Matthieu Legre

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.

Distribution of time-bin entangled qubits over 50 km of optical fiber

We report experimental distribution of time-bin entangled qubits over 50 km of optical fibers. Using actively stabilized preparation and measurement devices we demonstrate violation of the Clauser-Horne-Shimony-Holt Bell inequality by more than 15 standard deviations without removing the detector noise. In addition we report a proof-of principle experiment of quantum key distribution over 50 km of optical fibers using entangled photon.

Long-term performance of the SwissQuantum quantum key distribution network in a field environment

In this paper, we report on the performance of the SwissQuantum quantum key distribution (QKD) network. The network was installed in the Geneva metropolitan area and ran for more than one-and-a-half years, from the end of March 2009 to the beginning of January 2011. The main goal of this experiment was to test the reliability of the quantum layer over a long period of time in a production environment. A key management layer has been developed to manage the key between the three nodes of the network. This QKD-secure network was utilized by end-users through an application layer.

Quantum key distribution and 1 Gbps data encryption over a single fibre

We perform quantum key distribution (QKD) over a single fibre in the presence of four classical channels in a C-band dense wavelength division multiplexing (DWDM) configuration using a commercial QKD system. The classical channels are used for key distillation and 1 Gbps encrypted communication, rendering the entire system independent of any other communication channel than a single dedicated fibre. We successfully distil secret keys over fibre spans of up to 50 km. The separation between the quantum channel at 1551.72 nm and the nearest classical channel is only 200 GHz, while the classical channels are all separated by 100 GHz. In addition to that, we discuss possible improvements and alternative configurations, e.g. whether it is advantageous to choose the quantum channel at 1310 nm or to opt for a pure C-band (1530–1565 nm) configuration.

Experimental study of polarization properties of highly birefringent photonic crystal fibers.

We analyze experimentally the polarization properties of highly nonlinear small-core photonic crystal fibers (PCFs) with no intentional birefringence. The properties of recently emerged polarization maintaining PANDA PCFs are also investigated. The wavelength and temperature dependence of phase and group delay of these fibers are examined in the telecommunications wavelength range. Compared to a standard PANDA fiber, the polarization characteristics and temperature dependence are found to be qualitatively different for both types of fibers.

SiPM for fast Photon-Counting and Multiphoton Detection

We demonstrate fast counting and multiphoton detection abilities of a Silicon Photo Multiplier (SiPM). In fast counting mode we are able to detect two consecutive photons separated by only 2.3 ns corresponding to 430 MHz. The counting efficiency for small optical intensities at lambda= 532 nm was found to be around 16% with a dark count rate of 52 kHz at T= -5 masculine C. Using the SiPM in multiphoton detection mode, we find a good signal discrimination for different numbers of simultaneously detected photons.

Photon Counting OTDR: Advantages and Limitations

In this paper, we provide a detailed insight into photon-counting optical time-domain reflectometer (¿-OTDR) operation, ranging from Geiger-mode operation of avalanche photodiodes (APD), analysis of different APD bias schemes, to the discussion of OTDR perspectives. Our results demonstrate that an InGaAs/InP APD-based ¿-OTDR has the potential of outperforming the dynamic range of a conventional state-of-the-art OTDR by 10 dB, as well as the two-point resolution by a factor of 20. Considering the trace acquisition speed of ¿-OTDRs, we find that a combination of rapid gating for high photon flux and free running mode for low photon flux is the most efficient solution. Concerning dead zones, our results are less promising. Without additional measures, e.g., an optical shutter, the photon counting approach is not competitive.

High resolution optical time domain reflectometer based on 1.55μm up-conversion photon-counting module

We implement a photon-counting Optical Time Domain Reflectometer (OTDR) at 1.55mum which exhibits a high 2-point resolution and a high accuracy. It is based on a low temporal-jitter photon-counting module at 1.55mum. This detector is composed of a periodically poled Lithium niobate (PPLN) waveguide, which provides a wavelength conversion from near infrared to visible light, and a low jitter silicon photon-counting detector. With this apparatus, we obtain centimetre resolution over a measurement range of tens of kilometres.

A fast and versatile quantum key distribution system with hardware key distillation and wavelength multiplexing

We present a compactly integrated, 625 MHz clocked coherent one-way quantum key distribution system which continuously distributes secret keys over an optical fibre link. To support high secret key rates, we implemented a fast hardware key distillation engine which allows for key distillation rates up to 4 Mbps in real time. The system employs wavelength multiplexing in order to run over only a single optical fibre. Using fast gated InGaAs single photon detectors, we reliably distribute secret keys with a rate above 21 kbps over 25 km of optical fibre. We optimized the system considering a security analysis that respects finite-key-size effects, authentication costs and system errors for a security parameter of eQKD = 4 × 10−9.

Investigation of the ratio between phase and Group birefringence in optical single-mode fibers

A measurement technique for the phase and group birefringence of an optical fiber is thoroughly investigated. It is based on differential group delay measurements of twisted fibers and is capable of giving in a simple and elegant way the intrinsic birefringence values in the absence of twist. Analyzing various fibers with this method, we find that phase and group birefringence can be quite different for certain fiber types. Consequently, the commonly used assumption that in an optical fiber, phase and group birefringence are equal-and the resulting carelessness in distinguishing between these two a priori separate physical effects-is to be employed cautiously.