Berlin 2008 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 9: Transport: Quantum Coherence and Quantum Information Systems
TT 9.3: Hauptvortrag
Montag, 25. Februar 2008, 14:30–15:00, H 2053
Coherent Oscillations in Josephson Phase Qubits — •Jürgen Lisenfeld, Alexander Lukashenko, and Alexey V. Ustinov — Physikalisches Institut III, Universität Erlangen, D-91058 Erlangen
A phase qubit uses as its logical states discrete energy eigenstates of the Josephson phase of a current-biased tunnel junction. By embedding the junction in a superconducting loop, the qubit can be controlled by a magnetic bias flux, while its state is then read out by measuring the magnetic moment of the loop using a dc-SQUID. A maximum readout contrast is hereby achieved by strong coupling to the dc-SQUID detector, which acts as a source of decoherence.
We present a new method of data evaluation to restore the full 100 % readout contrast for a weakly coupled detector. The same technique is used to compensate for a loss of contrast induced by thermal fluctuations in the dc-SQUID. We applied this method to measure the temperature dependence of coherence times in phase qubits of different origins and featuring different materials. By observing Rabi oscillations, energy relaxation and Ramsey fringes, we find that the coherent qubit response of a sample operated in the two-level limit vanishes rapidly as soon as the thermal energy kBT becomes larger than the energy level spacing. In contrast, a sample which was operated in the multi-level limit displayed semi-classical oscillations similar to Rabi oscillations, but showed qualitatively different temperature dependence. Our experimental data shed new light on the origin of decoherence in superconducting qubits and suggest that contemporary phase qubits can be operated at temperatures of up to several 100 mK.