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.

Many applications of quantum communication crucially depend on reversible transfer of quantum states between light and matter. Motivated by rapid recent developments in theory and experiment, we review research related to quantum memory based on a photon-echo approach in solid state material with emphasis on use in a quantum repeater. After introducing quantum communication, the quantum repeater concept, and properties of a quantum memory required to be useful in a quantum repeater, we describe the historical development from spin echoes, discovered in 1950, to photon-echo quantum memory. We present a simple theoretical description of the ideal protocol, and comment on the impact of a non-ideal realization on its quantum nature. We extensively discuss rare-earth-ion doped crystals and glasses as material candidates, elaborate on traditional photon-echo experiments as a test-bed for quantum state storage, and describe the current state-of-the-art of photon-echo quantum memory. Finally, we give a brief outlook on current research.The picture shows a Europium doped Y2SiO5 crystal surrounded by electrodes in the setup used for the first proof-of-principle demonstration of the novel, photon-echo based quantum memory protocol.

We describe a new experimental approach to probabilistic atom-photon (signal) entanglement. Two qubit states are encoded as orthogonal collective spin excitations of an unpolarized atomic ensemble. After a programmable delay, the atomic excitation is converted into a photon (idler). Polarization states of both the signal and the idler are recorded and are found to be in violation of the Bell inequality. Atomic coherence times exceeding several microseconds are achieved by switching off all the trapping fields -including the quadrupole magnetic field of the magneto-optical trap -and zeroing out the residual ambient magnetic field.PACS numbers: 03.65. Ud,03.67.Mn,42.50.Dv Long-distance quantum cryptographic key distribution (QCKD) is an important goal of quantum information science. Extending the reach of quantum cryptography ideally involves the ability to entangle two distant qubits (two level quantum systems) [1,2], using the Bell inequality violation to verify the security of the quantum communication channel. Parametric down conversion is an established technology producing entangled photon pairs. Unfortunately, it is not directly applicable to longdistance QCKD, as the rate scales exponentially with the distance due to probabilistic nature of entangled photon pairs generation. It is necessary to provide a controllable delay between the two photons, that is, to have a means of photon storage. The latter requirement is problematical as photons are difficult to store for an appreciable period of time. By contrast atomic qubits are long lived and easily manipulated by laser fields, they are well suited for long term quantum information storage. Photonic qubits, however, can propagate for relatively long distances in fibers without absorption, making them excellent carriers of quantum information. Entangled systems of a single photon and a long-lived atomic qubit therefore offer an excellent building block for a quantum network.A quantum repeater architecture can overcome the limitations of photons by inserting a quantum memory qubit into the quantum channel every attenuation length or so [2]. The idea is to generate entanglement between two neighboring atomic qubits, which can be done efficiently since light will not be appreciably absorbed within the segment length. After entanglement between each pair of atomic qubits has been established, a joint measurement on each neighboring pair of qubits is performed. The quantum states of all the intermediate qubits are destroyed by the measurement, achieving entanglement swapping such that only the two atomic qubits at the two ends are entangled. These two qubits can be used for QCKD, either with the Ekert protocol, that directly uses the entangled pair of qubits, or the BB84 protocol that performs either remote state preparation or teleportation of a qubit [3,4,5,6,7,8]. The rate of QCKD using a quantum repeater protocol can scale polynomially with distance [2].In the microwave domain, single Rydberg atoms and single photons have been entangled [9]. An entangled st...

We report observations of entanglement of two remote atomic qubits, achieved by generating an entangled state of an atomic qubit and a single photon at site A, transmitting the photon to site B in an adjacent laboratory through an optical fiber, and converting the photon into an atomic qubit. Entanglement of the two remote atomic qubits is inferred by performing, locally, quantum state transfer of each of the atomic qubits onto a photonic qubit and subsequent measurement of polarization correlations in violation of the Bell inequality jSj 2. We experimentally determine S exp 2:16 0:03. Entanglement of two remote atomic qubits, each qubit consisting of two independent spin wave excitations, and reversible, coherent transfer of entanglement between matter and light represent important advances in quantum information science. Realization of massive qubits, and their entanglement, is central to practical quantum information systems [1][2][3]. Remote entanglement of photons can now be achieved in a robust manner using the well-developed technology of spontaneous parametric down-conversion [4], with propagation to remote locations by means of optical fibers. Photons, however, are difficult to store for any appreciable period of time, whereas qubits based on ground-state atoms have long lifetimes. Local entanglement of massive qubits has been observed between adjacent trapped ions [5] and between pairs of Rydberg atoms in a collimated beam [6]. In order to entangle qubits at remote locations, the use of photons as an intermediary seems essential [7][8][9][10]. Photons also offer some flexibility as information carriers as they can propagate in optical fiber with low losses. The creation, transport, storage, and retrieval of single photons between remote atomic ensembles located in two different laboratories were recently observed [11] [see also a related work on electromagnetically induced transparency (EIT) with single photon pulses [12] ]. The first step in creating remote entanglement between massive qubits is to entangle one such qubit with the mediating light field, which is then directed towards the second qubit via an optical fiber. There have recently been important advances towards this goal by demonstrating entanglement of a photon with a trapped ion [13], with a collective atomic qubit [14,15], and with a single trapped atom [16].A promising route towards the creation and application of long-lived qubit entanglement in scalable quantum networks was proposed by Duan, Lukin, Cirac, and Zoller [2,10]. These atomic qubits rely on collective atomic states containing exactly one spin excitation. In two recent experiments, collective atomic qubits were generated using cold atomic ensembles [14,15] While remote entanglement of atomic qubits has not been previously demonstrated, Refs. [14,15] realized two basic primitives of a quantum network: (a) entanglement of photonic and atomic qubits, and (b) quantum state transfer from an atomic to a photonic qubit. The crucial additional ingredient is the reverse operation, t...

We propose an original quantum memory protocol. It belongs to the class of rephasing processes and is closely related to two-pulse photon echo. It is known that the strong population inversion produced by the rephasing pulse prevents the plain two-pulse photon echo from serving as a quantum memory scheme. Indeed gain and spontaneous emission generate prohibitive noise. A second π-pulse can be used to simultaneously reverse the atomic phase and bring the atoms back into the ground state. Then a secondary echo is radiated from a non-inverted medium, avoiding contamination by gain and spontaneous emission noise. However, one must kill the primary echo, in order to preserve all the information for the secondary signal. In the present work, spatial phase mismatching is used to silence the standard two-pulse echo. An experimental demonstration is presented.

A quantum repeater at telecommunications wavelengths with long-lived atomic memory is proposed, and its critical elements are experimentally demonstrated using a cold atomic ensemble. Via atomic cascade emission, an entangled pair of 1.53 µm and 780 nm photons is generated. The former is ideal for long-distance quantum communication, and the latter is naturally suited for mapping to a long-lived atomic memory. Together with our previous demonstration of photonic-to-atomic qubit conversion, both of the essential elements for the proposed telecommunications quantum repeater have now been realized. PACS numbers: 42.50.Dv,03.65.Ud,03.67.Mn A quantum network would use the resources of distributed quantum mechanical entanglement, thus far largely untapped, for the communication and processing of information via qubits [1,2]. Significant advances in the generation, distribution, and storage of qubit entanglement have been made using laser manipulation of atomic ensembles, including atom-photon entanglement and matter-light qubit conversion [3], Bell inequality violation between a collective atomic qubit and a photon [4], and light-matter qubit conversion and entanglement of remote atomic qubits [5]. In each of these works photonic qubits were generated in the near-infrared spectral region. In related developments, entanglement of an ultraviolet photon with a trapped ion [6] and of a nearinfrared photon with a single trapped atom [7] have been demonstrated. Heterogeneous quantum network schemes that combine single-atom and collective atomic qubits are being actively pursued [8]. However, photons in the ultraviolet to the near-infrared range are not suited for long-distance transmission over optical fibers due to high losses.In this Letter, we propose a telecommunications wavelength quantum repeater based on cascade atomic transitions in either (1) a single atom or (2) an atomic ensemble. We will first discuss the latter case, with particular reference to alkali metals. Such ensembles, with long lived ground level coherences can be prepared in either solid [9] or gas [4] phase. For concreteness, we consider a cold atomic vapor confined in high-vacuum. The cascade transitions may be chosen so that the photon (signal) emitted on the upper arm has telecommunication range wavelength, while the second photon (idler), emitted to the atomic ground state, is naturally suited for mapping into atomic memory. Experimentally, we demonstrate phase-matched cascade emission in an ensemble of cold rubidium atoms using two different cascades: (a) at the signal wavelength λ s = 776 nm, via the 5s 1/2 → 5d 5/2 two-photon excitation, (b) at λ s = 1.53 µm, via the 5s 1/2 → 4d 5/2 two-photon excitation. We observe polarization entanglement of the emitted photon pairs and superradiant temporal profiles of the idler field in both cases.We now describe our approach in detail and at the end we will summarize an alternative protocol for single atoms.Step (A) -As illustrated in Fig. 1(a), the atomic sample is prepared in level |a , e.g., by mean...

We experimentally demonstrate the storage of 1060 temporal modes onto a thulium-doped crystal using an atomic frequency comb (AFC). The comb covers 0.93 GHz defining the storage bandwidth. As compared to previous AFC preparation methods (pulse sequences i.e. amplitude modulation), we only use frequency modulation to produce the desired optical pumping spectrum. To ensure an accurate spectrally selective optical pumping, the frequency-modulated laser is selflocked on the atomic comb. Our approach is general and should be applicable to a wide range of rare-earth doped material in the context of multimode quantum memory.

A source of deterministic single photons is proposed and demonstrated by the application of a measurement-based feedback protocol to a heralded single photon source consisting of an ensemble of cold rubidium atoms. Our source is stationary and produces a photoelectric detection record with sub-Poissonian statistics.PACS numbers: 42.50. Dv,03.65.Ud,03.67.Mn Quantum state transfer between photonic-and matterbased quantum systems is a key element of quantum information science, particularly of quantum communication networks. Its importance is rooted in the ability of atomic systems to provide excellent long-term quantum information storage, whereas the long-distance transmission of quantum information is nowadays accomplished using light. Inspired by the work of Duan et al.[1], emission of non-classical radiation has been observed in first-generation atomic ensemble experiments [2].In 2004 the first realization of coherent quantum state transfer from a matter qubit onto a photonic qubit was achieved [3]. This breakthrough laid the groundwork for several further advances towards the realization of a longdistance, distributed network of atomic qubits, linear optical elements and single-photon detectors [4,5,6,7,8]. A seminal proposal for universal quantum computation with a similar set of physical resources has also been made [9].An important additional tool for quantum information science is a deterministic source of single photons. Previous implementations of such a source used single emitters, such as quantum dots [10,11], color centers [12,13], neutral atoms [14,15], ions [16], and molecules [17]. The measured efficiency η D to detect a single photon per trial with these sources is typically less than 1%, with the highest reported measured value of about 2.4% [14], to our knowledge.We propose a deterministic single photon source based on an ensemble of atomic emitters, measurement, and conditional quantum evolution. We report the implementation of this scheme using a cold rubidium vapor, with a measured efficiency η D ≈ 1 − 2%. In common with the cavity QED system, our source is suitable for reversible quantum state transfer between atoms and light, a prerequisite for a quantum network. However, unlike cavity QED implementations [14], it is unaffected by intrinsically probabilistic single atom loading. Therefore, it is stationary and produces a photoelectric detection record with truly sub-Poissonian statistics.The key idea of our protocol is that a single photon can be generated at a predetermined time if we know that the medium contains an atomic excitation. The presence of the latter is heralded by the measurement of a scattered photon in a write process. Since this is intrinsically probabilistic, it is necessary to perform independent, sequential write trials before the excitation is heralded. After this point one simply waits and reads out the excitation at the predetermined time. The performance of repeated trials and heralding measurements represents a conditional feedback process and the duration of th...