Low-capacitance Josephson junctions, where Cooper pairs tunnel coherently while Coulomb blockade effects allow the control of the total charge, provide physical realizations of quantum bits (qubits), with logical states differing by one Cooper-pair charge on an island. The single-and two-bit operations required for quantum computation can be performed by applying a sequence of gate voltages. A basic design, described earlier [1], is sufficient to demonstrate the principles, but requires a high precision time control, and residual two-bit interactions introduce errors. Here we suggest a new nano-electronic design, close to ideal, where the Josephson junctions are replaced by controllable SQUIDs. This relaxes the requirements on the time control and system parameters substantially, and the two-bit coupling can be switched exactly between zero and a non-zero value for arbitrary pairs. The phase coherence time is sufficiently long to allow a series of operations.A quantum computer can perform certain tasks which no classical computer is able to do in acceptable times [2][3][4][5]. It is composed of a (large) number of coupled two-state quantum systems forming qubits; the computation is the quantum-coherent time evolution of the state of the system described by unitary transformations which are controlled by the program. Elementary steps are (i) the preparation of the initial state of the qubits, (ii) single-bit operations (gates), i.e. unitary transformation of individual qubit states, triggered by a modification of the corresponding one-qubit Hamiltonian for some period of time, (iii) two-bit gates, which require controlled inter-qubit couplings, and (iv) the measurement of the final quantum state of the system. The phase coherence time has to be long enough to allow a large number of these coherent processes. Ideally, in the idle period between the operations the Hamiltonian of the system is zero to avoid further time evolution of the states.Several physical realizations of quantum information systems have been suggested. Ions in a trap, manipulated by laser irradiation are the best studied system. However, alternatives need to be explored, in particular those which are more easily embedded in an electronic circuit as well as scaled up to large numbers of qubits. From this point of view mesoscopic and nano-electronic devices appear particularly promising [1,[6][7][8][9]. Normal-metal singleelectron devices are discussed in connection with classical digital applications and, in fact, constitute the ultimate electronic memory [10]. However, their use for quantum computation is ruled out, since, due to the large number of electron states involved, different tunneling processes are incoherent. Ultra-small quantum dots with discrete levels are candidates for qubits, but their strong coupling to the environment renders their phase coherence time short. More promising are systems built from Josephson junctions, where the coherence of the superconducting state can be exploited. Quantum extension of elements based on a sin...