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.

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. “The exposure of the fibres to realistic environmental conditions such as stress and strain during installation, as well as temperature changes were important features of this experiment,” says Poppe. “The successful operation of the system shows that it works in a realistic quantum cryptography scenario.”

The Vienna team has also recently demonstrated quantum teleportation over a distance of 600 metres along an optical fibre that runs beneath the Danube (R Ursin et al. 2004 Nature 430 849).