  We study the standard generic quantum computer model, which describes a
realistic isolated quantum computer with fluctuations in individual qubit
energies and residual short-range inter-qubit couplings. It is shown that in
the limit where the fluctuations and couplings are small compared to one-qubit
energy spacing the spectrum has a band structure and a renormalized Hamiltonian
is obtained which describes the eigenstate properties inside one band. The
studies are concentrated on the central band of the computer (``core'') with
the highest density of states. We show that above a critical inter-qubit
coupling strength, quantum chaos sets in, leading to quantum ergodicity of the
computer eigenstates. In this regime the ideal qubit structure disappears, the
eigenstates become complex and the operability of the computer is quickly
destroyed. We confirm that the quantum chaos border decreases only linearly
with the number of qubits n, although the spacing between multi-qubit states
drops exponentially with n. The investigation of time-evolution in the quantum
computer shows that in the quantum chaos regime, an ideal (noninteracting)
state quickly disappears and exponentially many states become mixed after a
short chaotic time scale for which the dependence on system parameters is
determined. Below the quantum chaos border an ideal state can survive for long
times and be used for computation. The results show that a broad parameter
region does exist where the efficient operation of a quantum computer is
possible.
