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The news is a bit old, about 2 months, but is very interesting: A group of scientists in Austria and Germany has installed an optical fibre quantum cryptography system under the streets of Vienna and used it to perform the first quantum secure bank wire transfer (A Poppe et al. 2004 Optics Express 12 3865).

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 kilometres of single-mode optical fibre.

Andreas Poppe at the University of Vienna and colleagues at ARC Seibersdorf Research in Austria and Ludwig-Maximillians University in Germany 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. Because they are entangled, measuring the polarization of one photon determines the polarization of the other.

At Alice, a 405-nm laser diode pumps a nonlinear crystal to produce entangled photon pairs with a wavelength of 810 nm using the process of “down-conversion”. One of the photons is locally analysed in Alice’s detection module, while the other is sent over the 1.45 kilometre link to the remote site (Bob). It took about 30 seconds to transfer the photons need to establish a secure “key” for the transaction.

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