Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
which restricts the dynamics to a feasible number of the leading Fourier
components. From a sequence of systematically improved dynamical approximations
we estimate the location of the quantum critical point.
TT 21.5 Mi 17:45 H18
On the quantum critical point in Sr2Ru1−xTixO4 — •Ralph
Werner 1 and Sam T. Carr 2 — 1 Institut f”ur Theorie der Kondensierten
Materie, Universit”at Karlsruhe, 76128 Karlsruhe — 2 Condensed
Matter Section, The Abdus Salam ICTP, Strada Costiera 11, 34014 Trieste,
Italy
Upon doping with 2.5% of Ti on the Ru site Sr2RuO4 develops incommensurate
static magnetic order which has been attributed to the nesting
of the bands that form the α and β Fermi surfaces [1]. We argue that a
standard Stoner mechanism modeled by RPA is insufficient to describe
the transition. Instead it is proposed that the quantum critical point is
driven by a weak-coupling to strong-coupling crossover which in turn
is modeled in a non-perturbative manner [2]. The expected dynamical
magnetic response is calculated.
[1] M. Braden et al., Phys. Rev. Lett. 88 (2002) 197002
[2] R. Werner and V. J. Emery, Phys. Rev. B 67 (2003) 014504
TT 22 Quantenkohärenz und Quanteninformationssysteme I
Zeit: Mittwoch 14:30–17:15 Raum: H19
TT 22.1 Mi 14:30 H19
Coherent oscillations in a single electron spin — •Iulian Popa,
Fedor Jelezko, Torsten Gaebel, Achim Gruber, Michael
Domhan, and Jörg Wrachtrup — 3.Phys. Institut Universität
Stuttgart 70563
Rabi nutations of a single electron spin in a single defect center in diamond
have been detected,at room temperature. The N-V defect center in
diamond is a defect consisting of a substitutional nitrogen atom adjacent
to a carbon-atom vacancy. The optical transition between 3A ground
state and 3E excited state has a very high quantum efficiency allowing
single defect spectroscopy. Because of paramagnetic nature of the ground
electronic state, single N-V defects are believed to be promising candidates
for the solid state quantum computation. The coherent evolution of
the spin quantum state is followed via optical detection of the spin state.
Coherence times up to several microseconds at room temperature have
been measured. The influence of the optical excitation on the coherence
time is analyzed.
TT 22.2 Mi 14:45 H19
Solid-state 2-qubit gate based on a single defect center in diamond
— •Torsten Gaebel, Michael Domhan, Iulian Popa,
Achim Gruber, Fedor Jelezko, and Jörg Wrachtrup — 3.
Physikalisches Institut, Universität Stuttgart
For spin-based quantum information processing, the readout and manipulation
of the single spin states are of an outmost importance. Recently,
single paramagnetic nitrogen vacancy (N-V)defects in diamond
have been investigated by the optically detected magnetic resonance
(ODMR)technique. Single spin ODMR approach is based on the optical
selection of a single paramagnetic defect by confocal optical microscopy.
Photon antibunching proves that ODMR signal is originating from the
single quantum system.
Pulsed ODMR experiments have been performed in order to demonstrate
the ability of the coherent manipulation of single spin states.
Coupling to the spin of a 13C nucleus provides a 2-qubit system, on
which the conditional rotation gate (CROT) was performed.
TT 22.3 Mi 15:00 H19
Open systems dynamics in a spin star — •Daniel Burgarth 1 ,
Heinz-Peter Breuer 1 , and Francesco Petruccione 1,2 —
1 Physikalisches Institut, Universitaet Freiburg, Germany — 2 Istituto
Italiano per gli Studi Filosofici, Naples, Italy
The study of strongly coupled spin systems is important for various
applications in quantum computation and quantum communication. The
dynamics of a spin coupled to a spin bath is strongly influenced by dissipation
and decoherence. Within the theory of open quantum systems
the dynamics is described in terms of a master equation for the density
matrix of the reduced spin degree of freedom.
A very simple, exactly solvable spin star system will be discussed and
the reduced non-Markovian dynamics of the spin will be derived using
different strategies of approximation. Particular attention will be devoted
to the comparison of the non-Markovian approximations obtained by the
Nakajima-Zwanzig and by the time-convolutionless projection operator
technique.
TT 22.4 Mi 15:15 H19
Quantum impurity approach to a two qubit problem — •Josef
Schriefl 1,2 , Sebastien Camamet 2 , Pascal Degiovanni 2 , and
Francois Delduc 2 — 1 Institut für theoretische Festkörperphysik,
Universität Karlsruhe, 76128 Karlsruhe, Germany — 2 Laboratoire de
Physique de l’Ecole Normale Supérieure de Lyon, UMR 5672, 69007
Lyon, France
We study the mutual influence of two qubits via a common environment.
By mapping our model on a system with two Kondo type interactions
we show its equivalence with the problem of a quantum wire
coupled at its boundaries to two resonant levels. Using non perturbative
techniques we obtain exact expressions for the equilibrium populations
and correlation of the qubits at a special point of the interaction strength.
The nontrivial behavior of both quantities can be explained within a simple
Kondo cloud picture. Depending on control-parameters each qubit
generates a Kondo cloud of a characteristic size in the environment. The
conditions for a non-zero correlation are found to be not only spatial
overlap of the clouds but also their action on the same environmental
modes. Furthermore, perturbation calculations suggest that this picture
is qualitatively valid nearby the exactly solvable point.
TT 22.5 Mi 15:30 H19
Quantum trajectory approach to the dynamics of spin qubits in
quantum dots — •Holger Schaefers and Walter T. Strunz —
Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-
Herder-Str. 3, 79104 Freiburg, Germany
We investigate continuous measurement of a single electron spin qubit
in a quantum dot as described in [1]. The dot is coupled to two leads
and we choose the sequential tunneling regime. Our approach is based
on ‘quantum trajectories’, widely used in quantum optics, here adapted
to describe conditional quantum dot dynamics in a fermionic environment.
We use the quantum trajectory approach to simulate the quantum
dynamics conditioned on the continuous measurement outcome, here the
electron current through the dot. We simulate so called ‘shelving’ experiments
and show that it is possible to measure directly the relaxation time
T1, in the time domain. In a modified parameter regime the decoherence
time T2 can be obtained as well. Further, using realistic parameters, we
calculate counting statistics of electrons tunneling through the dot and
show, that this device can be used to measure initial coherences of the
qubit state.
[1] H.-A. Engel, D. Loss, Phys. Rev. Lett. 86, 4648 (2001)
TT 22.6 Mi 15:45 H19
Dynamics of the spin-boson model with a structured environment
— •Michael Thorwart 1 , Elisabetta Paladino 2 , and
Milena Grifoni 3 — 1 Institut für Theoretische Physik IV, Heinrich-
Heine-Universität Düsseldorf — 2 Dipartimento Metodologie Fisiche e
Chimiche per L’ingegneria, Universitá di Catania, Italy — 3 Institut für
Theoretische Physik, Universität Regensburg
We investigate the dynamics of the spin-boson model when the spectral
density of the bath shows a resonance at a characteristic frequency
Ω but behaves Ohmically at small frequencies. A useful exact mapping
of this system to a system composed of a quantum two-state system
(TSS) coupled to an Ohmically damped harmonic oscillator (HO) with
frequency Ω is invoked. The dynamics is calculated by employing the
numerically exact quasiadiabatic path-integral technique. We find significant
new properties compared to the Ohmic spin-boson model. By
reducing the combined TSS+HO-system in the dressed states picture to
a three-level system, we calculate analytically the dephasing rates for