In principle, quantum key distribution (QKD) offers unconditional security based on the laws of physics. In practice, flaws in the state preparation undermine the security of QKD systems, as standard theoretical approaches to deal with state preparation flaws are not loss-tolerant. An eavesdropper can enhance and exploit such imperfections through quantum channel loss, thus dramatically lowering the key generation rate. Crucially, the security analyses of most existing QKD experiments are rather unrealistic as they typically neglect this effect. Here, we propose a novel and general approach that makes QKD loss-tolerant to state preparation flaws. Importantly, it suggests that the state preparation process in QKD can be significantly less precise than initially thought. Our method can widely apply to other quantum cryptographic protocols.PACS numbers: 03.67.Dd, 03.67.-a Introduction.-Quantum key distribution (QKD) [1] allows two distant parties, Alice and Bob, to distribute a secret key, which is essential to achieve provable secure communications [2]. The field of QKD has progressed very rapidly over the last years, and it now offers practical systems that can operate in realistic environments [3,4].Crucially, QKD provides unconditional security based on the laws of physics, i.e., despite the computational power of the eavesdropper, Eve. Indeed, the security of QKD has been promptly demonstrated for different scenarios [5][6][7][8][9][10][11][12]. Importantly, Gottesman, Lo, Lütkenhaus and Preskill [13] (henceforth referred to as GLLP) proved the security of QKD when Alice's and Bob's devices are flawed, as is the case in practical implementations. Unfortunately, however, GLLP has a severe limitation, namely, it is not loss-tolerant; it assumes the worst case scenario where Eve can enhance flaws in the state preparation by exploiting channel loss. As a result, the key generation rate and achievable distance of QKD are dramatically reduced [14]. Notice that most existing QKD experiments simply ignore state preparation imperfections in their key rate formula, which renders their results unrealistic and not really secure.In this Letter, we show that GLLP's worst case assumption is far too conservative, i.e., in sharp contrast to GLLP, we present a security proof for QKD that is loss-tolerant. Indeed, for the case of modulation errors, an important flaw in real-life QKD systems, we show that Eve cannot exploit channel loss to enhance such imperfections. The intuition here is rather simple: in this type of state preparation flaws the signals sent out by Alice are still qubits, i.e., there is no side-channel for Eve to exploit

In theory, quantum key distribution (QKD) offers information-theoretic security. In practice, however, it does not due to the discrepancies between the assumptions used in the security proofs and the behavior of the real apparatuses. Recent years have witnessed a tremendous effort to fill the gap, but the treatment of correlations among pulses has remained a major elusive problem. Here, we close this gap by introducing a simple yet general method to prove the security of QKD with arbitrarily long-range pulse correlations. Our method is compatible with those security proofs that accommodate all the other typical device imperfections, thus paving the way toward achieving implementation security in QKD with arbitrary flawed devices. Moreover, we introduce a new framework for security proofs, which we call the reference technique. This framework includes existing security proofs as special cases, and it can be widely applied to a number of QKD protocols.

The quantum internet holds promise for performing quantum communication, such as quantum teleportation and quantum key distribution, freely between any parties all over the globe. For such a quantum internet protocol, a general fundamental upper bound on the performance has been derived [K. Azuma, A. Mizutani, and H.-K. Lo, arXiv:1601.02933]. Here we consider its converse problem. In particular, we present a protocol constructible from any given quantum network, which is based on running quantum repeater schemes in parallel over the network. The performance of this protocol and the upper bound restrict the quantum capacity and the private capacity over the network from both sides. The optimality of the protocol is related to fundamental problems such as additivity questions for quantum channels and questions on the existence of a gap between quantum and private capacities.PACS numbers: 03.67. Hk, 03.67.Dd, 03.65.Ud, In the Internet, if a client communicates with a far distant client, the data travel across multiple networks. At present, the nodes and the communication channels in the networks are composed of physical devices governed by the laws of classical information theory, and the data flow obeys the celebrated max-flow min-cut theorem in graph theory. However, in the future, such classical nodes and channels should be replaced with quantum ones, whose network follows the rules of quantum information theory, rather than classical one. This network, called quantum internet, could accomplish tasks that are intractable in the realm of classical information processing, and it serves opportunities and challenges across a range of intellectual and technical frontiers, including quantum communication, computation, metrology, and simulation [1]. So far, the main interest in the quantum internet has been its realization [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. But, it must be one of the most fundamental trials from a theoretical perspective to grasp the full potential of the quantum internet. Along this line, recently, a general fundamental upper bound on the performance was derived [17] for its use for supplying two clients with entanglement or a secret key. Interestingly, this upper bound is estimable and applied to any private-key or entanglement distillation scheme that works over any network topology composed of arbitrary quantum channels by using arbitrary local operations and unlimited classical communication (LOCC). With this, for the case of linear lossy optical channel networks, it has been shown [17] that existing intercity quantum key distribution (QKD) protocols [18][19][20] and quantum repeater schemes [7,8,12,14,15] have no scaling gap with the fundamental upper bound. Moreover, in the case of a multipath network composed of a wide range of stretchable quantum channels (including lossy optical channels), it has been proven [21] to be optimal to choose a single path between two clients for running quantum repeater scheme, in order to minimize the number of times paths between them are used...

Selective neuronal death in the CA1 sector of the hippocampus [delayed neuronal death (DND)] develops several days after transient global cerebral ischemia in rodents. Because NGF plays a potential role in neuronal survival, it was decided to study its effect in DND. We report here that intraventricular injection of NGF either before or after 5 min forebrain ischemia in the Mongolian gerbil significantly reduced the occurrence of DND. The tissue content of NGF in the hippocampus was decreased 2 d after ischemia and recovered to the preischemic level by 1 week. By the Golgi staining technique, changes first began in the dendrites of affected neurons as early as 3 hr. Such changes could be ameliorated by NGF treatment. Although previous knowledge of NGF is limited to the survival of cholinergic neurons in the CNS, it is assumed that other mechanisms must be operating in the hippocampus, for example, postsynaptic modification at dendrites or aberrant expression of NGF receptors possibly at the initial excitation period by glutamate. Furthermore, because previous work has shown that inhibition of protein synthesis reduces the occurrence of DND, a program leading to cell death might also be operating via de novo synthesis of certain protein(s), collectively termed “killer protein,” because of a lack of NGF.