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Practical quantum key distribution with polarization entangled photons

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We present an entangled-state quantum cryptography system that operated for the first time in a real-world application scenario. The full key generation protocol was performed in real-time between two distributed embedded hardware devices, which were connected by 1.45 km of optical fiber, installed for this experiment in the Vienna sewage system. The generated quantum key was immediately handed over and used by a secure communication application.

©2004 Optical Society of America

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Figures (3)

Fig. 1.
Fig. 1. A quantum cryptography system is installed between the headquarters of a large bank (Alice) and the Vienna City Hall (Bob). The beeline distance between the two buildings is about 650m. The optical fibers were installed some weeks before the experiment in the Vienna sewage system and have a total length of 1.45 km.
Fig. 2.
Fig. 2. Sketch of the experimental setup. At the entangled state source a nonlinear BBO-crystal is pumped by a violet laser diode (LD) at 405nm and produces polarization-entangled photon pairs. Walk-off effects are compensated by the half-wave-plate (HWP) and the compensation crystals (BBO/2). One of the photons is locally analyzed in Alice’s detection module, while the other is sent over a 1.45 km long single-mode optical fiber (SMF) to the remote site (Bob). Polarization measurement is done in one of two bases (0° and 45°), by using a beam splitter (BS) which randomly sends incident photons to one of two polarizing beam splitters (PBS). One of the PBS is defined for measurement in the 0° basis, and the other in the 45° basis as the half wave plate (HWP) rotates the polarization by 45°. The final detection of the photons is done by passively quenched silicon avalanche photodiodes (APD). Once a photon is detected at one of Alice’s four avalanche photodiodes an optical trigger pulse is created (Sync. Laser) and sent over a standard telecommunication fiber to Bob to establish a common time basis. At both sites, the trigger pulses and the detection events from the APDs are fed into a dedicated quantum key generation device (QKD Electronics) for further processing. This QKD electronic is an embedded system, which is capable of autonomously running the classical protocol necessary for key generation via a standard TCP/IP connection.
Fig. 3.
Fig. 3. Results obtained during 18 minutes of the running experiment. That time was used to acquire 100 blocks of raw data that each consist of approximately 2500 bits after sifting. The blocks in this graph have been sorted by the estimated QBER and are not represented in the order of their acquisition. Each key block was further processed by the full quantum cryptography software. (a) Estimated QBER for the individual blocks and the real QBER determined by directly comparing the sifted key of each data block. This calculation was only done for evaluation of the system and is obviously not possible for a real key exchange. Additionally one can see the time it took to acquire the raw data of the given block. (b) The length of the final key, the number of bits disclosed by CASCADE and the number of bits discarded in privacy amplification. (c) The final secure bit rate produced by our system.


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