6 research outputs found
Decoherence in Crystals of Quantum Molecular Magnets
Decoherence in Nature has become one of the most pressing problems in
physics. Many applications, including quantum information processing, depend on
understanding it; and fundamental theories going beyond quantum mechanics have
been suggested [1-3], where the breakdown of quantum theory appears as an
'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories
cannot be tested until we have a handle on ordinary environmental decoherence
processes. Here we show that the theory for insulating electronic spin systems
can make accurate predictions for environmental decoherence in molecular-based
quantum magnets [5]. Experimental understanding of decoherence in molecular
magnets has been limited by short decoherence times, which make coherent spin
manipulation extremely difficult [6-9]. Here we reduce the decoherence by
applying a strong magnetic field. The theory predicts the contributions to the
decoherence from phonons, nuclear spins, and intermolecular dipolar
interactions, for a single crystal of the Fe8 molecular magnet. In experiments
we find that the decoherence time varies strongly as a function of temperature
and magnetic field. The theoretical predictions are fully verified
experimentally - there are no other visible decoherence sources. Our
investigation suggests that the decoherence time is ultimately limited by
nuclear spins, and can be extended up to about 500 microseconds, by optimizing
the temperature, magnetic field, and nuclear isotopic concentrations.Comment: Submitted version including 11 pages, 3 figures and online supporting
materials. Appeared on Nature Advance Online Publication (AOP) on July 20th,
2011.
(http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10314.html
Quantum nanomagnets and nuclear spins: an overview
This mini-review presents a simple and accessible summary on the fascinating
physics of quantum nanomagnets coupled to a nuclear spin bath. These chemically
synthesized systems are an ideal test ground for the theories of decoherence in
mesoscopic quantum degrees of freedom, when the coupling to the environment is
local and not small. We shall focus here on the most striking quantum
phenomenon that occurs in such nanomagnets, namely the tunneling of their giant
spin through a high anisotropy barrier. It will be shown that perturbative
treatments must be discarded, and replaced by a more sophisticated formalism
where the dynamics of the nanomagnet and the nuclei that couple to it are
treated together from the beginning. After a critical review of the theoretical
predictions and their experimental verification, we continue with a set of
experimental results that challenge our present understanding, and outline the
importance of filling also this last gap in the theory.Comment: 14 pages, 3 figures. Chapter in the Proceedings of the 2006 Les
Houches summer school "Quantum Magnetism", ed. B. Barbara & Y. Imry, Springer
(2007
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Towards the solution of the many-electron problem in real materials: Equation of state of the hydrogen chain with state-of-the-art many-body methods
We present numerical results for the equation of state of an infinite chain of hydrogen atoms. A variety of modern many-body methods are employed, with exhaustive cross-checks and validation. Approaches for reaching the continuous space limit and the thermodynamic limit are investigated, proposed, and tested. The detailed comparisons provide a benchmark for assessing the current state of the art in many-body computation, and for the development of new methods. The ground-state energy per atom in the linear chain is accurately determined versus bond length, with a confidence bound given on all uncertainties
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Solutions of the two-dimensional hubbard model: Benchmarks and results from a wide range of numerical algorithms
Numerical results for ground-state and excited-state properties (energies, double occupancies, and Matsubara-axis self-energies) of the single-orbital Hubbard model on a two-dimensional square lattice are presented, in order to provide an assessment of our ability to compute accurate results in the thermodynamic limit. Many methods are employed, including auxiliary-field quantum Monte Carlo, bare and bold-line diagrammatic Monte Carlo, method of dual fermions, density matrix embedding theory, density matrix renormalization group, dynamical cluster approximation, diffusion Monte Carlo within a fixed-node approximation, unrestricted coupled cluster theory, and multireference projected Hartree-Fock methods. Comparison of results obtained by different methods allows for the identification of uncertainties and systematic errors. The importance of extrapolation to converged thermodynamic-limit values is emphasized. Cases where agreement between different methods is obtained establish benchmark results that may be useful in the validation of new approaches and the improvement of existing methods