Ken-ichiro Yoshino

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.

Ultra fast quantum key distribution over a 97 km installed telecom fiber with wavelength division multiplexing clock synchronization

We demonstrated ultra fast BB84 quantum key distribution (QKD) transmission at 625 MHz clock rate through a 97 km field-installed fiber using practical clock synchronization based on wavelength-division multiplexing (WDM). We succeeded in over-one-hour stable key generation at a high sifted key rate of 2.4 kbps and a low quantum bit error rate (QBER) of 2.9%. The asymptotic secure key rate was estimated to be 0.78- 0.82 kbps from the transmission data with the decoy method of average photon numbers 0, 0.15, and 0.4 photons/pulse.

High-Speed Quantum Key Distribution System for 1-Mbps Real-Time Key Generation

A high-speed quantum key distribution (QKD) system has been developed with the goal of a 1-Mbps final secure key generation rate under 10-dB transmission loss, which corresponds to 50 km of standard single mode fiber. For the purpose of speeding-up all processes in QKD sequence, we apply a wavelength-division-multiplexing (WDM) technique using the colorless interferometric technique and a key distillation hardware (HW) engine. We establish a novel WDM scheme, sharing interferometers and their temperature regulators over multiple channels, which enables us to increase the number of channels with a small impact on system cost and size. To generate a secure key while satisfying both high speed and high security, we develop a key distillation HW engine which enables us to execute key distillation with 1-Mbit code length in real time. We have experimentally evaluated the performance of the developed system through installed fiber. By operating three wavelength channels, a new, world leading key generation rate of greater than 200 kbps over a 14.5-dB transmission loss has been achieved.

High-speed wavelength-division multiplexing quantum key distribution system

A high-speed quantum key distribution system was developed with the wavelength-division multiplexing (WDM) technique and dedicated key distillation hardware engines. Two interferometers for encoding and decoding are shared over eight wavelengths to reduce the systems size, cost, and control complexity. The key distillation engines can process a huge amount of data from the WDM channels by using a 1 Mbit block in real time. We demonstrated a three-channel WDM system that simultaneously uses avalanche photodiodes and superconducting single-photon detectors. We achieved 12 h continuous key generation with a secure key rate of 208 kilobits per second through a 45 km field fiber with 14.5 dB loss.

Efficient and low-noise single-photon avalanche photodiode for 1.244-GHz clocked quantum key distribution

An efficient and low-noise 1.244-GHz gating InGaAs single-photon avalanche photodiode (SAPD) was developed for a high-speed quantum key distribution (QKD) system. An afterpulsing probability of 0.61% and a dark count probability per gate of 0.71 ×10-6 were obtained at a detection efficiency of 10.9% for 1.55-µm photons. Furthermore, our SAPD successfully coped with high detection efficiency (≤ 25%) and quite low afterpulsing noise (≤ 3% for ≤ 25% efficiency) at the same time. Its potential was verified using the actual QKD setups installed over a metropolitan area network.

Transmission Experiment of Quantum Keys over 50 km Using High-Performance Quantum-Dot Single-Photon Source at 1.5 µm Wavelength

We have developed a high-performance single-photon source (SPS) operating at 1.5 µm wavelength. The source is an InAs/InP quantum dot with a horn-shaped nanostructure. A resonant excitation to the p-shell state helps achieve a single-photon efficiency of 5.8% after coupling into a single-mode fiber with a second-order correlation value of g(2)(0)~0.055. The performance of the source has been assessed by integrating it into a conventional quantum key distribution system. We have successfully transmitted secure keys over a 50 km commercial fiber, exceeding the previously reported range for an SPS operating below 1.3 µm.

Quantum encoder and decoder for practical quantum key distribution using a planar lightwave circuit

To launch quantum key distribution (QKD) into the commercial market, it is important to develop a system that is simpler and more reliable using current technology. This report proposes quantum encoders and decoders using a passive planar lightwave circuit (PLC) that is useful for implementing optical-fiber-based QKD systems. Our encoders and decoders are based on an asymmetric Mach–Zehnder interferometer and allow us to prepare and analyze various photonic time-bin qubits reliably. The system can be stable and polarization-insensitive merely by stabilizing and controlling the device temperature. Our PLC-based devices enables us to simplify the QKD system and increase its reliability.

Maintenance-free operation of WDM quantum key distribution system through a field fiber over 30 days

Maintenance-free wavelength-division-multiplexing quantum key distribution for 30 days was achieved through a 22-km field fiber. Using polarization-independent interferometers and stabilization techniques, we attained a quantum bit error rate as low as 1.70% and an estimated secure key rate as high as 112.4 kbps for a record-breaking 291.3 Gbits of estimated secure keys accumulated over an uninterrupted operation period.

Afterpulse-like phenomenon of superconducting single photon detector in high speed quantum key distribution system

We discuss our estimates of the performance of a superconducting single photon detector (SSPD) in a high speed quantum key distribution (QKD) system. We find that at high repetition operation reflections from the readout circuit at room temperature causes an afterpulse-like phenomenon, and drastically increases the quantum bit error rate (QBER). Such effects are not seen during low frequency operation. By using an amplifier with a small reflection coefficient S11, we succeed in reducing the afterpulse-like phenomenon and increasing a secure key rate.

Quantum Photonic Network: Concept, Basic Tools, and Future Issues

We present practical GHz-clocked QKD systems, next generation entanglement QKD technologies, and QKD platform to manage the secure keys and to support a variety of applications. We then show the intrinsic limit of QKD, i.e., a key rate bound, and discuss how to realize the provable (information theoretic) security with a larger secrecy capacity over longer distances. In particular, we present a basic theory of physical layer cryptography, which characterizes the secrecy capacity, and engineers the tradeoff between the efficiency of reliable transmission and secrecy of communication. We introduce a concept to unify these schemes in photonic network, referred to as quantum photonic network. Future issues for realizing this new network paradigm are discussed.