Anomalous Switching Curves in a dc SQUID Phase Qubit

We have observed anomalous switching curves (s-curves) in a dc SQUID phase qubit. The SQUID has two Al/AlO_x/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 μm², the critical current is 0.08 μA and the junction has an added low-loss SiN_x shunting capacitor. The 1.37 μA 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.