  The quantum adder is an essential attribute of a quantum computer, just as
classical adder is needed for operation of a digital computer. We model the
quantum full adder as a realistic complex algorithm on a large number of qubits
in an Ising-spin quantum computer. Our results are an important step toward
effective modeling of the quantum modular adder which is needed for Shor's and
other quantum algorithms. Our full adder has the following features: (i) The
near-resonant transitions with small detunings are completely suppressed, which
allows us to decrease errors by several orders of magnitude and to model a
1000-qubit full adder. (We add a 1000-bit number using 2001 spins.) (ii) We
construct the full adder gates directly as sequences of radio-frequency pulses,
rather than breaking them down into generalized logical gates, such as
Control-Not and one qubit gates. This substantially reduces the number of
pulses needed to implement the full adder. [The maximum number of pulses
required to add one bit (F-gate) is 15]. (iii) Full adder is realized in a
homogeneous spin chain. (iv) The phase error is minimized: the F-gates generate
approximately the same phase for different states of the superposition. (v)
Modeling of the full adder is performed using quantum maps instead of
differential equations. This allows us to reduce the calculation time to a
reasonable value.
