Berlin 2008 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 9: Transport: Quantum Coherence and Quantum Information Systems

TT 9.4: Talk

Monday, February 25, 2008, 15:00–15:15, H 2053

Coherent oscillations in a superconducting flux qubit without microwave pulses — •Stefano Poletto¹, Jürgen Lisenfeld¹, Alexander Lukashenko¹, Maria Gabriella Castellano², Fabio Chiarello², Carlo Cosmelli³, Pasquale Carelli⁴, and Alexey V. Ustinov¹ — ¹Physikalisches Institut III, Universität Erlangen-Nürnberg, Germany — ²Istituto di Fotonica e Nanotecnologie del CNR, Roma, Italy — ³Dipartimento di Fisica and INFN, Universita’ di Roma La Sapienza, Italy — ⁴Universita’ degli Studi dell’Acquila, Italy

We report on observation of coherent oscillations in a superconducting flux qubit by using no microwave excitation but only nanosecond-long dc flux pulses. The investigated circuit is a double-SQUID consisting of a superconducting loop interrupted by a small dc-SQUID, which we control via two bias fluxes φ_c and φ_x. The potential energy profile of the qubit has the shape of a double well, where the flux φ_c controls the height of the barrier between the two minima and the flux φ_x changes the potential symmetry. The two computational states of the qubit are identified with the two energy minima and physically correspond to clockwise or anticlockwise circulating currents in the double-SQUID main loop. We observed coherent oscillations, in the frequency range between 8 and 20 GHz, induced by fast pulses of the control flux φ_c modulating the barrier between the two potential wells. The quantum dynamics that leads to this kind of oscillations is composed of a non-adiabatic and adiabatic evolution of the two lowest energy states.