Nanocomputing - The qubit

A qubit is a bit of information represented by a quantum object, such as a single atom, ion, or photon. Just as a bit is the basic unit of information in a classical information system, a qubit is the basic unit of information of a quantum information system. The power of a qubit is that it is not limited to a value of 0 or 1: it can be in a superposition state of any combination of 0 or 1. This is the difference between a classical and a quantum computer, which theoretically allows quantum computers to be much more efficient at solving certain types of problems. This is what we get from http://p23lanl.gov/Quantum/qubit.html.

Other researchers say that a crucial question determining whether a quantum computer could be built or how efficiently it would work is how quickly the wavefunctions of the qubits lose their quantum-mechanical coherence. Quantum decoherence, the collapse of the quantum superposition mentioned above, was first measured in 1996 by the French scientist Serge Haroche (and co-workers) of the École Normal Superieur in Paris. They sent individual rubidium atoms - each one of them in a superposition of two states - through a cavity containing a microwave field. In the experiment, each of the two quantum states shifts the phase of the microwave field by a different amount - so the field falls into a superposition of two states. While the cavity field exchanges energy with its surroundings, the superposition can collapse into a single state. They stated this decoherence by measuring correlations between the energy levels of pairs of atoms sent through the cavity with various time delays between the atoms. The ENS team discovered that decoherence, says a note from the American Institute of Physics Bulletin of Physics News, "proceeds at a faster rate with time, and also when the differences between the two phase shifts are increased and therefore made more distinguishable from one another". This may be the reason why Chuang stated that quantum computation will take some time to be available for all.

Now we can begin to evaluate the importance of that first quantum computation described here earlier (read here: "Nanocomputing -For Real"). The impact on cryptography will be immense, demanding a whole new system of encoding.

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