We study a generic model of quantum computer, composed of many qubits coupled
by short-range interaction. Above a critical interqubit coupling strength,
quantum chaos sets in, leading to quantum ergodicity of the computer
eigenstates. In this regime the noninteracting qubit structure disappears, the
eigenstates become complex and the operability of the computer is destroyed.
Despite the fact that the spacing between multi-qubit states drops
exponentially with the number of qubits n, we show that the quantum chaos
border decreases only linearly with n. This opens a broad parameter region
where the efficient operation of a quantum computer remains possible.Comment: revtex, 4 pages, 5 figures, more details and data adde
