Robust two-qubit gates for donors in silicon controlled by hyperfine interactions

We present two strategies for performing two-qubit operations on the electron spins of an exchange-coupled pair of phosphorus donors in silicon, using the ability to set the donor nuclear spins in arbitrary states. The effective magnetic detuning of the two electron qubits is provided by the hyperfine interaction when the $^{31}$P nuclei are prepared in opposite spin states. This can be exploited to switch on and off SWAP operations with modest tuning of the electron exchange interaction. Furthermore, the hyperfine detuning enables high-fidelity conditional rotation gates based on selective resonant excitation. The latter requires no dynamic tuning of the exchange interaction at all, and offers a very attractive scheme to implement two-qubit logic gates under realistic experimental conditions.Comment: 8 pages, 3 figure

Radio frequency readout of electrically detected magnetic resonance in phosphorus-doped silicon MOSFETs

We demonstrate radio frequency (RF) readout of electrically detected magnetic resonance in phosphorus-doped silicon metal-oxide field-effecttransistors (MOSFETs), operated at liquid helium temperatures. For the first time, the Si:P hyperfine lines have been observed using radio frequency reflectometry, which is promising for high-bandwidth operation and possibly time-resolved detection of spin resonance in donor-based semiconductor devices. Here we present the effect of microwave (MW) power and MOSFET biasing conditions on the EDMR signals.Comment: 2 pages, 3 figure

Transport of Spin Qubits with Donor Chains under Realistic Experimental Conditions

The ability to transport quantum information across some distance can facilitate the design and operation of a quantum processor. One-dimensional spin chains provide a compact platform to realize scalable spin transport for a solid-state quantum computer. Here, we model odd-sized donor chains in silicon under a range of experimental non-idealities, including variability of donor position within the chain. We show that the tolerance against donor placement inaccuracies is greatly improved by operating the spin chain in a mode where the electrons are confined at the Si-SiO$_2$ interface. We then estimate the required timescales and exchange couplings, and the level of noise that can be tolerated to achieve high fidelity transport. We also propose a protocol to calibrate and initialize the chain, thereby providing a complete guideline for realizing a functional donor chain and utilizing it for spin transport.Comment: 19 pages, 12 figure

Estrategias hacia la sostenibilidad en el Mercosur

Se analiza brevemente el proceso de avance de la economía ecológica en el marco de la conformación de sociedades regionales en América Latina. La creación de la Sociedad Brasileña de Economía Ecológica, la Asociación Argentino Uruguaya de Economía Ecológica y la Red Iberoamericana de Economía Ecológica. Asimismo, se presenta información sobre las Segundas Jornadas de Economía Ecológica, desarrolladas durante el año 2004 y el enfoque de algunos de los documentos expuestos en la reunión. Finalmente, se realiza un análisis a nivel regional de la situación de la biodiversidad en el plano del Mercosur, las amenazas y las políticas alternativas necesarias para un ordenamiento territorial bajo un paraguas de desarrollo sostenible con base local

Pair-wise decoherence in coupled spin qubit networks

Experiments involving phase coherent dynamics of networks of spins, such as echo experiments, will only work if decoherence can be suppressed. We show here, by analyzing the particular example of a crystalline network of Fe8 molecules, that most decoherence typically comes from pairwise interactions (particularly dipolar interactions) between the spins, which cause `correlated errors'. However at very low T these are strongly suppressed. These results have important implications for the design of quantum information processing systems using electronic spins.Comment: 4 pages, 4 figures. Final PRL versio

Estrategias hacia la sostenibilidad en el Mercosur

Se analiza brevemente el proceso de avance de la economía ecológica en el marco de la conformación de sociedades regionales en América Latina. La creación de la Sociedad Brasileña de Economía Ecológica, la Asociación Argentino Uruguaya de Economía Ecológica y la Red Iberoamericana de Economía Ecológica. Asimismo, se presenta información sobre las Segundas Jornadas de Economía Ecológica, desarrolladas durante el año 2004 y el enfoque de algunos de los documentos expuestos en la reunión. Finalmente, se realiza un análisis a nivel regional de la situación de la biodiversidad en el plano del Mercosur, las amenazas y las políticas alternativas necesarias para un ordenamiento territorial bajo un paraguas de desarrollo sostenible con base local.

Exploring quantum chaos with a single nuclear spin

Most classical dynamical systems are chaotic. The trajectories of two identical systems prepared in infinitesimally different initial conditions diverge exponentially with time. Quantum systems, instead, exhibit quasi-periodicity due to their discrete spectrum. Nonetheless, the dynamics of quantum systems whose classical counterparts are chaotic are expected to show some features that resemble chaotic motion. Among the many controversial aspects of the quantum-classical boundary, the emergence of chaos remains among the least experimentally verified. Time-resolved observations of quantum chaotic dynamics are particularly rare, and as yet unachieved in a single particle, where the subtle interplay between chaos and quantum measurement could be explored at its deepest levels. We present here a realistic proposal to construct a chaotic driven top from the nuclear spin of a single donor atom in silicon, in the presence of a nuclear quadrupole interaction. This system is exquisitely measurable and controllable, and possesses extremely long intrinsic quantum coherence times, allowing for the observation of subtle dynamical behavior over extended periods. We show that signatures of chaos are expected to arise for experimentally realizable parameters of the system, allowing the study of the relation between quantum decoherence and classical chaos, and the observation of dynamical tunneling.Comment: revised and published versio