Hiroki Takesue

Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors

We report the first quantum key distribution (QKD) experiment to enable the creation of secure keys over 42xa0dB channel loss and 200xa0km of optical fibre. We used the differential phase shift QKD (DPS-QKD) protocol, implemented with a 10-GHz clock frequency and superconducting single-photon detectors (SSPD) based on NbN nanowires. The SSPD offers a very low dark count rate (a few Hz) and small timing jitter (60xa0ps, full width at half maximum, FWHM). These characteristics allowed us to achieve a 12.1xa0bitxa0s–1 secure key rate over 200xa0km of fibre, which is the longest terrestrial QKD over a fibre link yet demonstrated. Moreover, this is the first 10-GHz clock QKD system to enable secure key generation. The keys generated in our experiment are secure against both general collective attacks on individual photons and a specific collective attack on multiphotons, known as a sequential unambiguous state discrimination (USD) attack.

Field test of quantum key distribution in the Tokyo QKD Network

A secure communication network with quantum key distribution in a metropolitan area is reported. Six different QKD systems are integrated into a mesh-type network. GHz-clocked QKD links enable us to demonstrate the world-first secure TV conferencing over a distance of 45km. The network includes a commercial QKD product for long-term stable operation, and application interface to secure mobile phones. Detection of an eavesdropper, rerouting into a secure path, and key relay via trusted nodes are demonstrated in this network.

Differential phase shift quantum key distribution experiment over 105 km fibre

We report a quantum key distribution experiment based on the differential phase shift keying (DPSK) protocol with a Poissonian photon source, in which secure keys were generated over >100u2009km fibre for the first time. We analysed the security of the DPSK protocol and showed that it is robust against strong attacks by Eve, including a photon number splitting attack. To implement this protocol, we developed a new detector for the 1.5u2009μm band based on frequency up-conversion in a periodically poled lithium niobate waveguide followed by an Si avalanche photodiode. The use of detectors increased the sifted key generation rate up to >1u2009Mbitu2009s−1 over 30u2009km fibre, which is two orders of magnitude larger than the previous record.

Security of differential-phase-shift quantum key distribution against individual attacks

We derive a proof of security for the differential-phase-shift quantum key distribution protocol under the assumption that Eve is restricted to individual attacks. The security proof is derived by bounding the average collision probability, which leads directly to a bound on Eves mutual information on the final key. The security proof applies to realistic sources based on pulsed coherent light. We then compare individual attacks to sequential attacks and show that individual attacks are more powerful.

Single-photon detection using magnesium diboride superconducting nanowires

We fabricated 10 nm thick MgB2 nanowires with a width down to 100 nm using the liftoff process. The I-V characteristics of the nanowire show hysteresis and a sharp voltage jump at Ic. Though a 150 nm wide nanowire exhibits the capacity for detecting a single photon at 405 nm wavelength, the nanowire is too wide to detect a single photon at 1560 nm. A 100 nm wide nanowire exhibits the capacity for detecting single photons in the 405–1560 nm wavelength range. This indicates a possible application of MgB2 as a high-performance superconducting nanowire single-photon detector.

Long-distance distribution of time-bin entangled photon pairs over 100 km using frequency up-conversion detectors.

We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-mum non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bells inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km.

High-rate quantum key distribution over 100 km using ultra-low-noise, 2-GHz sinusoidally gated InGaAs/InP avalanche photodiodes

We have demonstrated quantum key distribution (QKD) over 100 km using single-photon detectors based on InGaAs/InP avalanche photodiodes (APDs). We implemented the differential phase shift QKD (DPS-QKD) protocol with electrically cooled and 2-GHz sinusoidally gated APDs. The single-photon detector has a dark count probability of 2.8 × 10(-8) (55 counts per second) with a detection efficiency of 6 %, which enabled us to achieve 24 kbit/s secure key rate over 100 km of optical fiber. The DPS-QKD system offers better performances in a practical way than those achieved using superconducting single-photon detectors. Moreover, the distance that secure keys against the general individual attacks can be distributed has been extended to 160 km.

Megabits secure key rate quantum key distribution

Quantum cryptography can provide unconditional secure communication between two authorized parties based on the basic principles of quantum mechanics. However, imperfect practical conditions limit its transmission distance and communication speed. Here, we implemented the differential phase shift (DPS) quantum key distribution (QKD) with an up-conversion-assisted hybrid photon detector (HPD) and achieved a 1.3u2009Mbits per second secure key rate over a 10u2009km fiber, which is tolerant against photon number splitting (PNS) attack, general collective attacks on individual photons and any other known sequential unambiguous state discrimination (USD) attacks.

Performance of various quantum-key-distribution systems using 1.55-{mu}m up-conversion single-photon detectors

We compare the performance of various quantum-key-distribution (QKD) systems using a single-photon detector, which combines frequency up-conversion in a periodically poled lithium niobate waveguide and a silicon avalanche photodiode (APD). The comparison is based on the secure communication rate as a function of distance for three QKD protocols: the Bennett-Brassard 1984, the Bennett-Brassard-Mermin 1992, and the coherent differential-phase-shift keying protocols. We show that the up-conversion detector allows for higher communication rates and longer communication distances than the commonly used InGaAs/InP APD for all three QKD protocols.

Generation of energy-time entangled photon pairs in 1.5-μm band with periodically poled lithium niobate waveguide

We report the generation of 1.5-μm-band energy-time entangled photon pairs using a periodically poled lithium niobate (PPLN) waveguide. We performed a two-photon interference experiment and obtained coincidence fringes with 77.3% visibilities without subtracting accidental coincidences.