Monday, November 07, 2005

Some Limitations of Current Quantum Devices

An extract on some of the limitations of current devices as well as the problems which need to be solved in the future.

"The technology behind quantum cryptography exists today. In fact, you can purchase your own quantum cryptography system today from a company called MagiQ for a mere $50,000. The current problem that prevents quantum cryptography from being implemented on a large scale is rooted in the same laws of quantum mechanics that make it an unbreakable cryptosystem in the first place. Eve's detectors alter the polarization of the light waves that Alice and Bob are using to communicate. In reality, many factors besides Eve exist that could alter the polarization of the light waves. Even simple physical occurrences like water vapor in the air could alter the polarizations. For this reason, it is very difficult to transmit the light over long distances. A year and a half ago, MagiQ announced that its commercially available quantum cryptosystem could transmit messages "an unprecedented 120 km distance." That's a far cry from the many thousands of miles that would need to be traversed between, say, Washington DC and Moscow. Over distances that long, the curvature of the earth becomes a limiting factor. Currently, a straight path is required to limit unwanted changes in polarization.

An even more severe restriction on the power of quantum cryptography also arises from the need to eliminate unwanted changes in polarization. All of the quantum cryptography systems in existence today require a direct connection between two machines in order to transmit data. This is because there is no currently known way to develop "quantum routers" or "quantum gateways" analogous to the routers and gateways of the Internet. The act of routing a polarized light wave from one line to another would alter its polarization. For this reason, quantum encryption might never come into personal use - there is no way that one person's home computer could be directly connected to every Internet server that the person could ever be interested in. Commerce over the Internet may continue to be based on RSA and similar cryptosystems far into the future, even if or when a quantum computer was built to crack RSA. Quantum cryptography might become a tool solely of governments and large banking institutions."


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