Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
pair in an annular Josephson junction is experimentally observed and
theoretically analyzed [1]. VAV pair is confined in a pinning potential
controlled by external magnetic field and bias current. The dissociation
of the pinned VAV pair manifests itself by a switching of the Josephson
junction from the superconducting to the resistive state. The observed
temperature and field dependence of the switching current distribution is
in agreement with our analysis. In an extremely narrow long JJs (a small
width of a JJ allows to decrease the ”electromagnetic” effective mass of
a VAV pair) the crossover from the thermal to the macroscopic quantum
tunneling mechanism of dissociation occurs at a temperature of about
100 mK. We also predict the specific magnetic field dependence of the oscillatory
energy levels of the pinned VAV state. The quantum-mechanical
dynamics of a VAV pair can be used for quantum information processing.
[1] M.V. Fistul et al., to appear in Phys.Rev.Lett. (cond-mat/0307705)
TT 29.3 Do 15:00 H19
Multi-photon transitions and subharmonic-like resonances in
a superconducting flux qubit device — •M. Thorwart 1,2 , S.
Saito 2 , H. Tanaka 2 , H. Nakano 2 , M. Ueda 2 , K. Semba 2 , and H.
Takayanagi 2 — 1 Institut für Theoretische Physik IV, Heinrich-Heine-
Universität Düsseldorf — 2 NTT Basic Research Laboratories, Atsugi-shi,
Japan
We investigate both experimentally and theoretically the quantum mechanical
dynamics of superposition states of a superconducting loop with
three Josephson junctions in the presence of an external RF-field. Readout
of the state of this driven macroscopic quantum two-state system
is performed by an additional dc-SQUID. For strong enough driving,
we find up to three-photon transitions in spectroscopy measurements.
The width of the n-photon resonance scales with the n-th Bessel function.
This is in agreement with theoretical predictions starting from the
driven spin-boson model and applying real-time path integral methods.
In addition, we report on subharmonic-like resonances in spectroscopy
measurements. They are related to excitations in the combined driven
flux-qubit-SQUID system and are explained in terms of avoided level
crossings in the Floquet spectrum.
TT 29.4 Do 15:15 H19
Dynamic Features of a Superconducting Ring-Type Charge
Qubit — •Detlef Born 1 , Wolfram Krech 1 , Vladimir
Shnyrkov 1 , Thomas Wagner 2 , Evgeni Il’ichev 2 , Uwe Hübner 2 ,
and Hans-Georg Meyer 2 — 1 FSU, Institut für Festkörperphysik,
07743 Jena — 2 IPHT e.V., 07702 Jena
We study dynamic properties of a special Josephson charge qubit consisting
of a single-Cooper-pair transistor with capacitive gate closed by
a superconducting loop.
Within the framework of a two-band model we consider analytically
the features of the oppositely circulating ring currents corresponding to
ground and upper quantum state, respectively, in terms of gate charge
and total Josephson phase across the transistor. We determine basic device
characteristics, e.g. the critical current, current-phase relationships
as well as the critical loop parameter and the oppositely acting Josephson
inductances belonging to the separate bands.
We observed experimentally dynamic features of the described quantum
object. For this purpose, we investigated the total impedance of a
configuration consisting of the ring qubit inductively coupled to the coil
of a high-quality tank circuit. Measuring the phase shift between tank
current and voltage (MHz range), we demonstrated the modulation of
the qubit ground state impedance with respect to both electric gate voltage
and magnetic flux bias. Illuminating the qubit by means of an UHF
source (GHz range), we detected the effect of the (negative) inductance
of the upper state.
TT 29.5 Do 15:30 H19
Investigation of a superconducting flux qubit with SQUID readout
— •S. Linzen 1 , B.L.T. Plourde 1 , T.L. Robertson 1 , T. Hime 1 ,
P.A. Reichardt 1 , C.E. Wu 1 , F. Wilhelm 2 , and John Clarke 1
— 1 Department of Physics, University of California, Berkeley, USA —
2 Sektion Physik, Ludwig-Maximilians-Universität, München, Germany
We have studied large inductance flux qubits based on submicron
Al/AlOx/Al tunnel junctions. A dc SQUID is used to distinguish the
two fundamental states of the three-junction qubit by measuring the
switching probability with a pulsed bias current. We have optimized the
qubit flux signal and the coupling to the SQUID to minimize decoherence.
Single-shot readout has been reached by on-chip shunting of each of the
two SQUID junctions with a resistor and capacitor in series. We can still
resolve the qubit state with 60% fidelity with a single measurement without
the RC shunts. We measure the qubit energy levels spectroscopically
using microwave pulses in the wide frequency range 100 MHz to 20 GHz
while adjusting the fluxes in the SQUID and qubit by means of static
currents in two inductively coupled lines on-chip. The repetition rate of
the readout pulses is limited by hot electron generation in the SQUID
tunnel junctions when the SQUID switches out of the supercurrent state.
The influence of the repetition rate on the resonance peak widths and
the relaxation time will be discussed. We observe long relaxation times,
more than 20 µs, and dephasing times of about 5 ns when the repetition
rate is lower than 2 kHz.
This work was supported by Alexander von Humboldt-Foundation,
AFOSR, ARO, and ARDA.
TT 29.6 Do 15:45 H19
Decoherence of superconducting qubits due to phonons —
•Christian Helm 1 , L.B. Ioffe 2 , V.B. Geshkenbein 1 , and G.
Blatter 1 — 1 ETH Hönggerberg, Institut fuer theoretische Physik,
Zürich — 2 Department of Physics and Astronomy, Rutgers University,
Piscataway, USA
Decoherence is the main adversary of the unitary time evolution governing
the quantum systems, which provide the hardware for a future
quantum information technology based on solid state devices. Here, we
discuss a fundamental limitation for the coherent operation of superconducting
quantum bits based on Josephson junctions due to phonon
radiation (Such an effect was observed e.g. in C. Helm at al. PRL 79
(1997),737). We find that recently reported quality factors may be explained
in terms of decoherence from phonon radiation, which therefore
might be the limiting factor in current experiments. We show the way to
reduce the impact of this source of decoherence, e.g. by a choice of materials
with appropriate elastic properties or by comparing various qubit
designs.
16:00 Pause
TT 29.7 Do 16:15 H19
Controllable coupling of two charge qubits — •C. Bruder 1 and
D.V. Averin 2 — 1 Department of Physics and Astronomy, University of
Basel, Klingelbergstrasse 82, CH-4056 Basel — 2 Department of Physics
and Astronomy, University of Stony Brook, SUNY, Stony Brook, NY
11794-3800
We propose and investigate a novel method for the controlled coupling
of two Josephson charge qubits by means of a variable electrostatic transformer.
The value of the coupling capacitance is given by the discretized
curvature of the lowest energy band of a Josephson junction, which can
be positive, negative, or zero. We calculate the charging diagram of the
two-qubit system that reflects the transition from positive to negative
through vanishing coupling. We also discuss how to implement a phase
gate making use of the controllable coupling.
[1] D.V. Averin and C. Bruder, Phys. Rev. Lett. 91, 057003 (2003).
TT 29.8 Do 16:30 H19
Enhancing the Gate Performance of Superconducting Qubits
by DFS Encoding — •Markus J. Storcz 1 , Frank K. Wilhelm 1 ,
Jiri Vala 2 , Kenneth R. Brown 2 , Julia Kempe 2 , and K. Birgitta
Whaley 2 — 1 Sektion Physik und CeNS, Ludwig-Maximilians-
Universität, Theresienstr. 37, 80333 München — 2 Department of Chemistry
and Pitzer Center for Theoretical Chemistry, University of California,
Berkeley, California 94720
Solid state qubits such as superconducting qubits are potentially extremely
scalable. However, strong decoherence may be caused or transferred
to the qubits by various elements of the circuit that couples individual
qubits, in particular when couplings over long physical distance
are implemented. We propose encoding into a subspace protected against
collective phase errors for a chain of superconducting qubits that are
coupled to nearest neighbors with ˆσ (i)
x ˆσ (i+1)
x + ˆσ (i)
y ˆσ (i+1)
y -type of interaction.
We show by numerical simulation of the encoded operations for an
ohmic environment that, even if the mathematical preconditions for full
DFS-protection are not fully satisfied, the encoding results in significant
improvement of the gate performance on the logical qubits at low temperatures.
Moreover, for a super-ohmic environment perfect protection
and gate performance of the logical qubits can be achieved.