Regensburg 2007 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 3: Quantum Coherence and Quantum Information Systems I

TT 3.2: Talk

Monday, March 26, 2007, 10:00–10:15, H20

Quasiparticle transitions in Josephson charge-phase qubits with radio frequency read-out — •Jens Könemann, Hermann Zangerle, Brigitte Mackrodt, Ralf Dolata, and Alexander Zorin — Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig

Tunneling of single quasiparticles in Josephson qubits operating on single Cooper pairs presents a serious problem. This tunneling instantly changes the working point of the qubit and, therefore, its charge state and transition frequency, which leads to decoherence. Moreover, one can expect both relaxation and excitation of the qubit induced by quasiparticle tunneling. We have investigated Al Josephson charge-phase qubits of SQUID-configuration inductively coupled to a radio-frequency tank circuit with a resonant frequency of about 77 MHz, thus enabling the readout of the state by measuring the Josephson inductance of the qubit. Depending on the flux and charge bias and the amplitude of rf-oscillations we have probed either a plain 2e-behavior in the dependence on gate charge corresponding to the ground state or a dynamic change of ground and excited states. The latter behavior is explained in terms of stochastic single quasi-particle tunneling onto and off the island of the qubit and, hence, possible transfer of energy from the quasiparticles to the qubit system. For this process we derive a selection rule which explains the observed suppression of the quasiparticle-induced transitions in the qubit operating in the magic point q=e