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.

Interfacing a single photon with another quantum system is a key capability in modern quantum information science. It allows quantum states of matter, such as spin states of atoms [1,2], atomic ensembles [3,4] or solids [5], to be prepared and manipulated by photon counting and, in particular, to be distributed over long distances. Such light-matter interfaces have become crucial to fundamental tests of quantum physics [6] and realizations of quantum networks [7]. Here we report non-classical correlations between single photons and phonons -the quanta of mechanical motion -from a nanomechanical resonator. We implement a full quantu protocol involving initialization of the resonator in its quantum ground state of motion and subsequent generation and read-out of correlated photon-phonon pairs. The observed violation of a Cauchy-Schwarz inequality is clear evidence for the non-classical nature of the mechanical state generated. Our results demonstrate the availability of on-chip solid-state mechanical resonators as light-matter quantum interfaces. The performance we achieved will enable studies of macroscopic quantum phenomena [8] as well as applications in quantum communication [9], as quantum memories [10] and as quantum transducers [11,12].Over the past few years, nanomechanical devices have been discussed as possible building blocks for quantum information architectures [9,13]. Their unique feature is that they combine an engineerable solid-state platform on the nanoscale with the possibility to coherently interact with a variety of physical quantum systems including electronic or nuclear spins, single charges, and photons [14,15]. This feature enables mechanics-based hybrid quantum systems that interconnect different, independent physical qubits through mechanical modes.A successful implementation of such quantum transducers requires the ability to create and control quantum states of mechanical motion. The first step -the initialization of micro-and nanomechanical systems in their quantum ground state of motion -has been realized in various mechanical systems either through direct cryogenic cooling [16,17] or laser cooling using microwave [18] and optical cavity fields [19]. Further progress in quantum state control has mainly been limited to the domain of electromechanical devices, in which mechanical motion couples to superconducting circuits in the form of qubits and microwave cavities [15]. Recent achievements include single-phonon control of a micromechanical resonator by a superconducting flux qubit [16], the generation of quantum entanglement between quadratures of a microwave cavity field and micromechanical motion [20], * This work was published in Nature 530, 313-316 (2016 Interfacing mechanics with optical photons in the quantum regime is highly desirable because it adds important features such as the ability to transfer mechanical excitations over long distances [9,24]. In addition, the available toolbox of single-photon generation and detection allows for remote quantum state control [7]. However...

Several vulnerabilities of single-photon detectors have recently been exploited to compromise the security of quantum-key-distribution (QKD) systems. In this Letter, we report the first proof-of-principle implementation of a new quantum-key-distribution protocol that is immune to any such attack. More precisely, we demonstrated this new approach to QKD in the laboratory over more than 80 km of spooled fiber, as well as across different locations within the city of Calgary. The robustness of our fiber-based implementation, together with the enhanced level of security offered by the protocol, confirms QKD as a realistic technology for safeguarding secrets in transmission. Furthermore, our demonstration establishes the feasibility of controlled two-photon interference in a real-world environment and thereby removes a remaining obstacle to realizing future applications of quantum communication, such as quantum repeaters and, more generally, quantum networks.

Future multi-photon applications of quantum optics and quantum information science require quantum memories that simultaneously store many photon states, each encoded into a different optical mode, and enable one to select the mapping between any input and a specific retrieved mode during storage. Here we show, with the example of a quantum repeater, how to employ spectrallymultiplexed states and memories with fixed storage times that allow such mapping between spectral modes. Furthermore, using a Ti:Tm:LiNbO3 waveguide cooled to 3 Kelvin, a phase modulator, and a spectral filter, we demonstrate storage followed by the required feed-forward-controlled frequency manipulation with time-bin qubits encoded into up to 26 multiplexed spectral modes and 97% fidelity.PACS numbers: 03.67. Hk, 42.50.Ex, 32.80.Qk, 78.47.jf Further advances towards scalable quantum optics [1,2] and quantum information processing [3,4] rely on joint measurements of multiple photons that encode quantum states (e.g. qubits) [3][4][5]. However, as photons generally arrive in a probabilistic fashion, either due to a probabilistic creation process or due to loss during transmission, such measurements are inherently inefficient. For instance, this leads to exponential scaling of the time required to establish entanglement, the very resource of quantum information processing, as a function of distance in a quantum relay [6]. This problem can be overcome by using quantum memories, which are generally realized through the reversible mapping of quantum states between light and matter [7,8]. For efficient operation, these memories must be able to simultaneously store many photon states, each encoded into a different optical mode, and subsequently (using feed-forward) allow selecting the mapping between input and retrieved modes (e.g., different spectral or temporal modes). This enables making several photons arriving at a measurement device indistinguishable, thereby rendering joint measurements deterministic. For instance, revisiting the example of entanglement distribution, a quantum relay supplemented with quantum memories changes it to a repeater and, in principle, the scaling from exponential to polynomial [4,9].Interestingly, for such multimode quantum memories to be useful, it is not necessary to map any input mode onto any retrieved (output) mode, but it often suffices if a single input mode, chosen once a photon is stored, can be mapped onto a specific output mode (e.g. characterized by the photon's spectrum and recall time) [4,10]. This ensures that the photons partaking in a joint measurement, each recalled from a different quantum memory, are indistinguishable, as required, e.g., for a Bell-state measurement. We emphasize that it does not matter if the device used to store quantum states also allows the mode mapping, or if the mode mapping is performed after recall using appended devices -we will refer to the system allowing storage and mode mapping as the memory.To date, most research assumes photons arriving at different times at the m...