Berlin 2008 – wissenschaftliches Programm

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TT: Fachverband Tiefe Temperaturen

TT 22: Transport: Poster Session

TT 22.21: Poster

Mittwoch, 27. Februar 2008, 14:00–18:00, Poster B

Dispersive Readout of Josephson Phase Qubits — •Tobias Wirth¹, Jürgen Lisenfeld¹, Alexander Lukashenko¹, Alexander Zhuravel², and Alexey V. Ustinov¹ — ¹Physikalisches Institut III, Universität Erlangen-Nürnberg, Erlangen, Germany — ²B. Verkin Institute for Low Temperature Physics & Engineering, National Academy of Sciences of Ukraine, Kharkov, Ukraine

Solid-state quantum bits based on current-biased Josephson junctions require appropriate isolation from the bias leads which can be achieved by using superconducting transformers. A superconducting loop with a tunnel junction allows to prepare an asymmetric double-well potential, where the discrete quantum levels in the shallow well form the qubit states. State-dependent tunneling to the deeper well changes the magnetic flux in the qubit, which is measured by a dc-SQUID. In our ongoing experiments, we use a high-frequency readout based on a non-dissipative measurement of the dc-SQUID Josephson inductance, which in turn depends on the qubit flux. The measurement is done by detecting either the resonance shift of the LC-circuit, which consists of the dc-SQUID with a shunt capacitor, or the phase change of the reflected probe signal. We observed the periodic dependence of the reflected microwave amplitude and phase on externally applied magnetic flux. By using a low temperature scanning laser microscope, we map the microwave current distribution in our circuit. Current experiments are focused on a pulsed rf-readout and the use of an IQ-mixer to detect the phase change of the reflected signal.