Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
relevant role. Prominent examples were charged impurities in the disordered
substrate of a Josephson qubit which produce typically telegraph
noise. In order to demonstrate the possibility of the active suppression
of the disturbance from a single fluctuator, we theoretically implement
an elementary bang-bang control protocol. We numerically simulate the
appearing random walk on the Bloch sphere with and without bang-bang
control and compare it with analytical results found by use of appropriate
Langevin equations in the long-time limit. Hereby we find out, that
the bang-bang control mechanism mostly compensates the slow noise,
which indicates, how the influence of 1/f-noise ubiquitous to the solidstate
world could be reduced. We also derive two generic and analytically
solvable random walk models to describe imperfect pulses and evaluate
the impact of these aberrations on the qubit.
TT 17.19 Di 14:30 Poster A
Periodically controlled qubit system coupled to an environment
— •Marcus Stollsteimer und Günter Mahler — Universität
Stuttgart, Institut für Theoretische Physik 1, Pfaffenwaldring 57,
D-70550 Stuttgart
We study the Schrödinger dynamics of a small system of coupled spin-
1/2 particles (qubits) which is subject to periodic external parameter
control (Floquet-type system). This system proper is coupled to another
quantum system that acts as environment or ”bath” (no bath approximations
are made).
When the control is switched off, the system relaxes into an equilibrium
state (except for finite size effects); for non-zero control the system
reaches a different ”quasi-equilibrium” state. We are interested on properties
of this new equilibrium state, and how they can be manipulated
by the external control.
TT 17.20 Di 14:30 Poster A
Superconducting flux qubits — •Heribert Knoglinger, Jürgen
Schuler, Chiara Coppi, Matteo Mariantoni, Georg Wild,
Christian Probst, Achim Marx, and Rudolf Gross — Walther-
Meissner-Institut für Tieftemperaturforschung der Bayerische Akademie
der Wissenschaften, 85748 Garching
Superconducting quantum bits (qubits) based on Josephson junctions
where the Josephson coupling energy is larger than the charging energy
are usually called flux qubits. Starting from our well-established technology
used for fabricating single charge transistors based on Al/Al2O3
tunnel junctions we have developed a modified process for fabricating
Josephson junctions and superconducting qubit structures. We have used
this process to produce and characterize different flux qubit structures.
We discuss different design variants for flux qubits and give a short
overview of the process technology. Measurements on various test structures
(Josephson junctions, SQUIDs, qubits) are used to analyze and further
optimize the system parameters and to compare the different qubit
variants. The measurement results and the limits of our technology are
the basis for a discussion of the usefulness of the individual elements to
realize superconducting qubits.
TT 17.21 Di 14:30 Poster A
Superconducting Qubits: Experimental setup for electrical
characterization — •Chiara Coppi, Georg Wild, Heribert
Knoglinger, Matteo Mariantoni, Jürgen Schuler, Christian
Probst, Achim Marx, and Rudolf Gross — Walther-Meissner-
Institut für Tieftemperaturforschung der Bayerischen Akademie der
Wissenschaften, 85748 Garching, Germany
An experimental setup for electrical characterization of superconducting
flux Qubits has been established. The setup consists of a 3 He/ 4 He
dilution refrigerator and a specially designed sample housing which provides
shielding against environmental electromagnetic radiation. Special
attention has been paid on filtering the wiring of the sample (wire-inpowder
filters at base temperature, low-pass filters at room temperature).
Magnetic shielding of the samples is obtained using cryoperm and double
µ- metal shields. The whole setup is placed in a high frequency shielded
room.
The setup has been used for measurements of switching current distributions
on low-TC Josephson junctions to evaluate the measurement
setup, the filtering, and the shielding. The temperature dependence of
the escape temperature, determined from the switching current distributions,
is compared to the theoretical predictions of thermal escape and
macroscopic quantum tunneling.
TT 17.22 Di 14:30 Poster A
Scaleable single-spin based quantum processor — •Michael
Domhan, Torsten Gaebel, Iulian Popa, Achim Gruber, Fedor
Jelezko, and Jörg Wrachtrup — Universität Stuttgart, 3.
Physikalisches Institut, Pfaffenwaldring 57, 70550 Stuttgart
Readout and manipulation of the single spin states are crucial basics
for spin-based quantum information processing. As formerly shown optically
detected magnetic resonance (ODMR) techniques can be used to
investigate single paramagnetic nitrogen-vacancy (N-V) defect centers
in diamonds. As two-qubit operations on single NV-defect centers have
also been performed, the effort will now be concentrated on coupling single
spins between NV-centers. Potential scemes for scaling-up single-spin
based quantum processor are presented.
TT 17.23 Di 14:30 Poster A
Characterization of qubits based on current-biased Josephson
junctions — •J. Lisenfeld 1 , C. Coqui 1 , A. Kemp 1 ,
A. Lukashenko 1 , A. Wallraff 2 , and A.V. Ustinov 1 —
1 Physikalisches Institut III, Universität Erlangen-Nürnberg, 91058
Erlangen, Germany — 2 Dept. Applied Physics, Yale University, New
Haven, CT 06520, USA
The discrete energy levels of small current-biased Josephson junctions
can be used as logical states for quantum computation [1]. Using microwave
spectroscopy, we characterize both the level spacing and their
dissipation-limited lifetimes. We report the observation of multi-photon
transitions [2] between levels at photon energies being integer fractions of
the transition energy, as it is expected for an anharmonic oscillator. For a
capacitively coupled two-junction system, which we currently study, entangled
macroscopic quantum states should be observable using the same
technique. Our experimental results available to date will be presented.
[1] J.M. Martinis et. al., Phys. Rev. Lett. 89, 117901 (2002)
[2] A. Wallraff et. al., Phys. Rev. Lett. 90, 037003 (2003)
TT 17.24 Di 14:30 Poster A
Coherent and incoherent motion in spin-boson systems —
•Wolfgang Pfersich and Walter T. Strunz — Albert-Ludwigs-
Universität Freiburg, Physikalisches Institut, Hermann-Herder-St. 3,
79104 Freiburg, Germany
We investigate the non-Markovian dynamics of the spin-boson system.
Our approach is based on ’quantum paths’ widely used in quantum optics.
We use an expansion in the coupling strength for the non-Markovian
stochastic Schrödinger equation based on Heisenberg’s equation of motion.
We have a special interest in the evolution of the reduced system for
short times compared to the correlation time of the bath and the decay
of the coherence.
Therefore we solve numerically the second and fourth order equation
of motion for the stochastic wave function of the reduced system.
TT 17.25 Di 14:30 Poster A
Microwave spectroscopy of long Josephson junctions in the
quantum regime — •A. Kemp 1 , A. Lukashenko 1 , Y. Koval 1 , J.
Lisenfeld 1 , M.V. Fistul 1 , A. Wallraff 2 , and A.V. Ustinov 1 —
1 Physikalisches Institut III, Universität Erlangen-Nürnberg, 91058 Erlangen,
Germany — 2 Dept. Applied Physics, Yale University, New Haven,
CT 06520, USA
We experimentally investigate the quantum escape of the phase in fluxfree
narrow annular Josephson junctions under influence of microwaves.
A measurement technique developed earlier [1] based on ramp-type measurements
enables us to resolve the populations of the ground state and
the first excited state. We probe the level structure of the junction with
small magnetic fields applied and at currents close to the fluctuation-free
critical current. The data obtained are compared to the theory [2] describing
the escape as dissociation of a vortex-antivortex pair and predicting
the specific scaling of the energy level spacing with the bias current. A
rather good agreement is found, while the detailed behavior of the observed
resonances leaves few open questions.
[1] A. Wallraff et al., Rev. Sci. Inst. 74, 3740 (2003)
[2] M.V. Fistul et al., to appear in Phys.Rev.Lett. (cond-mat/0307705)
TT 17.26 Di 14:30 Poster A
2-Qubit quantum gate with single spins in a solid state system —
•Achim Gruber, Michael Domhan, Thorsten Gaebel, Iulian
Popa, Fedor Jelezko, and Jög Wrachtrup — 3. Physikalische Institut,
Universität Stuttgart, 70550 Stuttgart, Germany