Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 22: Transport: Poster Session

TT 22.20: Poster

Wednesday, February 27, 2008, 14:00–18:00, Poster B

Observation of coherent oscillations in a superconducting qubit with π-junction — •Jürgen Lisenfeld¹, Stefano Poletto¹, Alexey Feofanov¹, Anna N. Rossolenko², Vladimir A. Oboznov², Vitaly V. Bolginov², Alexander Lukashenko¹, Valery V. Ryazanov², and Alexey V. Ustinov¹ — ¹Physikalisches Institut III, Universität Erlangen-Nürnberg, D-91058 Erlangen, Germany — ²Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, 142432 Russia

Flux qubits consist of superconducting loops interrupted by Josephson junctions. It has been theoretically proposed that embedding a π-junction into the loop allows to realize a ’quiet’ qubit, which does not require applying any external magnetic field for biasing at the optimum operation point. When the phase drop across the π-junction with high critical current is set to π, a circulating current spontaneously emerges and stabilizes the qubit at the working point.
We incorporated a superconductor-ferromagnet-superconductor π-junction in the loop of a phase qubit made by conventional Nb technology. We observed Rabi oscillations and measured the qubit decay from the excited state. We found no difference between the decoherence times of otherwise identical qubits with and without π-junctions. The short decay of the order of several nanoseconds is limited in both cases by the decoherence sources in the on-chip dielectrics rather than by dissipation induced by the ferromagnetic π-junction phase shifter. This promising first result supports the feasibility of designing self-biased ’quiet’ flux qubits.