Quantum communication is a method that offers efficient and secure ways for the exchange of information in a network. Large-scale quantum communication (of the order of 100 km) has been achieved; however, serious problems occur beyond this distance scale, mainly due to inevitable photon loss in the transmission channel. Quantum communication eventually fails when the probability of a dark count in the photon detectors becomes comparable to the probability that a photon is correctly detected. To overcome this problem, Briegel, Dür, Cirac and Zoller (BDCZ) introduced the concept of quantum repeaters, combining entanglement swapping and quantum memory to efficiently extend the achievable distances. Although entanglement swapping has been experimentally demonstrated, the implementation of BDCZ quantum repeaters has proved challenging owing to the difficulty of integrating a quantum memory. Here we realize entanglement swapping with storage and retrieval of light, a building block of the BDCZ quantum repeater. We follow a scheme that incorporates the strategy of BDCZ with atomic quantum memories. Two atomic ensembles, each originally entangled with a single emitted photon, are projected into an entangled state by performing a joint Bell state measurement on the two single photons after they have passed through a 300-m fibre-based communication channel. The entanglement is stored in the atomic ensembles and later verified by converting the atomic excitations into photons. Our method is intrinsically phase insensitive and establishes the essential element needed to realize quantum repeaters with stationary atomic qubits as quantum memories and flying photonic qubits as quantum messengers.

. Here we present an experimental investigation into extending the storage time of quantum memory for single excitations. We identify and isolate distinct mechanisms responsible for the decoherence of spin waves in atomic-ensemble-based quantum memories. By exploiting magnetic-field-insensitive statesso-called clock states-and generating a long-wavelength spin wave to suppress dephasing, we succeed in extending the storage time of the quantum memory to 1 ms. Our result represents an important advance towards long-distance quantum communication and should provide a realistic approach to large-scale quantum information processing.The quantum repeater with atomic ensembles and linear optics has attracted broad interest in recent years, as it holds promise to implement long-distance quantum communication and the distribution of entanglement over quantum networks. Following the protocol proposed in ref. 3 and the subsequent improved schemes 4-7 , significant experimental progress has been accomplished, including the coherent manipulation of the stored excitation in one 10,11 or two 14-16 atomic ensembles, the demonstration of memory-built-in quantum teleportation 17 and the realization of a building block of the quantum repeater 13,18 . In these experiments, the atomic ensembles serve as the storable and retrievable quantum memory for single excitations.Despite the advances achieved in manipulating atomic ensembles, long-distance quantum communication with atomic ensembles remains challenging owing to the short storage time of the quantum memory for single excitations. For example, for direct generation of entanglement between two memory qubits over a few hundred kilometres, we need a memory with a storage time of a few hundred microseconds. However, the longest storage time reported so far is of the order of only 10 µs (refs 10-13).It has long been believed that the short coherence time is mainly caused by the residual magnetic field 19,20 . Thereby, storing the collective state in the superposition of the first-order magnetic-field-insensitive states 21 , that is, the 'clock states', is suggested to inhibit this decoherence mechanism 19 . A numerical calculation shows that the expected lifetime is of the order of seconds in this case.Here we report on our investigation of prolonging the storage time of the quantum memory for single excitations. In the experiment, we find that using only the 'clock state' is not sufficient to obtain the expected long storage time. We further analyse, isolate and identify the distinct decoherence mechanisms, and thoroughly investigate the dephasing of the spin wave (SW) by varying its wavelength. We find that the dephasing of the SW is extremely sensitive to the angle between the write beam and detection mode, especially for small angles. On the basis of this finding, by exploiting the 'clock state' and increasing the wavelength of the SW to suppress the dephasing, we succeed in extending the storage time from 10 µs to 1 ms.The illustration of our experiment is depicted in Fig. 1a,b....

Roton-type excitations usually emerge from strong correlations or long-range interactions, as in superfluid helium or dipolar ultracold atoms. However, in a weakly short-range interacting quantum gas, the recently synthesized spin-orbit (SO) coupling can lead to various unconventional phases of superfluidity and give rise to an excitation spectrum of roton-maxon character. Using Bragg spectroscopy, we study a SO-coupled Bose-Einstein condensate of ^{87}Rb atoms and show that the excitation spectrum in a "magnetized" phase clearly possesses a two-branch and roton-maxon structure. As Raman coupling strength Ω is decreased, a roton-mode softening is observed, as a precursor of the phase transition to a stripe phase that spontaneously breaks spatially translational symmetry. The measured roton gaps agree well with theoretical calculations. Furthermore, we determine sound velocities both in the magnetized and in the nonmagnetized phases, and a phonon-mode softening is observed around the phase transition in between. The validity of the f-sum rule is examined.

ethanol) were found to play an important role in reducing the concentration of Fe-Li antisite defects. However, the concentration of Fe-Li antisite defects reduced is still limited (higher than 0.99%), which cannot improve the rate performance of LIB efficiently. [9,10] Herein, we report on the synthesis of an ideal crystalline LFP/carbon hybrid microtube (LFP/CMT) with the lowest Fe-Li antisite defects reported (<0.3%) to boost the lithium ion storage. The key concept for suppressing the Fe-Li antisite defects is through the use of the strong chelation interaction with Fe 3+ and absorbs interaction with Li + of alginate. This is able to efficiently control the prior occupancy of Fe at the beginning of the crystal formation, thus reduce the concentration of Fe-Li antisite defects in subsequent pyrolysis process and produce well-crystallized LFP nanoparticles (NPs). Meantime, the carbohydrate framework of alginate fiber was converted to highly porous carbonaceous hybrid microtube (CMT) after pyrolysis in nitrogen (N 2 ) atmosphere, in which the LFP NPs with low Fe-Li antisite defects are embedded. This can highly enhance the electrical conductivity of the LFP NPs. As a consequence, the LFP/CMT displays superior discharge capacity of 165 mAh g −1 at 0.5 C, excellent capacity retention of 91% after 1000 cycle numbers, and outstanding rate capacity of 99.7 mAh g −1 at 100 C.Protonated alginate fiber [11][12][13] was used to prepare the LFP electrodes with low Fe-Li antisite defects (see details in Supporting Information). The protonated alginate fiber was soaked into the mixed aqueous solutions of ferric nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O), ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ), and lithium nitrate (LiNO 3 ) to form yellow Li-Fe-P-alginate fibers. The reduction of Fe-Li antisite defects is achieved by unique "egg-box" structure in alginate macromolecule. As shown in Scheme 1b, the Fe 3+ was immobilized into "egg-box" via coordination with G blocks in alginate, and the PO 4 3− was adsorbed by the Fe 3+ cations. As described in Figure S1 of the Supporting Information, the diffraction peak at 2θ = 21.18° was observed from the X-ray diffraction (XRD) pattern of Fe-P-alginate fiber, ascribed to a typical "egg-box" structure in G-rich Fe-alginate junction zones. [14,15] However, the monovalent Li + ions can only be adsorbed by the carboxyl (M block) via the electrostatic force. [16,17] The diffraction peaks at 2θ = 38.05° and 44.22° are ascribed to Fe-rich Li-alginate junction zones (see Figure S1, Supporting Information). [16] Obviously, the unexpected Fe-Li mixing could be avoided at the initial ion-exchange stage.The Li-Fe-P-alginate fibers were calcined at different temperatures (350-850 °C) in N 2 atmosphere to obtain a series of porous LFP/CMT with low concentration of Fe-Li antisite Olive structured LiFePO 4 (LFP) is a good candidate for lithiumion battery (LIB) cathode material due to its high theoretical capacity of 170 mAh g −1 , high electrochemical potential, good thermal stability, environmenta...

We report the realization of a robust and highly controllable two-dimensional (2D) spin-orbit (SO) coupling with topological non-trivial band structure. By applying a retro-reflected 2D optical lattice, phase tunable Raman couplings are formed into the anti-symmetric Raman lattice structure, and generate the 2D SO coupling with precise inversion and C4 symmetries, leading to considerably enlarged topological regions. The life time of the 2D SO coupled Bose-Einstein condensate reaches several seconds, which enables the exploring of fine tuning interaction effects. These essential advantages of the present new realization open the door to explore exotic quantum many-body effects and non-equilibrium dynamics with novel topology.