A critical requirement for diverse applications in Quantum Information Science is the capability to disseminate quantum resources over complex quantum networks [1,2]. For example, the coherent distribution of entangled quantum states together with quantum memory to store these states can enable scalable architectures for quantum computation [3], communication [4], and metrology [5]. As a significant step toward such possibilities, here we report observations of entanglement between two atomic ensembles located in distinct apparatuses on different tables. Quantum interference in the detection of a photon emitted by one of the samples projects the otherwise independent ensembles into an entangled state with one joint excitation stored remotely in 10 5 atoms at each site [6]. After a programmable delay, we confirm entanglement by mapping the state of the atoms to optical fields and by measuring mutual coherences and photon statistics for these fields. We thereby determine a quantitative lower bound for the entanglement of the joint state of the ensembles. Our observations provide a new capability for the distribution and storage of entangled quantum states, including for scalable quantum communication networks [6].Entanglement is a uniquely quantum mechanical property of the correlations among various components of a physical system. Initial demonstrations of entanglement were made for photon pairs from the fluorescence in atomic cascades [7,8] and from parametric down conversion [9]. More recently, entanglement has been recognized as a critical resource for accomplishing tasks that are otherwise impossible in the classical domain [1]. Spectacular advances have been made in the generation of quantum entanglement for diverse physical systems [1, 2], including entanglement stored for many seconds in trapped ions for distances on the millimeter scale [10,11], long-lived entanglement of macroscopic quantum spins persisting for milliseconds on the centimeter scale [12], and remote entanglement carried by photon pairs over distances of tens of kilometers of optical fibers [13].For applications in quantum information science, entanglement can be created deterministically by way of precise control of quantum dynamics for a physical system, or probabilistically by way of quantum interference in a suitable measurement with random instances of success. In the latter case, it is essential that success be heralded unambiguously so that the resulting entangled state is available for subsequent utilization. In either case, quantum memory is required to store the entangled states until they are required for the protocol at hand.There are by now several examples of entanglement generated "on demand," [1] beginning with the realization of the EPR paradox for continuous quantum variables [14] and the deterministic entanglement of the discrete internal states of two trapped ions [15]. Important progress has been made towards measurement-induced entanglement on various fronts, including the observation of entanglement between a trapped...

We demonstrate entanglement distribution between two remote quantum nodes located 3 meters apart [1]. This distribution involves the asynchronous preparation of two pairs of atomic memories and the coherent mapping of stored atomic states into light fields in an effective state of near maximum polarization entanglement. Entanglement is verified by way of the measured violation of a Bell inequality, and can be used for communication protocols such as quantum cryptography. The demonstrated quantum nodes and channels can be used as segments of a quantum repeater, providing an essential tool for robust long-distance quantum communication.In quantum information science [2], distribution of entanglement over quantum networks is a critical requirement, including for metrology [3], quantum computation [4,5] and communication [4,6]. Quantum networks are composed of quantum nodes for processing and storing quantum states, and quantum channels that link the nodes. Significant advances have been made with diverse systems towards the realization of such networks, including ions [7], single trapped atoms in free space [8,9] and in cavities [10], and atomic ensembles in the regime of continuous variables [11].An approach of particular importance has been the seminal work of Duan, Lukin, Cirac, and Zoller (DLCZ) for the realization of quantum networks based upon entanglement between single photons and collective excitations in atomic ensembles [12]. Critical experimental capabilities have been achieved, beginning with the generation of nonclassical fields [13,14] with controlled waveforms [15] and extending to the creation and retrieval of single collective excitations [16,17,18] with high efficiency [19,20]. Heralded entanglement with quantum memory, which is the cornerstone of networks with efficient scaling, was achieved between two ensembles [21]. More recently, conditional control of the quantum states of a single ensemble [22,23,24] and of two distant ensembles [25] has also been implemented, as are likewise required for the scalability of quantum networks based upon probabilistic protocols.Our interest is to develop the physical resources that enable quantum repeaters [6], thereby allowing entanglement-based quantum communication tasks over quantum networks on distance scales much larger than set by the attenuation length of optical fibers, including quantum cryptography [26]. For this purpose, heralded number state entanglement [21] between two remote atomic ensembles is not directly applicable. In- * Current address: Group of Applied Physics, University of Geneva, Geneva, Switzerland † Current address: Departamento de Física, Universidade Federal de Pernambuco, Recife-PE, 50670-901, Brazil stead, DLCZ proposed to use pairs of ensembles (U i , D i ) at each quantum node i, with the sets of ensembles {U i }, {D i } separately linked in parallel chains across the network [12]. Relative to the state of the art in Ref.[21], the DLCZ protocol requires the capability for the independent control of pairs of entangled ensembles...

Ultrashort laser pulses have thus far been used in two distinct modes. In the time domain, the pulses have allowed probing and manipulation of dynamics on a subpicosecond time scale. More recently, phase stabilization has produced optical frequency combs with absolute frequency reference across a broad bandwidth. Here we combine these two applications in a spectroscopic study of rubidium atoms. A wide-bandwidth, phase-stabilized femtosecond laser is used to monitor the real-time dynamic evolution of population transfer. Coherent pulse accumulation and quantum interference effects are observed and well modeled by theory. At the same time, the narrow linewidth of individual comb lines permits a precise and efficient determination of the global energy-level structure, providing a direct connection among the optical, terahertz, and radio-frequency domains. The mechanical action of the optical frequency comb on the atomic sample is explored and controlled, leading to precision spectroscopy with an appreciable reduction in systematic errors.Ultrashort laser pulses have given a remarkably detailed picture of photophysical dynamics. In studies of alkali atoms (1) and diatomics (2) in particular, coherent wave packet motion has been observed and even actively controlled. However, the broad bandwidth of these pulses has prevented a simultaneous high-precision measurement of state energies. At the expense of losing any direct observation or control of coherent dynamics, precision spectroscopy enabled by continuous wave (cw) lasers has been one of the most important fields of modern scientific research, providing the experimental underpinning of quantum mechanics and quantum electrodynamics.This trade-off between the time and frequency domains might seem fundamental, but in fact it results from pulse-to-pulse phase fluctuations in laser operation. The recent introduction of phase-stabilized, widebandwidth frequency combs based on modelocked femtosecond lasers has provided a direct connection between these two domains (3, 4). Many laboratories have constructed frequency combs that establish optical frequency markers directly linked to a microwave or optical standard, covering a variety of spectral intervals. Atomic and molecular structural information can now be probed over a broad spectral range, with vastly improved measurement precision and accuracy enabled by this absolute frequency-based approach (5). One of the direct applications is the development of optical atomic clocks (6-8). To date, however, traditional cw laserbased spectroscopic approaches have been essential to all of these experiments, with frequency combs serving only as reference rulers (9).Here we take advantage of the phasestable femtosecond pulse train to bridge the fields of high-resolution spectroscopy and ultrafast dynamics. This approach of direct frequency comb spectroscopy (DFCS) uses light from a comb of appropriate structure to directly interrogate a multitude of atomic levels and to study time-dependent quantum coherence. DFCS allows time-resol...

Abstract:We report significant improvements in the retrieval efficiency of a single excitation stored in an atomic ensemble and in the subsequent generation of strongly correlated pairs of photons. A 50% probability of transforming the stored excitation into one photon in a well-defined spatio-temporal mode at the output of the ensemble is demonstrated. These improvements are illustrated by the generation of high-quality heralded single photons with a suppression of the two-photon component below 1% of the value for a coherent state. A broad characterization of our system is performed for different parameters in order to provide input for the future design of realistic quantum networks.