t b a b a

Different methods of implementing quantum computers have been proposed and many
exciting achievements have been made, such as using photons, trapped ions or neutral atoms,
cavity QED and nuclear magnetic resonance. At this time it is very difficult to assess in full
the relative merits and potential of each of these approaches. Thus no one can tell which
one will be the ”winning” technology in the coming future.
Up to date, small quantum computing devices became reality, quantum cryptography
has been demonstrated and is even at the level of real-world applications. However, some
potentially large obstacles still remain that prevent us from making full operational quantum
computers. Among these difficulties, error correction, decoherence, and hardware architec-
ture are probably the most formidable. The experimental physics involved is very demanding.
It still remains a great challenge to physicists and computer engineers to develop such full
quantum computers in a near future.
2.2 Basic Concepts in Quantum Mechanics
Although classical computers have become more compact and considerably faster in per-
forming their task, the task remains the same: to manipulate and interpret an encoding of
binary bits into a desired computational result. In classical computer, a bit is the fundamen-
tal unit of information, 0 or 1. In a quantum computer, the fundamental unit of information
(called a quantum bit or qubit) is not binary but rather more n-ary in nature. A qubit can
exist in a state corresponding to the logical state 0 or 1 as in a classical bit, or it can be
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