical domain. Cavity QED is a field which undeniably traces its origins to Einstein's seminal work on the statistical theory of light and the nature of its quantized interaction with matter. In this paper, we emphasize the development of techniques for the confinement of atoms strongly coupled to high-finesse resonators and the experiments which these techniques enable.(Some figures in this article are in colour only in the electronic version) From Einstein to cavity QEDIn the years prior to his seminal 1905 papers, Albert Einstein had given much thought to the statistical properties of electromagnetic fields [1], especially with regard to the theory of black-body radiation developed by Max Planck [2]. Einstein realized that the quantization of light-particularly the creation and annihilation of 'light quanta'-is something more fundamental than a tacit consequence of the assumption that the total energy of a black-body is discretely distributed between a set of microstates. Beginning in 1905 with On a heuristic point of view about the creation and conversion of light [3] and in four subsequent papers on quantization [4][5][6][7], he laid the foundations of the 'old quantum theory ' [8], summarized in what is commonly referred to as the 'light quantization hypothesis':. . . the energy of a light ray emitted from a point [is] not continuously distributed over an ever increasing space, but consists of a finite number of energy quanta which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units [3].

Optical communication channels have redefined the purview and applications of classical computing; similarly, photonic transfer of quantum information promises to open new horizons for quantum computing. The implementation of light-matter interfaces that preserve quantum information is technologically challenging, but key building blocks for such devices have recently been demonstrated in several research groups. Here, we outline the theoretical framework for information transfer between nodes of a quantum network, review the current experimental state of the art, and discuss the prospects for hybrid systems currently in development.

We demonstrate the reversible mapping of a coherent state of light with a mean photon number n ' 1:1 to and from the hyperfine states of an atom trapped within the mode of a high-finesse optical cavity. The coherence of the basic processes is verified by mapping the atomic state back onto a field state in a way that depends on the phase of the original coherent state. Our experiment represents an important step toward the realization of cavity QED-based quantum networks, wherein coherent transfer of quantum states enables the distribution of quantum information across the network. DOI: 10.1103/PhysRevLett.98.193601 PACS numbers: 42.50.Pq, 03.67.ÿa, 32.80.Pj An important goal in quantum information science is the realization of quantum networks for the distribution and processing of quantum information [1], including for quantum computation, communication, and metrology [2 -5]. In the initial proposal for the implementation of quantum networks [6], atomic internal states with long coherence times serve as ''stationary'' qubits, stored and locally manipulated at the nodes of the network. Quantum channels between different nodes are provided by optical fibers, which transport photons (''flying'' qubits) over long distances [7]. A crucial requirement for such network protocols is the reversible mapping of quantum states between light and matter. Cavity quantum electrodynamics (QED) provides a promising avenue for achieving this capability by using strong coupling for the interaction of single atoms and photons [8].Within this setting, reversible emission and absorption of one photon can be achieved by way of a dark-state process involving an atom and the field of a high-finesse optical cavity. For classical fields, this adiabatic passage process was first considered 20 years ago [9,10], before being adapted to quantum fields [11] and specifically to the coherent transfer of quantum states between remote locations [6], with many extensions since then [12]. The basic scheme, illustrated in Fig.

Proposed quantum networks require both a quantum interface between light and matter and the coherent control of quantum states1,2. A quantum interface can be realized by entangling the state of a single photon with the state of an atomic or solid-state quantum memory, as demonstrated in recent experiments with trapped ions3,4, neutral atoms5,6, atomic ensembles7,8, and nitrogen-vacancy spins9. The entangling interaction couples an initial quantum memory state to two possible light–matter states, and the atomic level structure of the memory determines the available coupling paths. In previous work, these paths’ transition parameters determine the phase and amplitude of the final entangled state, unless the memory is initially prepared in a superposition state4, a step that requires coherent control. Here we report the fully tunable entanglement of a single 40Ca+ ion and the polarization state of a single photon within an optical resonator. Our method, based on a bichromatic, cavity-mediated Raman transition, allows us to select two coupling paths and adjust their relative phase and amplitude. The cavity setting enables intrinsically deterministic, high-fidelity generation of any two-qubit entangled state. This approach is applicable to a broad range of candidate systems and thus presents itself as a promising method for distributing information within quantum networks.