Solid-state quantum computer architectures with qubits encoded using single atoms are now feasible given recent advances in the atomic doping of semiconductors. Here we present a charge qubit consisting of two dopant atoms in a semiconductor crystal, one of which is singly ionized. Surface electrodes control the qubit and a radio-frequency single-electron transistor provides fast readout. The calculated single gate times, of order 50 ps or less, are much shorter than the expected decoherence time. We propose universal one-and two-qubit gate operations for this system and discuss prospects for fabrication and scale up. DOI: 10.1103/PhysRevB.69.113301 PACS number͑s͒: 03.67.Lx, 73.21.Ϫb, 85.40.Ry In the search for an inherently scalable quantum computer ͑QC͒ technology solid-state systems are of great interest. One of the most advanced proposals is based on superconducting qubits, 1 where coherent control of qubits has been demonstrated and decoherence times measured.2 The Kane scheme, 3 in which qubits are defined by nuclear spin states of buried phosphorus dopants in a silicon crystal, has also attracted considerable attention due to its promise of very long ͑ms or longer͒ decoherence times below 1 K. Recent advances in single-dopant fabrication, 4 -6 together with the demonstration of fast single-electron transistor ͑SET͒ charge detection, 7,8 bring the Kane Si:P architecture closer to reality. These important results notwithstanding, the demonstration of single-spin readout remains a major challenge. Here we consider a Si:P dopant-based qubit in which the logical information is encoded on the charge degrees of freedom. This system, which is complementary to the Kane concept, is not dependent on single-spin readout and, given the present availability of fabrication 4 -6 and readout 7,8 technologies, can now be built. A two-qubit gate based on the charge qubit scheme we describe will enable an experimental determination to be made of the key sources of decoherence and error in a nanoscale silicon QC architecture. Such devices therefore provide an important and necessary pathway towards the longer term goal of real-spin Si:P devices.Semiconductor quantum-dot charge-based qubits were first considered in 1995 by Barenco et al.,9 where quantum information was encoded in excitation levels, and later by Fedichkin et al. 10 for position-based charge qubits in GaAs. Very recently, coherent oscillations have been observed 11 in a GaAs double quantum dot providing realization of a chargebased qubit with coherence times above 1 ns, accessible by existing fast pulse technology. In this paper we assess the potential of Si:P donor-based charge qubits by calculating the energetics and gate operation times for realistic device configurations and gate potentials and find that both one-and two-qubit operations times are well within the relevant decoherence times for the system.The buried donor charge qubit is shown in Fig. 1 for the case of P dopants in Si, although a number of other dopantsubstrate systems could also be consi...

The 2D quantum system of electrons at a GaAs/GaAlAs heterojunction in high magnetic field at low temperature is shown to exhibit conduction typical of pinned charge-density waves. Crossover from Ohmic conduction occurs on the same boundary at which radio-frequency resonances signal the onset of transverse elasticity. A further small non-Ohmic region is isolated from the main area by a v-j quantum-Hall-effect phase. The relationship found between the threshold conduction field and the resonance frequency is well accounted for by a model of a pinned electron crystal.