We report the first experimental demonstration of quantum entanglement among ten spatially separated single photons. A near-optimal entangled photon-pair source was developed with simultaneously a source brightness of ∼12  MHz/W, a collection efficiency of ∼70%, and an indistinguishability of ∼91% between independent photons, which was used for a step-by-step engineering of multiphoton entanglement. Under a pump power of 0.57 W, the ten-photon count rate was increased by about 2 orders of magnitude compared to previous experiments, while maintaining a state fidelity sufficiently high for proving the genuine ten-particle entanglement. Our work created a state-of-the-art platform for multiphoton experiments, and enabled technologies for challenging optical quantum information tasks, such as the realization of Shor's error correction code and high-efficiency scattershot boson sampling.

We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A novel and economic way of synchronization method is designed and incorporated into the system, which allows to work at a low frequency of 40kHz and removes the use of highly precise clock. A final key rate of 15 Hz is distributed within the experimental time of 3089 seconds, by using super-conducting single photon detectors. This is longest decoy-state QKD yet demonstrated up to date. It helps to make a significant step towards practical secure communication in long-distance scope.

Abstract:We have demonstrated a metropolitan all-pass quantum communication network in field fiber for four nodes. Any two nodes of them can be connected in the network to perform quantum key distribution (QKD). An optical switching module is presented that enables arbitrary 2-connectivity among output ports. Integrated QKD terminals are worked out, which can operate either as a transmitter, a receiver, or even both at the same time. Furthermore, an additional link in another city of 60 km fiber (up to 130 km) is seamless integrated into this network based on a trusted relay architecture. On all the links, we have implemented protocol of decoy state scheme. All of necessary electrical hardware, synchronization, feedback control, network software, execution of QKD protocols are made by tailored designing, which allow a completely automatical and stable running. Our system has been put into operation in Hefei in August 2009, and publicly demonstrated during an evaluation conference on quantum network organized by the Chinese Academy of Sciences on August 29, 2009. Real-time voice telephone with one-time pad encoding between any two of the five nodes (four all-pass nodes plus one additional node through relay) is successfully established in the network within 60km. 793-795 (1997). 3. T. Nishioka, H. Ishizuka, T. Hasegawa, and J. Abe, "'Circular type' quantum key distribution," Photon. Technol.Lett., IEEE 14, 576-578 (2002). 4. F. Grosshans, G. V. Assche, J. Wenger, R. Brouri, N. J. Cerf, and P. Grangier, "Quantum key distribution using Gaussian-modulated coherent states," Nature 421, 238-241 (2003). 5. C. Gobby, Z. L. Yuan, and A. J. Shields, "Quantum key distribution over 122 km of standard telecom fiber," Appl. Phys. Lett. 84, 3762-3764 (2004 1155-1163 (1995). 14. P. D. Townsend, "Quantum cryptography on multi-user optical fibre networks," Nature 385, 47-49 (1997).

Quantum mechanics provides means of generating genuine randomness that is impossible with deterministic classical processes. Remarkably, the unpredictability of randomness can be certified in a self-testing manner that is independent of implementation devices. Here, we present an experimental demonstration of self-testing quantum random number generation based on an detection-loophole free Bell test with entangled photons. In the randomness analysis, without the assumption of independent identical distribution, we consider the worst case scenario that the adversary launches the most powerful attacks against quantum adversary. After considering statistical fluctuations and applying an 80 Gb × 45.6 Mb Toeplitz matrix hashing, we achieve a final random bit rate of 114 bits/s, with a failure probability less than 10 −5 . Such self-testing random number generators mark a critical step towards realistic applications in cryptography and fundamental physics tests. 2Introduction.-Random numbers are widely used in applications ranging from numerical

Entanglement, the essential resource in quantum information processing, should be witnessed in many tasks such as quantum computing and quantum communication. The conventional entanglement witness method, relying on an idealized implementation of measurements, could wrongly conclude a separable state to be entangled due to imperfect detections. Inspired by the idea of a time-shift attack, we construct an attack on the conventional entanglement witness process and demonstrate that a separable state can be falsely identified to be entangled. To close such detection loopholes, based on a recently proposed measurement-device-independent entanglement witness method, we design and experimentally demonstrate a measurement-device-independent entanglement witness for a variety of two-qubit states. By the new scheme, we show that an entanglement witness can be realized without detection loopholes. 2Quantum entanglement plays an important role in the nonclassical phenomenons of quantum mechanics. Being the key resource for many tasks in quantum information processing, such as quantum computation [1], quantum teleportation [2], and quantum cryptography [3,4], entanglement needs to be verified in many scenarios. There are several proposals to witness entanglement and we refer to Ref.[5] for a detailed review. A conventional way to detect entanglement, the entanglement witness (EW), gives one of two outcomes: "Yes" or "No", corresponding to the conclusive result that the state is entangled or to failure to draw a conclusion, respectively. Mathematically, for a given entangled quantum state ρ, a Hermitian operator W is called a witness, if tr[W ρ] < 0 (output of 'Yes') and tr[W σ] ≥ 0 (output of 'No') for any separable state σ. Note that there could also exist an entangled state ρ ′ such that tr[W ρ ′ ] ≥ 0 (output of 'No'). In the experimental verification, one can realize the conventional EW with only local measurements by decomposing W into a linear combination of product Hermitian observables [5].Focusing on the bipartite scenario, a general illustration of the conventional EW is shown in Fig. 1(a), where two parties, Alice and Bob, each receive one component of a bipartite state ρ AB from an untrusted third party Eve. They want to verify whether ρ AB is entangled or not, by performing local operations and measurements onThe correctness of such witness relies on implementation details of W . An unfaithful implementation of W , say, due to device imperfections, would render the witness results unreliable. For example, the measurement devices used by Alice and Bob might possibly be manufactured by another untrusted party, who could collaborate with Eve and deliberately fabricate devices to make the real implementation W ′ = W + δW deviate from W , such that W ′ is not a witness any more,That is, with the deviated witness W ′ , a separable state σ could be identified as an entangled one, which is more likely to happen when tr[W σ] is near zero. There is a strong similarity between the EW and the quantum key distribution (QKD)...