Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
the special case at resonance. Finally, we apply our model to experimentally
realized superconducting flux qubits coupled to an underdamped
ds-SQUID-detector.
TT 22.7 Mi 16:00 H19
Coherence control for qubits — •Karen M. Fonseca Romero,
Sigmund Kohler, and Peter Hänggi — Institut für Physik, Universität
Augsburg, Universitätstr. 1, D-86135, Augsburg
We examine the possibility of coherence stabilization of qubits, weakly
coupled to a bath, by the application of periodic driving. For the description
of the qubit dynamics we derive a master equation, which makes use
of the periodicity of the driving force. The so-called Floquet-Born-Markov
master equation is valid for frequencies much larger than the inverse of
the typical qubit evolution and allows us to determine the decoherence
rates of the qubit system, under the influence of the driving. As a first
example we consider a single qubit under the influence of bit-flip noise
in two types of external harmonic driving fields (i) one that commutes
with the qubit-bath coupling Hamiltonian, in the coherent destruction of
tunnelling regime, and (ii) one that anticommutes with the qubit-bath
coupling Hamiltonian, which dynamically decouples the qubit from the
noise source. As a second example we consider two qubits performing a
CNOT gate operation using the Heisenberg exchange interaction, with a
decoherence source which couples to one spin component of the qubits.
We show that the influence of a high-frequency field leads to coherence
stabilized gate operation in the regime of low temperatures.
[1] K. M. Fonseca Romero, S. Kohler, and P. Hänggi, arxiv: quantph/0307136,
Chem. Phys. (in press).
TT 22.8 Mi 16:15 H19
Decoherence without T2 — •Frank Wilhelm — Sektion Physik
und CeNS, Ludwig-Maximilians-Universität, Theresienstr. 37, 80333
München
In order to achieve coherent manipulation of quantum states in solidstate
systems, it is crucial to analyze and engineer the decoherence of the
system. Traditionally, its impact is parameterized using the relaxation
and dephasing times T1 and T2. In particular, T2 is the time constant of
the exponential decay of quantum coherent oscillations at times longer
than the bath correlation time. Recent experiments on well-controlled superconducting
quantum bits show however, that T2 does not account for
the complete supression of the oscillations: The overall amplitude (visibility)
appears to be further reduced. In order to explain this effect, I
propose a consistent model invoking an environment whose spectrum is
gapped or at least supressed at low frequencies. It is shown from the exact
solution of the pure dephasing case, that the coherent oscillation amplitude
decays up to times of the order of the inverse infrared cutoff and
then remains constant, i.e. the system quickly decoheres to a constant
level although T2 defined from the long-time limit is infinite. Such baths
are ubiquitous in the experiments in question, e.g. in the quasiparticle
channel parallel to the Josephson junctions.
TT 22.9 Mi 16:30 H19
Quantum Dissipative Dynamics of the Magnetic Resonance
Force Microscope in the Single-Spin Detection Limit — •Hanno
Gassmann 1 , Mahn-Soo Choi 2 , Hangmo Yi 3 , and Christoph
Bruder 1 — 1 Department of Physics and Astronomy, University of
Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland — 2 Department of
Physics, Korea University, Seoul 136-701, Korea — 3 Korea Institute for
Advanced Study, 207-43 Cheongryang 2-dong, Seoul 130-722, Korea
We study a model of a magnetic resonance force microscope (MRFM)
based on the cyclic adiabatic inversion technique as a high-resolution tool
to detect single electron spins. We investigate the quantum dynamics of
spin and cantilever in the presence of coupling to an environment. To
obtain the reduced dynamics of the combined system of spin and cantilever,
we use the Feynman-Vernon influence functional and get results
valid at any temperature as well as at arbitrary system-bath coupling
strength. We propose that the MRFM can be used as a quantum measurement
device, i.e., not only to detect the modulus of the spin but also
its direction.
TT 22.10 Mi 16:45 H19
Towards mechanical entanglement in nano-electromechanical
devices — •Jens Eisert 1 , Martin B. Plenio 2 , and Sougato
Bose 3,4 — 1 Institut für Physik, Universität Potsdam, Am Neuen Palais
10, D-14469 Potsdam, Germany — 2 QOLS, Blackett Laboratory, Imperial
College London, Prince Consort Road, London SW7 2BW, UK
— 3 Department of Physics and Astronomy, University College London,
Gower Street, London, WC1E 6BT, UK — 4 Institute for Quantum Information,
California Institute of Technology, Pasadena, CA 91125, USA
We study arrays of mechanical oscillators in the quantum domain and
demonstrate how the motions of distant oscillators can be entangled
without the need for control of individual oscillators and without a direct
interaction between them. These oscillators are thought of as being
members of an array of nano-electromechanical resonators with a voltage
being applicable between neighbouring resonators. Sudden switching
of the interaction results in a squeezing of the states of the mechanical
oscillators, leading to an entanglement transport in chains of mechanical
oscillators. We discuss the spatial dimensions, Q-factors, and temperatures
that would be necessary to achieve entanglement in the canonical
coordinates in such a scheme, and discuss decoherence mechanisms in
some detail, and find a distinct robustness of the scheme under decoherence.
We also briefly discuss the challenging aspect of detection of the
generated entanglement.
TT 22.11 Mi 17:00 H19
Macroscopic quantum effects in driven nanomechanical wires
— •Vittorio Peano and Michael Thorwart — Institut für Theoretische
Physik IV, Heinrich-Heine-Universität Düsseldorf
We investigate the nonlinear response of a vibrating doubly clamped
suspended nanomechanical wire on an externally applied periodic driving.
The setup can be mapped onto the dynamics of a quantum particle moving
in a monostable quartic potential which is driven by an ac-field. The
quantum system is weakly damped due to interaction with a harmonic
bath. By solving the corresponding Born-Markovian master equation for
the reduced density operator, we calculate the response of the system
for varying driving frequencies. For strong driving, we observe characteristic
resonances which are absent in the corresponding classical model.
The resonances are explained in terms of tunneling transitions in the
dynamically induced effectively bistable system which is obtained within
the Floquet theory. Applications of the model to nanowires as well as to
driven SQUIDs are discussed.
17:15 Pause
TT 23 Nanoelektronik II: Spin-Elektronik
Zeit: Mittwoch 17:30–18:30 Raum: H19
TT 23.1 Mi 17:30 H19
Aharonov-Bohm Oscillations with Rashba Spin-Orbit Coupling:
Influence of Electron-Electron Interactions — •Maxim Trushin
and Alexander Chudnovskiy — 1.Institute of Theoretical Physics,
University of Hamburg, Jungiusstrasse 9, D-20355 Hamburg, Germany
We report a study of the Aharonov-Bohm effect, the oscillations of
the current through a one-dimensional quantum ring connected to two
reservoirs as a function of a perpendicular magnetic field, in presence of
Rashba spin-orbit coupling and Zeeman spin-splitting. In such a system
the geometric (Berry) phase is different for two chiral states and depends
on the parameters of the system such as the spin-orbit coupling constant
and Zeeman splitting. We discuss the quantum interference pattern arising
in the Aharonov-Bohm oscillations from the superposition of that
two chiral states. In the framework of the Tomonaga-Luttinger liquid approach
we derive an analytic formulae for the current and show that in
contrast to the usual situation without Rashba spin-orbit coupling the
electron-electron interactions can play a role here.