t b a b a

Up to date, there have been several serious proposals for quantum computer physical
implementation and they can be classified into six basic categories:
1. Nuclear Magnetic Resonance (NMR): The nuclear magnetic resonance (NMR)
was proposed by David Cory [CFH96] and by Neil Gershenfeld and Isaac Cuang [GC97]
as a working concept and a very attractive candidate for a quantum computer due to
the extremely long coherence times associated with the nuclear spins. A qubit can be
implemented using any quantum mechanical system with two basis states. In NMR
computers the two spin states of a spin-1/2 atomic nucleus in a magnetic field represent
the basis states of a qubit. The qubits are atomic nuclei within the same molecule.
Different atomic nuclei in a molecule can be distinguished and manipulated using
the nuclear magnetic resonance technique. So a molecule can be used as a quantum
computer. Simple logic gates which only affect a single qubit are easily implemented
using radio frequency fields. These interact strongly with nuclear spins, allowing them
to be controlled with great precision. Some quantum algorithms, such as a simple
version of Grover’s quantum search algorithm (N ≤ 10), have been implemented on
NMR quantum computers.
2. Ion Traps: The ion trap is the first technology proposed for quantum information
processing devices [CZ95]. Ions, or charged atomic particles, can be confined and sus-
pended in free space using electromagnetic fields. Qubits are stored in stable electronic
states of each ion, and quantum information is processed and transferred through the
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