74 research outputs found
The equilibrium shape of InAs quantum dots grown on a GaAs(001) substrate
The equilibrium shape of strained InAs quantum dots grown epitaxially on a
GaAs(001) substrate is derived as a function of volume. InAs surface energies
are calculated within density-functional theory, and a continuum approach is
applied for the elastic relaxation energies.Comment: 4 pages, 1 figure, to appear in "The Physics of Semiconductors
Interface dipoles of organic molecules on Ag(111) in hybrid density-functional theory
We investigate the molecular acceptors 3,4,9,10-perylene-tetracarboxylic acid
dianhydride (PTCDA), 2,3,5,6-tetra uoro-7,7,8,8-tetracyanoquinodimethane
(F4TCNQ), and 4,5,9,10-pyrenetetraone (PYTON) on Ag(111) using
densityfunctional theory. For two groups of the HSE(\alpha, \omega) family of
exchange-correlation functionals (\omega = 0 and \omega = 0.2\AA) we study the
isolated components as well as the combined systems as a function of the amount
of exact-exchange (\alpha). We find that hybrid functionals favour electron
transfer to the adsorbate. Comparing to experimental work-function data, we
report for (\alpha) ca. 0.25 a notable but small improvement over (semi)local
functionals for the interface dipole. Although Kohn-Sham eigenvalues are only
approximate representations of ionization energies, incidentally, at this value
also the density of states agrees well with the photoelectron spectra. However,
increasing (\alpha) to values for which the energy of the lowest unoccupied
molecular orbital matches the experimental electron affinity in the gas phase
worsens both the interface dipole and the density of states. Our results imply
that semi-local DFT calculations may often be adequate for conjugated organic
molecules on metal surfaces and that the much more computationally demanding
hybrid functionals yield only small improvements.Comment: submitted to New Journal of Physics (2013). More information can be
found at
http://th.fhi-berlin.mpg.de/site/index.php?n=Publications.Publication
Adiabatic quantum simulations with driven superconducting qubits
We propose a quantum simulator based on driven superconducting qubits where
the interactions are generated parametrically by a polychromatic magnetic flux
modulation of a tunable bus element. Using a time-dependent Schrieffer-Wolff
transformation, we analytically derive a multi-qubit Hamiltonian which features
independently tunable and -type interactions as well as local bias
fields over a large parameter range. We demonstrate the adiabatic simulation of
the ground state of a hydrogen molecule using two superconducting qubits and
one tunable bus element. The time required to reach chemical accuracy lies in
the few microsecond range and therefore could be implemented on currently
available superconducting circuits. Further applications of this technique may
also be found in the simulation of interacting spin systems.Comment: 11 pages, 6 figure
Time-resolved tomography of a driven adiabatic quantum simulation
A typical goal of a quantum simulation is to find the energy levels and
eigenstates of a given Hamiltonian. This can be realized by adiabatically
varying the system control parameters to steer an initial eigenstate into the
eigenstate of the target Hamiltonian. Such an adiabatic quantum simulation is
demonstrated by directly implementing a controllable and smoothly varying
Hamiltonian in the rotating frame of two superconducting qubits, including
longitudinal and transverse fields and iSWAP-type two-qubit interactions. The
evolution of each eigenstate is tracked using time-resolved state tomography.
The energy gaps between instantaneous eigenstates are chosen such that
depending on the energy transition rate either diabatic or adiabatic passages
are observed in the measured energies and correlators. Errors in the obtained
energy values induced by finite and times of the qubits are
mitigated by extrapolation to short protocol times.Comment: 5 pages, 4 figure
Improved precision scaling for simulating coupled quantum-classical dynamics
We present a super-polynomial improvement in the precision scaling of quantum
simulations for coupled classical-quantum systems in this paper. Such systems
are found, for example, in molecular dynamics simulations within the
Born-Oppenheimer approximation. By employing a framework based on the
Koopman-von Neumann formalism, we express the Liouville equation of motion as
unitary dynamics and utilize phase kickback from a dynamical quantum simulation
to calculate the quantum forces acting on classical particles. This approach
allows us to simulate the dynamics of these particles without the overheads
associated with measuring gradients and solving the equations of motion on a
classical computer, resulting in a super-polynomial advantage at the price of
increased space complexity. We demonstrate that these simulations can be
performed in both microcanonical and canonical ensembles, enabling the
estimation of thermodynamic properties from the prepared probability density.Comment: 19 + 51 page
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