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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.

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