PRACTICAL FREE-SPACE DECOY-STATE QUANTUM KEY DISTRIBUTION IMPLEMENTATION

IGNACIO H. LOPEZ GRANDE; AGUSTINA G. MAGNONI; MIGUEL A. LAROTONDA

Pucón

Conferencia; IX RIAO/OPTILAS Conference; 2016

RIAO (Red Iberoamericana de Óptica

In this work we introduce a complete Quantum Key Distribution system that implements a decoy-state protocol. Qubits are coded in the polarization state of weak optical pulses, produced by infrared LEDs. The link between Transmission (Alice) and Reception (Bob) stages is open air, which is the preferred channel that allows to establish quantum communications through low-orbit satellites. Both terminals are placed on different rooms, separated by approximately 8 meters. In order to compensate eventual misalignments that can be produced by small mechanic displacements or atmospheric thermal gradients, the system also implements a closed-loop for automatic alignment based on a counter-propagating laser, a servo-actuated mirror and a CMOS image sensor. The implementation of decoy states guarantees the security of the protocol against Photon Number Splitting (PNS) attacks, which can occur due to the statistical nature of currently available single photon sources. Signal and decoy states are obtained driving the LEDs with electronic pulses with different intensities.The apparatus runs as a whole in an autonomous way, and in the above describe conditions, it is able to generate raw cryptographic key at a rate of 185 bits/s, with a QBER of 6.15%. This value guarantees the generation of secure key by privacy amplification; other types of post-processing such as reconciliation and error correction can lower the bit error rate further down to 1% at the cost of reducing the key rate.