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  • Quantum Cryptography Used to Transfer Funds

Photonics Spectra
Jun 2004
Hank Hogan

Researchers at Universität Wien and the Austrian Academy of Sciences in Vienna, Austria, the Austrian Research Center in Seibersdorf and Ludwig Maximilians Universität in Munich, Germany, have demonstrated a new way to put valuables under lock and key. The group recently reported the transfer of €3000 over a 1.45-km link between Vienna City Hall and the headquarters of Bank-Austria Creditanstalt using quantum cryptography.

To accomplish this, the scientists generated a randomized encryption key via pairs of entangled photons. In doing so, they produced a code key immune to eavesdropping.

"In a sense, the key comes into existence at both sides simultaneously," explained Anton Zeilinger, a professor of physics at Universität Wien. "It does not have to be transported from A to B anymore."

Quantum Cryptography Used to Transfer Funds

A code key generated via quantum key distribution has been used to ensure a completely secure wire transfer. The setup relies on the production of entangled photon pairs by spontaneous parametric down-conversion in a nonlinear crystal.

In their demonstration, the scientists used a 16-mW, 405-nm diode laser to pump a nonlinear crystal. This produced 8200 polarization entangled pairs of 810-nm photons per second. The investigators generated the ones and zeros of the code key by observing the polarization of the entangled photons. The results, Zeilinger said, were used to encode the account numbers, amount of money and purpose of the transfer. One-time-use random keys have been proved theoretically to be absolutely secure.

Environmental stress

Because of the entanglement, the polarization of the two photons in a given pair matched. Quantum physics predicts that changing the state of one entangled photon instantly changes the state of the other -- no matter what the distance is between the two. In practice, environmental interactions destroy entanglement, given enough time.

In their demonstration, the researchers separated the pairs of photons by sending one down 1.45 km of fiber optic cable strung through Vienna's sewer system. This real-world environmental stress, they found, didn't damage the entanglement. The roundabout underground route resulted in one photon pair sitting at the bank while the other was at city hall.

At both ends, the scientists placed a prototype quantum key encryption device, which consisted of a detector and electronics. One side produced the entangled photon pairs and also had a synchronization controller. When an entangled photon was detected at that location, the synchronization controller fired a 1550-nm laser pulse to the other. The device at the other end then measured the pulse and photon, ensuring that the two devices were reading the polarization of the same entangled pair.

From strings of photon pairs, the researchers created encryption keys out of the equivalent series of ones and zeros. The system generated keys at a rate of about 80 bits per second, making it possible to distribute a randomized and secure key many times a minute. Such a key was used to encode the wire transfer.

Quantum cryptography systems that depend on attenuated laser pulses to simulate single-photon sources are commercially available, but they can produce two photons per pulse, making them prone to eavesdropping through beamsplitting. That's not the case with the entangled-photon approach. Partly as a result of this advantage, Zeilinger predicts that entanglement-based cryptography systems will be the technology of the future.

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