Monday, March 22, 2004		Feature

...and qubits will secure data
Pratibha Sharma

ONE of the fundamental problems in cryptography is how to get the encryption key (or password) to the receiver, secretly. There is no way to guarantee that the 'snooper' does not read the key in transit. One way to resolve is Public Key Cryptograph, which involves a shared public key that the whole world knows, and a hidden private key that only you can see. With quantum computers, it will be possible to crack even the larger keys. It will also be possible to infer the private key for a given public key. Another fundamental and counter intuitive property of quantum particles is the slipperiness. Measuring either the momentum or the location of a particle, but not both will alter the other. This means if Mr. A sends one quantum particle to Mr B in a known state and the snooper attempts to read that state, he will alter the particle in some measurable way.

The state of the art for key distribution is Quantum Key Distribution (QKD). QKD involves transmitting the bits of a key as qubits represented by quantum particles. Photons are the choice in current commercial products, since they can easily be carried over familiar media such as fibre optic cables. After all, a photon is just light. Magic and Quantique are pioneering companies with commercial QKD technology available now. Both claim to have been deployed in large companies and defence installations. Both are currently limited to tightly controlled fibre optics installations with a under 60 km range. British Researchers have managed to transmit usable photons across 23 km of open air and hope to eventually be able to bounce them off satellites hundreds of kilometres up for global coverage.

While there is a significant progress being made in the field of Quantum computing, researchers are still grappling with some basic problems in the fabrication of actual computers. One of the most difficult problems is that of decoherance. For a Qubit to function usefully it should be completely isolated from its surroundings or else the environmental noise will cause the decay of quantum state. As yet qubits have been maintained only for fractions of a second at a time. But this situation is improving steadily.

The second major problem is of fabrication. As quantum scales, light cannot be used to etch the control structures required. Electrobeam lithography has been used to create quantum structures on conventional semiconductors. This is a significant breakthrough, because it demonstrates the integration of quantum computing technology with existing semiconductor-based systems.