Using ultra-bright sources of pure-state entangled photons from parametric down conversion, an eight-photon interferometer and post-selection detection, we demonstrate the ability to experimentally manipulate eight individual photons and report the creation of an eight-photon Schr\"odinger cat state with an observed fidelity of $0.708 \pm 0.016$.Comment: 6 pages, 4 figure

The heralded generation of entangled states is a long-standing goal in quantum information processing, because it is indispensable for a number of quantum protocols. Polarization entangled photon pairs are usually generated through spontaneous parametric down-conversion, but the emission is probabilistic. Their applications are generally accompanied by post-selection and destructive photon detection. Here, we report a source of entanglement generated in an event-ready manner by conditioned detection of auxiliary photons. This scheme benefits from the stable and robust properties of spontaneous parametric down-conversion and requires only modest experimental efforts. It is flexible and allows the preparation efficiency to be significantly improved by using beamsplitters with different transmission ratios. We have achieved a fidelity better than 87% and a state preparation efficiency of 45% for the source. This could offer promise in essential photonics-based quantum information tasks, and particularly in enabling optical quantum computing by reducing dramatically the computational overhead.Comment: 24 pages, 4 figures, 1 tabl

. 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....