Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
TT 24.55 Mi 14:30 Poster A
Orbital ordering in manganites on the band approach. —
•Dmitry Efremov 1 and Daniel Khomskii 2 — 1 Technische
Universität Dresden, Institut für Theoretische Physik, 01062 Dresden
— 2 Universität zu Köln, 50937 Köln
We consider the orbital ordering in LaMnO3 and similar systems, proceeding
from the band picture. We show that for the realistic magnetic
structure of A-type there exists a complete nesting between two eg-bands.
As a result there occurs an instability towards Q = (π, π) an excitonic insulator
like state – an electron-hole pairing with the wave vector, which
opens a gap in the spectrum and makes the system insulating. In the
resulting state there appear an orbital ordering – orbital density wave
(ODW), the type of which coincides with these existing in LaMnO3.
TT 24.56 Mi 14:30 Poster A
Magnetic ordering in the trigonal chain compounds Ca3CoRhO6
and Ca3FeRhO6 — •Udo Schwingenschlögl, Volker Eyert, and
Ulrich Eckern — Theoretische Physik II, Institut für Physik, Universität
Augsburg, 86135 Augsburg
Low-dimensionality and frustration effects in compounds containing
magnetic ions have attracted a lot of attention since long owing to the
expectation of a variety of fascinating properties. Much interest has focused
on systems, where the chains are arranged in a triangular lattice.
Continuing recent work on the prototypical compound Ca3Co2O6
[1] we present the results of augmented spherical wave (ASW) electronic
structure calculations for the closely related compounds Ca3CoRhO6 and
Ca3FeRhO6. In accordance with experimental data we find strong intrachain
magnetic coupling of high-spin 3d-metal sites via the d-states of
the interjacent low-spin rhodium sites resulting in the observed ferromagnetic
and antiferromagnetic order in Ca3CoRhO6 and Ca3FeRhO6,
respectively. Considerable hybridization with the O 2p states leads to polarization
of the latter and the formation of extended magnetic moments,
which are well localized at the high-spin sites.
[1] V. Eyert. C. Laschinger, T. Kopp, and R. Frésard, submitted to
Chem. Phys. Lett.
TT 25 FV-internes Symposium ”Superconducting Qubits and π-junctions”
Zeit: Donnerstag 09:30–12:45 Raum: H20
Fachvortrag TT 25.1 Do 09:30 H20
Charge-phase Josephson qubit with radio frequency readout —
•Alexander Zorin — PTB, Bundesallee 100, 38116 Braunschweig
A solid state qubit presenting a macroscopic superconducting ring including
two small tunnel Josephson junctions is considered. A pulsed gate
voltage polarizing a small island between the junctions and a variable
magnetic flux applied to the ring make it possible quantum manipulations
of the system state. Readout of the state is performed by probing the
effective Josephson inductance of the qubit. This is done by inducing lowfrequency
harmonic oscillations in inductively coupled tank circuit and
measuring the resonance frequency shift. On one hand, this narrow-band
readout system requires sufficiently long time for a single measurement.
On the other hand, due to the qubit symmetry and its disconnection from
dc bias lines and, presumably, from microwave-frequency noise sources
it should be safely decoupled from environment and exhibit rather long
coherence and relaxation times. The issues related to optimization of the
qubit characteristics and regime of its operation will be addressed in the
talk. The results of first experiments will be also reported.
Fachvortrag TT 25.2 Do 10:00 H20
Exploring quantum dynamics of Josephson vortices —
•A.V. Ustinov 1 , A. Wallraff 2 , M.V. Fistul 1 , A. Kemp 1 , A.
Lukashenko 1 , J. Lisenfeld 1 , and Y. Koval 1 — 1 Physikalisches
Institut III, Universität Erlangen-Nürnberg, 91058 Erlangen, Germany
— 2 Dept. Applied Physics, Yale University, New Haven, CT 06520, USA
Engineering of an energy profile for a vortex in a Josephson junction
opens an opportunity for designing superconducting qubits based
on spatially distinct quantum states. The energy profile for the vortex
can be created, e.g., by making the junction of a particular geometrical
shape and applying an external magnetic field. In our first experiments
we have demonstrated novel vortex states in long Josephson junctions
with complex engineered potentials [1]. Recently, for the first time, we
have observed quantum tunneling of a single vortex in a long junction at
temperatures below 100 mK and performed microwave spectroscopy of
the vortex energy levels within a potential well [2]. The vortex behaves as
a macroscopic quantum particle (with a spatial extent of several micrometers)
which tunnels through a potential barrier created by a magnetic
field applied to the junction. In agreement with theory, the separation
between vortex energy levels is controlled by the field. We have also measured
quantum dissociation of a vortex ”molecule”, consisting of a bound
state of vortex and antivortex [3].
[1] A. Kemp et al. Phys.Stat.Sol.(b) 233, 472 (2002)
[2] A. Wallraff et al., Nature 425, 155 (2003)
[3] M.V. Fistul et al., to appear in Phys.Rev.Lett. (cond-mat/0307705)
Hauptvortrag TT 25.3 Do 10:30 H20
Quantum dynamics of persistent current qubits — •Kees Harmans
— Quantum Transport Group, Department of NanoScience/TNW,
Delft University of Technology,Delft, The Netherlands
Quantum bits employing superconductivity come in a few varieties,
comprising (Cooperpair) charge, (condensate) phase, and persistent current
or flux. In a flux-based qubit the loop-shape topology allows a persistent
current to circulate in opposite directions, leading to two quantum
states that are well separated from a higher lying manifold of states. The
properties of these two states can be fully engineered during the fabrication,
and largely controlled during the experiment. Employing flux-flux
coupling in addition allows a flexible qubit-qubit interaction scheme towards
multi-qubit systems. Using burst microwave magnetic excitation
resonant between the levels and pulsed fluxes, both of well controlled amplitude
and duration/interval, allows full quantum control of the qubit(s).
We will present a series of experiments demonstrating the quantum behavior
of a single qubit, comprising Rabi, Ramsey and echo-type oscillations.
In addition some results on a coupled two-qubit experiment will
be discussed. The role of decoherence in our quantum experiments will
be highlighted, including the effect of the qubit state detection.
11:00 Pause
Hauptvortrag TT 25.4 Do 11:15 H20
Superconducting Structures for Quantum Computing —
•Gianni Blatter 1 , Vadim Geshkenbein 1 , Mikhail Feigelman 2 ,
and Lev Ioffe 3 — 1 Theoretische Physik, ETH-Hönggerberg, CH-8093
Zürich — 2 Landau Institute, RAS, Moskow, Russia — 3 Department of
Physics and Astronomy, Rutgers University, Piscataway, USA
In a quantum computer the information is stored in arrays of quantum
two-level systems or ‘qubits’. Execution of a quantum algorithm involves
quantum gates, unitary operations rotating individual qubits and entangling
them pairwise. Superconducting solid-state qubits are promising
candidates for the hardware implementation of scalable quantum information
processors; quantum fluctuations are introduced through samllcapacitance
Josephson junctions and the frustrating drive is introduced
through a gate potential (charge-qubit) or a magnetic flux (phase- or flux
qubit). Recent experiments have achieved a breakthrough with quality
factors of the order of 10 4 . Upscaling to a real quantum computer will
require further improvements in noise reduction, manipulation, fault tolerance,
and simplifications in design/fabrication. Superconductors with
d-wave symmetry bear a number of benefits, such as the possibility to
construct π- or double-periodic 2φ-junctions providing a quiet driving
force and allowing for manipulation through switches. Further geometric
frustration can be used to enhance quantum fluctuations in a new
qubit design with tetrahedral symmetry, reducing charge noise and the
demands on the fabrication of ultra-small junctions.
Fachvortrag TT 25.5 Do 11:45 H20
Semifluxons as a base for classical and quantum digital circuits?
— •Edward Goldobin, Tobias Gaber, Albert Sterck, Dieter
Koelle, and Reinhold Kleiner — Physicalische Institut II, Universität
Tübingen