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Journal of Superconductivity and Novel Magnetism

Experiments on a Superconducting Qubit Manipulated by Fast Flux Pulses: The Issue of Non-adiabaticity

Maria Gabriella Castellano, F. Chiarello, G. Torrioli

Published Sep 23, 2010• 2 citations
Publisher
DOI
10.1007/s10948-010-0858-x

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A particular superconducting quantum interference device (SQUID) qubit, namely the double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses.In this system we observe coherent oscillations exhibiting non-exponential decay, indicating an unconventional decoherence mechanism.Moreover, via tuning the qubit in different conditions (different oscillation frequencies) by changing the pulse height, we observe a crossover between two distinct decoherence regimes and the existence of an 'optimal' point where the qubit is only weakly sensitive to intrinsic noise.We find that this behavior is in agreement with a model considering the decoherence caused essentially by low-frequency noise contributions, and we discuss the experimental results and possible issues.
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.