The reversible transfer of quantum states of light into and out of matter constitutes an important building block for future applications of quantum communication: it will allow the synchronization of quantum information, and the construction of quantum repeaters and quantum networks. Much effort has been devoted to the development of such quantum memories, the key property of which is the preservation of entanglement during storage. Here we report the reversible transfer of photon-photon entanglement into entanglement between a photon and a collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol, and increase the spectral acceptance from the current maximum of 100 megahertz to 5 gigahertz. We assess the entanglement-preserving nature of our storage device through Bell inequality violations and by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer. These measurements show, within statistical error, a perfect mapping process. Our broadband quantum memory complements the family of robust, integrated lithium niobate devices. It simplifies frequency-matching of light with matter interfaces in advanced applications of quantum communication, bringing fully quantum-enabled networks a step closer.

The realization of a future quantum Internet requires processing and storing quantum information at local nodes, and interconnecting distant nodes using free-space and fibre-optic links [1]. Quantum memories for light [2] are key elements of such quantum networks. However, to date, neither an atomic quantum memory for non-classical states of light operating at a wavelength compatible with standard telecom fibre infrastructure, nor a fibre-based implementation of a quantum memory has been reported. Here we demonstrate the storage and faithful recall of the state of a 1532 nm wavelength photon, entangled with a 795 nm photon, in an ensemble of cryogenically cooled erbium ions doped into a 20 meter-long silicate fibre using a photon-echo quantum memory protocol. Despite its currently limited efficiency and storage time, our broadband light-matter interface brings fibre-based quantum networks one step closer to reality. Furthermore, it facilitates novel tests of light-matter interaction and collective atomic effects in unconventional materials.The end of the last century witnessed the invention of, and important steps towards, several paradigm-shifting applications of quantum information science, including computers with unprecedented computational power [3], unbreakable secret key distribution [4], and measurement devices having ultimate precision [5]. Combining these applications in the so-called quantum Internet[1] requires transmitting quantum states encoded into photons between, and storage of quantum states in, nodes of the network. While the quantum Internet can leverage existing telecom fibre networks, standard (classical) repeater technology cannot be used to build large-scale networks, due to a fundamental restriction of quantum mechanics known as the no-cloning theorem [4]. Hence, classical repeaters, generally comprised of erbium-doped fibre amplifiers, need to be replaced with quantum repeaters, which include pairs of entangled photons, entanglement swapping, and light-matter interfaces that allow storing and manipulating quantum states of light [6].Despite enormous success in developing suitable lightmatter interfaces during the past decade (for recent reviews see [2, 6, 7]), a memory for non-classical states of light encoded into telecom-wavelength (i.e. approximately 1550 nm) photons -the most natural choice for a quantum network -still remains to be demonstrated. Considering the most popular materials -alkaline atoms (in particular caesium and rubidium), and rare-earth-ion doped crystals -the reasons for this challenge are twofold: First, Cs and Rb lack easily accessible atomic transitions, i.e. transitions starting at an electronic ground state, at around 1550 nm wavelength. Second, erbium (a rare-earth element and the seemingly obvious choice due to its telecom-wavelength transition and extensive use in fibre amplifiers) has so-far eluded all attempts to store non-classical states of light with a fidelity above the classical limit, albeit important steps towards this goal have recently been repor...

Multiparticle quantum interference is critical for our understanding and exploitation of quantum information, and for fundamental tests of quantum mechanics. A remarkable example of multipartite correlations is exhibited by the Greenberger-Horne-Zeilinger (GHZ) state. In a GHZ state, three particles are correlated while no pairwise correlation is found. The manifestation of these strong correlations in an interferometric setting has been studied theoretically since 1990 but no three-photon GHZ interferometer has been realized experimentally. Here we demonstrate threephoton interference that does not originate from two-photon or single photon interference. We observe phase-dependent variation of three-photon coincidences with (92.7±4.6) % visibility in a generalized Franson interferometer using energy-time entangled photon triplets. The demonstration of these strong correlations in an interferometric setting provides new avenues for multiphoton interferometry, fundamental tests of quantum mechanics and quantum information applications in higher dimensions.

We report the reversible transfer of photon-photon entanglement, generated by means of spontaneous parametric down-conversion, into entanglement between a photon and a collective atomic excitation in a thulium-doped lithium niobate waveguide cooled to 3 K.

Conditional detection is an important tool to extract weak signals from a noisy background and is closely linked to heralding, which is an essential component of protocols for long distance quantum communication and distributed quantum information processing in quantum networks. Here we demonstrate the conditional detection of time-bin qubits after storage in and retrieval from a photon-echo based waveguide quantum memory. Each qubit is encoded into one member of a photon-pair produced via spontaneous parametric down conversion, and the conditioning is achieved by the detection of the other member of the pair. Performing projection measurements with the stored and retrieved photons onto different bases we obtain an average storage fidelity of 0.885 ± 0.020, which exceeds the relevant classical bounds and shows the suitability of our integrated light-matter interface for future applications of quantum information processing.PACS numbers: 03.67. Hk, 42.50.Ex, 32.80.Qk, 78.47.jf Quantum memories are key elements for future applications of quantum information science such as long-distance quantum communication via quantum repeaters [1, 2] and, more generally, distributed quantum information processing in quantum networks [3]. They enable reversible mapping of arbitrary quantum states between travelling and stationary carriers (i.e. light and matter). This reduces the impact of loss on the time required to establish entanglement between distant locations [1], and allows the implementation of local quantum computers based on linear optics [4]. However, towards these ends, the successful transfer of a quantum state into the memory must be announced by a heralding signal. When using an individual absorber, such a signal can be derived through the detection of a change of atomic level population [5]. In atomic ensembles, this approach is infeasible. Instead, storage is derived from the detection of a second photon that either indicates the absorption [6], or the presence of the first at the input of the memory [7] (the first approach relies on spontaneous Raman scattering, the second on using pairs of photons). Furthermore, quantum memories must allow on-demand read-out after second-long storage with high efficiency [7,8], and, for viable quantum technology, should be robust and simple to operate (e.g. be based on integrated optics).A lot of progress towards these (and other) figures of merit has been reported over the past few years, including work that explores electromagnetically induced transparency (EIT), as well as photon-echo and cavity QEDbased approaches (see [2,5,[7][8][9][10][11][12][13][14] for reviews and latest achievements). Yet, strictly, most of these experiments did not report true heralding -either heralding was not actually implemented, or the 'heralding' signal was generated only after the stored photon left the memory, or the signal could, due to technical issues, only be derived once the stored photon was detected. Nevertheless, experiments that employ photon pairs [11][12][13]15] do gain from c...