Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
TT 29.9 Do 16:45 H19
Influence of Pulse Shape for Josephson Junction Quantum
Computation — •Carsten Hutter, Yuriy Makhlin, and Gerd
Schön — Institut für Theoretische Festkörperphysik, Universität Karlsruhe,
D–76128 Karlsruhe, Germany
Recently, a controlled NOT quantum gate employing coupled charge
qubits was demonstrated for the first time [1]. A pulse technique with
ideally steplike pulses was used to switch the field parameters. In reality,
the pulses have finite rise- and falltimes, which influence the time evolution
of the qubits. While most studies on this problem consider a single
qubit, we focus on the effects of the pulse shape on two coupled qubits.
Analytical and numerical results are presented, and based on them a
qualitative understanding of the suppression of oscillations in an earlier
experiment [2] is given.
[1] T. Yamamoto et al, Nature 425, 941 (2003)
[2] Yu.A. Pashkin et al, Nature 421, 823 (2003)
TT 29.10 Do 17:00 H19
Cavity QED using superconducting qubits - intermediate
temperatures — •Göran Johansson, Alexander Shnirman, and
Ileana Rau — Institut für Theoretische Festkörperphysik
Inspired by recent experiments we investigate the dynamics of a
Josephson qubit resonantly coupled to a damped harmonic oscillator.
We specifaclly investigate the temperature dependence of the cavity susceptebility,
which in solid state devices corresponds to the impedance
of the LC-circuit. At zero temperature the uncoupled resonant peak is
Rabi splitted by the qubit. At temperatures comparable to the oscillator
frequency we find an intricate multiple peak structure, while at high
temperatures the uncoupled single peak is recovered. We treat the dynamics
of the system numerically by exact diagonalizing of the qubit plus
up to 100 states in the oscillator. We then include the bath within the
Bloch-Redfield formalism, without making the secular approximation.
Specifically we investigate the break-down of the secular approximation
at finite temperatures, caused by the Liouvillian degeneray, i.e. two or
more transitions with approximately equal energy. In the impedance this
is seen as the overlap of nearby peaks, which cannot be calculated as the
sum of the two peaks.
TT 29.11 Do 17:15 H19
Bath assisted adiabatic following — •Christos Kostoglou,
Mathias Michel, and Günter Mahler — Institut für Theoretische
Physik 1, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart
We regard quantum systems for which adiabatic following is ”not
generic”, i.e. not only the eigenvalues but also the eigenstates of the
Hamiltonian are time dependent. A simple, pertinent model is a spin-
1/2-system in a rotating magnetic field, the adiabatic following of which
fades away with increasing rotation frequency. Here we test if the dephasing
influence of a microcanonically coupled environment supports
adiabatic following. The environment is modeled as a n-level system randomly
coupled to the spin without allowing for energy exchange. The
environment can be interpreted as an ubiquitous container of thermodynamic
machines; such a combination of spin-1/2-system and container
leads us to an analog to thermodynamic processes on quantum scale.
Time evolution is given by the time-dependent Schrödinger equation.
TT 29.12 Do 17:30 H19
Quantum Control: Mechanical Versus Thermodynamical
Aspects — •Harry Schmidt und Günter Mahler — Universität
Stuttgart, Institut für Theoretische Physik 1, Pfaffenwaldring 57,
D-70550 Stuttgart
External quantum control is usually taken to mean that certain classical
parameters entering the Hamiltonian of the controlled quantum system
can be changed from the outside. To exert such a control one may
be tempted to use another (mesoscopic) quantum system.
We investigate the conditions under which such a mesoscopic system
may, indeed, act like a classical (mechanical) control on the one hand or
as a bath on the other hand with respect to the state evolution of the
controlled microscopic system. Both control features can be used as working
parts of quantum thermodynamical machines. The actual control
mode depends on the state of the system as a whole, the state of the mesoscopic
object, in particular, and parameters of the interaction that are
easily accessible and adjustable in an experimental setup. These parameters
thus provide an external control of the behavior of the mesoscopic
system.
TT 29.13 Do 17:45 H19
Quantum versus classical temperature profiles — •Michael
Hartmann 1,2 , Günter Mahler 2 und Ortwin Hess 3 — 1 Institut
für Technische Physik, DLR Stuttgart, Pfaffenwaldring 38 - 40, D-70569
Stuttgart — 2 Institut für Theoretische Physik I, Universität Stuttgart,
Pfaffenwaldring 57, D-70550 Stuttgart — 3 School of Electronics & Physical
Sciences, University of Surrey, GU2 7XH, UK
We consider a quantum system consisting of a regular chain
of elementary subsystems with nearest neighbour interactions and
form groups of N subsystems each. Assuming that the total system
is in a canonical state ρ = exp(H/kT)/Z with temperature
T we analyse under what condition the state may be decompo-
sed into a product of canonical density matrices of the subgroups,
ρ ′ = �
i exp(Hi/kT)/Zi, with the same temperature T. We then compare
the classical and the quantum regime and apply our analysis to a
harmonic chain as an example.
TT 30 Postersitzung IV: Kritische Phänomene, Quantenstörstellen, niederdimensionale
Systeme
Zeit: Donnerstag 14:30–19:00 Raum: Poster A
TT 30.1 Do 14:30 Poster A
Lattice dynamics and electron-phonon interaction in carbon
nanotube — •K.-P. Bohnen 1 , R. Heid 1 , H.J. Liu 2 , and C.T. Chan 2
— 1 Institut für Festkörperphysik, Forschungszentrum Karlsruhe, P.O.B.
3640, D-76021 Karlsruhe, Germany — 2 Dept. of Physics, University of
Science and Technology, Clear Water Bay, Kowloon, Hongkong, China
Over the past decade the interest in understanding the physical properties
of carbon nanotubes has grown enormously. This is based not only
on their potential for applications but also due to their role as model systems
for studying properties in one dimension. Especially the electronphonon
interaction is of interest with respect to Peierls transition [1],
Kohn anomalies and superconductivity [2]. Using density functional perturbation
theory we have calculated the complete phonon dispersion for
(3,3) and (5,0) tubes. Both tubes show instabilities at q=2kF which can
be seen in certain phonon modes. Due to the special topology of the
Fermi surface for (n,n) tubes we find also phonon anomalies at q=0 for
the (3,3) tube. Part of these have already been seen in other studies
[3]. Since the instabilities at q=2kF show up usually at q vectors incommensurable
with the lattice periodicity it is exceptionally difficult to see
these effects in frozen-phonon calculations. We will present comparison
with Raman measurements and approximate theoretical treatments [4].
[1] M. Th. Figge et al., Phys. Rev. Lett 86, 4572 (2001)
[2] Z. K. Tang et al., Science 292, 2462 (2001)
[3] O. Dubay et al., Phys. Rev. B 67, 035401 (2003)
[4] R. Barnett at al., cond-mat/0305006v2 (2003)
TT 30.2 Do 14:30 Poster A
Numerical Renormalization Group Study for Bosonic Systems
— •Ninghua Tong 1 , Ralf Bulla 1 , and Matthias Vojta 2 —
1 Theoretische Physik III, Universität Augsburg — 2 Institut für Theorie
der Kondensierten Materie, Universität Karlsruhe
The physics of an impurity imbedded in a metallic fermionic bath is one
of the best studied fields in condensed matter physics. Wilson’s numerical
renormalization group (NRG) method has been very successfully applied
to such problems due to its non-perturbative nature and the ability to
access exponentially small energy scales. In recent years, the physics of
impurities in a bosonic bath has attracted much interest. Such systems
include impurities imbedded in a magnetic environment and qubits in
a dissipative environment. A non-perturbative and accurate method for
handling these problems is highly desirable. We propose to generalize
Wilson’s NRG method to bosonic systems [R. Bulla, N.-H. Tong, and
M. Vojta, Phys. Rev. Lett. 91, 170601 (2003)]. As a first application, we
study the spin-boson model, which describes a two-level system coupled