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