  It has been shown (Arxiv: quant-ph/0507236) that a universal quantum computer
could be powerful enough to solve efficiently the quantum search problem, and
the reversible and unitary halting protocol based on the state-locking pulse
field is the key component to construct the efficient quantum search processes,
while the state-locking pulse field is the key component to generate the
reversible and unitary halting protocol. In this paper the reversible and
unitary halting protocol and the generalized state-locking pulse field have
been extensively investigated theoretically. The basic principles to construct
the state-locking pulse field and design the reversible and unitary halting
protocol are studied in detail. A generalized state-locking pulse field is
generally dependent upon the time and space variables. It could be a sequence
of time- and space-dependent electromagnetic pulse fields and could also
contain the time- and space-dependent potential fields. Thus, the reversible
and unitary halting protocol built up out of the state-locking pulse field
generally consists of a sequence of time- and space-dependent unitary evolution
processes. It is shown how the quantum control process is constructed to
simulate efficiently the reversible and unitary halting protocol. An improved
subspace-reduction quantum program and circuit based on the reversible and
unitary halting protocol is proposed as the key component to construct further
an efficient quantum search process. A simple atomic physical system that is an
atomic ion or a neutral atom in the double-well potential field is proposed to
show how the state-locking pulse field is generated and how to implement the
reversible and unitary halting protocol.
