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Key generation efficiency, and security, in differential phase-shift quantum key distribution (DPS-QKD) improve with an increase in the number of optical delays or time-bin superpositions. We demonstrate the implementation of superposition states using time-bins, with two different approaches. In Type-A, we use an optical pulse and create superposition states with optical splitters and path delays. Similar superposition states are created, in Type-B, by applying direct phase modulation within a single weak coherent pulse. We establish the equivalence between both the approaches, and note that higher-order superposition states of Type-B are easier to generate for DPS-QKD. We set up DPS-QKD, over 105 km of single mode optical fiber, with a quantum bit error rate of less than 15% at a secure key rate of 2 kbps. With temporal guard bands, the QBER reduced to less than 10%, but with a 20% reduction in the key rate. [ABSTRACT FROM AUTHOR]

We investigate the practical network integration of differential phase shift quantum key distribution following a cost-optimized deployment scheme where complexity is off-loaded to a centralized location. User terminal equipment for quantum state preparation at 1 GHz symbol rate is kept technologically lean through use of a directly-modulated laser as optical encoder. Integration in a passive optical network infrastructure is experimentally studied for legacy and modern optical access standards. We analyze the implications that result from Raman scattering arising from different spectral allocations of the classical channels in the O-, S-, C- and L-band, and prove that the quantum channel can co-exist with up to 19 classical channels of a fully-loaded modern access standard. Secure-key generation at a rate of 5.1×10−7 bits/pulse at a quantum bit error ratio of 3.28% is obtained over a 13.5 km reach, 2:16 split passive network configuration. The high power difference of 93.8 dB between launched classical and quantum signals in the lit access network leads to a low penalty of 0.52% in terms of error ratio. [ABSTRACT FROM AUTHOR]

This paper reports experimental results for a long-term field trial of 1-GHz clock differential phase shift quantum key distribution that was incorporated in the test-bed optical fiber network installed in the Tokyo metropolitan area. A photon transmitter and a photon receiver were connected to a 90-km loop-back optical link with a 30-dB loss. Superconducting single photon detectors were employed to detect photons with a low dark count rate. Stable maintenance-free operation was achieved for 25 days, where the average sifted key generation rate and the quantum bit error rate were 11 kb/s and 2.6%, respectively. The average generation rate of a secure key after error correction and privacy amplification was 1.1 kb/s with a fluctuation of ± 0.5 kb/s. The achieved performance was examined using detailed bit error analysis. A comparison with meteorological data revealed the way in which the field environment influenced the key distribution performance. [ABSTRACT FROM PUBLISHER]

The fundamental quantum mechanics prohibitions on the measurability of quantum states allow secure key distribution between spatially remote users to be performed. Experimental and commercial implementations of quantum cryptography systems, however, use components that exist at the current technology level, in particular, one-photon avalanche photodetectors. These detectors are subject to the blinding effect. It was shown that all the known basic quantum key distribution protocols and systems based on them are vulnerable to attacks with blinding of photodetectors. In such attacks, an eavesdropper knows all the key transferred, does not produce errors at the reception side, and remains undetected. Three protocols of quantum key distribution stable toward such attacks are suggested. The security of keys and detection of eavesdropping attempts are guaranteed by the internal structure of protocols themselves rather than additional technical improvements. [ABSTRACT FROM AUTHOR]

Quantum-key distribution has been studied as an ultimate method for secure communications, and now it is emerging as a technology that can be deployed in real fiber networks. Here, we present our QKD experiments based on the differential-phase-shift QKD protocol. A DPS-QKD system has a simple configuration that is easy to implement with conventional optical communication components, and it is suitable for a high-clock rate system. Moreover, although the DPS-QKD system is implemented with an attenuated laser source, it is inherently secure against strong eavesdropping attacks called photon number-splitting attacks, which pose a serious threat to conventional QKD systems with attenuated laser sources. We also describe three types of single-photon detectors that are suitable for high-speed, long-distance QKD: an up-conversion detector, a superconducting single-photon detector, and a sinusoidally gated InGaAs avalanche photodiode. We present our record-setting QKD experiments that employed those detectors. [ABSTRACT FROM AUTHOR]

The aim of this study was to examine the complement system and the distribution of some human leukocyte antigen (HLA) class III alleles (C4, BF) in healthy aged people (77 centenarians and 89 elderly subjects). We have also studied the alleles of C3, a complement component genetically unrelated to HLA, the immunochemical levels of C4 and C3 and serum functional hemolytic activity for classical (CH50) and alternative (AP50) complement pathway. The levels of C3 and C4 and the CH50 and AP50 were found to be within the normal range. The frequencies of C3, BF, and C4A alleles were similar in the cohorts that have been studied. For C4B null allele (C4BQ0) a trend toward an increase in the older cohort was observed, although the differences were not significant after statistical correction. Our data suggest that the complement system is well preserved in centenarians and elderly subjects and class III HLA antigens are equally distributed in aged cohorts and in young healthy individuals. [ABSTRACT FROM AUTHOR]

The authors report a quantum key distribution experiment, in which they implemented a differential phase shift quantum key distribution protocol, using single-photon detectors based on InGaAs/InP avalanche photodiodes operated with a sinusoidal gating. The single-photon detectors were operated at a repetition frequency of 500 MHz with low after pulsing probabilities and low dark counts. A sifted key generation rate of 1.5 Mbit/s was achieved over a communication distance of 15 km. Taking account of the security of the protocol against general individual attacks, secure keys can be generated with a rate of 0.33 Mbit/s. [ABSTRACT FROM AUTHOR]

Realistic quantum key distribution (QKD) systems suffer from side-channel attacks, which manipulate single-photon detectors. Although measurement-device-independent QKD schemes were proposed to free QKD parties (Alice and Bob) from such measurement devices, these schemes are not easy to be implemented in practice because they require precise synchronization between signals from distant parties. On the other hand, differential phase shift (DPS) QKD is a simple system for practical implementation with current optical equipment. In this study, we propose a simple modification in DPS QKD to prevent side-channel attacks (control blinding and controlling attacks) such that Bob randomly attenuates the incoming signal. This modification allows Bob to utilize photon statistics during attenuated time slots in DPS-QKD systems, using which the side-channel attacks are revealed. [ABSTRACT FROM AUTHOR]

We report the fabrication of a superconducting nanowire single-photon detector (SSPD or SNSPD) with an ultralow dark count rate. By introducing optical band-pass filters at the input of the SSPD and cooling the filters at 3 K, the dark count rate is reduced to less than 1/100 at low bias. An SSPD with 0.1 cps dark count rate and 5.6% system detection efficiency at 1550 nm wavelength is obtained. We show that a quantum key distribution (QKD) over 300 km of fiber is possible based on a numerical calculation assuming a differential phase shift QKD protocol implemented with our SSPDs. [ABSTRACT FROM AUTHOR]

Since it was noted that quantum computers could break public key cryptosystems based on number theory, extensive studies have been undertaken on quantum cryptography, which offers unconditionally secure communication based on quantum mechanics. We investigate a quantum key distribution (QKD) scheme using macroscopic coherent light with optically pre-amplified direct differential detection. A transmitter “Alice” sends a series of two macroscopic nonorthogonal coherent states that partially overlap due to quantum noise. A receiver “Bob” amplifies and receives it with direct differential detection followed by a thresholding process. To avoid difficulties in detection, our scheme uses conventional direct differential photodetection, not single-photon detection or homodyne detection as in previous QKD protocols. System performance assuming some eavesdropping is evaluated, the results of which suggest that our scheme is usable for short or medium distance. [ABSTRACT FROM AUTHOR]