Anomalous Switching Curves in a dc SQUID Phase Qubit
H. Dharma Kwon, A. J. Przybysz, Benjamin K. Cooper, Hanhee Paik, Kevin D. Osborn, B. S. Palmer, Rangga P. Budoyo, J. R. Anderson, Christopher J. Lobb, F. C. Wellstood · IEEE Transactions on Applied Superconductivity · 2010
We have observed anomalous switching curves (s-curves) in a dc SQUID phase qubit. The SQUID has two${\rm Al/AlO}_{\rm x}/{\rm Al}$Josephson junctions in a 1.5 nH loop. One junction as a phase qubit and the other acts as a detector junction. The qubit junction area is 2$\mu{\rm m}^{2}$, the critical current is 0.08$\mu{\rm A}$and the junction has an added low-loss${\rm SiN}_{\rm x}$shunting capacitor. The 1.37$\mu{\rm A}$detector junction is isolated from the bias leads by an on-chip inductor-capacitor (LC) network with a 130 MHz cut-off. We measure the state of the system at 20 mK by sending a short current pulse to the device and monitoring whether it switches to the voltage state. The s-curves for the ground state (0) and excited state (1) look much as expected. However, unlike previous results on phase qubits, when the device is pumped to a superposition state, the resulting s-curve is not a weighted sum of the s-curves for the 0 and 1 state. Instead, the s-curve appears to shift to lower pulse current with increasing amplitude to be in the 1 state. We examine this behavior and discuss implications for measurement of quantum superposition states in this system.