t b a b a

2.4 Physical Implementations of Quantum Computer
Implementations of quantum computers will be a difficult experimental challenge. Several
physical implementations were proposed and at this time it is very difficult to assess in full
the relative merits and potential of each of these approaches. Significant progress is reported
in each of these areas, but a real breakthrough has not been achieved in any of them.
David DiVincenzo(IBM) criteria gave the five (plus two) essential characteristics of the
physical implementations of quantum computation and information processing [Div00]:
• A physical system has well characterized qubits and is scalable. The embodiment of
a qubit is a quantum two-level system, such as the two spin states of a spin 1/2
particle, or the ground and some excited state of an atom, or the vertical and horizontal
polarization of a single photon. A “well characterized” qubit should have accurately
known physical parameters, including: the internal Hamiltonian (which determines the
energy eigenstates of the qubit); the presence of and couplings to other states of the
qubit; the interactions with other qubits; and the couplings to external fields that might
be used to manipulate the qubit. If the qubit has a third, fourth, and other higher
energy levels, the probability of the transitions to such states from the characteristic
states must be very low and under the control of the physical system.
• The ability to initialize the state of the qubits to a simple state, such as | 000...〉. As in
classical computing, the registers must be initialized to a known value before the start of
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