Quantum networks are distributed quantum many-body systems with tailored topology and controlled information exchange. They are the backbone of distributed quantum computing architectures and quantum communication. Here we present a prototype of such a quantum network based on single atoms embedded in optical cavities. We show that atom-cavity systems form universal nodes capable of sending, receiving, storing and releasing photonic quantum information. Quantum connectivity between nodes is achieved in the conceptually most fundamental way: by the coherent exchange of a single photon. We demonstrate the faithful transfer of an atomic quantum state and the creation of entanglement between two identical nodes in independent laboratories. The created nonlocal state is manipulated by local qubit rotation. This efficient cavity-based approach to quantum networking is particularly promising as it offers a clear perspective for scalability, thus paving the way towards large-scale quantum networks and their applications.Connecting individual quantum systems via quantum channels creates a quantum network with properties profoundly different from any classical network. First, the accessible state space increases exponentially with the number of constituents. Second, the distribution of quantum states across the whole network leads to nonlocal correlations. Further, the quantum channels mediate long-range or even infinite-range interactions which can be switched on and off at will. This makes quantum networks tailor-made quantum many-body systems with adjustable degrees of connectivity and arbitrary topologies, and thus powerful quantum simulators. Open questions like the scaling behaviour, percolation of entanglement [1], multi-partite entanglement [2,3] and quantum phase transitions [4-6] make quantum networks a prime theme of current theoretical and experimental research. Similarly, quantum networks form the basis of quantum communication and distributed quantum information processing architectures, with interactions taking the form of quantum logic gates [7][8][9][10].The physical implementation of quantum networks requires suitable channels and nodes. Photonic channels are well-advanced transmitters of quantum information. Optical photons can carry quantum information over long distances with almost negligible decoherence and are compatible with existing telecommunication fibre technology. The versatility of quantum networks, however, is largely defined by the capability of the network nodes. Dedicated tasks like quantum key distribution can already be achieved using send-only emitter nodes and receive-only detector nodes [11]. However, in order to fully exploit the capabilities of quantum networks, functional network nodes are required which are able to send, receive and store quantum information reversibly and efficiently.The implementation and connection of quantum nodes is a major challenge and different approaches are currently being pursued. An intensely studied example are ensembles of gas-phase atoms [12][13][...

The faithful storage of a quantum bit (qubit) of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must be able to receive and recreate the photonic qubit; additionally, it must store an unknown quantum state of light better than any classical device. So far, these two requirements have been met only by ensembles of material particles that store the information in collective excitations. Recent developments, however, have paved the way for an approach in which the information exchange occurs between single quanta of light and matter. This single-particle approach allows the material qubit to be addressed, which has fundamental advantages for realistic implementations. First, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection; this can be used to combat inevitable losses and finite efficiencies. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory, by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is tested with weak coherent pulses and analysed using full quantum process tomography. The average fidelity is measured to be 93%, and low decoherence rates result in qubit coherence times exceeding 180  microseconds. This makes our system a versatile quantum node with excellent prospects for applications in optical quantum gates and quantum repeaters.

We demonstrate teleportation of quantum bits between two single atoms in distant laboratories. Using a time-resolved photonic Bell-state measurement, we achieve a teleportation fidelity of (88.0± 1.5) %, largely determined by our entanglement fidelity. The low photon collection efficiency in free space is overcome by trapping each atom in an optical cavity. The resulting success probability of 0.1 % is almost 5 orders of magnitude larger than in previous experiments with remote material qubits. It is mainly limited by photon propagation and detection losses and can be enhanced with a cavity-based deterministic Bell-state measurement.The faithful transfer of quantum information between distant memories that form the nodes of a quantum network is a major goal in applied quantum science [1]. One way to achieve this is via direct transfer, e.g., by the coherent exchange of a single photon [2]. Over large distances, however, the inevitable losses in any quantum channel render this scenario unrealistic, as its efficiency decreases exponentially with the distance between the network nodes. For any classical information, the solution is simple: It can be amplified at intermediate nodes of the network. It can also be copied before transmission, allowing for a new transmission attempt should the previous one have failed. For a quantum state, the no-cloning theorem states that this is impossible. Therefore, quantum repeater schemes have been proposed to establish long-distance entanglement using photons and memories [3,4]. This entanglement can then be used as a resource for the transfer of quantum information via teleportation [5].The underlying principle of teleportation was first realized with photonic qubits [6][7][8] and since then has been exploited in many experiments [9,10]. Teleportation between matter qubits was first achieved with trapped ions [11,12], albeit over a distance limited to a few micrometers owing to the short-range Coulomb interaction. Teleportation between distant material qubits, however, requires photons distributing entanglement, as was demonstrated with two single ions separated by about 1 m [13]. The low photon-collection efficiency in free space, however, prevents scaling of that approach to larger networks. We eliminate this obstacle by trapping two remote single atoms each in an optical cavity. This allows for an in principle deterministic creation of atom-photon entanglement and atom-to-photon state mapping using a vacuum-stimulated Raman adiabatic passage (vSTI-RAP) technique [14][15][16]. To teleport the stationary qubit at the sender atom, encoded in two Zeeman states of the atomic ground-state manifold, we map it onto a photonic qubit and perform a Bell-state measurement (BSM) between this photon and that of an entangled atom-photon state originating from the receiver atom [17,18]. Compared to realizations with atoms in free space, the use of cavities boosts the overall efficiency by almost 5 orders of magnitude [13].In our experiment, single 87 Rb atoms trapped in highfinesse optical ...

A single neutral atom is trapped in a three-dimensional optical lattice at the center of a high-finesse optical resonator. Using fluorescence imaging and a shiftable standing-wave trap, the atom is deterministically loaded into the maximum of the intracavity field where the atom-cavity coupling is strong. After 5 ms of Raman sideband cooling, the three-dimensional motional ground state is populated with a probability of (89±2)%. Our system is the first to simultaneously achieve quantum control over all degrees of freedom of a single atom: its position and momentum, its internal state, and its coupling to light.

A single rubidium atom trapped within a high-finesse optical cavity is an efficient source of single photons. We theoretically and experimentally study single-photon generation using a vacuum stimulated Raman adiabatic passage. We experimentally achieve photon generation efficiencies of up to 34% and 56% on the D1 and D2 line, respectively. Output coupling with 89% results in record-high efficiencies for single photons in one spatiotemporally well-defined propagating mode. We demonstrate that the observed generation efficiencies are constant in a wide range of applied pump laser powers and virtual level detunings. This allows for independent control over the frequency and wave packet envelope of the photons without loss in efficiency. In combination with the long trapping time of the atom in the cavity, our system constitutes a significant advancement toward an on-demand, highly efficient single-photon source for quantum information processing tasks.Comment: 7 pages, 5 figure