The Curious Case Of The Quantum Cardinal
Published: 13 Jul 2005 18:15 BST
Quantum mechanics (QM) is one of modern physics' best theories. Since it was established in a blaze of intellectual discovery during the first half of the last century, it has proved itself able to predict and describe a huge range of phenomena. Although its influence on IT is less than might be imagined — most semiconductor theory would work if expressed in older ways — we are rapidly advancing into technologies where QM is essential.
Of late, there are plenty of advances to report, some of them already out of the labs and in commercial products. Quantum encryption — where any attempt to copy data is impossible to hide — has already reached the second generation of products from companies such as id Quantique and MagiQ Technologies, with established names like NEC and Toshiba also demonstrating successful systems working on a wide range of data types.
Quantum encryption works thanks to Heisenberg's Uncertainty Principle, which declares rigidly defined areas of doubt and uncertainty — the better you measure one aspect of a quantum object, the less you can know about one other. Photons of light have orientation, the waves of light either going up and down or slanted at plus or minus 45 degrees — and data can be encoded in them by simply deciding that up is one and down is zero, or similarly with left and right slants.
But, says Heisenberg, if you measure a photon looking for whether it's up or down, you'll get a result — but you won't be able to tell from that measurement whether that photon really was up or down or whether it was in fact slanted. Some of your results will be accurate, and some wrong. Quantum encryption establishes a second channel between the sender and receiver, so as well as encoding the data on the photons on the first stream the sender also tells the receiver in an independent message which ones are slanted and which ones up/down — but not what the data in them was.
Thus, the receiver knows which of its incoming photons have been correctly received and can discard those which were wrong. Any attempt to monitor the traffic streams — which means the photons must be destroyed and regenerated — will create a recombined data stream where the values are right but the polarisation orientation is randomly wrong. Statistical analysis of the data stream by the intended recipient will show a characteristic divergence, and the data can be discarded as untrustworthy.
In practice, this method is used to distribute keys with the receiver telling the sender what photons were correctly received, thus synchronising the keys at both ends. A truly secure key distribution system removes the reliance on the security of an encryption algorithm, as once you can rely on your keys you can use the simplest of mathematics of combine key and data in an unbreakable way. This all works, is already deployed for financial and government users, and is being actively developed to work over long distances and over wireless. Radio signals are photons too.
Quantum computing is further away from real life, however. It uses superposition, a related aspect of QM, which states that before you measure an aspect of a quantum system that system exists in all possible states simultaneously. Each state is superimposed on the others. When you measure the state, the other possibilities vanish — the state is said to collapse — but until that point, there are multiple parallel realities. By maintaining a situation where those multiple parallel realities can interact with each other, you can simultaneously generate all possible outcomes. Encode an algorithm into the quantum system, and instead of having to laboriously work through all the possibilities in turn you create all the answers at once. When you look at the system, it collapses and you're left with a result which is probably right. Do it enough times, and you can be sure.
There are plenty of problems here, with perhaps the biggest one being the small detail that if the system interacts with anything else during the process it will prematurely collapse, a process known as decoherence. Nevertheless, quantum computing has been demonstrated — most famously when IBM implemented a small experiment that finds the factors of a number through a process called Shor's Algorithm. This has profound implications for classic encryption, which relies heavily on such factorisation being impracticable for large numbers, but also opens up many fascinating areas for mathematical analysis of systems where the numbers are beyond sequential searching. This stuff works.
So why would this upset the Roman Catholic church?
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6 comments
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There are two things I would like to see disappear... John Loperson -
your article says scientists believe that all meas... RCA
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So nice of RCA to illustrate.
I try hard to... John Loperson -
the heart of scientific progress is to be sce... peter cicman
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Although I don’t accept the lack of some involveme... Louis
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Only religion claims to have 'absolute' answe... Jon
