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.

Squeezing of quantum fluctuations by means of entanglement is a well-recognized goal in the field of quantum information science and precision measurements. In particular, squeezing the fluctuations via entanglement between 2-level atoms can improve the precision of sensing, clocks, metrology, and spectroscopy. Here, we demonstrate 3.4 dB of metrologically relevant squeezing and entanglement for 10 5 cold caesium atoms via a quantum nondemolition (QND) measurement on the atom clock levels. We show that there is an optimal degree of decoherence induced by the quantum measurement which maximizes the generated entanglement. A 2-color QND scheme used in this paper is shown to have a number of advantages for entanglement generation as compared with a single-color QND measurement. N A for the case of independent atoms also referred to as a coherent spin state (CSS). The CSS minimizes the Heisenberg uncertainty product so that, e.g., (δJ z ) 2 (δJ x ) 2 = 1 4| J y | 2 where J y is the expectation value of the spin projection operator. At the expense of an increase in (δJ x ) 2 , it is possible to reduce (δJ z ) 2 (or vice versa) below the projection noise limit while keeping their product constant. This constitutes an example of a spin squeezed state (SSS), for which the atoms need to be correlated. This correlation is ensured to be nonclassical ifwhere ξ defines the squeezing parameter. Under this condition, the atoms are entangled (3) and the prepared state improves the signal-to-noise ratio in spectroscopical and metrological applications (1). Systems of 2 to 3 ions have successfully been used to demonstrate spectroscopic performance with reduced quantum noise and entanglement (4, 5). The situation is somewhat different with macroscopic atomic ensembles where spin squeezing has been an active area of research in the past decade (6-13). To our knowledge, no results reporting ξ < 1 via interatomic entanglement in such ensembles have been reported so far, with a very recent exception of the paper (14) where entanglement in an external motional degree of freedom of 2 · 10 3 atoms via interactions in a Bose-Einstein condensate is demonstrated. Spin Squeezing by Quantum Nondemolition (QND) MeasurementsIn this article, we report on the generation of an SSS fulfilling Eq. 1 in an ensemble of ≈10 5 atoms via a QND measurement (7, 15-17) of J z . We show how to take advantage of the entanglement in this mesoscopic system by using Ramsey spectroscopy (1)-one of the methods of choice for precision measurements of time and frequency (18) (Fig. 1A). The figure presents the evolution of the pseudospin J whose tip is traveling over the Bloch sphere. The Ramsey method allows using the atomic ensemble as a sensor for external fields where the perturbation of the energy difference between the levels ΔE ↑↓ is measured, or as a clock where the frequency of an oscillator is locked to the transition frequency between the two states Ω = ΔE ↑↓ / . Fig. 1 B illustrates how a suitable SSS can improve the precision of the Ramsey measurement pr...

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...

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...