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Sewers host cryptography system

19 Aug 2004

The first bank transfer performed using quantum cryptography based on entangled photons takes place in Vienna's sewers.

A quantum cryptography system is the last thing you would expect to find in a sewage system. But under the streets of Vienna in Austria, a group of scientists has installed optical fiber and performed a quantum secure bank wire transfer. (Optics Express 12 3865)

"The exposure of the fibers to realistic environmental conditions such as stress and strain during installation, as well as temperature changes were important features of this experiment," Andreas Poppe and his colleagues report in their paper. "The successful operation of the system shows that it works in a realistic quantum cryptography scenario."

The quantum cryptography system consisted of a transmitter (Alice) at Vienna's City Hall and a receiver (Bob) at the headquarters of an Austrian bank. The sites were linked by 1.45 km of singlemode optical fiber.

The team used an entangled-state quantum cryptography system that relies on entangled photon pairs. Entangled photons are unpolarized while they travel and only assume a polarization state when measured. Due to their entangled nature, a measurement of the polarization of one photon determines the polarization of the other.

At Alice, a 405 nm laser diode pumps a nonlinear BBO crystal which produces entangled photon pairs at 810 nm using type II downconversion. One of the photons is locally analyzed in Alice's detection module (four silicon avalanche photodiodes), while the other is sent over the 1.45 km link to the remote site (Bob).

"There was also a synchronization signal indicating every sent photon," Poppe told Optics.org. "The exchange of needed photons was about 30 seconds and then the key was ready."

Poppe and colleagues are now working on a 1550 nm system. "We used a lossy wavelength of 810 nm where we had 6 dB overall loss from the transmission and the connectors," he said. "We also want to put the overall system in a 19-inch rack, at the moment is a little bit too large."

According to the authors, one advantage of their system is that the key comes into existence at both Alice and Bob and does not have to be transferred between the two. With the key safely in hand, the team was able to securely wire money from the City Hall to the bank.

This work was carried out by scientists from the University of Vienna and ARC Seibersdorf Research in Austria and Ludwig-Maximillians University, Germany. The fibers were installed by WKA.

Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.

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