Superconducting qubit manipulated by fast pulses: experimental observation of distinct decoherence regimes

A particular superconducting quantum interference device (SQUID)qubit, namely the double SQUID qubit, can be manipulated by rapidly modifyingits potential with the application of fast flux pulses. In this system weobserve coherent oscillations exhibiting non-exponential decay, indicating anunconventional decoherence mechanism. Moreover, via tuning the qubit indifferent conditions (different oscillation frequencies) by changing the pulseheight, we observe a crossover between two distinct decoherence regimesand the existence of an ‘optimal’ point where the qubit is only weaklysensitive to intrinsic noise. We find that this behavior is in agreement with amodel considering the decoherence caused essentially by low-frequency noisecontributions, and we discuss the experimental results and possible issues.