An elementary quantum network operation involves storing a qubit state in an atomic quantum memory node, and then retrieving and transporting the information through a single photon excitation to a remote quantum memory node for further storage or analysis. Implementations of quantum network operations are thus conditioned on the ability to realize such matter-to-light and/or light-tomatter quantum state mappings. Here, we report generation, transmission, storage and retrieval of single quanta using two remote atomic ensembles. A single photon is generated from a cold atomic ensemble at Site A via the protocol of Duan, Lukin, Cirac, and Zoller (DLCZ) [1] and is directed to Site B through a 100 meter long optical fiber. The photon is converted into a single collective excitation via the dark-state polariton approach of Fleischhauer and Lukin [2]. After a programmable storage time the atomic excitation is converted back into a single photon. This is demonstrated experimentally, for a storage time of 500 nanoseconds, by measurement of an anticorrelation parameter α. Storage times exceeding ten microseconds are observed by intensity cross-correlation measurements. The length of the storage period is two orders of magnitude longer than the time to achieve conversion between photonic and atomic quanta. The controlled transfer of single quanta between remote quantum memories constitutes an important step towards distributed quantum networks.A quantum network, consisting of quantum nodes and interconnecting channels, is an outstanding goal of quantum information science. Such a network could be used for distributed computing or for the secure sharing of information between spatially remote parties [1,3,4,5,6,7]. While it is natural that the network's fixed nodes (quantum memory elements) could be implemented by using matter in the form of individual atoms or atomic ensembles, it is equally natural that light fields be used as carriers of quantum information (flying qubits) using optical fiber interconnects.The matter-light interface seems inevitable since the local storage capability of ground state atomic matter cannot be easily recreated with light fields. Interfacing material quanta and single photons is therefore a basic primitive of a quantum network.

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Long-distance quantum communication via distant pairs of entangled quantum bits (qubits) is the first step towards secure message transmission and distributed quantum computing. To date, the most promising proposals require quantum repeaters to mitigate the exponential decrease in communication rate due to optical fiber losses. However, these are exquisitely sensitive to the lifetimes of their memory elements. We propose a multiplexing of quantum nodes that should enable the construction of quantum networks that are largely insensitive to the coherence times of the quantum memory elements.

We study the emergence of collective scattering in the presence of dipole-dipole interactions when we illuminate a cold cloud of rubidium atoms with a near-resonant and weak intensity laser. The size of the atomic sample is comparable to the wavelength of light. When we gradually increase the number of atoms from 1 to ∼450, we observe a broadening of the line, a small redshift and, consistently with these, a strong suppression of the scattered light with respect to the noninteracting atom case. We compare our data to numerical simulations of the optical response, which include the internal level structure of the atoms. DOI: 10.1103/PhysRevLett.113.133602 PACS numbers: 42.50.Ct, 03.65.Nk, 32.80.Qk, 42.50.Nn When resonant emitters, such as atoms, molecules, quantum dots, or metamaterial circuits, with a transition at a wavelength λ, are confined inside a volume smaller than λ 3 , they are coupled via strong dipole-dipole interactions. In this situation, the response of the ensemble to near-resonant light is collective and originates from the excitation of collective eigenstates of the system, such as super-and subradiant modes [1][2][3]. Dipole-dipole interactions affect the response of the system and the collective scattering of near-resonant light differs from the case of an assembly of noninteracting emitters [4]. It has even been predicted to be suppressed for a dense gas of cold twolevel atoms [5].Following the recent measurement of the collective Lamb shift [6] in a Fe layer [7], in a hot thermal vapor [8], and in arrays of trapped ions [9], it was pointed out [10] that the collective response of interacting emitters is different between ensembles exhibiting inhomogeneous broadening, such as solid state systems or thermal vapors, and those free of it, such as cold-atom clouds. In particular, inhomogeneous broadening suppresses the correlations induced by the interactions between dipoles, leading to the textbook theory of the optical response of continuous media [10,11]. In the absence of broadening, however, this theory fails and should be revisited to include the lightinduced correlations [12][13][14][15][16][17][18][19]. Several recent experiments aiming at studying collective scattering with identical emitters used large and optically thick ensembles of cold atoms [20][21][22][23]. However, the case of a cold-atom ensemble with a size comparable to the optical wavelength has not been studied experimentally, nor has the transition between the well-understood case of scattering by an individual atom [24] to collective scattering. In particular, the suppression of light scattering when the number of atoms increases in a regime of collective scattering has never been directly observed.Here, we study-both experimentally and theoreticallythe emergence of collective effects in the optical response of a cold-atom sample due to dipole-dipole interactions, as we gradually increase the number of atoms. To do so, we send low-intensity near-resonant laser light onto a cloud containing from 1 to ∼450 cold 87 Rb ato...

We show how strong light-mediated resonant dipole-dipole interactions between atoms can be utilized in a control and storage of light. The method is based on a high-fidelity preparation of a collective atomic excitation in a single correlated subradiant eigenmode in a lattice. We demonstrate how a simple phenomenological model captures the qualitative features of the dynamics and sharp transmission resonances that may find applications in sensing. DOI: 10.1103/PhysRevLett.117.243601 Resonant emitters play a key role in optical devices for classical and quantum technologies. Atoms have particular advantages because of an excellent isolation from environmental noise with well-specified resonance frequencies and no absorption due to nonradiative losses. At high densities, however, they exhibit strong light-mediated resonant dipoledipole (DD) interactions that can lead to uncontrolled and unwanted phenomena, such as resonance broadening, shifts, and dephasing. According to common wisdom, these are considered as a design limitation in quantum and classical light technologies, e.g., in quantum metrology [1,2], sensing [3], information processing [4], in the storage of light, and in the implementations of quantum memories [5][6][7][8]. DD interactions also receive significant attention, e.g., in Rydberg gases [9][10][11][12][13]. Here, we show how strong radiative interactions can be harnessed in engineering long living collective excitations that open up avenues for utilizing resonant DD interactions in the control and storage of light, and in sensing. Our protocol is based on controlled preparation of large, many-atom subradiant excitations, where the light-mediated interactions between the atoms strongly suppress radiative losses.Superradiance [14], where the emission of light is coherently enhanced in an ensemble of emitters has continued to attract considerable interest [15] with the recent experiments focusing on light in confined geometries [16], weak excitation regime [17][18][19], and the related shifts of the resonance frequencies [20][21][22][23][24]. Its counterpart, subradiance, describes coherently suppressed emission due to a weak coupling to the radiative vacuum. Because of the weak coupling, subradiant states are challenging to excite and have experimentally proved elusive. In atomic and molecular systems subradiance has been observed in pairs of trapped ions [25] and molecules [26], as well as in weakly bound ultracold molecular states [27,28]. In a large atom cloud, a subradiant decay was recently observed in the long tails of a radiative decay distribution [29] that indicated a small fraction of the atoms exhibiting a suppressed emission.In our model, an incident light excites a collective atomic state that exhibits a significant radiative vacuum coupling.The excitation is then transferred to a radiatively isolated cooperative state. The cold atoms that store the light excitation are confined in a planar lattice, providing a protection against nonradiative losses, which typically are a common hin...