Continous Variables

The system developed by CNRS-Institut d'Optique-Univ. Paris-Sud (Palaiseau, France), THALES Research & Technology (Palaiseau, France) and Université Libre de Bruxelles (Brussels, Belgium), implements a coherent-state reverse-reconciliated QKD protocol [1]. This protocol encodes key information on both quadratures of the electromagnetic field of coherent states.

The experimental setup is shown in Figure 1. Alice uses a laser diode, pulsed with a repetition rate of 500 kHz, and an asymmetric beam splitter to generate signal and local oscillator (LO) pulses. The signal pulses are appropriately modulated in amplitude and phase, and the desired output variance is selected with an amplitude modulator and a variable attenuator. The signal is then time-multiplexed with the delayed LO before propagating through the 25 km quantum channel. After demultiplexing, Bob uses an all-fiber shot-noise limited time-resolved pulsed homodyne detection system to measure the quadrature selected by his phase modulator. Alice and Bob share in this way correlated continuous data. These are transformed into a binary secret key using a discretization algorithm, efficient Low Density Parity Check error-correcting codes and privacy amplification. The two parties communicate via a synchronous automatic data processing software, which assures proper hardware operation and manages the key distribution protocol. The applied techniques include dynamic polarization control, feedforward loops, system parameter optimization, and real-time reconciliation. They enable a stable, automatic and continuous operation of the QKD system [2].



Figure 1: Experimental Setup

Figures 2 and 3 show photographs of the fully integrated devices, which are housed in 19'' racks.



Figure 2: Alice



Figure 3: Bob

1. F. Grosshans, G. Van Assche, J. Wenger, R. Brouri, N. J. Cerf and P. Grangier, Nature 421, 238 (2003).

2. J. Lodewyck, M. Bloch, R. Garcia-Patron, S. Fossier, E. Karpov, E. Diamanti, T. Debuisschert, N. J. Cerf, R. Tualle-Brouri, S. W. McLaughlin and P. Grangier, Phys. Rev. A 76, 042305 (2007).