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European Quantum Electronics Conference (EQEC)

Practical quantum key distribution with polarization entangled photons

Alessandro Fedrizzi, Andreas Poppe, Rupert Ursin, Thomas Lorünser, Momtchil Peev, T. Langer, Anton Zeilinger

Published Jan 1, 2005• 21 citations
Publisher

Abstract

The paper reports on the entangled-state quantum cryptography system practical operation. The full key generation protocol was performed in real-time over a fiber bundle of 1.45 km length, installed for this experiment in the Vienna sewage system. The generated quantum key was immediately handed over and used by a secure communication application of order for remittance.

DOI
10.1109/eqec.2005.1567469

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The paper reports on the entangled-state quantum cryptography system practical operation. The full key generation protocol was performed in real-time over a fiber bundle of 1.45 km length, installed for this experiment in the Vienna sewage system. The generated quantum key was immediately handed over and used by a secure communication application of order for remittance.
The secure distribution of the secret random bit sequences known as {open_quotes}key{close_quotes} material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single-photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over {open_quotes}open{close_quotes} multi-km node-to-node optical fiber communications links is possible. We have also constructed a quantum key distribution system for free-space, line-of-sight transmission using single-photon polarization states, which is currently undergoing laboratory testing. 7 figs.
The secure distribution of the secret random bit sequences known as `key' material, is an essential precursor to their use for the encryption and decryption of confidential communications. Quantum cryptography is an emerging technology for secure key distribution with single-photon transmissions: Heisenburg's uncertainty principle ensures that an adversary can neither successfully tap the key transmissions, nor evade detection (eavesdropping raises the key error rate above a threshold value). We have developed experimental quantum cryptography systems based on the transmission of non-orthogonal single-photon states to generate shared key material over multi-kilometer optical fiber paths and over line-of-sight links. In both cases, key material is built up using the transmission of a single- photon per bit of an initial secret random sequence. A quantum-mechanically random subset of this sequence is identified, becoming the key material after a data reconciliation stage with the sender. In our optical fiber experiment we have performed quantum key distribution over 24-km of underground optical fiber using single-photon interference states, demonstrating that secure, real-time key generation over `open' multi-km node-to-node optical fiber communications links is possible.
Entanglement-based quantum cryptography has the appealing advantage of intuitively more evident security. While originally, weak laser pulse schemes were implemented earlier as technologically simpler, it is now possible to build entanglement-based quantum key distribution systems on a technically equally advanced level. The existing polarization-based systems as developed in Vienna now cover distances of the order of 100 km in fiber and of 144 km in free space. In a recent fiber experiment, an asymmetric source is used such that one photon at the 1.550 nm telecom wavelength is transmitted to Bob, while the other photon at 810 nm is locally measured by Alice. It turns out that polarization entanglement is rather robust, certainly over distances of 100 km in fibers. In a recent long-distance free-space experiment, one photon was sent over 144 km from the Canary Island of La Palma to the island of Tenerife, while again the other photon was measured locally. The receiving station uses the OGS telescope operated by the European Space Agency ESA. This experiment opens up the possibility for future quantum key distribution using satellites.
In this paper we present our work demonstrating the transfer of quantum cryptographic information over a 78 km optical fiber, as a base for future long distance public quantum links, using a commercial quantum key distribution (QKD) system based on the BBM92 protocol. The system communicates with pairs of polarization-entangled photons generated by the nonlinear optical process of spontaneous parametric down-conversion. The system was put to the test in the laboratory using optical fiber of different lengths up to 112 km and with different link losses up to 29 dB. The QKD system was implemented on single mode standard optical fiber link of 78 km length deployed between the two cities of Braunschweig and Hannover. Characterizations and optimizations of the QKD system and the fiber link have been carried out in order to improve the synchronization, to reduce the quantum bit error rate, and to successfully establish the intercity quantum communication.
We present in this paper a quantum key distribution (QKD) system based on polarization entanglement for use in telecom fibers. A QKD exchange up to 50 km was demonstrated in the laboratory with a secure key rate of 550 bits s −1 . The system is compact and portable with a fully automated start-up, and stabilization modules for polarization, synchronization and photon coupling allow hands-off operation. Stable and reliable key exchange in a deployed optical fiber of 16 km length was demonstrated. In this fiber network, we achieved over 2 weeks an automatic key generation with an average key rate of 2000 bits s −1 without manual intervention. During this period, the system had an average entanglement visibility of 93%, highlighting the technical level and stability achieved for entanglement-based quantum cryptography.