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arXiv:cond-mat/0602464 · The European Physical Journal B

Double SQUID tunable flux qubit manipulated by fast pulses: operation requirements, dissipation and decoherence

F. Chiarello

Published Jan 1, 2007• 12 citations
arXivHTMLPDF
DOI
10.1140/epjb/e2007-00035-5

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Superconducting devices based on the Josephson effect are effectively used for the implementation of qubits and quantum gates. The manipulation of superconducting qubits is generally performed by using microwave pulses with frequencies from 5 to 15 GHz, obtaining a typical operating frequency from 100 MHz to 1 GHz. A manipulation based on simple pulses in the absence of microwaves is also possible. In our system, a magnetic flux pulse modifies the potential of a double SQUID qubit from a symmetric double well to a single deep-well condition. By using this scheme with a Nb/AlO x /Nb system, we obtained coherent oscillations with sub-nanosecond period (tunable from 50 to 200 ps), very fast with respect to other manipulating procedures, and with a coherence time up to 10 ns, of the order of that obtained with similar devices and technologies but using microwave manipulation. We introduce ultrafast manipulation, presenting experimental results, new issues related to this approach (such as the use of a compensation procedure for canceling the effect of 'slow' fluctuations) and open perspectives, such as the possible use of RSFQ logic for qubit control.
A double SQUID qubit (Superconducting Quantum Interference Device) can be handled by applying microwave trains, but also by using fast flux pulses. In this second case the manipulation is based on the fast and radical modification of the qubit potential shape that induces non-adiabatic transitions between the computational states (the two lowest energy eigenstates), still avoiding transitions to upper levels. This modality is interesting because it allows faster operations with respect to other techniques, but also because it gives access to interesting nontrivial physical features, concerning in particular decoherence and adiabaticity. About decoherence, we observed experimentally the existence of an “optimal” bias region and the transition between two distinct decoherence regimes. These results can be explained by considering the effect of first and second order slow fluctuations which dominate on high frequency noise contributions. This allows a deep insight in the qubit decoherence mechanisms.